Plastic Injection Molding

The injection molding process utilizes molds or tooling to produce the part or component correctly and to the quality standards you require. Molds are typically made from steel or aluminum depending on how many ‘shots’ or cycles you want to get from the mold. The mold consists of many components but can be split into two halves. Each half is attached to the inside of the injection molding machine and the rear half is on a hydraulic slide so that the mold opens and closes along the molds parting line. Two main components of the mold are the mold core and mold cavity that are filled with melted plastic to create the desired component or product. Multiple cavity molds are sometimes used in which the mold can have more than one component or product cavity in the mold.

The mold core and cavity are each mounted on the mold base which is then fixed to the platens inside the injection molding machine. The front half of the mold base also includes a support plate to which the mold cavity is attached, the sprue bushing which is where the material will flow from the nozzle of the bin holding the plastic material, a locating ring used in order to align the mold base with the nozzle. The rear half of the mold base will have the ejection system to which the mold core is attached and another support plate. When the clamping unit separates the mold halves the ejector bar will activate the ejection system. The ejector bar pushes the ejector plate forward inside the ejector box, which in turn pushes the ejector pins into the component or product. The ejector pins push out the finished part.

In order to get good flow rate into the mold cavities, several channels are integrated into the mold design. The melted plastic enters the mold through the sprue. Channels referred to as runners are added which carry the melted plastic from the sprue to all of the areas of the cavity that must be filled. At the end of each runner, the melted plastic must enter the cavity through a gate that directs the flow of the melted plastic. The melted plastic that solidifies inside the runners is attached to the finished part and must be separated after the part has been ejected from the mold. There are a couple of things that can change on this mold process regarding the runners: 1) Hot runner systems can be used which independently heat the channels and allows the material to be melted and detached from the part; or 2) a channel can be built into the mold that cools the channel. These channels allow water to flow through the mold walls and cool the melted plastic.

In addition to runners and gates, there are many other things that need to be considered in the design of the molds. The mold must allow the melted plastic to flow easily into all of the cavities. Also important is the removal of the finished part from the mold, so a draft angle must be applied to the mold walls. The mold must also accommodate any complex features of the part, such as undercuts or threads, which will also require additional mold pieces. Most of the pieces slide into the mold cavity through the side of the mold and are known as slides, or side actions. One of the most common side-action is a side-core which allows the external undercut to be molded. Other pieces enter through the end of the mold along the parting direction, such as internal core lifters which allow and internal undercut. To mold threads into the part an unscrewing device must be added which can rotate out of the mold after the threads are formed.