No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Abstract
The effect of molding parameters on material distribution and mechanical properties of co-injection molded plates has been studied using experimental design. The plates were molded with a polyamide 6 (PA 6) as skin and a 20% glass fiber-reinforced polybutyleneterephtalate (PBTP) as core. Five molding parameters—injection velocity, mold temperature, skin and core temperature, and core content—were varied in two levels. The statistical analysis of the results showed that three parameters—Injection velocity, core temperature, and core content—were the most significant in affecting skin/core distribution. A high core temperature was the most significant variable promoting a constant core thickness, while core content was the most significant factor influencing a breakthrough of the core. Mechanical properties, such as flexural and impact strength showed a high correlation with the skin/core distribution. The slight increase in falling weight impact strength of the sandwich molded plates, compared to similar plates molded from PBTP only, could be explained from the failure process, which initiates in the brittle core and propagates through the ductile skins.
... Specifically, one of the major lightweight technologies is utilizing fiber-reinforced especially for the breakthrough phenomena, and how to quantify that core material advancement has still not been completed. Moreover, although the skin to core interface variation has been studied by many scholars [14][15][16][17][18][19][20][25][26][27][28], the mechanism of the skin material breakthrough has not been fully understood yet. For example, Watanabe et al. [17] has proposed a more comprehensive description about skin material breakthrough. ...
... Specifically, when the skin breakthrough phenomena happens and the associated further influence on the co-injected product are very difficult to identify. One of the most significant factors to skin breakthrough is the skin to core ratio [16][17][18][19][20][25][26][27][28][29][30][31][32]. To study the skin tocore ratio effect on the location of the skin breakthrough, the 30SFPP/30SFPP material arrangement is first selected. ...
... This special "Core-Skin-Core" (CSC) structure is not the same as that the regular skin-core structure before breakthrough, or that of the core structure after breakthrough happens. In fact, from the previous literature [14][15][16][17][18][19][20][25][26][27][28][29][30][31][32], this kind of CSC structure has never been mentioned and discussed. ...
One of the main challenges in co-injection molding is how to predict the skin to core morphology accurately and then manage it properly, especially after skin material has been broken through. In this study, the formation of the Core-Skin-Core (CSC) structure and its physical mechanism in a two-stage co-injection molding has been studied based on the ASTM D638 TYPE V system by using both numerical simulation and experimental observation. Results showed that when the skin to core ratio is selected properly (say 30/70), the CSC structure can be observed clearly at central location for 30SFPP/30SFPP system. When the skin to core ratio and operation conditions are fixed, regardless of material arrangement (including 30SFPP/30SFPP; PP/PP; 30SFPP/PP; and PP/30SFPP systems), the morphologies of the CSC structures are very close for all systems. This CSC structure can be further validated by using μ-CT scan and image analysis technologies perfectly. Furthermore, the influences of various operation parameters on the CSC structure variation have been investigated. Results exhibited that the CSC structure does not change significantly irrespective of the flow rate changing, melt temperature varying, or even mold temperature being modified. Moreover, the mechanism to generate the CSC structure can be derived using the melt front movement of the numerical simulation. It is worth noting that after the skin material was broken through, the core material travelled ahead with fountain flow to occupy the flow front. In the same period, the proper amount of skin material with certain inertia of enough kinetic energy will keep going to penetrate the new coming core material to travel until the end of filling. It ends up with this special CSC structure.
... Recycled plastics, however, often suffer from deteriorated properties [17]. Putting recycled content into the core layer of sandwich-structured multi-layer products is a seemingly easy way to satisfy demands for recyclate utilization without compromising on product aesthetics and other surfacerelated performance characteristics [18][19][20][21][22][23]. ...
... The higher complexity of co-injection compared to conventional injection molding confers a number of consequences for the performance of the manufactured part. Seldén [20] analyzed the influence of different molding parameters including skin and core temperature as well as core content during co-injection of polyamide 6 and polybutylene terephthalate. While a strong correlation of mechanical properties such as flexural and impact strength with the skin-core distribution was observed, the prime determinant for a constant core thickness was found to be the core temperature [20]. ...
... Seldén [20] analyzed the influence of different molding parameters including skin and core temperature as well as core content during co-injection of polyamide 6 and polybutylene terephthalate. While a strong correlation of mechanical properties such as flexural and impact strength with the skin-core distribution was observed, the prime determinant for a constant core thickness was found to be the core temperature [20]. Different combinations of PP and short glass fiber reinforced polypropylene (PP-GF) in coinjection molding were investigated by Messaoud and co-workers [21]. ...
In pursuit of a circular economy of plastics, there is a need to use more recycled plastics for new products. Polypropylene (PP) constitutes a major fraction of post-consumer plastic wastes, and mechanical recycling is currently the most sustainable recovery strategy. Sandwich-structured multi-layer products with recyclate cores are a seemingly easy way to satisfy demands for recyclate utilization without compromising on product aesthetics. We present the case of a reusable plastic transport box with a recycled content of 45 wt% manufactured by a co-injection molding process. The box was characterized by spectroscopic and thermo-analytical methods. Mechanical performance was tested on both specimen and product levels. A comparison was made to transport boxes fabricated entirely from virgin or entirely from recycled PP, respectively. A number of contaminants including foreign polymers were identified within the recyclate core layer of the sandwich-structured material. While these contaminants had no deteriorative effect on stiffness-controlled performance, a strong influence on strength-controlled and impact-related properties was observed. We argue that the presence of inclusions of both polymeric and inorganic nature is an intrinsic quality characteristic of post-consumer recyclates. These need to be considered in any design-from-recycling philosophy to guarantee functionality, reliability, and safety of products with recycled content.
... Considering the innate advantages of SCIM, several efforts have been made to investigate this process [1][2][3][4][5][6][7][8][9][10][11]. These efforts can be roughly classified into three categories: (1) the material distribution of co-injection molded parts and its influence on mechanical properties by studying experimental methods [1][2][3], (2) theoretical modeling leading to the development of a computer code simulating the realistic development of the interface and material distribution during the molding process [4][5][6], and (3) evolution of the internal structure for a special material [7][8][9][10][11]. ...
