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Two-Component Injection Moulding of Thermoplastics with
Thermoset Rubbers: Process Development
G.J. Bexa, J. De Keyzerb, F. Desplenterec, A. Van Baela
aMaterials Technology TC, KU Leuven Technology Campus Diepenbeek,
Wetenschapspark 27, 3590 Diepenbeek, Belgium
bSustainable Chemical Process Technology TC, KU Leuven Technology Campus Diepenbeek,
Wetenschapspark 27, 3590 Diepenbeek, Belgium
cMaterials Technology TC, KU Leuven Technology Campus Ostend,
Zeedijk 101, 8400 Oostende, Belgium
Corresponding author: gertjan.bex@kuleuven.be
Abstract. Two-component injection moulding is a manufacturing process for combining polymers with different
properties in a single injection moulding process. The process is typically used to combine thermoplastics with another
thermoplastic or with a thermoplastic elastomer to create colour differences or hard and soft areas respectively. The
present study aims at the development of a two-component injection moulding process for the combination of a thermoset
rubber and a thermoplastic. Currently products that consist of those two materials (e.g. wheels, syringes and other
products with gaskets) are made by assembling separate components. Implementing the two-component injection
moulding technique for these products will result in better interphase properties, savings on rubber and avoiding costs
related to the assembly process. A technological challenge is posed by the fact that injection moulding of rubbers and
thermoplastics is fundamentally different. The injection moulding of a rubber requires a heated mould (140°C-180°C)
whereas thermoplastics need a relatively cold mould (20°C-100°C) for the polymer to solidify. In this study a versatile
two-component mould is proposed in which the mould cavities for the rubber and the thermoplastic are thermally
separated and equipped with facilities to control the temperature of both cavities individually. The design of the mould
also makes it possible to vary the sequence of injection. In this way it is possible to test several processes variations.
Preliminary test results will be presented for specific rubber-thermoplastic combinations.
Keywords: Two-component injection moulding, thermoset rubbers, thermoplastics.
PACS: 81.20.Hy
INTRODUCTION
Several applications require a combination of parts with different characteristics like hard/soft, conductive/non-
conductive or different colours. This diversity in requirements is only possible by combining different materials.
Two-component (2K) injection moulding is a manufacturing process that allows to combine different polymers with
different properties in a single injection moulding process. 2K injection moulding is often used to combine multiple
thermoplastics with colour differences. In other applications, like in toothbrushes, a combination of a soft rubberlike
material and a stiff material is required. In these cases the rubberlike material is a thermoplastic elastomer (TPE).
A large number of products combine thermoset rubbers with thermoplastics, e.g. wheels, syringes and other
products with gaskets. Currently these products are made by assembling separate components. Implementing the 2K
injection moulding technique for these products should result in better interphase properties and the avoidance of
costs related to the assembly process. Furthermore it is expected that a redesign will result in savings on rubber.
Such 2K process poses considerable challenges, since injection moulding of thermoplastics and thermoset rubbers
requires fundamentally different process parameters. Thermoset rubbers are injected in a heated mould (140°C-
200°C) to start the curing process whereas thermoplastics are injected in a relatively cold mould (20°C-100°C) to
solidify the product [1,2]. High-end thermoplastics like polyamide (PA) can withstand high temperatures making it
possible to insert the thermoplastic part in a completely heated mould and then overmould it with rubber [3, 4].
When this technique is applied with commodity thermoplastics like polyethylene (PE) or acrylonitrile butadiene
styrene (ABS), it is expected that the thermoplastic part will deform under the high temperatures and pressures
necessary for the injection moulding process of the thermoset rubber. This study aims at the development of a novel
2K injection moulding process that can be applied with commodity thermoplastics. Two possible solutions are
proposed. In the first, deformation of the thermoplastic part is prevented by cooling the thermoplastic part during
vulcanization of the rubber. This technique requires special moulds where the mould cavities for the rubber and the
thermoplastic are thermally separated and equipped with facilities to separately control the temperature of both
cavities. In the other solution, the thermoplastic material is injected after vulcanization of the rubber part.
