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Injection molding generally uses molds made in two parts. The first part is the cavity, which gives a concave part its external shape. The second part is the core, which gives a part its internal shape. The cost of the mold depends on the geometry of the part being made; as the part become complex, the mold increase in complexity and hence more expensive. During the injection molding process, an injection molding machine melts and inject thermoplastic into the mold under high pressure. The molten plastic then takes on the shape of the mold, cools and solidifies. The formed part is then ejected from the mold. An ejection system is needed to push the part off the mold because the part shrinks onto the core as it cools. This is accomplished with ejection pins and they are usually located in the core of the mold.

Injection Molding

Extrusion is suitable to manufacture long part with uniform cross section such as tubes and rods. Thermoplastic pellets are placed into a hopper to feed the extrusion barrel, which contains a rotating screw. A heater in the barrel melts the pellets and friction from the screw generates heat to maintain the pellets in molten state. The melt is then forced through a die hole of desired geometry to form the part. Unlike injection molding, where cooling and replenishing the mold with new material tend to slow down the process, extrusion manufacturing is a continuous process because material is constantly feed into the barrel and extruded. Therefore, extrusion manufacturing is extremely efficient.


Compression Molding Compression molding technique is used to form thermoset materials. The mold is mounted onto a press heated by steam or electricity. The raw material, usually called a charge, is place in the mold cavity where it softens. The mold is then closed and the material is forced to take the shape of the mold. The mold remains closed and pressurized until the part cures. The time can vary from 10 seconds to 24 hours and it depends on the wall thickness and geometry of the part.