The process of overmolding a soft thermoplastic elastomer (TPE) onto a rigid engineering thermoplastic (ETP) can sometimes be tricky; feel like an art, more than a science. But if we start with the fundamentals, like the basics of adhesion, and understand the science behind it, we can make more informed decisions concerning material selection and processing.
Adhesion of the TPE onto the ETP substrate occurs via “physical blending”. The three stages of physical blending are depicted below:
What can we learn from this?
1. Chemical Compatibility is Important
Physical blending between the TPE and ETP is greatly dependent on the chemical compatibility of the materials, including miscibility and interfacial tension. TPEs need to be chemically modified to adhere to a specific substrate material such that their polarity and solubility parameters match. Anything that interferes with the “blending”, like moisture or mold release agents, will adversely affect the chemical interactions at the interface. Teknor Apex portfolio includes specialty TPEs that are adhesion-modified for excellent bonding to a wide range of ETPs.
2. Heat Promotes Adhesion
Sufficient heat must be transferred from the melted TPE to the substrate to promote diffusion and entanglement of the polymer chains. Thus optimal adhesion is achieved when the substrate is softened or melted. This is why 2K or multi-shot molding is preferred over insert molding, and also why preheating the substrate is recommended for inserts molding.
3. Processing Conditions have a Significant Influence
Which processing parameters have the greatest impact on adhesion?
• Melt Temperature: The TPE melt temperature is determined by the substrate and the level of adhesion desired. Higher temperatures result in greater bond strength.
• Injection Speed: The ideal injection speed of the TPE is as fast as possible without warping or moving the substrate.
• Holding Pressure: The ideal holding time is however long it takes for the gate to freeze off. To do so, use a “Gate Freeze Analysis” to determine the optimum time, which involves weighing parts made using different holding times to determine when the part weight does not change.
How strong is your bond?
In the industry, several different test methods are used to determine the resistance-to-peel strength of adhesive bonds between flexible and rigid materials, so when looking at data, make sure you’re comparing apples to apples. At Teknor Apex, we use a 90 deg. C peel test, ASTM D6862, which determines the resistance to peeling in lbf/in or N/cm. Our research shows that excellent adhesion is achieved when the peel force is greater than 15 lbf/in, and poor adhesion is exhibited by values of 10 lbf/in and under.
There are two types of failure modes:
(1) Adhesive failure occurs when the TPE peels from the substrate (Material Strength > Peel Strength)
(2) Cohesive failure occurs when the TPE material breaks before it peels from the substrate (Material Strength < Peel Strength) Cohesive is the more desired mode of failure.
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