How to Machine Plastic Injection Molds

Plastic injection molds make plastic water bottles among other products.

Machining plastic injection molds is one of the most difficult jobs a machine shop can face. Because of the complexity of the molds, they almost always need to be created in a 3D computer-aided manufacturing program. If they are being repaired, the machinist must be sure to not make any mistakes that could render the mold useless. In these cases, thousands of dollars could be wasted, so the proper tools must be used as well as a keen attention to detail.

Things You'll Need

• Cutting tools
• Indicator
• Vise or fixture

Instructions

Plastic Injection Mold Repair

1. Set the injection mold in a fixture or tighten in a vise. Using a dial indicator, make sure that the mold is sitting flat in order to machine accurately. Place the dial indicator on the spindle of the milling machine and manually move it along the X and Y axes to make sure it is straight within .002 inch over 12 inches of movement. If not, tap the high side down with a rubber mallet and recheck until the mold is flat.

2. Bring in the tool you need to use to machine the plastic injection mold very slowly. If you need to make small cuts, use a micro end mill or smaller type end mill to avoid mistakes. Plastic injection molds can cost thousands of dollars, and one slip can be disastrous, causing the plastic parts to come out deformed or get stuck in the cavity.

3. Take small cuts, bringing the tool out of the part often to inspect your work. These molds often need very small fixes or cuts, so bring the tool out and blow the area off to make the measurements to note your progress. Always return to the spot you left off in order to complete the job with minimal mistakes.

Plastic Injection Mold Creation

1. Create the program to create the plastic injection mold in a CNC mill. Most molds are very complicated and require the use of a CNC mill. Create the program using a CAM, or computer-aided manufacturing, program so that you can also simulate the tool paths as well as get an accurate list of the tools needed for the job.

2. Set up the tooling as well as the program in the CNC milling machine. Make sure to indicate the raw material. Indicating is the process of setting and measuring the raw material to make sure that it is straight and flat along all axes. Set the tools in the tool turret in order of their use and place in the program.

3. Allow the machine to rough the cavity using the proper tools as well as finish the surfaces using new finish end mills. Plastic injection molds will go through a process that creates a mirror-like finish, but it is best to machine the cavities with new end mills to get the best finish possible, which makes the tool and die maker's job easier.

References

• CNC Concepts: What is CNC?