Silicone rubber is an inorganic polymer formed by silicon (Si), oxygen (O), carbon (C) and hydrogen (H). The main chemical chain, called the backbone, is formed by silicon and oxygen, which is why it is called siloxane. Carbon and hydrogen can be found in lateral chains as methyl (-CH3), vinyl (-CH=CH2), phenyl (-C6H5, also known as a benzene ring) or other groups (Figure 1). As with all polymers, silicone rubber needs a polymerization process to create the real material from SiOH monomers, which react with Me3SiOSiMe3 (where Me is a metal catalyst) to increase the chain length and with Me3SiO2 to end the chain. As a result of this process, the uncured silicone rubber contains chains with different lengths .
Silicone rubber is recognized for being inherently biocompatible with human tissues, resistant to bacteria, and it does not degrade in the presence of fluids like blood, saliva, and others. This material is used more and more in medical applications, but increasingly strict requirements make it necessary to modify the silicone in order to use it. Additives are a common option for increasing mechanical and physical properties, but silicone can also be combined with other materials such as thermoplastics, thermosets, thermoplastic elastomers, ceramics, and metals to comply with application requests. In the industrial processing of materials, the challenge is always to create high-performance products in the shortest time with the least amount of the components. There are two options for that: (i) select the best material that fits with all the requirements of the final application; in some cases, this can be almost impossible since it is common that application requests are opposing, for example, rigid in one area but flexible in another area of the part. There is also option (ii): design a single part made with multiple materials that fulfill the application requirements, which is manufactured using co-injection or over-molding, otherwise called two-shot or multi-shot injection molding; this process offers extended functionality, better appearance, and high quality of the product. In this special process, critical variables include mold temperature, shrinkage, and deformation of the second injected material, and they must be analyzed carefully because the right choice will determine the success of the product.
The simulation of polymer processes permits product designers to consider any geometric or processing condition, as crazy as it may be. It is known in the field of design that during the development of a product, the cost of fixing an error grows exponentially with the time it takes to detect the error. For that reason, the advantage of using simulation as a design tool can be very economical, but there are other reasons: such as decreasing the development time. Using a computer and software with material, process, and product data can lessen the need for tests (experiments) and reduce the costs related to material, staff, and machine time. For this, it is necessary to know as much as possible about the material variables, process conditions, and even the small details about the initial design of the product that will change during the process until the optimal design and/or processing result is obtained. The more information the designer has on it, the more accurate and more exact the simulation and the resulting values of the analysis will be.
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 . 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).
In the United States, the Food and Drug Administration (FDA) is the entity in charge of approving materials that come into contact with human body, based on the material’s biocompatibility and biostability properties. There are a variety of methods that can be used to verify a material’s properties. However not all materials will require testing, the FDA states “if there is sufficient knowledge about the biocompatibility and toxicity of the material, then it is not necessary for further biocompatibility testing”.