... Considering the innate advantages of SCIM, several efforts have been made to investigate this process [1][2][3][4][5][6][7][8][9][10][11]. These efforts can be roughly classified into three categories: (1) the material distribution of co-injection molded parts and its influence on mechanical properties by studying experimental methods [1][2][3], (2) theoretical modeling leading to the development of a computer code simulating the realistic development of the interface and material distribution during the molding process [4][5][6], and (3) evolution of the internal structure for a special material [7][8][9][10][11]. From the point of view of the macro-scale experiment, Young [1] studied the effects of viscosity ratio on the material distribution, and it was shown that the uniform thickness of skin occurred when the zero shear viscosities were approximately equal or slightly higher than in the core. ...
... Considering the innate advantages of SCIM, several efforts have been made to investigate this process [1][2][3][4][5][6][7][8][9][10][11]. These efforts can be roughly classified into three categories: (1) the material distribution of co-injection molded parts and its influence on mechanical properties by studying experimental methods [1][2][3], (2) theoretical modeling leading to the development of a computer code simulating the realistic development of the interface and material distribution during the molding process [4][5][6], and (3) evolution of the internal structure for a special material [7][8][9][10][11]. From the point of view of the macro-scale experiment, Young [1] studied the effects of viscosity ratio on the material distribution, and it was shown that the uniform thickness of skin occurred when the zero shear viscosities were approximately equal or slightly higher than in the core. ...
In this study, isotacitc polypropylene (iPP) samples were prepared by conventional injection molding (CIM) and sequential co-injection molding (SCIM), in which two kinds of polymer melt were injected into the mold cavity one after the other. The morphological structure of the samples was investigated by polarized light microscopy (PLM) and scanning electron microscopy (SEM). Results show that the structures of the samples prepared by CIM have a typical skin-core structure. This structure could be divided into three layers along the thickness direction of the samples: skin layer, transition region and core layer. However, the morphologies of the samples prepared by SCIM have a fascinating supermolecular structure that can not be roughly divided into three layers. A region of cylindritic structures, which is rare in CIM, is formed between the skin layer and the core layer of the second injected material. In particular, the cylindritic structures are more easily found when the melt temperature is relatively lower and the delay time is longer. The results were further interpreted based on the analysis and comparison of the thermo-mechanical history imposed on the melt during the CIM and SCIM processes.
... Along with the understanding and prediction of the melt flow behaviour in co-injection moulding, the selection of processing conditions is crucial to fully exploit the benefits of this technology: this activity is usually done in a trial and error fashion which is time and money consuming, even when computer simulations support or replace experiments. In literature, Seldén [19] suggests the use of the Design of Experiment (DOE) [20] statistical approach in the experimental moulding of simple co-injected plates in order to single out the most significant parameters affecting the core=skin distribution in these parts. This approach allows to obtain (statistically) significant information on the effect of several parameters on a specific process with a limited number of experiments: therefore it is very interesting to apply to complex and expensive technological processes, such as co-injection moulding, in which many factors have to be investigated and the testing costs are high. ...
... Another important parameter is the core temperature (CT) which displays a positive effect. This is due to the lowering of the core=skin viscosity ratio, as already observed by Seldén [19] . ...
... The effects of processing parameters on this quantity are reported in Figure 8, which shows that only the core material volumetric fraction (CC) and its injection rate (CIR) have a significant effect. Interestingly, the two parameters have opposite effects, as already noticed by Seldén [19] . As the core injection rate is increased, the skin material has less time to freeze on the cavity walls and is carried towards the end of the cavity by the core material. ...
In this work the effect of processing parameters on core breakthrough and material distribution during co-injection in a model mould has been studied. PMMA was used for the skin and ABS for the core. To identify the influences of the core volumetric fraction, its injection rate and temperature and the skin injection temperature a series of computer simulations and experimental tests were performed according to the design of experiments statistical approach. The results of statistical analysis performed on data coming from experiments and simulations are consistent, even if simulations overestimate the ABS content at which the breakthrough takes place.
... It provides foresight about the parameters such as filling time, filling temperature, injection and holding pressures, temperature after cooling, and production defect generations such as the air bubbles, sink marks, and weld lines. This foresight makes it possible to optimize the mold design before starting mold manufacturing [29][30][31][32]. ...
The classical plastic injection method is based on the principle of injecting a single color of a single polymeric material into the mold cavity under high pressure. In cases where the products are expected to have contrasted functional features and different colors, the classic injection process and the conventional injection molds are not sufficient. This paper proposes a new design approach for multi-component injection molds required by products containing different polymeric materials or different colors of the same polymeric material at the same time. It also presents a case study including the design of the hot runner, electromechanical rotary-cross, cooling, and ejection systems of a two-component, eight-cavity toothbrush mold. The polymeric materials are polypropylene for the first component, and styrene based thermoplastic elastomer for the second component, which exhibit good bonding properties with each other. In addition, an analysis study covering the filling parameters and production defect generations is also provided. Compared to existing methods, the results show that the proposed multi-component injection mold design method eliminates the need for particular injection machines and robotic systems, shortens the cycle time, and reduces energy consumption.
... To deal with this complicated process, there is some literature that can be used as guidelines. Seldén [8] monitored different process conditions and found out the skin-to-core material ratio is the main factor causing break-through problems. Moreover, the correlation between internal material distributions, process condition, and material property are discussed in many previous studies [9][10][11]. ...
