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Additive Manufacturing, Modeling Systems and 3D Prototyping, Vol. 34, 2022, 1–8
https://doi.org/10.54941/ahfe1001583
Design and Production of Hybrid
Products
Álvaro M. Sampaio1,2,3, André Lima1,2, Cátia Silva1,2,
Ana Miranda1,2, and António J. Pontes1,2
1IPC–Institute for Polymers and Composites, University of Minho, Guimarães, Portugal
2DONE Lab–Advanced Manufacturing of Products and Tools, University of Minho,
Guimarães, Portugal
3Lab2PT, School of Architecture, University of Minho, Guimarães, Portugal
ABSTRACT
Injection moulding (IM) is a well-known and widely employed fabrication process,
which allows the production of versatile and lightweight parts, with precise dimensio-
nal tolerance. Multi-material and multi-component products are achieved by means
of process variations of the conventional IM. Although the resulting products pre-
sent combined properties and functionalities, the available techniques require specific
machinery and complex tooling design. This paper presents a new processing appro-
ach, based in a hybrid method, for the fabrication of multi-material products, with
enhanced functionality, based in the combination of IM with additive manufacturing
(AM) technologies by means of overmoulding process.
Keywords: Design rules, Additive manufacturing, Injection overmoulding, Hybrid method
INTRODUCTION
Industrial processes and products are in constant development due to the
increasing requirements for the fabrication of time and cost competitive parts
with optimized performance, customizable in terms of design and materi-
als, lightweight, and integrating additional functionality. When employing
conventional technologies to solve these issues, typically moulding tools are
required, and when concerning complex shapes there are technologic limitati-
ons in regarding to the fabrication. In addition, the development of solutions
and tools is time consuming, costly, and products customization is limited.
Considering this, alternative solutions are required to guarantee the achie-
vement of such demands. For this reason, one possibility to solve this matter
consists of combining AM technologies with conventional technologies. Due
to constant advancements in regarding to AM processes optimization for
shorter lead times, cost reduction, improved design freedom, new materi-
als, tighter tolerances, larger building areas, among other enhancements,
layered processes are often employed as replacement technologies for conven-
tional processes (Pontes, 2021). This is related to the innovative aspects of
AM processes which, if wisely explored can improve and even revolutionize
conventional manufacturing efficiency. The combination of processing tech-
nologies or hybrid manufacturing is one possible answer to bypass obstacles
© 2022. Published by AHFE Open Access. All rights reserved. 1
2Sampaio et al.
by combining processes strengths and overcoming manufacturing limitations
(Lima et al., 2017; Sampaio et al., 2019). In this scope, a recent approach
being explored consists in the combination of alternative processes and pro-
ducts with conventional injection moulding, by means of overmoulding steps,
in order to facilitate the next-generation of customized products. Summarily,
the hybrid method encompasses the fabrication of an insert part by means of
AM processes, followed by the positioning and fixation of the insert within
the mould cavity, and a subsequent overmoulding step allows the genera-
tion of a multi-material product. When resorting to AM processes for the
fabrication of insert parts, it is possible to mass customize a part at relati-
vely low-cost (no tools, moulds or punches are needed) and short lead times
(Guo and Leu, 2013). Combining this possibility with injection moulding it
is assured design freedom to change products when necessary, and without
the need for the development of new tools and awaiting times.
ADDITIVE MANUFACTURING AND THE HYBRID METHOD
AM processes allow the generation of lightweight parts while maintaining,
and in some cases, improving the performance (e.g. structurally). This is rela-
ted to the ability to produce lattice structures (e.g. honeycomb, chiral truss)
(Guo and Leu, 2013) and also to perform topology optimization (Attaran,
2017). The possibility to integrate a lightweight insert part, with improved
structural performance, into a mould for an overmoulding step, facilitates
the fabrication of a lightweight and multi-material product. The integration
of lattice structures as reinforcing features in products obtained by IM opens
new possibilities for reinforcing products and for integrating shapes which
are difficult to obtain by conventional techniques. The reduction of wei-
ght ultimately leads to a reduction of material waste and overall cost, and
a minimized environmental impact.
