TPE elastomers are increasingly being used in product designs due to their excellent surface feel and the flexibility of adjusting their hardness through modification by manufacturers. From everyday items like toothbrushes and pens to industrial power tools and computer and mobile phone accessories, TPE overmolding has become ubiquitous. The rigid plastic components have also expanded—from the original PP and ABS to materials such as PC, PA, and POM. This process, where two different materials are combined via secondary injection molding—commonly referred to as overmolding—requires a detailed analysis of the injection-molding process to identify the root causes of poor adhesion.
From a process perspective, there are the following points:

1. Two-color injection molding (2K molding) delivers better results than two-step injection molding (2-step molding).

Reason: Simply put, the hard plastic is still hot when it’s first injection-molded and immediately enters the second cavity of the 2K injection molding machine. During the intermediate transfer process, it doesn’t absorb moisture, so it’s neither easily eroded by the ultra-thin layer formed on its surface by the high-temperature TPE melt, nor affected by the adsorption of moisture vapor on its surface.

2. Within the processing limits of TPE materials, select the highest possible barrel (melt) temperature whenever feasible; otherwise, TPE may easily fail to provide sufficient heat to ablate the surface of the rigid plastic material.

3. During the second step of rubber coating, the mold cavity for the rigid plastic insert should be set at as high a mold temperature as possible. The higher the temperature of the rigid plastic, the slower the high-temperature TPE melt will cool, allowing sufficient heat and time for the TPE to erode the rigid plastic and form an ultra-thin, mutually soluble layer.

4. During the second step of encapsulation, while avoiding TPE flash formation, use the highest possible injection speed to inject the molten TPE.

1) High firing rate—TPE spreads quickly on hard plastic surfaces, allowing for a longer duration of ablation on the hard plastic surface within the molding cycle.

2) High firing rate—friction between the TPE and the hard plastic surface generates heat, which slows down the cooling of the TPE melt temperature, allowing it to remain in contact with and ablate the hard plastic surface for a longer period.

3) High injection speed—most TPEs (except TPU, which exhibits greater temperature sensitivity in melt viscosity)—experience shear-thinning of melt viscosity, leading to a decrease in surface tension and facilitating better spreading on hard plastic surfaces.

5. To avoid moisture absorption or contamination on hard plastic surfaces—especially during two-step injection molding, where the hard plastic undergoes pauses and transfers in between steps—this issue is even more likely to occur. Polar hard plastics such as PC, PBT, PET, and POM, particularly highly polar ones like nylon 6 and nylon 66, absorb moisture on their surfaces, leading to hydrogen bonding between water molecules adsorbed onto the surface. As a result, TPE cannot effectively form intermolecular bonds with the surface molecules of the hard plastic, naturally reducing the adhesion strength of the encapsulation. Surface contamination of hard plastic parts—for example, if workshop workers wear unclean gloves or if cotton fibers from gloves adhere to the surface of polar hard plastic components—will also adversely affect the TPE melt’s ability to properly encapsulate the hard plastic. Furthermore, TPE formulations used for coating polar hard plastics are inherently polar themselves; if moisture protection is neglected, this will similarly impair the cohesion of the encapsulation bond.