Secondary injection molding: is a special plastic molding process, also known as encapsulation or encapsulation. It is similar to the two-color molding process, but there are relatively large differences from the two-color molding process. It is a process in which a certain plastic raw material is molded in a primary plastic mold, the molded parts are taken out, and the molded parts are placed in a secondary molding mold to be injected with the same or another plastic material for molding again. More complicated will be multiple molding, so that the appearance and process of the plastic product meet the desired requirements, which is called secondary injection molding.

Development Status

Secondary injection molding can make the surface of the product full of softness, and can also increase the functionality and added value of the product. In the past 10 years, secondary injection molding technology has completely changed the aesthetic standards, design ideas and functional requirements of consumer products. Secondary injection molding technology is famous for creating "soft surfaces", but it also has many other functions. For example, ergonomic design, two-color appearance, brand logo and feature improvement. Using this technology, the functions of the product can be increased, such as noise reduction, shock absorption, waterproofing, anti-collision and added value. It is currently widely used in mobile phone protective cases/mobile phone protective covers.

Two-shot molding, like co-shot molding, double shot molding and sandwich injection molding, is a multi-material injection molding technology. The basic idea of multi-material injection molding is to combine two or more materials with different properties to increase the value of the product. Generally speaking, the first injected material is called the substrate or base material; the second injected material is called the covering material.

Various two-shot molding technologies During the two-shot molding process, the covering material is injected above, below, around or inside the substrate to form a complete component. This process can be completed by multiple injection molding or insert injection molding. The covering material commonly used is an elastic material.

Precautions

In general, to enhance adhesion, the melting temperature of the covering material resin should be the same as that of the substrate. If the melting temperature of the covering material is too low, it cannot melt the surface of the substrate. The bond between the two is not strong enough. However, if the melting temperature is too high, the substrate will soften and deform. In severe cases, the covering material will penetrate the substrate, resulting in component processing failure. Therefore, choosing matching materials can ensure good adhesion. Generally speaking, matching materials should have similar chemical properties or contain matching composite components.

When the substrate and covering materials do not match, usually only mechanical interlocking can be formed. Rather than chemical bonding.

There are also some issues that need to be paid attention to in multi-material injection molding, the most common ones include:

1. The chemical or mechanical bonding strength between polymers is not enough, and the material filling of a single or multiple components is incomplete;

2. Flashing of burrs on a single or multiple material components.

3. The injection molding machine must maintain injection consistency.

In addition, the ratio of the injection volume of the injection molding machine barrel to the size of the molded part is also an important factor affecting the quality of injection molding. This ratio is critical for all injection molding operations, especially in secondary injection molding.

The backflow device can separate the covering material like a sluice. When the secondary injection molding materials are all metal, the backflow device is easier to operate. If it is a metal substrate and a more elastic plastic, the backflow device is more difficult to operate.

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