In recent years, due to the rapid development of industrial lightweight technology, composite materials based on fiber reinforced plastics (FRP) have been widely used in the industry. However, the environmental impact of the FRPs is higher each year. To overcome this impact, co-injection molding could be one of the good solutions. But how to make the suitable control on the skin/core ratio and how to manage the glass fiber orientation features are still significant challenges. In this study, we have applied both computer-aided engineering (CAE) simulation and experimental methods to investigate the fiber feature in a co-injection system. Specifically, the fiber orientation distributions and their influence on the tensile properties for the single-shot and co-injection molding have been discovered. Results show that based on the 60:40 of skin/core ratio and same materials, the tensile properties of the co-injection system, including tensile stress and modulus, are a little weaker than that of the single-shot system. This is due to the overall fiber orientation tensor at flow direction (A11) of the co-injection system being lower than that of the single-shot system. Moreover, to discover and verify the influence of the fiber orientation features, the fiber orientation distributions (FOD) of both the co-injection and single-shot systems have been observed using micro-computerized tomography (μ-CT) technology to scan the internal structures. The scanned images were further utilizing Avizo software to perform image analyses to rebuild the fiber structure. Specifically, the fiber orientation tensor at flow direction (A11) of the co-injection system is about 89% of that of the single-shot system in the testing conditions. This is because the co-injection part has lower tensile properties. Furthermore, the difference of the fiber orientation tensor at flow direction (A11) between the co-injection and the single-shot systems is further verified based on the fiber morphology of the μ-CT scanned image. The observed result is consistent with that of the FOD estimation using μ-CT scan plus image analysis.
... The first polymer injected will begin to cool and solidify on the mould surface while the inner will remain molten. Then, the second polymer is injected; forcing most of the first polymer to the mould surface [41]. Turng and Kharbas [42] have shown that co-injection moulding technology can be used in combination with MIM; where the first polymer is not gas infused. ...
Injection moulding (IM) is a well-established manufacturing process for cost-effective replication of polymer-based components. Advanced IM processes have been developed; modifying the final polymer part produced to create microcellular structures. Through the creation of microcellular materials, not only can the final weight be reduced but also, the near net shape can be improved. Microcellular Injection Moulding (MIM) shows high potential to improve near net shape of polymer parts and, the green aspect of polymer manufacturing platforms. This chapter aims to present the significant developments that have been achieved to enhance the MIM technology. Aspects covered include co-injection moulding, core back processing, gas counter pressure, vario-thermal moulding and mould coating. The resulting characteristics of each enhancing technique is presented.
... The thickness and homogeneity of the skin feedstock play a critical role in the properties and cost of the final part. From previous research on polymer co-injection moulding, it is known that the morphology of the interface is influenced by the incompatibility of the injection parameters of the two feedstock, which requires an overall study involving several moulding factors such as Powder Technology 305 (2017) 405-410 viscosity ratio, injection speed, injection timing of the core material, skin/core volume ratio and mould temperature [10][11][12]. Up to now, research on PCM is limited, and most of it focuses on the co-sintering behaviour and properties of the final product [13][14][15][16][17][18]. ...
Powder co-injection moulding (PCM) was carried out by using two feedstocks, 316L(60%) and 316L(40%), as core and skin feedstocks, respectively. The effects of processing parameters such as the pre-filling volume of skin feed-stock, the injection temperature of core feedstock and the injection rate of skin feedstock on the profiles of the two feedstock layers were studied. It was found that the interface between core and skin feedstocks exhibited an arched shape in the transverse plane and a V shape in the longitudinal plane. There is a " maximum thickness " of core feedstock in the longitudinal direction. The penetration length and maximum thickness depended on the injection parameters. The formation mechanism of the interface was studied on the basis of rheological theory. This research provided an experimental base for the simulation work of PCM processes.
... For the same reasons, numerical simulations are also critical for PCM, whose process is more sophisticated due to the application of two feedstocks with individual flowabilities, mechanical properties, thermal properties, etc. A simulation process prior to experimental use can further reduce costs, because the PCM facilities and its operation costs are higher than the traditional PIM [4,5]. Compared to the simulation process of PIM, research on the simulation of PCM process is still in the early stages. ...
... Unlike the sequential over-moulding which has a distinct interface, skin/core interfacial flow front of co-injection moulding cannot be controlled with ease. Past attempts have been made to associate material distribution with important factors of processing conditions and material properties (Garner and Oxley, 1971;Seldén, 2000;Love and Goodship, 2002;Watanabe et al., 2003;Ilinca et al., 2006;Gomes et al., 2011;Messaoud et al., 2002). In summary, the dynamic behaviour between the skin and the core materials will influence the quality of products significantly. ...
Multiple component moulding (MCM) is one of the great methods to fabricate the modern injection products. Due to many procedure combinations, it is very difficult to know the detailed process. Besides, because of its complicated nature and the unclear physical mechanism, using trial-and-error method cannot realise and manage the mechanism effectively. In this study, we will review various MCM technologies, including over-moulding, and co-injection moulding, and study these two applications in more details. Results show that the product geometries, process condition, and moulded materials will affect the product quality. The physical mechanism of warpage in a sequential over-moulded part is due to the unbalance between the volumetric shrinkage during filling/packing and the thermal unbalance from cooling. The warpage behaviour for co-injection moulding is more complicated because of the uncertain interface. However, one of the keys to manage the warpage is the control of the core penetration.
... To carry out the measurements of the skin and core thickness for each sample, specimens were cut at the mid-point (see Fig 3.16) and a milling machine was then used to smooth the cross section ( Fig. 3.17). An optical microscope, the wilder toolmakers microscope R.S. wilder INC, was used to do the measurements (see In order to characterize the material distribution which is influenced by a number of factors, such as the amount of core, viscosities of skin and core, injection velocity and melt temperature, we used the penetration and cross sectional area ratio of the core layer (synonymous to core volume ratio) as shown in Figure 3.19 [11,12]. The penetration of core material into the skin material is called "P" as shown in Figure 3.16 and penetration ratio is 100 × L P ...
... Polymers can be used to act as protective coating of many different substrates like on metals to prevent corrosion which has been a subject of great research effort. On the contrary, only little attention has been given to the effect of the processing parameters on the final performance of bi-layered structures [1,2]. The union between two polymers can be achieved by many different methods , but the different techniques derived from injection moulding, like over moulding, offer significant advantages either in cost or in the properties of the final article [3] . ...
