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Multi-Component Molding: Which Way Should You Go?

Jul. 25, 2018

Multishot molding adds significant value to plastic products: soft-touch enhancements (for hand tools, toothbrushes, razor handles, writing instruments); performance improvements via molded-in gaskets, o-rings, seals; permanent graphics through the use of additional colored resins; and part functionality with in-mold-assembly techniques. Consumers have demonstrated, through purchasing trends, the value of placing additional materials on common products. OEMs and brand owners appreciate the quality enhancements and overall production cost efficiencies of incorporating multishot molding techniques into their product lines.

Multishot or multi-component molding is certainly not a new technique. Methods of creating injection molded parts with more than one resin have been around for decades. Yet, the very nature of the process and the specialized molding equipment required preclude it from common use at the majority of both custom molders and OEMs’ captive molding facilities. That needs to change, because molders with the appropriate resources (multishot-enabled injection molding machines and trained personnel) realize significant opportunities with both existing and new customers.

There are many ways of referring to the multishot process and some of the names are actually misleading. Two-shot, twin-shot, dual-shot, 2K (from the German word “kunststoff” meaning plastics), two-color, double-shot, overmold, and bi-injection come to mind. Depending on who is doing the talking, you will hear several definitions of what “multishot” means. Related processes include:

Machines with multiple barrels to facilitate color changes.

Molding a base substrate in one mold in one press then transferring and inserting the first-shot part into a second mold in a different press to overmold the second shot.

Reducing cycle time for an unusually thick-walled part by creating a multishot mold but using the same resin in all shots as built-up layers.

Consecutively injecting two resins into the same injection point of the mold to create a two component part with a core and a surface layer—usually referred to as co-injection, bi-injection or sandwich molding.

For this article, we will define multishot molding as using one injection molding machine and one injection mold to create a part with two or more plastic resins within one cycle. We will also take the perspective that this is the molder’s first multishot project. Let’s concentrate on a two-shot process producing one finished part each cycle. We will need a one-cavity, two-shot mold and a press that is able to inject a specific amount of one material into one injection point of the mold and simultaneously inject a specific amount of another material into another injection point of the same mold. That holds true for the majority of multishot molding applications.

There is also a style of part design that requires injection to be consecutive—first shot followed by second shot—we will examine that a bit later in this article. Regardless of technique, we’ll need a press with two injection units and the controls to regulate the process. But first, we’ll discuss the multishot mold.

The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is

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