How injection-molded plastic pieces are made.
Chances are good that by the end of today, you will have used multiple products that have injection-molded plastic parts.
These parts are abundant—whether in toys, automotive parts, vacuums, printers or any number of items. They are cheap to manufacture in quantity and can be designed with complex shapes that are hard to achieve with other processes. Most of the products brought to market through Edison Nation have at least some injection-molded parts. We are well versed in the best practices to design them in an attractive yet functional way. In injection molding, liquid plastic is pumped into a mold and allowed to cool to form the shape of the desired part. Even though most injection-molded parts can fit in the palm of your hand, the machines used to make them are usually between 13 and 40 feet long.
The process starts at the material hopper, where plastic pellets are heated until liquefied. Then a large screw drives the molten plastic into a mold that is usually made from hardened steel. The mold has two halves that are tightly held together by a large hydraulic cylinder or motor. Once the mold is filled with plastic, it is cooled so that the plastic hardens. The clamp is released and the parts are pushed out of the mold with ejector pins. The mold closes and the cycle begins again. Cycle times can be as little as a few seconds to more than a minute, depending on the size of the part.
Injection molding requires parts be designed a certain way to take full advantage of the process and make the parts look and function well. Here are some common practices and features of injection-molded part design.
Constant wall thickness
The first tenet of injection mold part design is that parts should have the most constant wall thickness possible. Having a constant wall thickness lets the plastic flow through the mold consistently
and lessens the likelihood of an imperfect part. Consistent wall thickness also helps the aesthetics of the part. Thickened areas cool more slowly and cause the material to shrink and pull away
from the mold. The resulting divots, called sink marks, are usually in areas of increased thickness.
Draft describes the slightly angled surfaces found on injection-molded parts, required to allow the parts to be released from the mold without sticking to it. The amount of draft necessary on plastic parts depends on the surface finish but is usually between 1/2 and 3 degrees. Parts with deeper surface texture require more draft than smooth parts to prevent the mold from dragging on and causing surface imperfections.
In order for all of the features of a plastic part to be formed correctly, they need to be accessible to the steel of the mold. In a standard two-piece or single-pull mold, this means that there
can be no undercuts—the features perpendicular to the pull direction that would lock the part to the mold. The goal is to always design the part to have all of the functionality and aesthetics
while avoiding any undercuts.
Because that is not always feasible, injection molders have ways to deal with this. Secondary actions called side pulls can be added into the mold to form the geometry that is hidden from the main core and cavity of the mold. If the geometry is particularly tricky, a cam can be used to rotate the steel away from the part and unlock it from the mold.
Injection molding allows for some unique ways to fasten parts together. One of the most widely used techniques to join plastic parts is the snap fit.
Snap fits are long beams of plastic with angled heads that are molded into the part. As the parts come together, the beam bends and the head snaps into a cavity in the mating part to lock it in place. The geometry for a snap fit can be adjusted, depending on the material and the desired assembly and holding force.
A shell of a Hot Huez compact shows the snap fit feature that holds the lid closed. The head on this one is not too aggressive, so the user can easily open the compact.
Screws are also a popular option to join together injection-molded parts. Bosses can be designed into parts to allow screws to be threaded into the part without a nut. The threads of plastic screws have aggressive threads that cut into the screw boss to provide a strong connection.