The strength of adhesion between two over-moulded polymers, methylmethacrylate–butadiene–styrene copolymer (MABS) and thermoplastic polyurethane (TPU) that constitute a bi-component laminar system has been the subject of study. Results showed that at the bi-layer interface the adhesive fracture tough-ness increases as temperature and surface roughness are increased. Roughness has been demonstrated to be the most important parameter defining adhesion strength and failure. With the increase of roughness the failure was observed to change from adhesive towards cohesive type. An increase in either the temperature or the pressure applied to the samples caused a rise in adhesion energy. However, pressure seemed to have a minor effect in comparison to temperature. Significant increments in adhesion were obtained after applying the corona discharge treatment (CDT) to the attaching surfaces resulting in adhesion strengths almost double those of non-treated systems.
In this paper, poly(styrene-ethylene-butylene-styrene) (SEBS) triblock copolymer blends were used as the soft skin material and polypropylene (PP) was used as the hard core for co-injection molding. The effect of different SEBS thermoplastic elastomer skin materials on the filling state of the core layer of the co-injection sample was analyzed, and the co-injection molding was evaluated by the filling factor and shape distribution factor. When the core material flows at the matched skin interface, the core layer gradually changes from an uneven finger-like flow front to a smooth flow, and the shape distribution factor increases greatly. Both simulation and experimental results show that the shape distribution factor can correctly evaluate the filling effect of co-injection, and the corresponding samples have uniform skin-core interface, good compatibility and maximum skin thickness. According to the change of shearing rate, a dynamic viscosity ratio method was established to evaluate the co-injection process. When the dynamic viscosity ratio of the core/skin material is greater than and close to 1, the movement of the molecular chains on the interface matches, the flow front is smooth and the shape distribution factor is high. By changing the injection temperature of the material to adjust the dynamic viscosity ratio, the optimization of co-injection can also be achieved. The dynamic viscosity ratio method can be used to guide the flow forming and interface interaction analysis of heterogeneous materials.
Water-assisted co-injection molding (WACIM) is a complex and creative injection molding process. A three-dimensional model for WACIM was setup and a k – ω turbulence model was adopted to deal with the turbulent flow of the water. The volume of fluid (VOF) method was used to track the moving interfaces of the skin–inner melt and the inner melt–water. Numerical simulations for the filling stage of WACIM parts with four types of cross-sections were carried out with the computational fluid dynamics (CFD) method. Experiments were conducted to verify the simulation results. The results of the experiments were in agreement with those of the simulations. The cross-section shape of the cavity had a significant effect on the penetrating area of the inner melt, while the shape of the water permeation eventually becomes round. The penetration area of the water increased in the flow direction, and the residual wall thicknesses of the inner melt downstream was thinner than the melt upstream.
In co-injection molding, the properties and distribution of polymers will affect the application of products. The focus of this work is to investigate the effect of molding parameters on the skin/core material distribution based on three-dimensional (3-D) flow and heat transfer model for the sequential co-injection molding process, and the flow behaviors and material distributions of skin and core melts inside a slightly complex cavity (dog-bone shaped cavity) are predicted numerically. The governing equations of fluids in mold are solved by finite volume method and Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm on collocated meshes, and the domain extension technique is employed in numerical method for this cavity to assure that the numerical algorithm is implemented successfully. The level set transport equation which is used to trace the free surfaces in co-injection molding is discretized and solved by the 5th-order Weighted Essentially Non-Oscillatory (WENO) scheme in space and 3rd-order Total Variation Diminishing Runger-Kutta (TVD-R-K) scheme in time respectively. Numerical simulations are conducted under various volume fraction of core melt, skin and core melt temperatures, skin and core melt flow rates. The predicted results of material distribution in length, width and thickness directions are in close agreement with the experimental results, which indicate that volume fraction of core melt, core melt temperature and core melt flow rate are principal factors that have a significant influence on material distribution. Numerical results demonstrate the effectiveness of the 3-D model and the corresponding numerical methods in this work, which can be used to predict the melt flow behaviors and material distribution in the process of sequential co-injection molding.
The main objective of this chapter is to increase the existing knowledge in Incremental Sheet Forming (ISF), as a near net shape medical manufacturing process specifically for obtaining polymer of prostheses-parts, evaluating and defining the process parameters involved to improve the technology based on the analysis of quantitative outputs. This should help to provide process guidelines useful for manufacturing complex and customized parts, to be applied for example in the biomedical field. The chapter is divided into two main blocks: (i) the study of the influence of the process parameters on basic polymeric geometries manufactured by SPIF, and (ii) an analysis of some case studies of cranial implants manufactured by ISF using non-biocompatible and biocompatible polymers.
One of the drawbacks of microcellular injection molded parts is lower part tensile strength and stiffness than solid parts. This is caused by a reduction in the effective cross-section area as the microcellular structure is generated inside the part. This study investigated how microcellular co-injection molding can add a solid skin layer, encapsulating the foamed core layer, to increase both part strength and stiffness. In addition to PP, PP-GF (10-wt% GF) was used for the reinforcing effect. The experiment used constant injection parameters and varied material combinations, and conventional, MuCell, and co-injection molded parts acted as comparators. The weight reduction was measured to ensure successful microcellular structure generation. The results show that microcellular co-injection molded PP/PP-GF (skin/core) is the optimal combination, reducing weight by 4.2% over co-injection PP/PP-GF, improving yield strength by 18.2% and Young’s modulus by 2.5% over MuCell PP-GF, yet with a brittle strain at break of 0.084 mm/mm.
Injection moulding technologies are constantly evolving to meet changing customer requirements, improved cost effectiveness and to meet new environmental legislation. One such technology is in-mould decoration (IMD). This technique is attractive due to increased customer desire for multi-functional and decorative products and includes any process in which a fully, or partially, decorated component is produced directly from the moulding process.
A new patented IMD process has been developed to produce a painted component direct from the injection moulding tool. It uses thermoset powder coatings which are sprayed under pressure through a valve into a closed mould using a novel feed system. The heat of the tool softens the coatings which are then cured by the injection of thermoplastic polymer into a standard injection moulding system. In this first paper initial design features and considerations as well as the process itself are discussed.
