Injection molding is one of three main manufacturing technologies used to create parts. (The others are machining and additive manufacturing.)
Injection molding offers a predictable and scalable process for rapidly prototyping and/or producing products. The process begins when an injection molding provider creates a mold that will hold the material used to create the part. The mold will have a cavity that is shaped based on the design of the part to be produced. Making the mold can take a number of weeks, but once it is made, it can be used for hundreds of thousands to millions of parts. The cost of making the mold can be high, depending on the complexity of the part to be molded.
Once the mold is made, it is placed in an injection molding machine. Then material (either plastic resin or metal) is injected into the mold usually using a ram or screw-type plunger. The material fills every nook and cranny of the mold cavity and solidifies into the shape dictated by the mold.
The injection molding process works with a range of materials to make parts consistently and with tight tolerances. For most injection molded parts, tolerances range from ± 0.005 in. to ± 0.001 in.
In addition to the basic injection molding process, this technology offers several options that can transform a design. These options include overmolding, insert molding, and undercuts.
Overmolding is a process where one material is molded on top of another. A range of materials can be overmolded, including both hard and soft plastic resins. One of the more common reasons to use the overmolding process is to create a soft grip.
Overmolding can also be cleverly used to add rubber-like grips to clips designed to grab inanimate objects, and it can be used in more than one area of the same part for cosmetic or color contrast.
Insert molding is the process of injection molding molten thermoplastic around pieces placed in the injection-molding cavity resulting in a strong bond between integral pieces of the final part. Accurate mold design and construction is essential to insert molding to not only maintain part tolerances but also assure the tooling reliability.
Undercuts can be used to carry out complex forms of molding such as the overmolding process and insert molding process. Undercuts are used to create interlocking or snap and latch features, allowing for clamshell or housing designs to come together for quick and easy assembly, or capturing holes or ports for wiring, button features or assembly, and vertical threads and barb fittings typically used in medical device products.
Some of the benefits of injection molding include less scrap material and a highly repeatable process.
To remove a part from a mold quickly and easily, designers usually need to conform to several best practices. These practices deal with material selection, wall thickness, draft, runners and gates, ribs, bosses, corners and transitions.