Processes

Plastic Molding Overview

Plastic molding services use a number of different molding techniques to produce components.  A number of the most common styles are detailed below.

Thermoplastic Injection Molding is the most widely used of all plastic processing methods. The injection molding machine reduces pelletized raw material and colorants into a hot liquid. This "melt" is forced into a cooled mold under tremendous pressure. After the material solidifies, the mold is unclamped and a finished part is ejected. Injection molding offers the lowest piece prices available, but tooling prices are generally the highest.  An injection-molding machine preforms the entire process of plastic molding services.  These machines serve to both heat the plastic material and form it.  Using different molds the shape of the produced components may be changed. Injection molding machines have two basic parts, the injection unit, which melts the plastic and then injects or moves it into the mold, and the clamping unit, which holds the mold. The unit clamps the mold in a closed position during injection, opens the mold after cooling, and ejects the finished part.

Thermoset injection molding is similar to thermoplastic injection molding except that uncured thermoset resins are mixed, injected, and held in the mold until cured.  As with thermoplastic molding, the price per piece can be low, but the tooling prices are generally very high.

Blow Molding is similar to injection molding except that hot liquid plastic comes out of a barrel vertically, in a hollow molten tube or "parison." The mold closes on it and compressed air blows up the parison like a balloon, forcing it outward to conform to the inside shape of the mold. After cooling, a hollow part emerges. This method is typically used to create jars, bottles, containers, etc.  Part prices for this style of plastic and rubber services are generally higher than injection molded parts, but lower than rotationally molded parts. Tooling costs are moderate.

Rotational Molding (rotomolding) uses hollow molds filled with powdered resin, which are secured to pipe-like spokes that extend from a central hub. The hub swings the entire mold into an enclosed furnace room that melts the powder, which sticks to the mold as it cools. With the molds still tumbling slowly, the tools swing into a cooling room where sprayed water causing the plastic to harden into a hollow part.  Low tooling costs and high piece prices are typical of this process.

Compression plastic molding services use a slug of hard media pressed between the two halves of a heated mold. After being formed, the part is air-cooled. Both tooling and piece part prices are moderate. Thermoforming plastic molding services uses sheets of pre-extruded rigid plastic are heated horizontally and sucked down into hollow one-piece tools. After the hot plastic solidifies again, its shape conforms to that of the mold. Trimming is usually necessary to put the part in final form.