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Tips and Secrets of Injection Mold Polishing

injection mould polishing Ahhh... the curse of mold polishing. It is usually the last thing to be done, the job is usually behind schedule, it eats up precious hours of labor, and the finish is what the end user actually sees!
Funny, plastic injection mold making is a rather sophisticated, high-tech trade; 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 ultra-sonic 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.

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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.

Keeping the 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.
  • You need good equipment and 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.
Sending the work outside

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!
Using EDM to polish for you

  • This is my favorite method, because almost all the work is done for you! What more do you want? In order for this to succeed, you need a highly finished electrode(s), an available EDM machine, and lots of time. So, in spite of the fact that it is a great idea that does work, it is often more practical just to do it by hand.

If you can allow your EDM machine to run the dozens of hours required in order to polish for you, this is definitely the way to go. Plus, you need a number of electrodes because the wear factor is quite high. Nevertheless, with CNC milling and a little hand work on the electrode, it can certainly be a part of your strategy.

Some tips to improve your mold polishing

  • Cleanliness. Keep your polishing area away from grinding dust and chips. Do not allow it to become a disorganized mess of loose stones and accessories.
  • Separate your stones. It is amazing that people dump a 220 grit stone in a box or can with a 600 grit stone. Sometimes they even mix up polishing sticks with different grits of diamond! Let's see, if you think you are polishing with a fine finish diamond and you get a coarser diamond piece mixed in, what do you think will be the result?
  • Move your polishing area away from grinders and bead blasting equipment. For some reason many shops just keep polishing away, right next to the surface grinders. All it takes is one stray piece of grinding grit to ruin your finish.
  • Use a microscope. Even though it is disheartening to see your work under a microscope, this is the only way to see the tiny scratches and imperfections. Plus, you can avoid rolling edges and damaging critical molding details
  • Don't use unskilled help to do skilled work. Polishing is an art that takes a lot of time to learn. Don't expect everyone to have the same ability when it comes to fine finishes or detail. Yet this often happens when owners view polishing as a nuisance.
  • Make a chart of the diamond paste colors. For some reason, some shops never write down which diamond is which grit, and moldmakers are left trying to figure out the same problem over and over again. Figure it out and make a copy near the diamond compound!
  • Don't think that "cross-hatching" is some kind of religion. Sometimes you see people who think you must only polish in one direction for each grit. Horizontal with a 400 grit and vertical with a 600 grit, for example. You can and should go back and forth in any direction you need to, just make sure you end up with the lines in the direction of the ejection-draw polish it. Your speed will dramatically increase once you get over this misconception.

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