I finally decided to make a vacuum formed polystyrene cover for the encoder on the front of the big mill. Seems pretty straight forward. I've got my first vacuum box finished except for the port to put the vacuum hose in. I noticed a lot of the DIY guys make a hole for the smaller vacuum hose even though they have a full size shop vac with the big hose. Is there some reason for that? Seems to me that the larger port for the larger hose would evacuate the box faster and form the plastic quicker before it cools. Am I missing something obvious?
(I have some PETG on the way to play with in case I don't like the way polystyrene forms or holds up)
If this goes well, I have all kinds of ideas for vacuum forming...
I think the reason is that the time is about the same with either hose. The bigger hose might be faster but not enough faster to be able to tell the difference.
I went ahead and drilled it to fit my big hose for now. If I do much of it I know I'll have to make two or three vacuum boxes anyway. It certainly has plenty of hold for a simple suction test, but not doing any forming. I couldn't lift a board off the box. I was able to lift a sheet of plastic, but only because it would bend. I didn't make it particularly perfect either. I am sure it leaks everywhere. LOL.
I posted over it sci.polymers too, but that group looks like its totally over run by spam.
There are two issues here, ultimate vacuum and pumping speed. A shop vac (from reading on the Internet, so it must be true :-)) has an ultimate vacuum of about -5 or 6 psig, or roughly one third of an atmosphere, but it has a pretty high pumping speed. So when you first turn on the shop vac it moves lots of air, beginning the evacuation of the box very quickly, and starts to suck down the softened plastic onto the mandrel very quickly. However, with no leaks and no matter how long you let it go, it will base out at about -5 or 6 psig (or 9 or 10 psia), which means the forming force on the plastic will be 5 or 6 lbs. per square inch of plastic. Adding more shop vacs in series or parallel helps the ultimate a little, but not much. Using a standard one or two stage oil vacuum pump gives you much better ultimate vacuum, down to 1 torr or better which is essentially -14.7 psig or
0 psia. This gives three times the "forming" force, which sounds great, but the problem is that those pumps (at least affordable, readily available ones) don't move nearly as much air at the beginning as the shop vac will, so the plastic has lots of time to cool off and stiffen before it ever gets formed. So people seem to either live with the shop vac or use a large storage tank that they pump down with a vacuum pump and then have a large diameter pipe and valve between tank and vacuum box so the effective pumping speed is very high at the beginning to really suck down the plastic. I've also read of hybrid systems using both shop vacs and vacuum pumps, using the shop vac for the first second or two then switching over to a storage tank. It all depends on your needs and goals. Not having built mine yet (:-)) I think starting with a shop vac is the best way to get a simple system up and running, to get some experience and to see if a better vacuum system is needed for what you want to do. Then upgrade as needed.
Vacuum pump has more vacuum than a shop vac. Should suck down quick enough, and get better corner forming.
Good vacuum pumps aren't cheap. I remember how expensive the one we had at Qualcomm was. We used it all the time for holding thin parts on fixtures we made. It broke in the middle of a job I was doing and I tried to use a Shop-Vac. Learned a valuable lesson on why a good vacuum pump is needed.
Finally got around to the first attempt this evening using Plaskolite polystyrene.
1st attempt I didn't get the plastic hot enough and let it sag far enough.
2nd attempt was not bad over all. There was just a little too much radius at the junction between the mold and the table.
3rd attempt we spaced up from the table with popsicle sticks. It formed very well with the radius below the mold. I had an issue with webbing because I took the sheet down too fast, and let it vacuum down too quickly, but I have a feel for it now.
There will be no forth attempt with polystyrene for this project. The polystyrene is more than hard enough when formed, but its way to brittle. I have some PETG ordered that should arrive sometimes this week, so I'll give it a shot when it does.
I do have some ideas for the use of the polystyrene in the future though. I need to look into its chemical reaction a little more and find out what it reacts to, and what its stable with.
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