Twin-shot injection moulding, also known as overlay injection molding, is a type of multi-component injection molding. It is a process where a more elastic material (usually a polyurethane elastomer) is layered onto a rigid substrate. In twin-shot injection moulding, another component is overlaid on the rigid substrate to finally form a product with different mechanical properties, different two-color visual effects, and a permanently bonded polymer composition.
Twin-shot injection moulding technology originated in 1961 when Germany's Arburg developed a two-color injection molding machine. However, due to the high cost of early two-color machines and the complex and precise design requirements of two-color molds, many manufacturing plants opted for secondary injection molding. This process involves molding an item using a single plastic material and mold, then removing the molded part and placing it into another mold to inject the same or a different plastic material. With the maturation of the technology, twin-shot injection moulding has seen widespread application in recent years in almost all plastic fields, such as electronic products, power tools, medical products, home appliances, toys, and more. It meets the increasing demand for diversified, high-quality, and high-value-added products.
The twin-shot injection moulding machine consists of two plasticizing injection units with identical structures and specifications. The nozzles should have a special structure or be equipped with identical twin molding molds that can rotate if required by the production method. During plasticizing injection, the temperature of the melt, injection pressure, amount of injection melt, and other process parameters in the two plasticizing injection units should be identical, and fluctuations in these parameters should be minimized.
Compared to ordinary injection-molded plastic products, the melt temperature and injection pressure during twin-shot injection moulding need to adopt higher parameter values. The main reason is that the mold flow paths in twin-shot injection moulding are relatively long and complex, resulting in greater injection melt flow resistance.
Materials with good thermal stability and low melt viscosity should be selected for twin-shot injection moulding plastic products to avoid decomposition due to the high melt temperature and long retention time in the flow path. The commonly used plastics are polyolefin resins, polystyrene, and ABS materials.
To ensure that the two different colored melts fuse well in the mold and guarantee the molding quality of injection-molded products, higher melt temperatures, mold temperatures, injection pressures, and injection rates should be used during the injection molding process of twin-shot injection moulding plastic products.
