Overmolding, like co-injection, double injection and sandwich injection, are all multi-material injection technologies. The basic idea of multi-material injection molding is to combine two or more materials with different characteristics to increase product value. The first implant material is called a substrate or base material, and the second implant material is called a cover material.

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Precautions for multi-material injection molding
Written By Jasmine HL
Overmolding, like co-injection, double injection and sandwich injection, are all multi-material
injection technologies. The basic idea of multi-material injection molding is to combine two or
more materials with different characteristics to increase product value. The first implant material
is called a substrate or base material, and the second implant material is called a cover material.
1. Various secondary injection technologies
During the overmolding process, the covering material is injected above, below, around or inside
the substrate to form a complete part. This process can be done by multiple injection molding or
insert molding. A commonly used covering material is an elastic resin.
Multiple injection molding: If the construction 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, and the resin is
injected into the mold from one or more injection points.
When the same injection point is used, it is called co-plasticity, and the composite component
produced is a core resin material covered by an outer layer. When multiple injection points are
used, it is called overmolding, where one material is molded on top of another, creating a
multilayer structure.
But multiple injections are not suitable for all products. For secondary injection, the slider must
be moved or the mold core must be moved to another cavity. Another method is to send the
mold core to another injection molding machine.
Insert molding: To produce products such as fully covered injection molded handles, insert
molding is required. In order to achieve complete coverage, the substrate must be removed from

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the original cavity and placed into another core and cavity in order to inject the covering
material.
In the process, another mold should run simultaneously on the same or another injection
molding machine of a different size (depending on the size of the injection molded part). The
substrate is usually much larger than the cover material and may require preheating to bring the
surface temperature close to the melting point of the cover material to obtain the best bond
strength.
2. In-mold assembly
Overmolding is sometimes referred to as in-mold assembly because the two materials are finally
combined together, not just creating a layered structure. This technique can be used for either
individual parts or component materials. Regardless of the application, it is critical to ensure that
the substrate and cover material achieve the required mechanical or chemical bond strength.
3. Precautions for multi-material injection
In general, to enhance adhesion, the melting temperature of the covering resin should be the
same as that of the substrate. If the melting temperature of the covering material is too low, the
surface of the substrate cannot be melted, and the adhesion between the two is not strong
enough. However, or 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
component processing failure. Therefore, the choice of matching materials can ensure good
adhesion.
In general, matching materials should have similar chemical properties or contain matching
composite components. When the substrate and the cover material are not matched, usually
only mechanical interlocking can be formed, not chemical bonding.
There are some issues to be aware of in multi-material injection molding. The most common
include insufficient chemical or mechanical bonding strength between polymers, incomplete
filling of single or multiple component materials, and flashing of single or multiple material
components.
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 quality of injection molding. This ratio is critical for all
injection molding operations and is especially important in overmolding.
The check device can separate the covering material like a sluice. The check device is easier to
operate when the secondary injection molding material is metal. If it is a metal substrate and a
more elastic plastic, the check device is more difficult to operate.