Ahhh… the curse of injection mold polishing: it is usually the last thing to be done, the job is usually behind schedule, polishing eats up precious hours of labor, and the finish is what the end user actually sees!

It is ironic that the plastic injection mold making is a sophisticated, high-tech process; but, in the end, it gets down to somebody rubbing abrasive stones and diamond compound by hand on the finished cavities.

If you’re lucky, you can make use of things like ultrasonic polishers and hand grinders; but most of the time it gets down to hand work by a very skilled polisher. So far, there is just no way around it in the mold making process.

Injection mold polishing is not as easy as it looks

There are basically three ways to manage mold polishing: Do it in-house, send it out, or a combination of these two options. Most companies seem to do the quick and easy polishing in-house and send the complicated or mirror finish work outside. Each approach has it’s merits and drawbacks.

Pros and cons of mold polishing in-house

• Able to keep better control of the work and deadlines. You can also keep from over-polishing or under-polishing due to the difficulty in communicating all the details in emails, phone conversations and drawings.
• You must have at least one top-notch polisher on hand who can also do other mold work when there is no polishing, such as a qualified injection mold maker.
• You need good equipment and mold polishing supplies on hand. Some companies short-circuit themselves by under-supplying the polishing department.
• To properly attain a mirror, or number 1 finish, you really should have a dedicated room to prevent contamination of the diamond compound. It’s amazing how dirty some polishing areas are. All it takes is a tiny bit of coarse grit to land on your workpiece and soon you have big, ugly scratches that require massive amounts of labor to remove.
• Pros and cons of sending the mold polishing out
  • You can meet deadlines easier by scheduling the polishing to an outside company. This is especially true with multiple cavity work.
  • It can be very difficult at times to communicate just what needs to be polished and what must be left al one. More than one very expensive cavity has been completely scrapped because of this problem.
  • You don’t need to keep a top polisher in-house.
  • You sacrifice a lot of control over the work-flow. Most molds seem to enter a “crazy” stage at the end where a lot of fitting and small modifications take place. If your cavities are in another shop, you cannot work with them in the final stages as well. Plus, who wants to handle a highly polished cavity more than necessary!

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