Basics of Plastic Injection Molding

The basics of plastic injection molding process includes creating the product design, making a tooling a mold to fit the product design, melting the plastic resin pellets, and using pressure to inject the melted pellets into the mold.

See a breakdown of each step below:

1. Creating the Product Design

Designers (engineers, mold maker businesses, etc.) create a part (in the form of a CAD file or other transferrable format), following fundamental design guidelines specific to the injection molding process. Designers should try to include the following features in their designs to help increase the success of a plastic injection mold:

• Bosses for threaded inserts/fasteners
• Constant or near-constant wall thicknesses
• Smooth transitions between variable wall thicknesses
• Hollow cavities in thick sections
• Rounded edges
• Draft angles on vertical walls
• Ribs for supports
• Friction fits, snap-fit joints, and other non-fastener joining features
• Living hinges

Additionally, designers should minimize the following features to reduce defects in their designs:

• Non-uniform wall thicknesses or especially thin/thick walls
• Vertical walls with no draft angles
• Sudden geometrical changes (corners, holes, etc.)
• Poorly designed ribbing
• Undercuts/overhangs

2. Making a Tooling Mold to Fit the Product Design

Highly skilled machinists and toolmakers, using the product design, fabricate a tooling mold for the injection molding machine. A tooling mold (also known as simply a tool) is the heart and soul of the injection molding machine. They are carefully designed to contain the negative cavity for the product design and additional features such as sprues, runners, gates, vents, ejector systems, cooling channels, and moving components. Tooling molds are made out of specific grades of steel and aluminum that can withstand tens of thousands (and sometimes hundreds of thousands) of heating and cooling cycles, such as 6063 aluminum, P20 steel, H13 steel, and 420 stainless steel. The mold fabrication process takes upwards of 20 weeks to complete, including both fabrication and approval, making this step the most extended aspect of injection molding. It is also the most expensive part of injection molding, and once a tooling mold is fabricated, it cannot be drastically changed without incurring additional costs.

3. Melting the Plastic Resin Pellets

After operators obtain the finished mold, it is inserted into the injection molding machine, and the mold closes, starting the injection molding cycle.

Plastic granules are fed into the hopper and into the barrel. The reciprocating screw is drawn back, allowing materials to slip into the space between the screw and the barrel. The screw then plunges forward, forcing the material into the barrel and closer to the heater bands where it melts into molten plastic. The melting temperature is kept constant as per the material specifications so that no degradation occurs in the barrel or in the mold itself.

4. Using Pressure to Inject the Melted Pellets Into the Mold

The reciprocating screw forces this melted plastic through the nozzle, which is seated within a depression in the mold known as a mold sprue bushing. The moving platen pressure fits the mold and the nozzle together tightly, ensuring no plastic can escape. The melted plastic is pressurized by this process, causing it to enter all parts of the mold cavity and displacing cavity air out through the mold vents.