MATERIALS AND METHOD
Material
For this research HDPE (Sabic, grade M80064) is selected as thermoplastic material. HDPE is a commonly used
thermoplastic characterized by a rather low maximum service temperature (110°C-130°C) since it easily deforms at
high temperatures. The thermoset rubber selected for this research is EPDM (Hercorub, grade 005K), a frequently
used material for seals, gaskets, hoses, roofing and cable insulation. The selected EPDM rubber is a sulphur
vulcanizing rubber that needs approximately 20 minutes of curing at 160°C.
Method
Injection moulding is conducted on an Engel ES330H/80V/80HL-F 2K injection moulding machine with a
vertical rubber injection unit and a horizontal thermoplastic injection unit.
A versatile mould is required to enable both proposed 2K injection solutions. First of all it is designed in such a
way that the injection sequence can be reversed. For this purpose, the mould is based on a movable core system.
Instead of using a real moving core, a metal plate that can be manually inserted was used (figure 1). Moving the
metal plate from one mould cavity to another is required to enable a different injection sequence. Secondly the
mould cavities for the rubber and the thermoplastic are thermally separated and equipped with facilities to control
the temperature of both cavities separately. In this way, it was possible to cool the thermoplastic during the in-mould
vulcanization of the rubber. Thermal separation was achieved by using insulation and air gaps.
FIGURE 1. Mould system based on a movable core system. The metal plate, indicated in green, is used to change the order of
injection: (a) shows the configuration where the thermoplastic is injected first followed by the rubber, (b) shows the
configuration where the rubber is injected first. Both mould cavities are thermally separated. The left cavity is heated, the right
cavity is cooled.
Pressurized water is used both for heating the mould cavity for the thermoset rubber and for cooling the mould
cavity for the thermoplastic part. Design rules found in literature were used for the cooling channels [5] in order to
obtain evenly distributed temperatures.
To achieve even more flexibility a combination of two temperature control units and a valve system is used to
heat the mould cavity of the rubber (Figure 2). This set-up makes it possible to quickly cool the rubber after
vulcanization. The same set-up is used for rapid heat cycling of the mould in injection moulding of thermoplastics to
improve mouldability, strength of weld lines and surface quality [6].
FIGURE 2. Temperature control set-up with possibility for rapid heat cycling on the mould cavity for the rubber. (a) and (b) are
respectively temperature control units for the upper and lower temperatures of the mould cavity for the rubber. (d) is the valve
system that switches between hot and cold water. (c) is the temperature control unit for the mould cavity for the thermoplastic
part. (e) is the mould used in this study.
Temperature control units (a) and (b) determine respectively the upper and lower temperatures of the mould
cavity for the rubber. The valve system (d) allows either the hot or the cold water to flow through the mould cavity
for the rubber. In this way it is possible to vulcanize the rubber first and then quickly cool it to the temperature of the
thermoplastic part. Temperature control unit (c) controls the temperature of the mould cavity for the thermoplastic
part.
EXPERIMENTAL
Temperature Distribution Within the Mould
To verify thermal separation between the mould cavity of the thermoset rubber and the thermoplastic, an infrared
camera (OPTRIS PI400) is used. Black adhesive tape with an emissivity of 0.95 is used in order to avoid reflection
of the metal surface. The temperature control unit for the mould cavity of the rubber was set at 160°C. The
temperature control unit for the cavity of the thermoplastic was set at 100°C.
FIGURE 3. Thermal image of both mould cavities during heating and cooling. The left cavity is the colder cavity (100°C) for
the thermoplastic part. The right cavity is the hot (160°C) cavity for the thermoset rubber.
At the left side of the thermal image is the cavity for the thermoplastic. It can be seen that the temperature of this
cavity is successfully maintained at 100°C. At the right side of the thermal image is the cavity for the rubber. The
temperature of this cavity is 160°C. The thermal image shows a uniform temperature distribution within the cavity
for the rubber, which allows the rubber to vulcanize evenly and prevents warpage of the rubber part.
