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Principiante
03-16-2007, 01:34 PM
I´m about to start a new thermoforming business with a homemade vacuum machine.

But as the vacuum has to pass trough the mold, I must make little holes to the mold. I dont like it because it takes my time and it leaves marks in the finished parts. Those little marks, if manipuled, can be broken, and the parts is no more airtight.

With vacuum, can tiny holes be eliminated?

If not, I have heard you can pressure form the plastic, instead of vacuum form it. I think this way I elliminate the holes. But if I make a pressure box above the plastic sheet, there will be no space for the heater.

Please post any ideas or links about a pressure thermoforming machine diagram. Thank you

One of Many
03-16-2007, 04:11 PM
Some form of getting the air out of any sealed cavity is a must, regardless if it is vacuum or pressure formed. Pressure forming could make the problem worse because you can develop greater positive pressure than you can get from vacuum verses ambient pressure. It also takes more structural strength in the machine and related tooling to keep the air pressure sealed.

Theoretically, with materials less than 1mm thick, the vacuum hole size should not be any larger than one material thickness that you are forming. Most of the time a much smaller hole works best, but you might need more of them. If the holes end up too large, I have drilled the hole out larger, then fill with epoxy and re-drill smaller vac holes or beat in a small aluminum rod/rivet/plug and re-drill as needed.

On thin materials, too much heat allows the material to pull into the holes easier too. Getting the material to just the right temperature can be merely seconds from too much or not enough heat.

Machines that can do pressure forming have an oven that the material slides into for the heat cycle, then slides out to the forming station. The forming station consists of an upper and lower cylinder mounted platen that can be moved up or down into position. While one platen holds the die and the other platen holds the seal ring pressure box. Normally the whole cycle is automated with timer relays for each portion of the total cycle sequence. The automation of the timed sequences maintains consistent duplication of the formed parts which would be harder on manually operated forming machines.

In some instances you can use course sand to sand blast the surface of aluminum dies. This helps retain an air gap on the die surface so it is less likely to seal off around vacuum holes.

Some forethought on the die design, if it is built up from aluminum plate, can have vacuum ported between the parts. In other words, larger vacuum holes can be hidden under another blocks mounting surface that has band sawed slot on its underside before it is fastened to the main die.

Unfortunately, it comes down to better equipment and/or methods that work around undesired results.

DC

Principiante
03-18-2007, 05:59 PM
thanks a lot for your answer, One of many. Very complete.

I want perfect surface finish. So one question:

pill packaging molds hace vacuum holes? (Surface finish is perfect)
Or is other method?

Thank you

One of Many
03-18-2007, 06:58 PM
Not necessarily. Some Pill packaging may also be considered bubble packaging. Where there is no traditional die, just slots in a plate that the bubbles pull down into.

Other than that, it depends and whether it is a male or female die. A male aluminum die in this case could be polished with a few vac holes placed around the base of each cell feature, would never be visible on the finished part. A female die could present a problem in this application since the cavity size limits the amount of material available and could become too thin based on the cavity depth of draw.

Some of this is total speculation on my part, since I have never had experience at that type of forming.

DC

drcrash
04-02-2007, 04:39 PM
Are you forming into a cavity, or over a positive mold?

For small run stuff over a positive, you might try putting pantyhose-type nylon fabric over the mold, rather than actually roughening the mold by sandblasting. That may be enough to give enough fine-grained texture to do the trick without the texture showing up on the outside. (It might not even show up on the inside, but that might depend on a lot of variables.)

If you do try this, I'd be interested in hearing how it works out. I've heard of doing it, but never done it myself.

manyhobies
07-03-2007, 08:49 AM
Hello

I worked for a thermo former for 6 years. I developed a "micro" hole process for flat packages to eliminate the pooling (air trapped on the flat surface not allowing the plastic to be pulled down until it cooled causing rings in the plastic). I back drilled the mold and then used a .008” drill on a mill using a hand fed micro chuck. This gave us a clear flat surface. The visibility of the vacuum holes in the plastic was greatly reduced. However, the time to drill the larger back drill holes and then the smaller holes on the surface was greatly increased but the finished product’s quality was greatly increased.

Regards,

Dan

kayaker43
07-07-2007, 12:51 AM
There's always this stuff....

http://www.segen-online.com/th-rawb.html

If forming over a male mold, a fine texture from sanding, beadblasting etc.. usually does it. The holes (or slot) would only be needed around the base. Female molds would need holes, but you can usually hide them in corners.