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Introduction to Metal Insert Molding and Double Injection Molding

Points to Note in Metal Insert Molding Design

Metal insert molding refers to the formation process of injecting resin into a mold after preparing a pre-made insert made of a different material. The melted material is bonded and solidified with the insert, creating an integrated product. The formation of metal inserts can result in uneven shrinkage rates, and therefore, important parts should be tested for shape and size accuracy limits beforehand. During the injection process, metal inserts are prone to deformation and displacement, which should be considered when designing the mold to ensure that the shape of the metal insert is preserved. Pre-testing is essential for products in which the shape of the insert cannot be altered. Predictable factors that determine mold construction, such as gate positions and formation cycle, should be addressed or improved with corresponding measures in advance. It should be confirmed whether the metal inserts require preheating or drying treatment, with the aim of ensuring product quality and stable formation.

To avoid the accumulation of fine metal insert or forming product particles in the mold cavity, an air blowing device may be assembled if necessary. The rate of metal insert formation, productivity, and formation cost are influenced by various factors and technical tricks, such as the accuracy of the insert, the shape of the insert, whether the mold is conducive to insert molding, and the shape of the formed product. The effective combination of the injection machine, the mold, and the automation device and how to operate them efficiently in a short time are crucial in determining the automatic insert molding system.

Key Elements in Double Injection Molding Process Design

In the double injection molding process, coating materials are injected above, below, around, or inside the base material, forming a complete component. This process can be achieved through multiple injections or embedding injections. Elastic resin is typically used as a coating material. When performing double injection molding, the slider must be moved, or the core must be moved to another mold cavity. Another method is to send the core into another injection molding machine.

The reverse-stop device between the base material and the coating material is critical for adhesive effectiveness and should prevent the gradually thinning or frayed injection of the coating material. If the coating material is too thin, it may result in poor adhesion, debonding, and curling. An excellent reverse-stop device design should distinctively separate the coating material and the base material. Nozzle hole design is equally important for successful double injection molding. The length-to-wall thickness ratio of the flow path is the primary factor influencing the adhesive effect.

In order to shorten the process as much as possible, the nozzle hole should be placed in the position with the maximum wall thickness. When using TPE resin, attention should be paid to the size of the nozzle hole. Materials such as TPU require a larger diameter nozzle to adapt to higher viscosity to prevent material degradation due to excessive shear force. SEBS materials require a higher shear rate to achieve optimal flow velocity. A better approach is to use a smaller nozzle diameter in the initial stage, then adjust the nozzle size after the initial sampling.

Like the nozzle hole, the air outlet also plays an important role in adhesive effectiveness. How to control the air volume is a major challenge. If poorly controlled, inadequate adhesion, and filling fluffing may occur. The depth of the air outlet is critical to prevent fluffing. For some component designs, decorative surface textures may be used to facilitate the forming product popping out. Most TPE materials adhere easily to the mold surface due to their metal affinity, or because a vacuum is formed between the material and the mold surface during the demolding process. As many materials have not yet formed stable chemical adhesion after the component pops out, if it adheres to the mold surface, it will greatly affect the adhesive effectiveness. This implies that the formed parts must be carefully handled after processing, and if adhesive testing is needed, it should be conducted after 24 hours to allow the material to form stable chemical adhesion. If the tension is not enough when the surface of the mold is parallel to the demolding direction, adhesion may also occur. In addition, coating the mold surface will also help the component pop out.

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