With the growing use of co-injection molding process, it becomes essential to understand skin/core material distribution in the cavity for ensuring the quality of final product. In this study, warpage issue will be examined numerically and systematically by process effect and material combination in sequential co-injection development. Specifically, decreasing melt temperature or slowing down flow rate of 1st shot, which makes frozen layer thicker, can improve warpage problem. When the frozen layer is thicker, the core layer is more difficult to penetrate through the thickness direction. Thus, the melt is squeezed to advance in the flow direction, that forces melt go farther and better packing effect can be implemented accordingly. However, warpage becomes worse due to two used different materials with distinct properties mismatching while processing co-injection. To validate our simulation investigation, the experimental study will be performed in the near coming future. Copyright © (2014) by the Society of Plastics Engineers All rights reserved.
Based on filling simulation in co-injection molding and design of experiment (DOE) technology, Moldflow software was used to analyze the core thickness fraction distribution and the penetration length of the core melt. Injection flow rate, mold temperature, skin temperature, core temperature and volume skin were regarded as process parameters. The fractional factorial design method was introduced to investigate the significance of process parameters, and the effect of each factor on the quality response of mold part was also studied. Co-injection molding simulation analyses show that volume skin has the great impact on core thickness fraction distribution and the penetration length of the core melt, while the influences of mold temperature and melt temperature are comparatively inappreciable.
With the growing use of co-injection molding process, an understanding of skin/core material distribution in the cavity is essential. We presented numerical simulations concerning factors including filling ratio, material viscosity, and injection rate. Both core penetration behaviors and skin ratio uniformity from the material distribution profiles were compared. In addition to material distribution predictions, numerical simulation also revealed breakthrough points and cornering-effect locations. This is helpful for the part manufacturer to enhance the benefits of the co-injection process through the computer-aided simulation.
In the co-injection molding process, sometimes referred to as sandwich molding, two different polymer melts are either simultaneously or sequentially injected into a mold to form a part with a skin/core structure. Co-injection molding offers the flexibility of using the best properties of each material to reduce material cost and part weight. Particularly, it allows the use of recycled material in the core without an adverse effect on surface quality. The properties of a co-injection molded product depend on the individual properties of the skin and core layers, and the skin/core volume ratio. This paper presents a study of the effect of molding parameters on material distribution and mechanical properties of co-injection molded plates. Two virgin materials were tried: polypropylene (PP), and thermoplastic polyolefin (TPO) as well as ground TPO from plastic bumpers.
An experimental study of co-injection molding which involves sequential injection of dissimilar metal feedstocks into a mold has been carried out. The effect of delay time on the interface morphology of the co-injection molded specimens has been studied. It was found that, as the delay time is gradually increased, the extent of penetration of the core melt parallel to the flow direction becomes large, while the skin thickness uniformity deteriorated. The maximum core content can be achieved was 44. 6 vol. %. The results were analyzed by taking account of the rheological properties of the two feedstocks. Temperature of the skin feedstock encountered in experiments was calculated. It was demonstrated that the differences in rheological properties of the metal feedstocks involved are the primary variable determining the interface morphology of the molded parts.
Sequential co-injection moulding (SCIM) is a promising technique in industrial production. Because of the complex hydrodynamic effect of air on polymer melts in a moulding process, it is difficult to accurately design and control the process. Corresponding theoretical investigations are very limited, especially for the transient free surface flows in the filling stage. In this paper, a three-dimensional (3-D) unified model is proposed for simulating fluids flow in SCIM. The melted polymer and air in the cavity can be regarded as a continuous fluid. The evolution of the melt front interface and skin/core melt interface are captured simultaneously at any moment by the level set method; this method has been widely-used in two-phase flow for its ability to track interfaces. The finite volume method is combined with a domain extension technique to deal with 3-D flows in an irregular cavity. The model is validated by simulating the SCIM process for a cavity with a block insert, and a distinct corner effect is observed. For a widely-used plastic toothbrush made by SCIM, the evolutions of transient free surfaces in the filling stage are investigated. All numerical results are consistent with corresponding experimental results, and demonstrate the capability of the model with the domain extension technique to simulate multiphase flows in SCIM.
The fluid-assisted co-injection molding (FACIM) process can be used to produce hollow plastic products with outer and inner layers. It can be divided into two categories: water-assisted co-injection molding (WACIM) and gas-assisted co-injection molding (GACIM). An experimental study of penetration interfaces in overflow FACIM was carried out based on a lab-developed FACIM system. High-density polyethylene and polypropylene were used as the outer layer and inner layer plastics, respectively, in the experiments and the injection sequence was reversible. Six cross-section cavities were investigated in the experiments. The penetration behaviors of water and gas in different sequences and cavities were compared and analyzed. The penetration interfaces were characterized by the residual wall thickness (RWT). The experimental results showed that the RWT of the inner layer in WACIM fluctuated along the flow direction, while that in GACIM was more even. The difference of viscosity between the outer and inner layer melts affected the stability of the interface between them. The penetration sections of the inner layer and the gas were closer to the cavity sections in GACIM, while the penetration sections of the inner layer and the water were closer to the circular forms in WACIM.
The morphology of polystyrene (PS)/polyethylene (PE) molded by multi-melt multi-injection molding (MMMIM) was investigated using scanning electronic microscopy (SEM). The results show that an interesting double skin-core structure is formed in the parts molded by MMMIM. No matter in the skin layer or the core layer, there exist significant differences between the near gate ends and the far gate ends: (a) amount of fibers formed in the near gate ends, (b) few ellipsoidal and rodlike structures existing in the far gate ends. The morphology study of injection-molded PE/PS blends based on scanning electron microscope (SEM) explores the penetrating effect of the second melt on the formation and distribution of the morphology in the molded parts.
The residual wall thicknesses (RWT) of the skin and the inner layers are important quality indicators of water-assisted co-injection molding (WACIM) parts. The influences of the shape of the cavity cross section and the processing parameters, including the water pressure, water delay time, inner melt temperature, and inner melt flow rate, on the penetration of the inner melt and water were explored via experiments. The results showed that the shape of the penetration section of the inner melt was closer to the cavity section with round corners, while that of the water ended up being round. Both the penetration ratios of the inner melt and the water increased proportionally with increasing circle ratio. Both the minimum values of the total RWT and the inner melt RWT increased with increasing circle ratio. Both the maximum values of the total RWT and the inner melt RWT increased with increasing Max_D, which is the maximum distance between the inscribed circle center and the wall. Both the penetration ratios of the inner melt and the water increased with increasing water pressure, decreased with increasing water delay time, and increased with increasing inner melt flow rate and increasing inner melt temperature. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42866.
