Understanding True and False Overmolding Techniques in TPE Applications
Time of issue:2024-08-23
TPE overmolding, also known as over-moulding, involves the process of using injection molding equipment to encapsulate TPE soft rubber material onto other materials (typically plastics). This is typically achieved through either a two-shot injection molding process using a bi-color injection machine or a secondary injection molding process with a dedicated overmolding mold. Overmolding is a crucial forming method and application for TPE, showcasing its diverse functional properties. These include soft touch (e.g., handles and grips), slip resistance (e.g., casters, coasters, mats), elasticity (e.g., buttons), sealing capabilities (e.g., composite seals), and other specialized functions (e.g., foldable kitchenware). Depending on the product's structure and desired overmolding effects, there are two distinct overmolding mechanisms: true overmolding and false overmolding.
I. True Overmolding
True overmolding refers to the fusion of the soft TPE material with the hard plastic at the interface where overmolding occurs. This fusion mechanism is possible due to the similar polarity and a degree of compatibility between TPE and the hard plastic. When TPE is injected in its molten state onto the surface of the hard plastic, it melds with the plastic at the interface (similar to metal welding). Upon cooling, the TPE and hard plastic become a firmly integrated unit, making it difficult to separate them without damaging the TPE material.
True overmolding relies on chemical compatibility and compounding, while false overmolding employs physical and mechanical bonding. In terms of adhesion strength, true overmolding surpasses false overmolding. However, the TPE materials required for true overmolding are typically more costly. Common soft materials used in true overmolding include TPE and TPR, which may vary in properties based on the type and characteristics of the hard plastic (e.g., ABS, PA, PC, PC/ABS, PP, PS). Other soft materials like TPV, TPU, and PVC are also used, but TPE and TPR are often preferred for their unique soft touch properties.
II. False Overmolding
False overmolding does not require the fusion between TPE and the hard plastic or other materials. Instead, it utilizes specific overmolding structures, such as full encapsulation of the hard component or structural modifications to the hard part (e.g., adding holes or grooves at the overmolding edges) to enhance the bond between TPE and the hard component (also known as insert injection molding). The overmolding of TPE onto metal belongs to false overmolding, and its feasibility depends on the overmolding structure. Higher hardness TPE favors metal overmolding.
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