One of the advantages of manufacturing with polymers in general is that several functions can be integrated in one component. For example, if the part was made with metal, different materials should be used for the different functions, leading to additional costs related to the assembly [1]. For example, some functions can be integrated in one plastic component using snap fits, pipe connectors, seals, sliding bearings, threads, gear racks, and reinforcements (ribs). In some cases, it is impossible to fulfill all the requirements with one polymeric material, so it is very usual to find products with two materials that are processed at the same time or in the same mold in a special type of injection molding called co-injection molding or multishot injection molding (MSM).
The process occurs in one machine with two plasticating unit, and the injection is sequential, done in one cycle. In the first part of the cycle, the first material is injected into a mold. Then, there is a modification of the mold prior to injecting the second material. It can be said then that the mold is special. It must include the channels and cavities for two materials, and to do that in sequence as the definition says, several techniques can be employed: rotary platen, movable core, and overmold. They have in common that the moving part corresponds to the final part contour while the fixed part corresponds to the semi-finished part or the elements correspond to the first material [2]. The rule of thumb suggests that the material with the lower melting temperature be injected first. Although the mold is more complex than a conventional mold, the costs related with production are reduced considerably because the need to include a mechanical assembly is no longer necessary. Additionally, the quality of the product is better because, with the process done in the same place, there is better dimensional control of the part and a strong bond between the materials.
When working with silicone rubber products or products that have one component made with silicone rubber, there is normally a combination of hard and soft materials. For example, a hard housing is made with a thermoplastic and a silicone rubber seal is then injected in a second step. The challenge that must be overcome in multi-shot processing is to ensure that there will be excellent adhesion between the silicone component and the other material (thermoplastic or other silicone part). Otherwise, the advantages of that process would be totally nullified. Another option is when the silicone rubber has two different colors in the same product (multi-color injection). Different products for different applications can be manufactured by this method, and this article will focus on automotive applications; but before explaining this, it is necessary to understand the role that silicone rubber plays in the automotive industry.
LEARN MORE ABOUT LSR IN THE AUTOMOTIVE INDUSTRY
Silicone rubber properties preferred for the automotive industry are strength, resistance, and durability. One of the biggest problems in automotive industry is finding material that can maintain its properties even at the high temperatures normally found inside the engine compartment. Until a few years ago, the only materials that achieved this were metals, but they have the disadvantage of high weight, which meant cars with high fuel costs and low control over the safety of the driver and passengers. With advances in the study of materials science, ceramic and polymeric materials began to replace metals and could greatly reduce fuel consumption. However, the same consumers also started to demand greater power and performance in automobiles, which meant an exponential increase in the internal temperatures, and new materials had to be modified again to fulfill this function, which now can be achieved fully. Because of that, the use of silicone rubber increased and replaced EPDM because its performance temperature limit is about 140°C. In addition to engine applications, silicone rubber can be found in battery seals, gaskets, headlamps, ignition cables, high tension cables, grommets, radiator seals, hoses, vibration dampeners, shock absorbers, spark plug boots, and ventilation flaps. Gaskets and seals, for example, use silicone rubber with resistance to oil, waxes, and coolant agents in combination with high temperatures, and the resulting degradation behavior cannot be catastrophic. As with cable, tubes, and hoses, sometimes it is necessary to reinforce with textile or metals in order to withstand medium to high pressures, so the adhesion between both materials must be very good. Flexibility must be maintained under dynamic loads, high temperatures, and chemical agents that are transported through them. The main objective of all these applications inside the engine is to reduce the maintenance, which means an increase in warranty times of the vehicles and reliability of the machine.
In the exterior of the vehicle, silicone rubber can be found in airbags, load bearings, vibration suppressors, and shock absorbers to improve the comfort and driving experience through silicone rubber damping elements. Figure 1 shows a diagram of where silicone rubber applications can found in an automobile.
Figure 1. Silicone rubber applications in an automobile [3]
Now, which silicones and combinations with silicones are most popular in automotive applications? PA (Polyamide, Nylon)/LSR, even fiber glass reinforced, is used as an engine housing, and in some cases, it also includes electric circuits; here, silicone rubber replaces thermoplastic elastomers (TPE) due to its lifetime at high temperatures. Another combination is PC (Polycarbonate)/LSR which has its most important use in the integration of the headlights with the seals inside and outside the assembly. The advantage of that is having perfect control of the adhesion because bonding occurs during the cooling of the PC lamp, and this avoids problems with the ingress of water, other liquids, and dirt around the system that may affect its performance or operation. Similar to PC/LSR combinations, PMMA (Polymethyl methacrylate)/LSR is used in lightning applications, but in this case in the interior of the car; the application criteria are stricter for the PC/LSR because it must maintain a perfect transmittance of light during the whole service life. This assembly is mainly affected by environmental conditions such as rain, UV light, and air contamination which can be better resisted by the PC than the PMMA. For that reason, PMMA/LSR is used in interior lights and in LED devices for the board. The advantage here is the great variety of shapes, color and styles that can be generated with the PMMA with the addition of the stability of the LSR. This combination avoids, among other things, the so-called stick-slip noise (the noise that you hear but you do not know where it comes) increasing the comfort sensation for the passengers. A PBT (Polybutadiene)/LSR combination has one of the newest and most interesting applications on this list: rain and light sensors. This device has the ability of turn on the windshield wipers when it rains, turn on the lights when it senses darkness, and activate windows in case they are open when the rain starts. The surface is made with a photodiode protected with a PBT housing, and a silicone rubber diaphragm is attached below the assembly. The photodiode detects the presence of water (raindrop) on the windshield and absorbs light in the presence of natural or artificial light; sometimes it is included as a freeze or irrigating sensor as well. The objective of the silicone rubber is to permit an optimal attachment to the car windshield, otherwise the system could have bad performance, losing its position or even getting lost during car movement.
There are a lot to studies related to these four well-known combinations, and it is also necessary to include more combinations of rigid and flexible materials with LSR, which should be analyzed carefully. This detailed study will help to broaden the field of applications for silicone rubber and will find new niches.