Vacuum forming pickup covers with my new vacuum table [Pictures] - created 12-25-2008
Searcy, Clint - 12/25/2008.14:50:08
MIMForum Staff, Nashville
Ok... so I'm trying to make some new pickup covers for some very non standard pickups. But I'm having some trouble. I know some of you here have experience with this so I figure time to ask the experts.
Here is my air hockey converted into a vacuum form table.
This is the mess I keep getting. See the corners? Can't get them to form up right.
I figure I have one of the fallowing problems.
1. Not enough vacuum. I have seen guys on YouTube doing this with 5 hp shop vacs. Mine is a 5.5hp
2. Plastic is too heavy. I'm using EverGreen Sheet Styrene that is .040" (1.0 mm) thick. Maybe that's too thick for the corners?
3. Dummy needs some relief under it to get it up off the table?
I don't know.... what say you ?
Hold up! An air hockey table converted into a downdraft/vacuum table?!?! That's friggin' genius!
Oh, and uh, I have no idea how to help you with your vacuum forming problems.
I'll be on craigslist...
Yah, If you haven't noticed Clint is a savant.
(1)Try cutting some ways in the mold/dummy on your table saw. These will ,or should, allow more air passage ways as the styrene yields to the vacuum.
(2)You might want to keep a heat gun handy if the material is cooling to fast in the thinner spots formed as the material srtetches over the form.
(3) try a taller form. The taller form may allow the material to follow the form longer as it yields to the vacuum. The wrinkling should usually appear at approximately the same relative distance from the table. The taller form will give you more distance from the table which should stave off the wrinkling long enough to allow for the pickup depth. Did that make any sense?
Yea, One other thing I tried was making my vacuum surface larger by removing some of the tape. I also raised the model about 1/2" off the table. Oh.... I also pulled the filter off my shop vac to get a little more pull.
We're getting further away from K-Mart.... yea...yea... almost time for Wapner.
The shop vac does not pull enough vacuum. You need a real vacuum pump capable of pulling upward of 20 inches of mercury vacuum. Also a 20 mil vinyl plastic would probably stretch more and have less folding.
I would suspect that a Shop Vac wouldn't last very long with that kind of load for extended periods of time. Most vacuums don't like to be plugged up for long, and it burns up the seals.
I claim "observer status" only, but... it seems like you'd want to try to find a way to make the plastic sorta "bunch up" along the middle of the sides and the ends, you know, sorta stretching it around the corner into the available space on either side, as opposed to the natural tendency for the sides to stay "straight" and have all the excess material stuck at the corners with no place to go. Does that make any sense?
And of course, I have no idea how to
do
that... Maybe something in the way the material is originally laid out around the mold? I'm just thinking... suppose you stapled the material down near the corners, on the angle, so that material was somewhat taught there; you'd have a "bunch" left over in the middle of the sides, where it would make a little awning. I'm imagining the excess material forming the undesirable anomalies in the middle of the sides, now, which would be, at the very least, different.
I suggest you switch from plastic to a rubber membrane. more stretch, more confirmity. I'm using an old refrigerator compressor with no problems. LMI sells the membrane or you can buy from a local rubber products supplier.
A 5 HP "peak" vac can only develop 7-10 inches of mercury vacuum. You would do better with a tiny Ghast diaphragm pump or rotary vane pump which will get you around 24-27" of Hg vacuum. I don't think the air hockey table will survive that sort outside pressure without a lattice framework underneath to support it. Most of the vacuum formed PU covers I've seen have been ABS plastic. Styrene doesn't always hold up very well over the long run.
Hi Clint,
As far as the pattern itself goes, one thing that will help is adding a little draft to the sides, just angling them in towards the top by a degree or two. Putting a bit bigger roundover at the top will help it pull better, and will also lessen the thinning that happens in that top edge. Raising the pattern up often helps; probably 1/8" is enough. Drilling extra holes in the baseplate right along the perimeter of the pattern is also a good idea.
I'd agree with the comment about the polystyrene; it's cheap and easy to experiment with, but it cracks easily. I've never vac-formed ABS before, but it is certainly worth a try. Lexan is really tough and forms well, and you can paint the inside. Good luck; it's a bit fussy but you'll get there I'm sure. Timing is everything; having the vacuum hooked up to a footswitch or an assistant makes a big difference.
