Methods of Manufacturing: Plastic Molding
By Mary Iannuzzi on June 2, 2020
Plastic Molding is a manufacturing process that forms parts by melting plastic and then solidifying it into a desired shape.
There are many types of plastic molding, such as injection molding, extrusion molding, rotational molding, and compression molding. Injection molding is by far the most commonly used plastic molding process.
To make an injection molded part, melted plastic is injected into a steel mold. Each steel mold can contain a single cavity or multiple cavities. The cavity reflects the final shape of the part. Having multiple cavities on a single mold allows for multiple identical parts to be produced in a single "shot" or injection of plastic. Injection molding is a very quick and affordable process, especially at high volumes. Multiple cavity molds speed up the production process many times over.
Thanks to its versatility, injection molding can produce a wide range of shapes and sizes from many different types of plastic. Injection molding can be particularly well-suited for intricate parts. It also caters to engineering innovation, allowing designers to make a highly customized parts with various resin and additives.
Though the per-part price can be extremely low and the production process very quick, the tooling can be costly and have a long lead time. The upfront costs and time of an injection molded part can be a barrier to low-volume projects.
Extrusion molding is used to produce lengths of parts of a continuous cross section. Rather than pushing melded plastic into a static die, the melted plastic is extruded in the shape of the final product. Some examples of extruded parts are plastic straws, weather stripping, and wire insulation.
The extrusion process works by melting plastic pellets from a feeder and then pushing the melted plastic through a die. The plastic is usually melted using heaters and turning screws within a barrel. The screw provides both mechanical energy towards the melting process as well as the directional force that pushes the melted plastic through the die.
Compression molding uses heated molds to press heated plastic into a desired shape. The plastic is placed in between a top and bottom mold which apply significant amounts of pressure to force the plastic into it's final shape. Heat is added in addition to pressure in order to cure the plastic. This ensures that the plastic will retain its final shape after cooling.
Compression molding is often used for high-strength plastics. This makes it an applicable process across many industries, from automotive parts to appliances to shirt buttons. The process is relatively inexpensive compared to injection molding and doesn't waste much material. Flashing and inconsistency are two possible downsides of compression molding.
Rotational molding uses a heated mold, usually filled with a powdered material. The mold rotates, coating the interior walls of the mold. The continuous rotation provides an even walled part of a chosen thickness. This process is best suited for large, hollow parts such as barrels and tanks. However, small to large parts with complicated shapes can also be made using rotational molding. Rotation occurs on two axes.
Production can be slow, but is consistent, cost effective, and wastes very little material. It also provides superior toughness and durability over injection molding.
Michigan Manufacturing International provides plastic molding as well as many other methods of production to American OEMs. Our engineers are available to help you ensure the best production process for your custom part. Learn more on our website, or download our brochure.