The processes of plastic molding, also known as injection molding, and machining plastic components are very different. How are they different? What process is best for your custom plastic items?

Let’s define them first.

What is Plastic Injection Molding?

Molds, also known as tools, are used for injection molding to produce parts by injecting molten plastics into them. The plastic is heated in the barrel before being injected into the mold, moving along channels called runners, and then being injected into the part. Upon completion, the mold is opened, and the part is ejected, ready for the process to begin again. Well-designed molds and the right plastic resin can produce high-precision parts very quickly when loaded into hydraulic or electric machines.

What is CNC Machining?

While injection molding is about filling, CNC machining is all about subtraction, much like whittling away at a tree branch with a pocket knife. In CNC machining (which stands for computer numerical control) the material is removed from a solid block a layer at a time using computer-controlled cuts. The information is derived from a CAD or similar digital file, allowing for alterations between productions. Using a computer, these files are broken down into a series of commands that are sent to one or more milling tools that work on the material to produce the final product. Using this method, it is possible to quickly translate digital designs into physical objects.

How Do They Compare?

Plastic machining can be 25 times more expensive than plastic molding. However, sometimes machining plastic parts is cheaper. How is this possible?

In order to injection mold parts, the mold has to be created, which costs money. Depending on its complexity, its price can range from $5,000 to $200,000 or more. Molding plastic is a less expensive process, but if you only need a few parts-and spend $10,000 on the initial mold-you spend more than you would if you simply machined them. Remember that once you create a plastic mold, it cannot be easily changed, making it impractical if your processes include frequent changes.

When you only need a few hundred parts or fewer, machining is your best option. Mold creation costs almost always exceed machining costs at these levels. Molding is the best option when larger orders are involved. However, there are a few exceptions.

Plastic machining is a more precise process than plastic molding. You can expect results within .005 inches of specifications when molding. You can count on results within .001 inches of specifications when machining and some newer machines hold tolerances even tighter than that. For certain industries and projects, such precision is vital.

There is also an exception if your parts require uniform surface smoothness. During the process of transferring plastic into a mold, surface imperfections appear on the finished product. Plastic is introduced into a cavity through a gate that meters the flow as it fills the part during injection molding.

Consequently, when the finished parts are removed from the mold, the gates leave a slight cosmetic imperfection. Further, the part is usually pushed out of the cavity after cooling using knockout pins. Where these knockout pins contact the part, a slight depression is left behind.

Consider plastic machining if you need parts free of blemishes.

Moreover, plastic machining offers faster turnaround times, which can be important if your project is behind schedule.

We hope the following tips will help you decide between plastic molding and machining:

  • If you need a few hundred or fewer parts, plastic machining is likely your best option.
  • Consider the costs of creating the initial mold versus the higher costs of machining if you are considering molding.
  • If (1) your project demands a high level of precision and/or (2) your parts must have uniform surfaces, consider machining.