I figure I have one of the fallowing problems.
1. Not enough vacuum. I have seen guys on YouTube doing this with 5 hp shop vacs. Mine is a 5.5hp
2. Plastic is too heavy. I'm using EverGreen Sheet Styrene that is .040" (1.0 mm) thick. Maybe that's too thick for the corners?
3. Dummy needs some relief under it to get it up off the table?
I don't know.... what say you ?
Clint I don't see in here or your descricption where you applied heat or how much heat. I think that is part of your problem, but the shape and depth of the part is likely part of the problem as well. This part looks like a good candidate for casting using high durometer urethane molds.
Weren't you also building or buying a small cnc recently? This could help in cover production as well.
Mike
A couple of thoughts--Like Mike mentioned, it looks to me like you may not have the plastic hot enough/even enough for vacuum forming. But I think that uneven vacuum looks more likely. For vacuum bagging veneers, there is a plastic mesh that you can use to allow all of the air to escape from the bag--I think that some of this mesh underneath the pickup form might be just the ticket. The mesh really does help keep that kind of bunching up from taking place in a vacuum bag--it seems like it should do the same thing here. Joe Woodworker sells the mesh, and I'm sure others do too.
Dave
AHA! I run a thermoforming shop as a day job.. So here are my observations. 1) you have webbing at the part corners. This is usually caused by having the plastic sag too much or heating it too long. Remember that thermoforming is a stretching process, in the at the plastic thins a bit as it stretches over the mold. Webs are caused by plastic piling up and not being able to conform to the tool face. 2) The plastic is stapled to the top of the carrier frame. This standoff is a bad idea, it promotes webbing and low sheet tension over the model at time of contact. It is always better to have the sheet seal off against the sheet face instead of the carrier. 3) There are small bubbles in the plastic, this is usually a sign of not drying the material before use. I can't tell what you were pulling, but it looks suspiciuosly like Vivak or PETG. Both need to be dried in an oven on low for 3 or 4 hours on thin plastic. These plastics are hygroscopic and absorb moisture from the air over time and elevating the sheet temp over 212 f can result in the trapped moisture blistering the sheet. 4) you are sealing the model off against the platen. I suggest sticking thumbtacks in the model to make a vac path under the part to pull a tight corner. 5) there is a 90 degree corner at the bottom of the model. Assuming you have excess height in the model to let you trim the part off easily, I'd add runout to the model. 45 degree angles around the model help kill webbing and make it easier to form. 6) insufficient vacuum. A shop vac can and will pull this part, but a better vacuum motor for simple home parts is this
http://surpluscenter.com/item.asp?UID=2008122912411722&item=16-1234&catname=air
sorry the format is poor.. cut and paste it to your browwser. Forget a gast pump to pull small parts unless you want to go to the trouble of using a plenum tank. You need volume and vacuum to make it work. Shop vacs have volume, vac pumps have the vacuum. The hombrew choice has devolved into shop vacs on platens full of holes.
You should be able to pull this part cleanly, but if it pulls down and isnt tight to the parting line, you can leave the vac on, grab a heat gun and warm the plastic. It takes a while, but you can make it work.. Even better is the use of a backpacking infared heater on a cmall propane tanl. There is a 4500 btu one that screws onto the top of the 16 oz bottles. This is a process that most guys mess up because they have insufficient vacuum and they overheat the sheet to adjust the process. Be patient, make a few of the changes I recommended and write back..... Scott Correa.
Good stuff Scott. I'm sure I was over heating the plastic at this point. I bet I had 3 inches of sag when I took the sheet out of the 450 degree oven.
The reason I'm doing this is that I need to make some Parker Fly covers and molds. I have made rubbers molds for countless other projects and that is just to expensive a process for ever strange order I get. I need to add this style of cover making to my bag of tricks.
I'll try a few more tricks and get back with you.
Clint I have done some vacuum forming and I suggest you use a hair dryer or heat gun to warm your material right before you put the suction on it. It will help ease those folds.