The residual thicknesses of the skin and the inner layers are important quality indicators of water-assisted co-injection molding (WACIM) process or overflow WACIM (O-WACIM) parts. At the curved section, the residual thicknesses change significantly. A numerical simulation program based on the computational fluid dynamics method was developed to simulate the O-WACIM process. After the numerical simulation program was validated with the experimental results, it was used to study the effects of the bending radii and bending angles on the residual thicknesses of the skin and inner layers of O-WACIM parts. The results showed that the penetration of the inner melt and water was always close to the inner concave side due to the higher local pressure gradient and temperature. The effects of processing parameters on the residual thicknesses of the skin and inner layers were investigated using the orthogonal simulation method. It was found that the residual thicknesses of the skin/inner layer at the inner concave/outer convex side are mainly influenced by different parameters. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42468.
Co-injection molding (CIM) is an advanced technology which was developed to meet quality requirements and to reduce the material cost. Theoretical investigations concerning it are very limited, especially for simultaneous CIM. The interactions of air, skin and core polymer melt in the process are very complex, which makes it more challenging to simulate free surface flows in the mold. Thus, this article presents a mathematical model for it. The extended Pom-Pom (XPP) model is selected to predict the viscoelastic behavior of polymer melt. The free surface is captured by the level set method. The article vividly shows the simultaneous CIM process by means of a visual numerical simulation technique. Both two-dimensional (2D) and 3D examples are presented to validate the model and illustrate its capabilities. The 3D flow behaviors of simultaneous CIM process are hard to predict numerically. To our knowledge, this is the first attempt at simulating melt flow behaviors in 3D simultaneous CIM based on the XPP constitutive equation and visual technique. The numerical results are in good agreement with the available experiment results, which establish the capability of the multiphase flow model presented in this article to simulate the flow behaviors of polymer melt in simultaneous CIM process.
In the present study, the effect of injection temperature, velocity and delay time on the interface morphology of the co-injection molded plates was studied. The results showed that the core penetration parallel to the flow direction becomes less as the skin injection velocity and temperature increases and delay time decreases. Among the parameters, temperature was the most significant in affecting the interface morphology, followed by delay time, while injection velocity seemed to play no significant role. The results were analyzed by taking account of rheological properties of the two feedstocks. Calculations and comparisons of viscosity ratios encountered in experiments were made. It was demonstrated the differences in the rheological properties of the metal feedstocks involved are key factors in determining the interface morphology of the molded parts.
An experimental study of co-injection molding which involves sequential injection of dissimilar metal feedstocks into a mold has been carried out. The effect of skin temperature and injection velocity on the material distribution of co-injection molded plates has been studied. It was found that the molding temperature was important in controlling skin-core distribution, while injection velocity seemed to play no significant role. The experimental results were analyzed by taking account of the relative viscosity of the two melts. It was demonstrated that the differences in rheological properties of the metal feedstocks involved are the primary variable determining the phase distribution of the molded parts.
In order to explore the coupling effects of second shear flow and temperature fields on the formation as weH as the evolution of crystal morphologies of high-density polyethylene ( HDPE) , the crystal morphologies of the parts obtained by multi-fluid multi-injection molding ( MFMIM) were investigated. The main feature of MFMIM is that the polymer melts in two different injection units are injected sequentiaHy into the cavity to form an encapsulated sandwich-structure product. Polarizing light microscopy ( PLM ) results showed that some peculiar microstructures ( e. g. cylindritic structures and ring-banded spherulites) were generated in the skin layer of the MFMIM parts. With the aid of scanning electronic microscopy (SEM) ,a detailed description of cylindritic structure was obtained , which showed that the cylindritic structure consisted of some ring-banded spherulites. For comparison , the crystaHine morphology of the sample molded by conventional injection molding (CIM) was also investigated. Different from the MFMIM parts, there were not observed any ring-banded spherulites or cylindritic structures in CIM parts except the common spherulites. And there was not a significant skin-core structure formed due to the higher temperature of the mold. Based on the comparison between the experimental results of the MFMIM parts and those of the CIM counterparts , it was concluded that the main reasons for ring-banded spherulites and cylindritic structures arising in the skin layer of MFMIM parts were the shearing and the complex thermo-mechanical fields provided by the secondary melt penetration process. Therefore, on the basis of the previous results, the mechanisms of ring-banded spherulites and cylindritic structures formation were discussed.
