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What is Plastic Injection Molding?

Plastic Injection Molding

The primary process for manufacturing plastic parts and components is Plastic Injection Molding. Plastic injection molding is a cost effective plastic part manufacturing process designed for high-volume production in order to achieve a low cost per part. It's the most common method for producing plastic parts today. Plastic is an economical material used in countless applications. Its versatility sets it apart from other moldable materials. Plastic components, versus other moldable material components, bring you the economical versatility of plastic, at a low cost.

Plastic injection molding is the process of "pushing" or "injecting" melted plastic (resin) into a mold under extreme pressure. The process starts by heating tiny granules of plastic until they liquefy. The molten plastic is then injected into a multi-part mold that is held together with many tons of pressure. Once the plastic has been injected into the mold, the plastic quickly cools and hardens. When the plastic has hardened, the mold is opened and the newly formed parts are "ejected" out of the mold. The parts are then trimmed and packaged accordingly. This process is accomplished using injection molding machines. Injection molding machines vary in size. The size of a machine is usually indicated by its clamping force which is measured in tons. Injection molding machines typically range in size from 50 tons to 1100 tons in order to handle both small and large jobs.

The molding process is generally run at a high number of cycles to mass-produce the plastic parts. While the tooling required to create a single mold can be costly, the price per part is generally very low due to high volume production.

Design of the plastic parts is inextricably tied to the nature of the production method; injecting molten plastic into split molds. Part design is impacted because the parts are produced with molds, which have unique nuances that must be considered when molding parts from molten plastic. For example, the plastic must flow into the mold through a "gate". Gates must be strategically located in the mold to allow for the uniform flow of molten plastic into the various mold cavities in order to ensure the structural integrity of the part. Parts must also be designed with angled surfaces, or draft features, to allow for easy removal from the mold. There is much more that goes into part design, such as material selection, surface finish, wall thickness and more.

What makes up a mold?

Molds (commonly referred to as tooling) are made up of two halves, an "A" half and "B" half. Tooling is usually constructed out of steel, but can also be made out of aluminum for shorter run applications. When each half is clamped together, it forms a cavity making up the form of the part. Each half of the mold is placed inside the injection molding machine. When the machine clamps the two halves together, the plastic is injected into the mold. The mold will generally have "runners" which are little more than "plastic highways" used to move the plastic out to all sections of the mold. Some molds require water passages or "plumbing" which is used to cool the plastic parts more quickly. The more quickly the parts cool, the faster the machine can run. Faster molds equals cheaper parts. Finally, most molds have what are called "knock-out pins" or "ejector pins." As the name implies, these pins kick the part out of the mold to speed up production and assist the operator in removing the parts.

This technical information has been contributed by
Custom Plastics

Click on Company Name for a Detailed Profile

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