Regarding material possibilities, AM processes present a portfolio of
available materials that can only be processed in each specific technology.
This is related to the unique properties and additives of these materials’
composition, meaning that materials with such properties are not availa-
ble for other types of processing techniques. In addition, possibilities range
from employing a single building material, to generating variations of a
same polymer formulation during the building process of the insert part
(e.g. colour gradients, hardness shore gradients), and also combine different
materials during the manufacture (in the same technology or by combining
technologies).
The portfolio of materials adequate for injection moulding is very exten-
sive as research for new materials has been going on for many decades.
This includes e.g. thermoplastics, foams, elastomers, and reinforced pla-
stics with various types of additives and fillers. Also, it is possible to create
multi-material products based in variants techniques of the injection moul-
ding, which expands possibilities. Considering the numerous possibilities
for material combination within each processing technology, and also by
combing technologies, the process to generate products which vary material
composition along its structure is simplified and improved. With the hybrid
Design and Production of Hybrid Products 3
Figure 1: CAD representation of a hybrid: (a) cup with detail of the mould and
(b) toothed gear wheels.
method, it is possible to assure specific properties at pre-defined locations of a
multi-material product and therefore, improve its overall performance. Ano-
ther benefit consists in the fact that there are specific plastic materials which
are difficult and expensive to process conventionally (e.g. PEKK), and that
may be easily processed by already optimized AM processes, and afterwards
overmoulded.
ON-GOING RESEARCH
Based in the presented possibilities and advantages when resourcing to the
hybrid method, several analysis and case-studies are under analysis to eva-
luate processing possibilities and difficulties such as, e.g. material chemical
compatibility, possible integration of structural snap-fits for enhanced adh-
esion, control dimensional tolerances, and avoid conventional processes
complications and defects (e.g. flash, warpage). For example, Figure 1 (a)
depicts a customized cup, based on the same mould tool, by simply using an
insert part built by AM with a different material and design to be overmoul-
ded. This way, one can change a product configuration and aesthetics without
changing the mould. A toothed wheel (Figure 1 (b)) is presented as exam-
ple of a product combining a lightweight and optimized structure produced
by AM with a teeth profile with dimensional accuracy defined by injection
overmoulding.
These examples present some of the possibilities that combining AM with
IM enables, which can greatly evolve the actual workflow of plastic industry
for the production of versatile and customizable products (Miranda, 2019;
Pontes, 2021). In order to be possible to apply this method, the joint between
materials may occur through welding based in the materials’ chemical com-
patibility, or by resorting to structural interlocks for dissimilar materials. If
intended, one may combine both approaches, since AM processes allow the
fabrication of structural interlocks.
This possibility simplifies the bonding between compatible and dissimilar
materials in a single product. As this step of the method is of paramount
importance, this paper presents the research undertaken regarding bounding
strategies in terms of material compatibility and joint design. For that it was
used a hybrid specimen composed by a half produced by AM and another
half by overmoulding.
4Sampaio et al.
HYBRID SPECIMEN PRODUCTION
A hybrid tensile test specimen type B (ISO 527-2) was defined based in an
existent mould tool with a half insert part produced by Fused Deposition
Modelling (FDM) and an overmoulded half. A UV-stable acrylonitrile styrene
acrylate (ASATM), and an electrostatic dissipative acrylonitrile butadiene sty-
rene (ABS-ESD7TM) from Stratasys Ltd. were used to manufacture the insert
parts with a Fortus 900mc. Main process conditions, for both building mate-
rials, include: flat (XY) build orientation; 100 % infill; raster angle of ±45°
and layer height of 0.254 mm.
