Secondary injection moulding can not only make the surface of instruments soft, but also increase product functionality and added value.
In the past 10 years, secondary injection technology has thoroughly changed consumer aesthetic standards, design concepts and functional requirements. Medical device manufacturers have also recognized the potential advantages of this technology and continue to expand its application in the medical field. Thesecondary injection mouldingtechnology is known for creating "soft surfaces", but it also has many other functions, such as ergonomic design, two-color appearance, brand logo, and feature improvement. Using this technology, product functionality (such as noise reduction, shock absorption, waterproofing, and collision prevention) and added value can be increased.
Secondary injection moulding, like co-injection moulding, two-color injection moulding, and sandwich injection moulding, belongs to multi-material injection moulding technology. The basic idea of multi-material injection moulding is to combine 2 or more different materials with different properties to improve product value. In this article, the first injected material is called the substrate or base material, and the second injected material is called the cover material.
During the secondary injection moulding process, the cover material is injected above, below, around or inside the substrate, combining to form a complete part. This process can be completed through multiple injections or embedded injections. The cover material commonly used is elastic resin.
Multiple injections: If the structure of the cover material allows, multiple injections are a good medical device processing method. This technology requires a special injection moulding machine equipped with multiple barrels to inject different resins into an injection mold. The barrels should be placed side by side or in an L shape, and the resin should be injected into the mold through one or more injection points. When using the same injection point, it is called co-injection, and the produced composite part is a core resin material wrapped by an outer layer. When using multiple injection points, it is called secondary injection moulding, and one material is formed on top of another, producing a multi-layer structure.
However, multiple injections are not suitable for all products. During secondary injection moulding, the slide must be moved or the mold core must be moved to another mold cavity. Another method is to send the mold core to another injection moulding machine.
Embedded injection: In order to produce fully covered injection-molded handles and other products, embedded injection must be used. To achieve complete coverage, the substrate must be removed from the original mold cavity and placed in another mold core and mold cavity to inject the cover material. During this process, another mold should also run on the same or a different sized injection moulding machine (depending on the size of the injection molded part). Usually, the substrate is much larger than the cover material and may need to be preheated to bring the surface temperature close to the melting point of the cover material to obtain the best bonding strength.
Secondary injection moulding is sometimes called in-mold assembly because the two materials are completely combined at the end, rather than just producing a layered structure. This technology can be used for both individual parts or component materials. It is critical to ensure that the substrate and cover materials achieve the required mechanical or chemical bonding strength, regardless of the application.
Generally speaking, if you want to strengthen adhesion, the melting temperature of the cover material resin should be the same as that of the substrate. If the melting temperature of the cover material is too low, the substrate surface cannot be melted, and the bonding between the two is not strong enough. However, if the melting temperature is too high, the substrate will soften and deform. When severe, the cover material can penetrate the substrate, resulting in component processing failure. Therefore, selecting matching materials can ensure good bonding. Generally speaking, the matching materials should have similar chemical properties or