A Course in Plastic Design and Injection-Molded Parts
From toys to automotive parts, vacuums to printers, injection-molded plastic parts are everywhere. 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 and Enventys have at least some injection-molded parts in them, and we are well-versed in the best practices to design attractive yet functional parts.
Injection molding is a process by which 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 your palm, the machines are large and are usually between 13 and 40-feet long. The process starts at the material hopper where plastic pellets are heated until they are 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 held together tightly 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 over a minute depending on the size of the part.
The injection molding process requires that parts be designed a certain way to take full advantage of the process and make the parts look and function well. The following 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 as constant wall thickness as 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 are called sink marks and are usually in areas of increased thickness.
Draft is the name used to describe the slightly angled surfaces found on injection-molded parts. Draft is 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 half 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, which are 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. However, that is not always feasible, and fortunately 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 together 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 locking it into place. The geometry for a snap fit can be tuned depending on the material and the desired assembly and holding force.
Screws are also a popular option to join injection molded parts together. Bosses can be designed into the parts to allow screws to be threaded into the part without the need for a nut. The threads of plastic screws have aggressive threads that cut into the screw boss to provide a strong connection.
Overmolding is a process where an injection molded part is put into a second tool and a different material molded on top of the original part. This is a common design feature on products that require a rigid substrate and a soft touch area. Power tools and toothbrushes are products where overmolding is common.
Overmolding gives designers many more options to incorporate multiple textures and colors into a product, but it is not without consequence. Overmolding requires a second set of tooling which makes the production more costly. There is also a higher rate of rejected parts as the overmold can bleed into areas where it is not supposed to be.
Of course, as with any set of rules, there are times when they have to be broken. Sometimes overmolded parts will have variable wall thicknesses or really thick areas. Usually the soft touch material is more expensive than the rigid substrate and it is more cost effective to fill the volume with the cheaper material. Undercuts are also very common, as is it often more economical to use a mold with side actions than to design out the undercuts. How far we break the rules depends on each details of the product being designed.
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Also published on Medium.