multi-material injection molding technology includes two-shot injection molding, like co-injection, double injection molding and sandwich injection molding, etc. The basic idea of multi-material injection molding is to combine 2 or more materials with different properties to increase product value. The first implant material is called the substrate or base material, and the second implant material is called the over molding material.
1. Various two-shot injection molding technologies
During the overmolding process, the cover material is injected over, under, around, or into the substrate to form a complete part. This process can be done by multiple injection molding or insert injection molding. A commonly used covering material is elastic resin.
Multi-injection: Multi-injection is a good method of medical device processing if the construction of the covering material allows it. This technique requires specially designed injection molding machines equipped with multiple barrels in order to inject different resins into one injection mold. Barrels should be placed side-by-side or in an L-shape, with one or more injection points to inject resin into the mold.
When using the same injection point, known as co-molding, the resulting composite part is a core resin material covered by an outer layer. When multiple injection points are used, called double shot injection molding, one material is molded on top of another, creating a multi-layered structure.
But multiple injection molding is not suitable for all products. When over-molding, the slider must be moved or the core must be moved to another cavity, and another method is to feed the core by another shot barrel of the multi-shot injection molding machine.
Insert injection molding: To produce products such as fully covered injection handles, insert injection molding is required. To achieve full coverage, the substrate must be removed from the original cavity and placed into another core and cavity for injection of the covering material.
During this process, another mold should be running at the same time on the same or another different size injection molding machine (depending on the injection part size). Typically the substrate is much larger than the cover material and may require preheating to bring the surface temperature close to the melting point of the cover material for optimum bond strength.
2. Double shot injection moulding
Double shot injection moulding is know as overmolding because the two materials end up fully combined, rather than just creating a layered structure, and can be used for individual parts or component materials. Regardless of the application, it is critical to ensure the desired mechanical or chemical bond strength between the substrate and cover material.
3. Cautions for multi-material injection molding
In general, to enhance adhesion, the cover material resin should have the same melting temperature as the base material. If the melting temperature of the cover material is too low, the surface of the substrate cannot be melted and the bond between the two is not strong enough. However, if the melting temperature is too high, the base material will soften and deform. In severe cases, the covering material will penetrate the base material, resulting in the failure of parts processing. Therefore, matching materials are selected to ensure good bonding.
In general, matching materials should have similar chemical properties or contain matching composite components. When the substrate and cover material do not match, there is usually only a mechanical interlock, not a chemical bond.
There are also some issues to be aware of in multi-material injection molding, the most common of which include: insufficient chemical or mechanical bond strength between polymers, incomplete material filling of single or multiple parts, and flash flashing of single or multiple material parts.
The injection molding machine must 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 also an important factor affecting the injection quality. This ratio is critical for all injection molding operations and is especially important in overmolding.
The anti-return device can separate the covering material like a sluice. It is easier to operate the anti-return device when the secondary injection material is metal. If the metal base material and relatively elastic plastic are used, the operation of the anti-return device is more difficult.
