Precautions for multi-material injection molding

Secondary injection molding, like double injection molding and sandwich injection molding, is a multi-material injection molding technology. The basic idea of multi-material injection molding is to combine 2 or more materials with different characteristics to increase the value of the product. The first injected material is called the substrate or base material, and the second injected material is called the covering material.

1. Various secondary injection molding technologies

Automotive Plastic Injection Molding Companies during the secondary injection molding process, the covering material is injected above, below, around or inside the substrate to form a complete part. This process can be completed by multiple injection molding or insert injection molding. The covering material commonly used is elastic resin.

Multiple injection molding: If the structure of the covering material allows, multiple injection molding is a good method for medical device processing. This technology requires a special injection molding machine equipped with multiple barrels to inject different resins into one injection mold. The barrels should be placed side by side or in an L-shape, with one or more injection points injecting the resin into the mold.

When the same injection point is used, it is called co-molding, and the composite part produced is a core resin material covered by an outer layer. When multiple injection points are used, it is called overmolding, and one material is molded on top of another material to produce a multi-layer structure.

However, overmolding is not suitable for all products. For overmolding, the slide must be moved or the core must be moved to another cavity. Another option is to feed the core into another injection molding machine.

Insert molding: To produce products such as fully covered injection molded handles, insert molding is required. To achieve complete coverage, the substrate must be removed from the original mold cavity and placed in another core and cavity to inject the covering material.

During this process, another mold should be running simultaneously on the same or another injection molding machine of a different size (depending on the size of the injection molded part). Usually the substrate is much larger than the covering material and may need to be preheated to bring the surface temperature close to the melting point of the covering material to achieve the best bond strength.

2. In-mold assembly

Secondary injection molding is sometimes called in-mold assembly because the two materials are finally fully combined, rather than just producing a layered structure. This technology can be used for both individual parts and component materials. Regardless of the application, it is critical to ensure that the substrate and the covering material achieve the required mechanical or chemical bonding strength.

3. Precautions for multi-material injection molding

In general, to enhance the bonding, 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 will not melt the surface of the substrate, and the bond between the two will not be 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, causing the component processing to fail. Therefore, choosing matching materials can ensure good bonding.

In general, 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, not chemical bonding.

Automotive Plastic Injection Molding Companies there are also some issues to be aware of in multi-material injection molding, the most common of which include: insufficient chemical or mechanical bonding strength between polymers, incomplete filling of single or multiple parts, and flash on single or multiple parts.

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

The backstop can separate the covering material like a sluice. The backstop is easier to operate when the secondary injection molding materials are all metal. If the metal substrate and the more flexible plastic, the backstop is more difficult to operate.