Injection molding uses a ram or screw-type plunger to force molten plastic material into a mold cavity. This material solidifies into a shape that has conformed to the contour of the mold. Injection molding is most commonly used to process thermoplastic and thermosetting polymers. Thermoplastics are by far the most prevalent, due to characteristics that make them suitable for injection molding — such as recyclability, versatility for various applications,  and ability to soften and flow on heating.

How Injection Molds are Made

Molds can contain a single cavity or multiple cavities. In multi-cavity molds, each cavity can form identical parts or multiple unique geometries during a single cycle. Molds are generally made from Tool Steels, but Stainless Steels and Aluminum molds are suitable for certain applications. Aluminum molds are typically used for lower quantities and precision, due to certain mechanical properties that make Aluminum more prone to wear, damage, and deformation during injection and clamping. However, Aluminum molds are cost-effective in low-volume applications, as mold fabrication costs and time are considerably reduced. Many Steel molds are designed to process millions of parts during their lifetime and can cost hundreds of thousands of dollars to fabricate.

Insert Molding, Overmolding, and Two-Shot Molding

Pre-molded or machined components can be inserted into the cavity while the mold is open, allowing the material injected in the next cycle to form and solidify around them. This process is known as insert molding and allows single parts to contain multiple materials. Insert molding is often used to create plastic parts with protruding metal screws so they can be fastened and unfastened repeatedly.

Certain molds allow previously molded parts to be reinserted to allow a new plastic layer to form around the first part. This process is referred to as overmolding and allows, for example, production of one-piece tires and wheels.

Two-shot or multi-shot molds are designed to "overmold" within a single injection cycle. Two-shot molding requires specialized injection molding machines with two or more injection units. This process is actually an injection moulding process performed twice and therefore has a much smaller margin of error. In the first step, the base color material is molded into a basic shape, which contains spaces for the second shot. Then the second material, a different color, is injection-molded into those spaces.

Scientific Injection Molding

In traditional molding, the entire injection process occurs at one constant pressure to fill and pack the cavity. For high-precision parts, this method can allow variation in dimensions between cycles. More recently a method called scientific injection molding has gained market adoption. In scientific molding, the injection of the plastic is "decoupled" into stages for better dimensional control and shot-to-shot repeatability.

First, the cavity is filled ~98% under tight velocity controls. Once the cavity is 98% full, the molding press switches from velocity control to pressure control — where the cavity is "packed out" at constant pressure and sufficient velocity.