For the overmoulding, a standard polypropylene (PP) ISPLEN® PP070
G2M (Repsol) and an ABS with 15% of carbon fibre (LNP™ STAT-KON™
COMPOUND AE003), supplied by SABIC were used. Main processing con-
ditions, for a switchover volume of 10 cm3include: injection temperature of
180-238 °C and 220-270°C, mould temperature of 40 °C and 80 °C and a
holding pressure of 140 bar and 80 bar for 8s for PP and ABS, respectively.
CRITICAL ASPECTS DEFINITION
To assure a quality hybrid part, certain critical aspects, such as, insert part gap
to fit the mould, temperature and material compatibility need to be determi-
ned. For the gap to fit the mould, several gaps bellow the nominal dimension
were defined for the insert part, varying from 0.00 to 0.50 mm in steps of
0.05. Overmoulding with both IM materials has shown that flash occurre-
nce is reduced with tighter gaps (up to a gap of 0.15 mm when overmoulding
with PP) and, that it is smaller when overmoulded with ABS due to its higher
viscosity associated with the filler content (up to 0.35 mm flash occurrence is
negligible). Surface defects (e.g. crushing marks) were more visible for tigh-
ter gaps, between 0.00 mm and 0.10 mm, corresponding also to the hardest
manual fitting in the mould. Based on the results, 0.15 mm was the most sui-
table gap that combined the ease of fitting the mould with the least defects.
During this study material compatibility was evaluated showing that both
AM materials are compatible with ABS and non-compatible with PP.
Temperature influences molecular diffusion which improves adhesion
between compatible materials. For amorphous polymers, such as ABS,
diffusion occurs close to the glass transition temperature (Emblem and
Hardwidge, 2012). Considering this, insert parts were pre-heated at three
temperatures that were comparatively tested, for compatible materials only:
(i) 80 °C, equal to the mould temperature for ABS; (ii) 110 °C, closest to the
glass transition of both AM materials (108 °C) (Stratasys Ltd., 2022); 140 °C,
in order to consider a much higher temperature. Joint adhesion strength was
evaluated by tensile testing (ISO 527-1), with an Instron 5969 Dual Column
Tabletop Testing System with video extensometer considering a tensile test
speed of 5 mm/min and a cell load of 50kN. Based in Figure 2, for both over-
moulded ASATM and ABS-ESD7TM, the load at maximum tensile strength
(~1100 N to 1220 N), tensile stress (~27 MPa to ~28 MPa) and strain (~1 %
to 1,1 %) at break are higher when the insert part temperature is 140 °C.
Besides joint strength, the evaluation of the most suitable insert part tem-
perature considered the structural stability of the insert part during handling
Design and Production of Hybrid Products 5
Figure 2: Typical stress-strain curves obtained for the hybrid specimens of ASATM/ABS
and ABS-ESD7TM/ABS for different insert part temperatures.
Figure 3: Joint designs for compatible (A, B, C) and non-compatible (D, E, F) materials.
and ease of fit in the mould. A temperature of 110°C was the most suitable
by direct comparison because, although the highest temperature improves
adhesion strength it was rather complex to manually handle the insert part
and place it in the mould cavity.
JOINT ANALYSIS
The mechanical behaviour of a bonded structure is influenced by material
compatibility and joint design (Emblem and Hardwidge, 2012). Therefore,
several joint designs were defined (Figure 3) to improve adhesion between
compatible materials, by increasing the contact area and promoting a uni-
form stress distribution, and also, to create bond between non-compatible
materials by defining interlocks to create mechanical connection. All the desi-
gns account mass balance between the halves of the hybrid specimen while
contemplating the manufacturing reproducibility.
Joint design was validated based in simulation analysis of the overmoul-
ding process with the software Moldex3D R16. Joints A, B and C were not
simulated as no major issues were expected due to design simplicity. Main
conditions include an injection temperature of 235 °C and 270 °C and a
mould temperature of 40 °C and 80 °C for PP and ABS, respectively. Simu-
lation results (Figure 4) indicate that the filling was complete and the filling
time is dependent on the geometry of the joint because, for the same geometry,
it presents approximately the same filling time, for both injection materi-
als. Small air traps occur for all the interlocks which may be diminished by
controlling processing speed and pressure.
