What is Silicone Overmolding?
Liquid silicone overmolding is a process for molding silicone rubber onto a base material or object such as a housing, wire, or handle to create a composite component. When referring to overmolding thermoplastics or other silicones, overmolding may also be called two shot molding.
Why overmold silicone?
The primary reasons for overmolding are quality and performance such as simplifying assembly and improving sealing quality between components. Reasons vary by application but include lower assembly cost and achieve device functionality such as in electronics encapsulation. Silicones act as protective covers against dust, water, impact, heat and electrical shock. For implantable devices, silicone creates a bio-friendly barrier between the medical device and the patient. This leads them to use in a wide range of overmolding applications.
- Medical Encapsulation of Electronics
- Overmolded Gaskets
- Surgical Handles
- Radio Opaque Markers for Positioning Medical Devices
- Electrical and Thermal Insulation
Silicones have been used for reinforcing, changing mechanical behavior, and they demonstrate good compression sets giving gaskets long term sealing quality. Silicones are stable through a wide range of temperatures and demonstrate high surface finish reproducibility for texturing. Silicone can act as a protective cover against dust, water, impact, heat and electrical shock since silicone materials are naturally electrical and thermal insulating. Specialty silicones may be formulated to transfer heat or conduct electricity for electrical contacts. Additionally, medical grade silicone materials have favorable biocompatibility and support being sterilized.
What about bonding?
Liquid silicone is typically bonded to metal, thermoplastics and silicone. Bond development is a typical step in most overmolding projects as part of the R&D process. Silicone overmolding bonds through either a mechanical bond, where the silicone interlocks “grabs” with features on the substrate shown in Figure 1 below or through chemical bonding at the surface between the silicone and substrate. Chemical silicone bonding is typically stronger and more reliable than mechanical bonding, since it takes place on a molecular level and covers a greater surface area.
Mechanical bonding is pretty straight forward. One major consideration is that silicone materials are often very deformable so simply filling in a plug hole may not be sufficient to achieve desired bonding. Mesh designs and interlocks that connect to great mechanical loops often are more effective as shown in Figure 1.
Chemical bonding falls into two categories. The first is adhesive where the bonding is not greater than the strength of silicone or substrate shown in Figure 2. The second is cohesive bonding where the bonding is great enough that bonding failure causes ripping of the materials before bonding shown in Figure 3. In practice both types of bonding are often seen. When materials are not fully compatible, surface treatments including primers or surface activations such as corona or plasma are used.
Primers are applied to the substrate prior to molding silicone to act as a tie layer. The primer bonds to the substrate and the silicone bonds to the primer. Most of our silicone suppliers offer platinum primers to support their line of liquid silicones. Some primers target specific substrate materials such as metals, plastic families, etc. Primers can be a robust solution to some bonding challenges. Unfortunately, primers do add a step to production and in medical silicone applications, the primer can affect biocompatibility.
Plasma and corona treatment for bonding liquid silicone to thermoplastic substrates is used in some
medical silicone applications because there are no added chemicals. Alternatively, silicones with a bonding promotor also called self-bonding silicones may provide a solution to some applications. There are adhesion promoters manufactured for different thermoplastics. The challenge with self-bonding silicones is that the material tends to stick to everything, including the mold tooling. Additionally, self-bonding silicones have adhesion promoters that can affect biocompatibility.
Testing should be performed in order to determine the best liquid silicone and substrate match for bonding. A common qualitative test molds a section of silicone onto a substrate sample. The silicone is then pulled off to see if total bonding failure may be ruled out quickly. This often provides quick insight. For quantitative testing: lap shear, tensile strength tests, and cyclic fatigue tests are often used.
What about substrates?
A major risk factor in all substrates is mold release, oils, or chemicals that inhibit bonding such as sulfur. Even trace amounts of these substances may cause additional challenges or failure in an overmolding project.
Substrate tolerances have a significant impact on molding and should be considered. Silicone flashes at 0.0001in (0.0025mm) or less under molding pressures. The Tips Section of this page discusses shut-off considerations directly, but here are few points to remember:
- Remember that silicone is deformable and so tolerances are often over-specified increasing both the cost and challenge of building it.
