Plastic injection molding is a common manufacturing process that is desirable for many parts production projects because of its low cost and wide-ranging selection of materials.
Plastic injection molding is a manufacturing method utilized for high volume production of identical plastic parts. In this plastic injection process, molten plastic is inserted into a mold to produce a component in the form of the mold cavity. Plastic injection molding is often used to meet the following criteria:
Plastic injection molding is extremely cost effective at high production volumes—usually volumes of thousands to hundreds of thousands of components. An additional benefit of plastic injection molding is that it can be done using a wide range of materials and colors, which gives product designers more freedom in their designs.
Plastic injection molding is a highly effective manufacturing process for products in many different industries. Here are some of the most common examples:
The plastic injection molding process injects molten plastic into a mold, which subsequently cools and solidifies into a part. There are four basic steps in the injection molding process:
Plastic injection molding machines are made up of three main parts, which are the injection unit, the mold, and the clamping/ejector unit. The injection mold tool itself contains the sprue and runner system, the gates, two halves of the mold cavity, and optional side actions.
Mold cavities have an A side and B side. The B side—also known as the “core”—is usually the non-cosmetic, interior side that has the ejection pins used to push the finished part out of the mold. The A side—also known as the “cavity”—is the other side of the mold that gets filled with molten plastic. Mold cavities contain vents that enable hot air to dissipate so the parts don’t overheat.
The runner system channels the molten plastic from the screw feed to the cavity. For cold runner molds, the plastic solidifies in the runner system and the part cavities. This means that the runners are ejected with the parts. Many cold runner systems will automatically eject the part and runners separately via a three-plate mold. With a three-plate mold, the runner is separated using an extra plate between the injection point and the part gate. By contrast, hot runner molds don’t create runners attached to the part since the feed material is maintained in a molten state up to the part gate. Hot runner systems are advantageous because they decrease waste and improve molding control.
Sprues channel the molten plastic from the nozzle. Sprues intersect with a runner, which leads to the gate where the molten plastic enters the cavities. The sprue channel has a larger diameter than the runner channel, which allows a sufficient amount of material to pass from the injection unit.
A gate is a small opening that enables molten plastic to enter the mold cavity. You can usually see the gates on the molded part because they appear as a small rough patch or depression called a gate vestige.
The parting line of an injection molded plastic part is seen where the halves of the mold close together for injection. It’s a thin line of plastic that covers the outside diameter of the part.
Side actions consist of inserts placed in molds that cause the material to flow around them to create undercut features. It’s important to remember that side actions must be designed to allow for proper ejection of the part.
There are three types of plastic that are most commonly used in injection molding because of their low cost and durability: polycarbonate, polypropylene, and nylon. Let’s take a closer look at these three plastics, as well as others used in injection molding.
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