Two shot molding Description
Two shot molding is one of many two-component processes available today. Unlike other multi material molding processes, the co-injection process is characterized by its ability to completely encapsulate an inner core resin with an outer skin material. This encapsulation of one resin by another provides product in which only one material is evident, unless you section the product. A cross section of a molding shows a three-layer effect: skin, core, and skin (which is why co-injection is sometimes referred to as “sandwich molding”), two shot molding is similar to overmolding, and sometimes we use overmolding instead of 2 shot molding since 2K molding machine is very exxpensive.
Co-injection process mechanics rely on the sequential injection of two different materials through the same gate(s), and usually with some amount of simultaneous injection. In general, a majority of skin material is injected into a cavity, followed by a combination of both skin and core materials flowing into the same cavity, and then followed by the balance of the core material to fill the cavity. Restated, the injection sequence is: skin, followed by skin and core concurrently, followed by core. (Sometimes an additional very small amount of skin injection will “cap” the end of the sequence described.)
Accurate sensors, sensitive electronics, and fast response valves permit excellent process repeatability. Proper shut-off of the materials prevent cross contamination. Laminar polymer flow and cavity cooling promote well-defined and predictable boundary layers of the materials.
Two Shot Molding of Market Overview
Co-injection has found its way into a selection of market categories in the United States, even though its current use is a small fraction of all molded products. In every case, it is simple economics that steer applications to the co-injection process. Part performance and part cost are the drivers that lead to the selection of co-injection molding as the process of choice in the applications and markets discussed below. Any novel manufacturing technique that fails to prove itself as cost effective, does not survive.
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