why can't TPE overmolding with PP firmly? 

Why can't TPE overmolding with PP firmly?

By using two-shot injection molding or dual-color injection molding, TPE materials can be bonded to the surface of PP products, which not only enhances the tactile feel and aesthetics of the product but also adds functions such as sealing, shock absorption, anti-slip, and noise reduction. However, during the encapsulation of TPE on PP, there can sometimes be issues with poor adhesion. The main reasons are as follows:

• Firstly, the selection of TPE material is crucial. TPE overmolding with PP differs from that for other materials (such as ABS, PC, PA). PP is a non-polar material with a solubility parameter of 7.8-8. According to the principle of "like dissolves like," we need to choose materials with similar polarity and solubility parameters to achieve a good bonding effect.
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• Secondly, the processing temperature is also crucial. If the processing temperature is too low, the TPE material and the PP product surface will not form a good interfacial compatibility, resulting in poor adhesion. Conversely, if the processing temperature is too high, it will affect the performance of the TPE material. Generally, we recommend that the injection molding temperature for TPE overmolding with PP products be between 180-210 degrees Celsius. Special cases may require adjustments based on the size of the product or the content of glass fibers.
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• Thirdly, the product size can also affect the bonding effect. If the product size is large, the TPE material will take longer to overmolding PP in the mold cavity. This extended time can cause the material's temperature to gradually decrease during flow, leading to the phenomenon of cold adhesion, which is not conducive to encapsulation bonding.

The solution is to increase the processing temperature and mold temperature, enhance the injection pressure and speed, or adjust the position and number of gates and other processes, so that the TPE material can quickly encapsulate the surface of the PP product, thereby achieving a good encapsulation effect.