Here is a mold-build in more detail. The parts are for a dental application and are fairly small; a few grams apiece. The mold is 5″ x 6″ and about 7″ tall. This is a “simple” mold because it is essentially two flat blocks of steel with the proper shapes cut into them into which molten plastic is squirted. There is no complicated mechanical action in this tool other than the ejector system that allows the cooled parts to be pushed free. This is the least expensive style of mold; it is smaller and much simpler than the two-slide mold shown below even though the parts are larger and there are two more cavities in this mold.
This is a fully hardened, high volume production tool. All the cavities were made with sinker and wire EDM so everything was burned after hardening and grinding. The small size of this mold meant that the main mold plates could be made from solid blocks of tool steel…it’s far more common to machine rectangular pockets in the mold set and bolt in smaller tool steel cavity inserts. It’s a lot less work to do it in the solid, but the tool steel gets very expensive in larger blocks.
After the mold was almost finished, the inventor decided on a new color scheme for his parts that required each to be molded separately, and also added a new part to his design that needed to be accommodated in this tool. A valve had to be added to allow a single pair of parts to be molded without filling the other cavities, and two new cavities had to be squeezed in too. This decision added cost…almost a third of the final build price, mostly because the blocks were already hardened and ground by the time of the new decision, and the four original cavities had been burned in and couldn’t be moved without welding and re-burning. All of the preliminary work (like drilling and tapping holes) that could have been done in minutes in the soft steel blocks took hours in the hardened ones. Additional parts also had to be made, and some of the original work had to be scrapped.
||This is how a mold starts; a bunch of rough sawcut blocks on the bench. The two upper blocks are the main mold plates which will be hardened. Everything else is 4140 prehard.|
|Here is the trode for the second burn.|
|The finished mold on the bench and ready for testing.|
Quality and tooling cost are a subject I’m asked to comment on frequently, particularly with the decline of North American manufacturing and the rise of overseas manufacturing to take its place. As with all things, there are tradeoffs to be made, particularly since it’s quite difficult to be assured of quality tooling when it’s commissioned from far away.
That still means your best hope for success is to commission North American tooling, unless you can find a vendor to partner with, who has lots of experience overseeing its tooling procurement from abroad and can help you pick sensible circumstances under which you can benefit from the cost advantage. Only a seasoned molding company can do this reliably: we’ve partnered with a very well-regarded, quality conscious Vancouver company, and have them commission tooling abroad whenever it’s the best way to meet our customers needs. However, all of our logistically complex projects are tooled in-house to mitigate the problems of long supply chains, intellectual property protection and uncertain communication.
Quality is not cheap, and there are plenty of competitors who offer extremely inexpensive tooling and like to pretend that it’s just as good as a quality tool and can give you what you need. That’s not quite true if your needs are more sophisticated than the roughest of plastic parts and if you can’t accept production delays from tool breakdown.
I’ve shown a few shots of some typical problems that have crossed my desk over the years and are worth thinking about when you’re setting tooling goals and budgets, as well as when you’re choosing a tooling and molding vendor.
Shown is a mold I’m just finishing up (as of August 2009). It makes a family of small dental parts using interchangeable cores and slides. I’m going to run one experimental cavity first, measure the finished plastic parts, and then cut the remaining cavities as needed to get the very tight tolerances demanded in this application.
This is a good example of a medium to high volume production mold and has fully hardened tool steel parts wherever high wear or strain will be encountered. Everything is fitted very precisely and the finished mold is expected to produce parts at the rate of 4 parts every ten seconds. A mold like this costs around $20,000.00 to build.
|All that to make this tiny part!! This is the same photo that is shown in the miniature machining section, and is, of course, a picture of the machined prototype part, not a molded part. I’ll update this shot when I get parts out of the mold.|
|The molded parts ready for testing. There are 6 different configurations of this part. All the mold cores will now need to be tuned to make these parts fit perfectly to their mates.|