Mould cavity rubber:
160°C
Mould cavity
Thermoplastic: 100°C
Rubber – Thermoplastic injection sequence
In this section the process of first injecting and vulcanizing the rubber is described. The rubber is vulcanized for
20 minutes. After cooling to a temperature of 40°C, the polyethylene is injected. Slow injection speeds (34 cm³/s)
and a low holding pressure (274 bar) were used for the polyethylene in order to minimize the deformation of the
rubber part. In figure 4 (a) it can be seen that the rubber part is largely deformed during the injection of the
polyethylene. The deformation is caused by the elastic behaviour of the rubber. Due to the deformation of the
rubber, it is impossible to accurately control the dimensions of the finished product. Adhesion between the rubber
and the polyethylene was found to be very poor. The stress within the rubber, caused by the deformation of the
rubber, was large enough to break the adhesion.
(a)
(b)
FIGURE 4. 2K injection moulded test samples of thermoset EPDM rubber and polyethylene. In (a) the thermoset rubber was
injected first. Due to the elastic properties of the rubber, the rubber part is largely deformed. In (b) the injection moulding process
started with the injection of the polyethylene.
Thermoplastic – Rubber injection sequence
In a second solution the thermoplastic part is moulded first and kept at a lower temperature during the
vulcanisation process of the thermoset rubber. The result is shown in figure 4 (b). It is important to mould the
thermoplastic part with minimal shrinkage by selecting optimal process parameters. Thanks to thermal separation
between the two cavities, the cavity for the thermoplastic part can be kept at a lower temperature than the cavity for
the rubber. When the thermoplastic part is smaller than its the cavity due to shrinkage, the interface between the
rubber and the thermoplastic will be in the colder cavity. Because of the lower temperatures, the rubber at the
interface will not vulcanize. The next step in the process is the moulding of the rubber part. It is important to keep
the temperature of the thermoplastic part at an intermediate temperature during the moulding of the rubber part. The
intermediate temperature should not be too high in order to prevent deformation of the thermoplastic part. On the
other hand when the temperature is too low, the thermoplastic part will absorb heat at the interface resulting in an
incomplete vulcanisation of the rubber at the interface. In this research it was found that for the specific type of
polyethylene, 100°C is a good value for the temperature of the thermoplastic part. The rubber is now vulcanized for
20 minutes. After vulcanisation the rubber is quickly cooled down till 100°C using the valve system (d) in figure 2.
The last cooling step of the rubber part is necessary for the solidification of the polyethylene at the interface.
Without this cooling step the interface is weak and fails when ejected.
CONCLUSION
In this study a versatile mould is developed for 2K injection moulding of thermoset rubbers and thermoplastics.
The sequence of injection is controllable by moving a metal plate. The cavities for the rubber and the thermoplastic
are thermally separated in order to control the temperatures of both cavities separately. With the aid of a valve
system it is possible to quickly cool the rubber after vulcanisation. Several 2K injection moulding variants have been
tested. In this research it was found that the deformation of the rubber part is too large when the rubber is injected
first. Adhesion between the rubber and the thermoplastic part was very weak for this injection sequence. By first
injecting the thermoplastic it was possible to successfully make two-component products out of thermoset rubber
and thermoplastics. In this process it was necessary to minimize shrinkage of the thermoplastic part by selecting
optimal process parameters, in order to completely vulcanize the rubber at the interface. The thermoplastic part was
kept at an intermediate temperature during the vulcanisation process of the rubber. After vulcanisation the rubber
was quickly cooled down before ejection in order to solidify the thermoplastic at the interface. In future research this
process will be optimized in order to maximize the adhesion and minimize the deformation. In order to further
validate the proposed 2K injection moulding process, more combinations of different thermoset rubbers and
thermoplastics will be tested.
ACKNOWLEDGMENTS
G.J. Bex acknowledges Research Foundation – Flanders (FWO) for funding his PhD grant Strategic Basic
Research. The authors also acknowledge the company Hercorub NV for providing the rubber material.
REFERENCES
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