The reason why TPE overmolding is not strong, in short, one is that the material is not selected correctly (compatibility and fluidity of the compounding system); the second is that the temperature required for overmolding is not controlled well!

　　According to the philosophical point of view, the internal cause refers to the internal reason for the development and change of things, that is, the internal basis; external cause refers to the external reason for the development and change of things, that is, external conditions. tpe overmolding effect, or the mechanism of tpe overmolding plastic, there are also two aspects of internal and external causes.

　　Internal factors: the polarity and compatibility of TPE and encapsulated materials

　　Theoretical basis

　　This is the internal basis for effective bonding. According to the principle of similar solubility and fluid solubility parameter (SP), TPE and the covered material should have close polarity and good compatibility in order to achieve smooth overmolding.

　　Extension content: mastering the solubility parameter is to master the degree of compatibility between different polymers, which provides the basis for the success and use. The more similar the solubility parameters of two polymers are, the better the blending effect. If the difference between the two exceeds 0.5, it is generally difficult to blend uniformly, and additional solubilizers are needed to do so. The role of the solubilizer is to reduce the surface tension of the two phases, so that the surface at the interface is radicalized, thereby increasing the degree of compatibility. The solubilizer is often a polymer that acts as a bridge intermediary. In addition, when designing a formulation, the choice of liquid additives for a certain adhesive must also take into account the proximity of the two SPs to ensure that the components are evenly dispersed.

　　Practical analysis

　　TPE (SEBS base) and PP, TPR (SBS base) and PS, the materials themselves have good compatibility and close polarity, no need to add compatibilizer or polarity modifier to the blending system.

　　TPE and PC, ABS, PA, PE and other plastics have a certain gap in polarity and are less compatible. By adding compatibilizer in the compounding system, the bonding effect of TPE and PC, ABS, PC, etc. can be improved.

　　POM material has high density of internal cohesion, and the solubility parameter is very different from TPE, while POM has a large surface tension. At present, most of the TPE blending system, including domestic TPE, can not effectively reduce the polarity difference between TPE and POM, as well as reduce the surface tension of POM materials. (Plastic Bo's humble opinion, TPE blending system can introduce a kind of alkaline substances, because POM will be corroded by alkaline substances). I can also understand why TPE coating POM, become a technical problem difficult to solve in the industry.

　　External factors: the temperature required for bonding between TPE and the covered material

　　Theoretical basis

　　The surface tension of the material, compatibility is only relevant in the liquid or molten state. In overmolding, sufficient temperature must be provided for the TPE material with overmolding performance to have the proper overmolding effect. If the temperature is too low, the material will not be compatible at the interface of the two phases (fusion and interpenetration at the phase structure and even molecular level) and chemically, and no change can be made to the surface tension of the two phases, then the overmolding phase will not achieve good results.

　　Practical analysis

　　TPE and PP, TPR and PS have good compatibility, as well as TPE and ABS, PC, PA, etc. Due to the addition of compatibilizers, only sufficient overmolding temperature is needed to provide the required energy for the compatible bonding of the two materials at the contact interface.

　　Even if we use the expected overmolding temperature, there are times when the product structure (such as the size of the overmolded surface), the overmolded flow length ratio, and the material fluidity will prolong the flow filling time of the TPE melt in the mold cavity, resulting in a significant drop in the temperature of the TPE melt, which is detrimental to overmolding. Therefore, it is necessary to choose different TPE materials (flowability) and design suitable product gates (flow length ratio) for different overmolding product structures.

　　For nylon material, because the melting points of different raw materials and modified materials are different, in order to ensure the effective compatibility of TPE and nylon two-phase interface, TPE and nylon materials with similar melting points should be chosen to match, generally the melting point of TPE is slightly lower than that of nylon material by 5~10℃ is good. In addition, for the overmold, the nylon parts should be pre-baked to enhance the surface temperature of the nylon parts to ensure that the overmold has sufficient two-phase interface temperature to obtain good overmold bonding effect. It is recommended that the baking temperature is 130~140℃ and the baking time is 3~4 hours.