The sequential coinjection molding (SCIM) process has always been regarded as a challenging multiphase flow problem, which includes skin and core polymer melts together with gas. Thus, this article presents a 3D mathematical model for it, in which the governing equations of gas, skin, and core melts are united into a system namely the generalized Navier-Stokes equations. By doing this, the model can be solved simply by applying the finite volume SIMPLE method on the collocated grid. The core penetration process is simulated by level set method, which can capture two different types of moving interface simultaneously at different time. By simulating the SCIM process of a centrally gated rectangular plate and then comparing the numerical results with available experiment results, the proposed mathematical model is validated and the influences of skin/core volume ratio, injection temperature, and core injection delay on the depth of core penetration are analyzed in detail. Then, the sequential coinjection of a line-gated plate is investigated numerically, obtaining some important information in the gap-wise direction, which cannot be caught by 2.5D model. All the numerical results show that the multiphase flow model proposed in this article is effective and can be used to describe the flow behaviors of polymer melt in the SCIM process. POLYM. ENG. SCI., 2014. © 2014 Society of Plastics Engineers
Modelings of the interface distribution and flow-induced residual stresses and birefringence in the sequential co-injection molding (CIM) of a center-gated disk were carried out using a numerical scheme based on a hybrid finite element/finite difference/control volume method. A nonlinear viscoelastic constitutive equation and stress-optical rule were used to model the frozen-in flow stresses in disks. The compressibility of melts is included in modeling of the packing and cooling stages and not in the filling stage. The thermally induced residual birefringence was calculated using the linear viscoelastic and photoviscoelastic constitutive equations combined with the first-order rate equation for volume relaxation and the master curves for the relaxation modulus and strain-optical coefficient functions of each polymer. The influence of the processing variables including melt and mold temperatures and volume of skin melt on the birefringence and interface distribution was analyzed for multilayered PS-PC-PS, PS-PMMA-PS, and PMMA–PC–PMMA molded disks obtained by CIM. The interface distribution and residual birefringence in the molded disks were measured. The measured interface distributions and the gapwise birefringence distributions in CIM disks were found to be in a fair agreement with the predicted interface distributions and the total residual birefringence obtained by the summation of the predicted frozen-in flow and thermal birefringence. POLYM. ENG. SCI., 2014. © 2014 Society of Plastics Engineers
This work studies the effect of processing parameters on mechanical properties and material distribution of co-injected polymer blends within a complex mold shape. A partially bio-sourced blend of poly(butylene terephthalate) and poly(trimethylene terephthalate) PTT/PBT was used for the core, with a tough biodegradable blend of poly (butylene succinate) and poly (butylene adipate-co-terephthalate) PBS/PBAT for the skin. A ½ factorial design of experiments is used to identify significant processing parameters from skin and core melt temperatures, injection speed and pressure, and mold temperature. Interactions between the processing effects are considered, and the resulting statistical data produced accurate linear models indicating that the co-injection of the two blends can be controlled. Impact strength of the normally brittle PTT/PBT blend is shown to increase significantly with co-injection and variations in core to skin volume ratios to have a determining role in the overall impact strength. Scanning electron microscope images were taken of co-injected tensile samples with the PBS/PBAT skin dissolved displaying variations of mechanical interlocking occurring between the two blends. © 2014 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41278.
En este trabajo se han estudiado las características de la adhesión entre dos polímeros que conforman un sistema laminar bicomponente. Como sustrato rígido se ha empleado un copolímero de acrilonitrilo butadieno-estireno y metacrilato de metilo (MABS) y como recubrimiento se ha utilizado un poliuretano termoplástico (TPU). Ambos polímeros dispuestos en forma de bicapas han sido obtenidos utilizando diversos métodos de transformación (compresión, sobreinyección y coextrusión). En esta primera etapa se han realizado estudios mecánicos para la caracterización de la interfase en función de diversas variables: temperatura, presión y rugosidad de las superficies, modificando estas variables de acuerdo al método de transformación. Los resultados preliminares muestran un incremento de la energía de adhesión a
medida que aumentan la temperatura y la rugosidad en la en muestras inyectadas, ha evidenciado que el incremento de la fractura de adhesiva hacia adhesiva-cohesiva.
A novel concept of developing green composites with improved performance using coinjection
molding technique has been established. Poly (butylene succinate) (PBS), poly
(butylene adipate-co-terephtalate) (PBAT), and their blends were investigated as skin material.
Composite core material was made from poly (3-hydroxybutyrate-co-hydroxyvalerate) (PHBV)
and miscanthus. Co-injected samples with good surface finish were achieved at a skin/core
volume ratio of 40/60 and mold temperature
of 45 8C. Notched impact strength of
140 J/m, four times higher than that of
neat core material and unnotched impact
strength of almost 400 J/m with modulus
of more than 1 GPa was achieved for the
co-injection molded green composites.
Good adhesion at the skin/core interface
was observed under scanning electron
microscopy.
In sandwich injection molding, two polymeric materials are sequentially injected into a mold to form a multilayer product with a skin and core structure. Different properties of these polymers and their distribution in the cavity greatly affect the applications of the moldings. In an ideal situation, the core material should be entirely encapsulated in the skin material. When the flow front of the core material overtakes that of the skin material, breakthrough occurs, resulting in a defective part. The focus of this study is to determine the effect of molding parameters on the skin/core material distribution. The commercial simulation package (Moldflow) has been extensively compared with experiments. Both simulated and measured results suggest that in order to obtain the optimum encapsulated skin/core structure in the sandwich injection molded parts, it is necessary to select a proper core volume fraction and suitable processing parameters. A good agreement between simulation and experimental results indicates that the Moldflow program can be used as a valuable tool for the prediction of melt-flow behavior during the sandwich injection process.
Automotive plastic components are often required to withstand impact loadings and dissipate energy in automotive collisions, protecting occupants and pedestrians. The design of plastic components against impact loading is not a trivial engineering task, but still a challenging activity. The optimisation of the impact behaviour of plastic products requires a global approach involving material properties, processing methods and geometrical design solutions. This communication presents solutions to develop plastic components requiring high impact performance, based on a highly interrelated triad: polymeric material systems with improved impact toughness, processing methods for plastic products with enhanced toughening performance and design solutions for plastic components with superior impact behaviour.
In mould decoration (IMD) is attractive because a fully, or partially, decorated component is produced directly from the moulding process, with reduced emissions at lower process costs when compared to traditional techniques. A new IMD process has been developed to produce a painted component direct from the injection moulding tool. This incorporates the pressure spraying of thermoset powders through a valve into a closed mould. The residual heat of the tool initially softens the thermoset. The high temperature of thermoplastic polymer injected in a standard injection moulding subsequently cures the thermoset. The resultant product combines both thermoplastic and thermoset in a single injection moulding cycle. This paper presents frames from high speed video capture of powder mould filling and the results of INSPIRE (in mould spray painting, impact reduced on the environment) initial injection moulding using thermoset polyester and acrylonitrile butadiene styrene (ABS). The parameters that affect material distribution are examined and discussed. Similarities to the coinjection moulding process are noted.