- A loose substrate flashes and a tight substrate crashes causing damage to the part, tool, or both. Designs should carefully consider how the substrate interacts with the mold.
Thermoplastics – Plastic components are common as a base substrate. Bonding is often achieved through the use of any of the bonding techniques listed including surface treatment, primers,
mechanical bonding, etc. It should also be noted that many plastic substrates have low softening and melting temperatures. Two common reference points for thermoplastics include Vicat softening temperature or alternatively Heat Deflection Temperature which may be an indicator that the material will not be rigid enough to overmold silicone using standard processes. When possible, it is best to select a substrate material with a softening temperature above 300°F. We at Albright have developed processes for specialty liquid silicone overmolding at low temperatures in order to support temperature sensitive substrate projects. We have successfully bonded to a number of thermoplastics including Polyetherimide, Nylon, Polycarbonate, and others. Chemical bonding in Olefin materials such as polyethylene and polypropylene are not recommended due to their low softening temperature and low bonding probability.
Metals – Bonding silicone to metals tends to be more straightforward and robust due to the temperature stability of most metals. Aluminum and common steels can often be bonded with mechanical means or with surface treatments especially primer. More exotic metals such as titanium may be possible but require more development effort and special processing to be successful.
Silicone to Silicone – This process is known as two shot silicone molding. Unlike thermoplastics, silicone on silicone generally has temperature stability and favorable bonding as long as the silicone substrate is clean and the cure system such as platinum is matched. Post-baking of the first shot before overmolding should be avoided so as to leave open as many chemical bonding sites as possible. Post-baking the silicone part after silicone overmolding will promote the best bond. The challenge becomes managing the deformation of the first shot when the second shot is processed.
Electronics – We have successfully overmolded flex circuits, LEDs, batteries, and other circuit components. The primary challenges are locating the substrate in place during molding and overcoming temperature restrictions. Some electronic components’ sensitivity to pressure and temperature limit material and process selection. These are less efficient resulting in higher cost techniques based on electrical component design limitations.
Tips and Advice:
- Remember not all substrates are created equal so changing grades or finishes may significantly impact the ability to bond.
- Shut off is a region where the mold contacts the substrate to prevent silicone from flowing into key areas as shown in Figure 4. Vertical shut offs are typically more challenging to create high quality edges.
- Substrate location – features may be needed to control the location of the substrate during molding. Keep in mind that as material flows, ‘floating’ substrates will be prone to moving causing variability in the process and in the final product.
For real live help with your challenging silicone overmolding applications, call Albright’s engineering and processing experts for your next product. Contact us today at 978.466.5870 or via email. Feel free to send your silicone overmolding part designs to us via our RFQ page to ensure your design is suitable for production manufacturing and to receive a quote.
- Multicomponent Injection Molding of Thermoplastics and Liquid Silicone Rubber (LSR) – Either Cured by Heat or UV Light, Christof Schlitt, Michael Hartung, Annette Rüppel, SPE ANTEC 2016, http://leaders.4spe.org/spe/conferences/antec2016/papers/322.pdf.
- Wacker Chemie – Solid and Liquid Silicone Rubber Material and Processing Guidelines, revision 6709e/09.16, https://www.wacker.com/cms/media/publications/downloads/6709_EN.pdf
- The Effect of Primer on Bond Strength of Silicone Prosthetic Elastomer to Polymethylmethacrylate: an in vitro study, Harsh Patel, Ravi Kumar C. N., Ponnanna A.A., Arvind Singh Bithu, Kelvin Shah, Soham Prajapati, Journal of Clinical and Diagnostic Research, 2015 Mar; 9(3): ZC38–ZC42, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4413153/
- LIMS Self-adhesive Liquid Silicone, Shin-Etsu Silicones, Revision 2012.8, http://www.shinetsusilicones.com/files/literature/LIMS%20Self-Adhesive%202012.pdf
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