Plastic molding, also known as “injection molding,” and machining plastic components are very different processes. But how are they different? And which process is right for your custom plastic fasteners?

First, let’s define what they are. During the injection molding process, plastic pellets—rigid when cool but viscous at higher temperatures—are heated to their melting point and then injected into a cavity. As they cool, they harden in that cavity’s shape.

Plastic machining, however, is the process of whittling plastic down from a larger shape—like shaving slivers from a branch with a knife.  

So how do they stack up?

Plastic machining can cost as much as 25 times plastic molding. Yet, sometimes it is cheaper to machine plastic parts. How can this be?

The reason is the cost of creating the initial mold in which the parts are manufactured. Its price ranges anywhere from $5,000 to $200,000, depending on the complexity. Molding plastic may be a less expensive process, but if you only need a few parts—and you spend $10,000 to make the initial mold to create them—you spend more than if you had just machined those parts.

And keep this in mind: once you create a plastic mold, you cannot easily change it. Wholesale changes cannot be made.

Typically, when you need a few hundred or fewer parts, machining is the answer. At these levels, the cost of creating the mold almost always outweighs the expense of machining. When you’re dealing with larger orders, molding is likely the answer.

There are, however, a few exceptions. 

As a process, plastic machining is more precise than plastic molding. When molding, you can expect results within .005 inches of specifications. When machining, you can bank on results within .001 inches of specifications. This level of precision is necessary for some industries and projects.

Another exception arises if your parts require uniform surface smoothness. The process of transferring plastic into a mold leaves behind surface imperfections on the finished product. During injection molding, plastic flows through runners and is introduced into a cavity through a gate which meters the flow as it fills the part.

As a result, when the finished parts are removed from the mold, the gates leave a slight cosmetic imperfection. In addition, most molds use knockout pins to push the part out of the cavity after it cools. These knockout pins leave a slight depression in the place where they contact the part.

If your plastic parts must be blemish free, strongly consider plastic machining.

Also, another benefit of plastic machining is its faster turnaround time, which may be important if your project is behind schedule.  

So, if you or your company is choosing between plastic molding and machining, we hope you’ll keep the following in mind:

  • If you require a couple hundred or fewer parts, the best choice is very likely plastic machining.
  • If you are considering molding, make sure to weigh the cost of creating the initial mold against the higher expense of machining.
  • Give machining serious thought if (1) your project requires a high degree of precision and/or (2) your parts must have uniform surfaces smoothness.

NB:  Keep in mind that not all plastic materials can be injection molded!

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