The instrumented falling weight perforation impact behavior of coinjection-molded multipolypropylene sandwich plaques of various compositions having a β-phase isotactic polypropylene (PP) homopolymer core was studied at room temperature and respectively. The results were compared with those achieved on injection-molded monomaterial plaques of α- and β-polymorphs. It was found that the coinjection strategy is beneficial only if the skin-forming α-phase PP has a markedly lower molecular weight than the β-nucleated core PP. Sandwich plaques with PP copolymer skins exhibited an outstanding perforation impact resistance. The coinjection strategy for extrusion grade PPs yielded no improvement. Perforation impact at showed a prominent toughness reduction of the multipolypropylene plaques which became comparable with that of the monolayer plaques.
Two-dimensional simulation and experimental studies of flow-rate-controlled coinjection molding were carried out. Skin polymer was injected first, and then both skin and core polymers were injected simultaneously into a center-gated disk cavity through a two-channel nozzle to obtain an encapsulated sandwich structure. The physical modeling and simulation developed, reported in Part I of this series, were based on the Hele–Shaw approximation and the kinematics of the interface to describe the multilayer flow, and the interface development was used to predict the skin/core distribution in the moldings. The effects of rheological properties and processing conditions on the material distribution, penetration behavior, and breakthrough phenomena were investigated. The predicted and measured results were found to be in a good agreement. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2310–2318, 2003
This article demonstrates using sandwich injection molding in order to improve the mechanical properties of short glass fiber-reinforced thermoplastic parts by investigating the effect of fiber orientation, phase separation, and fiber attrition compared to conventional injection molding. In the present case, the effect of short glass fiber content (varying from 0–40 wt%) within the skin and core materials were studied. The results show that the mechanical properties strongly depend not only on the fiber concentration, but also on the fiber orientation and the fiber length distribution inside the injection-molded part. Slight discrepancies in the findings can be assumed to be due to fiber breakage occurring during the mode of processing. POLYM. COMPOS., 26:823–831, 2005. © 2005 Society of Plastics Engineers
In the sandwich injection molding process (co-injection), two different polymer melts are sequentially injected into a mold to form a part with a skin/core structure. Sandwich molding can be used for recycling, improving barrier and electrical properties, or producing parts with tailored mechanical properties. In this study the evaluation of flexural modulus and impact strength of co-injected plaques have been investigated. Virgin and short glass fiber reinforced (10 and 40%) polypropylene were used in six different combinations of sandwiched layers. The skin and core thicknesses were measured by optical microscopy and used to calculate the theoretical flexural modulus, which was compared to the experimentally measured modulus. Fiber orientation states were also observed by scanning electronic microscopy (SEM) at some specific locations and their effect on mechanical properties discussed. The experimental results indicate that an important improvement in transverse modulus, near the gate, is obtained when the virgin polypropylene (PP) is used as a skin and 40% short glass fiber polypropylene (PP40) as core. When both skin and core are made of PP40, the flexural moduli are slightly higher than conventionally injected PP40. POLYM. COMPOS. 26:265–275, 2005. © 2005 Society of Plastics Engineers.
The significance of using simulation programs in the design of injection was augmented with the advent of computer aided engineering. In this paper, the latest developments and extensions of the programs delineating the special issues dealt with in injection moulding were described. First, an improvement of the model characterizing the 2-component process was shown. In addition, a method of computing the topography of welding lines was elucidated. The results from this calculation is of great import to weld line strength. Finally, the developments on the computation of shrinkage and warpage reaction in injection moulded parts were provided in details.
This paper describes the microstructure of injection molded, short glass fiber reinforced, thermoplastic polyethylene terephthalate plaques as a function of fiber loading and plaque thickness. The influence of the microstructural variations on the fracture mechanical properties of this composite system were measured. The fracture toughness was higher for cracks perpendicular to the main fiber orientation and increased with weight fraction of fibers. Additional effects on fracture mechanical behavior due to differences in matrix toughness, strength of fiber/matrix bond quality and plaque thickness are discussed.
The influence of microstructure on the stiffness and toughness of ‘long’ glass fibre reinforced nylon 66 mouldings was studied. Using the results of previous work which established the influence of processing conditions on moulding microstructure, the effects of processing on resulting mechanical properties are inferred. Whilst impact toughness is shown to be insensitive to processing conditions, slow crack propagation toughness is greatest in situations where the processing produces a pronounced core region of long fibres orthogonally aligned to the direction of crack propagation. Model studies of the influence of microstructure on stiffness suggests little sensitivity of modulus to likely variations in the processing parameters.
An experimental study of sandwich injection molding is reported which involves sequential injection of polymer melts with differing melt viscosity into a mold. In isothermal injection molding the relative viscosity of the two melts is the primary variable determining the phase distribution in the mold. Generally the most uniform skin-core structure occurs when the second melt entering the mold has a slightly higher viscosity than the first melt injected. Large viscosity inequalities lead to nonuniform skin thicknesses. The influence of blowing agents and non-uniform temperature fields on the extent of encapsulation is described. Temperature fields are very important especially if the first polymer melt injected has a greater activation energy of viscous flow (or a greater temperature dependence of the viscosity function).
In response to increasing ecological and economic pressures, a two-shot molding process has been developed to recycle molding plastics. This process buries scrap plastic under a skin of virgin plastic in the molded part, resulting in a laminate that has the appearance of and similar mechanical properties to the skin material only. Two injection units and a special nozzle design are used to achieve the desired lamination. Theoretical and experimental studies have been conducted to determine the effects of parameters on the amount of scrap material that can be buried and its effect on impact strength. With conventional production molding dies, scrap plastic comprising approximately 40 percent of the total shot has been molded beneath virgin plastic in parts having stringent appearance requirements.
Co-injection molding of calcium carbonate filled polypropylene, short glass-fiber-filled polypropylene, or unfilled high-density polythylene melts is studied using a mediumsize injection-molding machine and a center-gated disc mold. Injection molding is carried out under non-isothermal conditions. Order of injection of the melts, injection speed, and mold temperature is changed in order to understand the mold filling in general and to investigate the type of skin/core structure and mechanical interlocking of the phases in the moldings. It is found that the order of injection is not significant in obtaining a skin/core structure but it is important in obtaining extensive phase interlocking, which is reduced if the flow rafe and the mold temperature are low. Presence of fillers appears to result in more mechanical interlocking of the phases.