Beginner moldmaking questions

[ckirchen]

Glad to help.

In Alberta, the plastic molding trade needs a boost. The local technical school cancelled their program about two years ago, and we've definitely noticed the lack of new skilled workers. We're working with the school to get it started again, but everyone we've talked to (including members of the Canadian Plastics Association) is hesitant because of all the work being sourced overseas. I don't know what the situation is like in the Boston area, but any growth is exactly what we need to maintain North America's presence in the global mold making/plastic injection industry.

As for your questions:

The half of the mold on the nozzle side of the machine is called the 'fixed half' or 'hot half'. The other half is the 'moving half', 'cold half' or 'ejector half' (FYI: The cavity usually refers to the hot half and the core is usually the cold half--special circumstances may dictate otherwise).

The top surface of the mold has a locating ring which fits into a locating hole in the fixed platen. The mold is installed as one piece, using the locating hole as a locator. Once each half is clamped to its platen, the mold can be opened (by opening the machine).

Unless you have special circumstances, you always want the part to stick to the core (moving half). Sometimes, it's a no-brainer. The plastic will shrink as it cools, and it will grip the core and pull away from the cavity (a drinking glass is a perfect example). Other times, the part will not release from the cavity and some sort of small undercut will be required on the core (i.e. texturing, shallow grooves, etc.). But, in both cases, since the part sticks to the core, the ejector system will also be in the core.

Once the resin is injected and the holding cycle is complete, the press (and mold) will open. Then, the ejector plate in the mold is pushed by the knockout system (a system behind the moving platen). In it's simplest form, it is just a round rod that protrudes through the moving platen, through a hole in the mold's base plate, and pushes the ejector plate forward as the platen moves. All ejector pins are held in the ejector plate, so they all push forward, and the part falls into the bin below. Return pins are also attached to the ejector plate, and as the press (and mold) closes, the fixed half presses the ejector plate back into place. Then the cycle is ready to start again. The knockout system is built into and powered by the injection press. Most molds should not require extra equipment to eject the part. But, you'll soon find out that no mold is regular...

The press's knockout system is retracted by itself, but it is up to the mold to take care of retracting the ejector pins. Sometimes, you can attach the knockout system directly to the ejector system (so that the ejector system retracts with the knockout system). Or, the return pins I described above will do the job. Springs sometimes also help. Usually, it is the molded part that dictates the path you take.

We use D-M-E (www.d-m-e.com) parts in our molds, but the link you gave shows similar parts. The D-M-E catalog has all sorts of devices for more complicated ejection. I'd suggest requesting a copy and read it cover-to-cover. It's full of technical drawings--a lot of things will be explained.

If the part wall is flat, the end of the ejector pin needs to be flat, and it's a simple matter of facing the end. If the part wall is curved or angled, the ejector pin needs to be located in the ejector plate (i.e. to prevent all spinning) and machined to match. Sometimes it's tricky, but if the core side of the part is not seen by the end-user, special efforts are not necessarily required. Unless you're a perfectionist...

As for part shot size, we try to stay within 20% to 60% of the press's shot size for smooth running. If the shot size is too small, the resin will degrade because it's in the barrel too long. If the shot size is too large, the machine is working hard and the cycle might include a waiting period where the barrel refills. As for a maximum, we try not to go above 80%.

To look at used machines, I would suggest you talk to press manufacturers. Make it known that you are not looking for a new press, and the salesmen will do the some digging. They have contacts all over their sale's area and they might know of a company trying to get rid of a used machine. Sometimes, they even take used machines back to the factory as a trade on a new machine. And, more often than not, you can see them running.

Based on past experiences, I would tend to stay away from uncertified used machines. There will always be bargains out there, but once the press is installed, you'll see why it was a bargain in the first place. At that point, the press is often not worth the steel it's made out of.

However, if getting a beater up and running would be a task you willing to undertake, you can save a bunch of money and, at a minimum, you'll learn the inner workings of an injection press. Just be careful what needs to be fixed--hydraulic hoses are one thing, but if the press needs a new screw for the barrel, it might cost as much as the machine itself. And, I will admit, there are diamonds out there. Do your homework and you might come across a great deal.

Chris Kirchen

[skippy]

"In Alberta, the plastic molding trade needs a boost. The local technical school cancelled their program about two years ago, and we've definitely noticed the lack of new skilled workers. We're working with the school to get it started again, but everyone we've talked to (including members of the Canadian Plastics Association) is hesitant because of all the work being sourced overseas. I don't know what the situation is like in the Boston area, but any growth is exactly what we need to maintain North America's presence in the global mold making/plastic injection industry."
It's scary to think that in 20 - 30 years time, people like you (mold making/plastic injection industry) and lots of other clever people in other industries might cease to be. Sometimes ignorance is bliss, better not to think about it.
Skippy

[rsecret]

I find this to be an interesting topic. Its nice to get some insight considering we just had a number of AL molds and ABS parts made for us last year. When we sent the job out for bid the prices where all over the place. Being are parts are small the bigest dicision was the number of mold cavities to use. As the cavity count went up so did the mold but the price per part went way down. So from an econmical stand point multi cavity molds can save you money if running a large number of parts.

The cost of are molds where not to bad as they where AL inserts or mud based. I was not to happy with the final parts as many had flash around the parting line and others had nibs. We had to clean up a large number of them in house which was a pain. Being a small start up company we had to go cheap so as the story goes you get what you pay for.

We have a number of other parts in the works so I will again need to find a good shop to make the molds and run the job. I considered having the molds and parts made by different vendors but I think its best to have it all done under one roof. This way I dont have to get in to the blame game if something goes wrong.

If anyone's interested or knows of a good shop I would be happy to here about it. We should be done with our drawings in a couple of weeks.

Thanks, RJS

[trkulja]

A USEFUL LINK FOR ME WAS
http://www.polymers-usa.bayer.com/resources/index.cfm
THERE ARE LOTS OF REFERENCES THAT HELPED ME AND I HOPE THAT YOU FIND THEM USEFUL...

CHEERS

[Dave47]

I Build Quaulity molds in the USA at reasonable prices , Send part prints or iges files for a Quote Thanks Dave 731 513 0396

We can build in mud sets or standard mold base. located in Tn.

Originally Posted by rsecret I find this to be an interesting topic. Its nice to g et some insight considering we just had a number of AL molds and ABS parts made for us last year. When we sent the job out for bid the prices where all over the place. Being are parts are small the bigest dicision was the number of mold cavities to use. As the cavity count went up so did the mold but the price per part went way down. So from an econmical stand point multi cavity molds can save you money if running a large number of parts. The cost of are molds where not to bad as they where AL inserts or mud based. I was not to happy with the final parts as many had flash around the parting line and others had nibs. We had to clean up a large number of them in house which was a pain. Being a small start up company we had to go cheap so as the story goes you get what you pay for. We have a number of other parts in the works so I will again need to find a good shop to make the molds and run the job. I considered having the molds and parts made by different vendors but I think its best to have it all done under one roof. This way I dont have to get in to the blame game if something goes wrong. If anyone's interested or knows of a good shop I would be happy to here about it. We should be done with our drawings in a couple of weeks. Thanks, RJS

[rkonnen]

We also do molds and molding and would not mind a chance to bid on your job. You can contact us by e-mail at rkonnen@neandertech.com or phone 941-722-1174.
Thanks, Richard Konnen