Plastic injection molding is a fast and efficient process that produces parts with nice surface finishes. It’s also cost effective for producing large quantities of parts in a short amount of time. Injection molding is used for manufacturing everything from medical devices to garbage cans. The initial cost for a plastic injection mold can be high but the investment is well worth it due to the speed of production and lower cost of raw materials.
The first step in the plastic injection molding process is to create the tooling. Often times the tooling is a block of aluminum that’s CNC milled into a negative image of the final plastic part. The next step is to select the thermoplastic that’s going to be injected into the mold and then test it. The plastic selection process is important because the choice will impact the performance of the finished product. Polypropylene (PP) is a good thermoplastic for injection molding as it’s tough and has excellent fatigue and heat resistance. Other common thermoplastics include polyethylene (PE), acrylonitrile butadiene styrene (ABS) and polycarbonate (PC).
Once the tools are ready to go the injection molding process can begin. The injection molding cycle is incredibly fast, typically lasting only ten seconds to two minutes. Because of the rapid cycle times it’s possible to produce multiple duplicates of a part in an hour at very low costs. This is a huge advantage over other manufacturing processes.
After the injection molding cycle is complete the part is ejected from the mold and falls onto a conveyor belt or into a holding container. The A side of the part is typically the cosmetic or functional side while the B side contains the hidden structural elements such as ribs, snap-fits and bosses. The B side can also contain a number of holes such as vents and threaded inserts.
It’s common to add threaded fasteners to injection molded parts. There are 3 ways to achieve this in an injection molded part: by modeling the thread directly on the part, by adding a boss where the screw can be attached or by using a metal threaded insert. Modelling a thread directly on the part is not recommended as it’s difficult to do and will create a lot of material waste. Bosses are more common and can be designed as cylindrical projections with a hole that’s sized to fit the screw or insert.
A sprue is a channel in the mold that all of the melted plastic initially flows through during the injection molding cycle. It’s important to design the sprue so that it doesn’t interfere with the final part’s geometry and surface finish. When designing a sprue it’s best to keep wall thickness uniform and avoid thick sections as this can interfere with the flow of the melted plastic. It’s also recommended to include a chamfer on the sprue to ensure that screws or inserts can easily slide into place. Text is commonly added to injection molded parts and can be a great way to display logos or provide instructions to users. When designing text on a injection molded part it’s usually best to choose embossed over engraved text as it’s easier to CNC machine and more economical. It’s also a good idea to raise the text 0.5 mm above the surface of the part so it can be clearly read.