Secondary injection molding, like co-injection molding, dual-injection molding, and sandwich molding, is all part of 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 base material or substrate material, and the second injected material is called the cover material.
1. Various secondary injection molding techniques
In the secondary injection molding process, the overlay material is injected above, below, around, or inside the base material to combine into a complete part. This process can be completed by multiple injection molding or insert molding. Usually, the cover material used is an elastomeric resin.
Multiple Injection Molding: Multiple injection molding is a good method of processing medical devices if the construction of the cover material allows it. This technique requires special injection molding machines 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 to inject the resin into the mold.
When the same injection point is used, it is called co-molding and the composite part produced is the core resin material covered by the outer layer. When multiple injection points are used, it is called secondary injection molding, where one material is molded on top of another, producing a multi-layer structure.
However, multiple injection molding does not apply to all products. For secondary injection, the ram must be moved or the core must be moved to another cavity, and another method is to feed the core into another injection molding machine.
Insert molding: To produce products such as injection molded handles with complete coverage, insert molding is used. To achieve complete coverage, the base material must be moved out of the original mold cavity and into another core and cavity to inject the covering material.
During this process, the other mold should run simultaneously on the same or another injection molding machine of a different size (depending on the size of the molded part). Usually, the base material is much larger than the overlay material and may need to be preheated so that the surface temperature is close to the melting point of the overlay material to obtain the best bond strength.
2、In-mold assembly
Secondary injection molding is sometimes referred to as in-mold assembly, because the two materials end up completely combined, rather than just producing a layered structure, whether it is a separate part or component material, this technique can be used. Regardless of the application, it is critical to ensure that the substrate and overlay materials achieve the required mechanical or chemical bonding strength.
3, the considerations of multi-material injection molding
In general, to strengthen the bonding, the melting temperature of the covering material resin should be the same as the base material. If the melting temperature of the cover material is too low, it can not melt the surface of the base material, the bond between the two is not strong enough. However, if the melting temperature is too high, the base material will be softened and deformed, and in severe cases, the covering material will penetrate the base material, failing the part processing. Therefore, choosing a matching material is the only way to ensure a good bond.
Generally speaking, matching materials should have similar chemical properties or contain matching compound components. When the base material is not matched to the overlay material, only a mechanical interlocking effect is usually formed, not a chemical bond.
Multi-material injection molding also requires attention to several problems, the most common of which include insufficient chemical or mechanical bonding strength between polymers, incomplete filling of the individual or multiple parts, and flashing of the individual or multiple material parts.
Injection molding machines are required to maintain injection consistency. In addition, the ratio of the shot volume of the injection molding machine barrel to the size of the injection molded part is an important factor in injection quality. This ratio is critical for all injection molding operations and is especially important in secondary molding.
The stopper can act like a sluice gate to separate the cover material. It is easier to operate the stopper when the second molding material is metal, but more difficult to operate when the base material is metal and the plastic is more flexible.
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