The production of the hybrid specimens with the new joint configurati-
ons considered the processing conditions previously presented and occurred
without difficulties.
Hybrid specimens were analysed by computed tomography (CT) with a
Metrotom 800 from ZEISS. Transversal and longitudinal analysis enabled
6Sampaio et al.
Figure 4: Moldex3D simulation results for joint D (left image), E (middle image), and F
(right image).
Figure 5: Tomography images of representative hybrid specimens for the majority of
joints in analysis.
Figure 6: Hybrid tensile test specimens: (a) ABS-ESD7TM overmoulded with ABS;
(b) ASATM overmoulded with ABS; (c) ABS-ESD7TM overmoulded with PP; (d) ASATM
overmoulded with PP.
to verify the quality of the joints (Figure 5). It is noticeable the presence of
voids (blue and green dots) in all insert parts which is related to the FDM
process. Joint A (similar to B), C and F present an interface between materials
with no anomalies or voids. Joint D presents no voids or anomalies when
overmoulded with ABS while a significant volume of voids (blue, green and
red dots) are present when overmoulded with PP. This may be related to
material compatibility. Finally, joint E presents a deflection of a thinner zone
that allowed the injected material to cross over.
Hybrid specimens were then subjected to tensile testing, with the same con-
ditions as previously presented, in order to make a comparative evaluation
of joint strength. The typical stress-strain curve obtained are presented in
Figure 6. All hybrid specimens fractured at the joint, typically at the zone of
more fragility (e.g. thinner sections) of the insert part. For compatible mate-
rials, joints B and C provided the highest values of load at maximum tensile
strength (~1250 to ~1300 N), tensile stress (~30 to ~33 MPa) and strain
(~1.3 to ~1.7 %) at break. Regarding the joints defined for non-compatible
Design and Production of Hybrid Products 7
materials, joints D and F presented the best behaviour. Values ranging betw-
een ~950 to ~1200 N and ~245 to ~270 N for load at maximum tensile
strength, ~23 to ~30 MPa and ~3.8 to ~6.5 MPa for tensile stress at break
and, ~0.7 to ~1.1 % and ~0.9 to ~3.3 % for tensile strain at break were
obtained when overmoulding with ABS and PP, respectively. For all joint
designs, hybrid specimens produced with ABS-ESD7TM provided slightly
stronger bond than ASATM, when overmoulded with ABS. However, when
overmoulded with PP, mechanical performance is rather similar for both AM
materials varying mostly based in the joint design. Also, when overmoul-
ding with PP, each joint presents a different behaviour varying from tough to
brittle behaviour.
CONCLUSION
A hybrid method for the fabrication of customizable products based on the
combination of AM processes with conventional IM through overmoulding
steps was presented. This paper briefly presents the possibilities and advanta-
ges of such combination (i.e. production of customizable products presenting
complex geometries combined with advanced material properties and enha-
nced functionality). This paper also presented the hybrid manufacturing of
a test specimen based in the combination of AM and IM. Critical aspects
were assessed indicating that the most suitable insert part gap was 0.15 mm
due to less surface defects and negligible flash. Also, the insert part tempe-
rature, for the materials in study, was 110 °C, a value closest to the glass
transition temperature of both AM materials used and only necessary for
compatible materials. Several joint designs were defined and analysed for
compatible and non-compatible materials. Based in joint strength and defe-
cts, joints B and C were most suitable for compatible materials while D and F
were more suitable for non-compatible materials. Future work encompasses
the fabrication of case-studies based in the studied materials and selected joint
designs.
ACKNOWLEDGMENT
This work is supported by: European Structural and Investment Funds in
the FEDER component, through the Operational Competitiveness and Inter-
nationalization Programme (COMPETE 2020) [Project n° 039334; Funding
Reference: POCI-01-0247-FEDER-039334]
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