When thermoplastic elastomer (TPE) is co-injected with metal inserts, interfacial bonding and stress concentration are key factors affecting product performance. TPE materials combine the elasticity of rubber with the processability of plastics, but their non-polar properties create a natural compatibility difference with the polar surfaces of metals, leading to insufficient interfacial wetting and low bond strength. Metal surfaces often have oxide layers or oil stains; if not thoroughly cleaned, these form a physical barrier layer, hindering the microscopic contact between the TPE melt and the metal substrate, thus weakening the mechanical interlocking effect. Furthermore, insufficient surface roughness on the metal surface makes it difficult for the TPE melt to penetrate the microscopic uneven structure, failing to form an effective anchoring effect and further reducing interfacial bonding strength.
To improve interfacial bonding performance, synergistic optimization is needed in both metal pretreatment and TPE formulation modification. The metal surface needs to undergo degreasing, pickling, and rinsing processes to remove contaminants, and plasma treatment or flame treatment should be used to introduce active functional groups such as hydroxyl and carboxyl groups to increase surface energy and enhance the wettability of the TPE. In TPE formulations, maleic anhydride-grafted SEBS can be added as a compatibilizer. Its polar groups can form chemical bonds with the metal surface and simultaneously entangle with the TPE matrix molecular chains, significantly improving interfacial adhesion strength. Furthermore, adding hydrogenated C5/C9 copolymer petroleum resin can reduce the surface tension of the TPE melt, promoting its spreading on the metal surface, filling microscopic depressions, and enhancing physical adsorption.
Stress concentration issues mainly stem from the difference in thermal expansion coefficients between the metal insert and the thermoplastic elastomer (TPE) and structural design defects. The different shrinkage rates of the metal and TPE during cooling easily generate internal stress at the interface, leading to debonding or cracking. If the metal insert has sharp corners, thin walls, or abrupt structural changes, it will become a stress concentration point, increasing the risk of local stress exceeding the TPE's tolerance limit. Mold design needs to optimize the gate position and number to ensure uniform melt filling and avoid local over- or under-pressure due to uneven filling. Simultaneously, sufficient venting channels should be provided around the metal insert to prevent trapped air bubbles from forming and weakening interfacial bonding.
The preheating temperature of metal inserts is crucial for interfacial quality. If the insert temperature is too low, the TPE melt cools rapidly upon contact, causing a sharp increase in viscosity. This prevents sufficient flow and wetting of the metal surface microstructure, resulting in decreased adhesion. Preheating the insert reduces the temperature difference, prolongs melt flow time, and improves wetting. The preheating temperature needs to be adjusted according to the type of TPE and the size of the metal, generally recommended to be between 80℃ and 120℃. Large inserts or high-viscosity TPEs require higher preheating temperatures. Furthermore, using magnetic adsorption or precision positioning posts to fix the insert can prevent displacement caused by melt impact, ensuring uniform coating thickness.
The coordinated control of injection molding process parameters plays a decisive role in reducing stress concentration and improving interfacial adhesion. The melt temperature needs to be high enough to reduce viscosity and improve flowability, but excessively high temperatures can lead to thermal degradation. A medium-to-high injection speed is recommended to allow the melt to quickly overcome surface tension and achieve good spreading, but jetting or air entrapment must be avoided. The holding pressure and time must be sufficient to compensate for TPE cooling shrinkage, enhance interfacial density, and reduce internal stress. Mold temperature needs to balance cooling rate and stress release. Higher mold temperatures can slow down cooling and facilitate molecular chain relaxation, but will prolong the cycle time.
Primers are an effective means of solving difficult bonding problems. When the polarity difference between thermoplastic elastomer (TPE) and the metal is too large or the surface activity is insufficient, a special primer can act as a chemical bridge, forming a strong bond with the metal on one end and being compatible or covalently bonded to the TPE on the other, significantly improving the bonding strength. When selecting a primer, its compatibility with both the TPE and the metal must be considered, and the bonding effect must be verified through testing. Furthermore, lubricants or release agents that easily migrate to the interface should be avoided in the formulation to prevent them from weakening the bonding force.
Under long-term use, the aging and stress relaxation of TPE materials will gradually reduce the interfacial bonding strength. Ultraviolet radiation, high temperature, or humidity may trigger material degradation, leading to crack propagation or debonding. Therefore, antioxidants and light stabilizers need to be added to the formulation to improve aging resistance. Simultaneously, annealing treatment can eliminate internal residual stress and improve the material's toughness. Regularly inspecting the product interface condition and assessing changes in adhesive strength can provide a basis for optimizing design and processes, ensuring product reliability throughout its entire lifecycle.
