Plastic Injection Molding Process Steps

Because plastics are a major component of many consumer products, and injection molding is one of the best ways to make plastics. There are three main stages in the custom plastic injection molding process.

Injection Molding Process, Basic Step 1: Product Design
Design is one of the most important aspects of the production process because it is the earliest opportunity to prevent costly mistakes later on. There are many design goals: functionality, aesthetics, manufacturability, assembly, and more. The right design is one that accomplishes the desired goal at a satisfactory level, but it may take a lot of creativity to achieve the goal.

Some specific ways to avoid costly mistakes during product design are to plan for uniform wall thicknesses whenever possible and to gradually transition from one thickness to another when thickness variations cannot be avoided. It is also important to avoid stresses in the design, such as corners of 90 degrees or less.

Injection Molding Process, Basic Step 2: Mold Design
After designs that look and feel similar have been tested and are planned for further production, a mold needs to be designed for injection mold manufacturing. Molds are typically made from the following types of metals:

Hardened steel: Typically the most expensive material used for molds, and is often the most durable (which can keep the unit price down). This makes hardened steel a good material choice for products that are to be produced in the hundreds of thousands.
Pre-hardened steel: Will not last as many cycles as hardened steel and is less expensive to manufacture.
Aluminum: Most often used for single-cavity “prototype tools” when a relatively small number of parts are needed for testing. Once the injection molded components of that tool are tested and approved, a multi-cavity steel production tool is produced. Thousands of parts can be obtained from an aluminum tool, but it is usually used for smaller quantities.
Beryllium copper alloy: Often used for areas of the mold where heat needs to be dissipated quickly or where shear heat is concentrated.
Like overall product design, mold design is another opportunity to prevent defects in the injection molding process. We have previously blogged about the top 10 injection molding defects and injection molding mistakes to avoid, but here are some examples of how poor mold design can be a costly mistake:

Not designing for proper draft: This refers to the angle at which the finished product is ejected from the mold. Inadequate draft can lead to ejection issues that cost a lot of time and money.
Improper gate placement or sizing: The gate is the opening in the mold through which the thermoplastic or thermoplastic material is injected. Each will leave a mark (scar) that can create aesthetic or functional issues if not placed correctly.
The number of parts required and the materials from which they are made will help drive decisions about how and what materials to use to make the mold.

Injection Molding Process, Basic Step 3: Manufacturing Process
Once the product has been properly designed, approved, and die cast, it is time to actually start manufacturing!

Thermoplastic or thermoplastic material in pellet form is fed into a heated barrel through a hopper. The plastic is heated to a predetermined temperature and driven through the gate into the mold by a large screw. Once the mold is filled, the screw is held in place to apply the proper pressure for a predetermined cooling time. Once this point is reached, the screw is withdrawn, the mold opens, and the part is ejected. The gate will either shear off automatically or be removed manually. This cycle will be repeated over and over again and can be used to create hundreds of thousands of parts in a relatively short period of time.