Process Spotlight: Injection Molding

Injection molding is the most common way to form a variety of plastics for mass production. In a nutshell, heated liquid plastic is squeezed into a mold cavity and then cooled to shape.

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New injection molding machine with a set of molds shown on the right.

The cavity is created using a two-piece mold, although more movable parts can be employed for more complex designs. Our injection molding factories require a precise 3D model of your product in order to design a set of molds that is the spatial negative of your part.

As long as we are talking thermoplastics, there is really no limit to what injection molding can do. Products or various size, shape, hardness can be produced using injection molding. Both hard plastics such as polycarbonate and soft plastics such as TPE can be injection molded. Acrylic, Nylon, PE, PP, PVC, ABS and TPU are all suitable for injection molding.

There is great amount of flexibility with surface finishes as well. The mold can be made with different degrees of polish which results in different surface finishes on the end product. Clear polycarbonate for example, will appear transparent when the mold is made with a mirror-like finish (A1-A3 below). The same polycarbonate can appear translucent and have a matte feel when the mold is rougher (D1-D3 below). Interestingly, various regions of the mold can be refined to different degrees to achieve very aesthetically pleasing results.

mold finishes, surface finish, finishing, roughness

Various mold finishes ranging from mirror-like to sandblast finishes.

It gets better. Since molds can represent a large investment. Customers often try to get as much value out of their tooling as possible. It’s not uncommon to find molds that can produce 12 parts at once!

The technology is all fine and dandy, but what does it mean for the ambitious and hopeful entrepreneur?

Designing the product

There are a few basic points to keep in mind when designing a plastic part for injection molding.

Wall thickness:  the majority of injection molding parts are constructed like shells. The thickness of this shell or wall thickness needs careful attention. It needs to be thick enough to provide structural integrity, but not too thick since too much plastic will not be cooled evenly. Walls should also be as uniform as possible since abrupt changes in thickness also cause uneven cooling and a weak part.

Complexity: seemingly small variations in design can mean a world of difference the injection molding process. Complex designs equates to complicated molds, often with moving parts, long cycle times, and overall increased cost.

Getting quotes

The price of an injection molded part largely depends on its mass and type of material. Two completely different parts might cost the same because they use the same amount of the same material. A secondary factor is surface finish. Especially strict requirements might necessitate another process after parts come out of the mold, but that is seldom the case.

You might be also very interested in the tooling cost. The cost of a set of injection molds depend on the amount of material, usually steel, and the machining required. This in turn is determined by the size and complexity of your designs and the number of cavities you want. Multiple cavities can reduce the unit price by cutting down labor and machine time required per part to a fraction of what is needed with one cavity. It generates extra tooling costs of course and is not advisable for parts of significant size or thickness, and you must weigh the trade-offs carefully.

Naturally, the best thing to provide during the quote process is a real-life sample and a 3D model. If that is not available, a 3D model of the part alone is a close second. Pictures alone are not solid bases to quote from and suppliers almost always raise the price in actual production due to “details not previous known”. Being as clear about your requirements as reasonably possible will work to your advantage. Before actual tooling begins, our injection molding partners always request a final, precise 3D document, with surface finishes well defined.

Moving onto production

Starting tooling for a product is always exciting, partially because there is little to no room for mistakes! It’s a lengthy process, easily taking up to 20 days from the start to the time first samples are made. Parameters such as temperature, cooling time and pressure all have to be adjusted until the process stabilized to one of acceptable quality. After a few revisions and the final approval of the product, mass production finally begins. The fight is not over however as quality always fluctuates through time and diligence must be paid to quality control.

Production speeds varies between products, with 500 injections per 8 hour shift being typical. After a run is finished, the mold is taken off the machine and stored with care for future use.

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