Are You Sure You Understand Injection Mold Polishing

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
Mold Polishing Stones

Should you polish in-house or send it out?

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.
Injection Mold Polishing
Mold Polishing A Helmet

The mold polisher is a key part of the team, don’t overlook this!

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 alone. 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!

More for the injection mold maker


  1. Where is Mold Making Resource located?

    Do you have knowledgable individuals available for ‘outside’ (in the U.S.) contract work with regards polishing existing cavities?

    Please email contact information: tele, email and name/s of people in your company to speak with.



  2. Located Southern Ca
    45 years of experience, small cavity inserts d
    Email me if interested

  3. We do a majority of polishing in house on our molds, I am always looking for ways to improve the skills of the technicians. Is there a class for teaching polishing techniques? If so where?

    • Great question! The only “class” I am aware of was put on by a mold polisher who used to work for Jack Courtemanche at GW Plastics in Bethel, VT. It was really very helpful; I was in the class.

  4. Mold maintenance technician, I specialize in high diamond finishes and am available for hire with experience in multiple sectors of the industry from lens and logo to medical and lighting applications. When it comes to quality mold finishing I take pride in providing this much needed service keeping minimum stock removal procedures as a key factor in uniform serface finishing. The human eye can see what sometimes only amounts to 2/10ths of serface variation, sometimes referred to as orange peel or (a wave in the molding serface) Which depending on your thermo plastic applications may show in production parts. To combat this various aspects come into play some of which include but are not limited to: Uniform Material Hardness (Rockwell), CNC finish of molding area, Standing material stock to required specification ratio, ect. There are many variables to take into consideration when a company takes on a new project and the two objectives that every company providing high grade polymer molded products come down to,
    #1 Efficient tool process, which ultimately comes down to mold design, build and operation.
    #2 Quality product production which require: Uniform serface height of molding area, gate/runner/vent efficiency, and molding area serface finish.
    I specialize in high diamond spi finishes from

    A-3 #9-#15 micron diamond buff: applied to projects using colored plastics or non-transparent applications requiring a smooth polished serface.
    A-2 #3-#6 micron diamond buff: applied to projects using near transparent plastics requiring near perfect mirror finish.
    A-1 #1-#3 micron diamond buff: applied to projects using transparent plastics requiring an absolute mirror finish.
    I also provide sub A-1 finishing for projects requiring specialized finishes down to #.25 micron diamond buff.
    If you require a knowledgeable mold finisher for mold maintenance in house, My contact is

  5. Can come to your location and do custom polishing 50 plus years experience cost is based on travel plus a base charge we are here to make sure that your deadlines are met we are available to give your firm with on call service

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