Avadon Posted September 20, 2009 Share Posted September 20, 2009 So i've asked in other places on the net and since this forum is full of fabricators, welders, and blacksmiths I thought this might be the best place of any. I want to build a vaccuforming machine for pulling thin plastic sheet (like hobby sheet) down over a part. Now I have a wooden box which is shoebox size with lotsa small holes in the top. You stick the shop vac in the end and with the part sitting over the holes you push the plastic down over the part. The plastic is held in a wooden frame with staples and it's heated in the oven till it gets saggy. This method is good for very small parts. But I now hav a need for larger parts. 1/32scale aircraft canopies is what I know want to make. These are more like the size of a pack of cigarettes. So how do I scale this up to where I can really get a good form over these slightly larger parts. I think one of the problems is that why it failed with my smaller setup is that when I took the plastic out of the oven it would cool to fast and the plastic wouldn't stretch that far. So maybe I need much larger sheets. Anyone have a good source for clear, thin, styrene? Also maybe i need to heat it while it's actually being placed onto the part? I guess there are a lot of what-if's still left to be figured out. If anyone knows of good plans or has built one successfully i'd love to hear about it. Quote Link to comment Share on other sites More sharing options...
Avadon Posted September 20, 2009 Author Share Posted September 20, 2009 If your unfamiliar with what i'm referring to above this is basically the style i've been using. Warmplastic Order Form These guys seem to have pretty big setups. So maybe I just need to build a bigger box and frame? Is it really that simple or should I evolve to something more advanced. I've got considerable fabrication experience. I once built my own plastic injection molding machine with electric band heaters and a digital thermacouple and electric piston so I'm no novice to crazy contraptions. However no reason in making something complex if it's unnecessary. Hence why i'd love opinions, comments, concerns. Quote Link to comment Share on other sites More sharing options...
David Einhorn Posted September 20, 2009 Share Posted September 20, 2009 ....These guys seem to have pretty big setups. So maybe I just need to build a bigger box and frame? Is it really that simple or should I evolve to something more advanced..... Your easiest option seems like it is to build a bigger box and frame and give it a try. At least that is what I would try first. :D Quote Link to comment Share on other sites More sharing options...
Dick L. Posted September 20, 2009 Share Posted September 20, 2009 (edited) Are you familiar with the Vacuform Machine that was sold in the late 50's or early 60's ? It was a toy but made fantastic parts. All the plastic sheets had holes around the periphery and the machine had a hinged frame. The plastic sheet would lay in the frame sandwiched within it and locked in place by small pins aligning with the holes. The frame would swing 180 degrees. At 0 degrees it would lay over an electric hot plate. When it go saggy you would swing it to 180 which would place it over the form and create a air tight seal. You would pump a hand pump right away creating a vacuum and forming the part. The short time to transition from hot plate to forming position and positive alignment really made it work well. Hope this makes sense . Dick Edited September 20, 2009 by Dick L. Quote Link to comment Share on other sites More sharing options...
arftist Posted September 20, 2009 Share Posted September 20, 2009 Have not seen your setup, but can throw a couple guesses based on experience. 1. Air leaks. How tight is the seal between the forming machine and the plastic to be shaped? 2. Hole size and placement. 3. Improper vacum pump. While it is called a "vacum", a shop vac is unlikely to create a strong enough vacum. This is just a guess, I could be wrong. Most of the setups I have seen use an actual vacum pump. A quick test would involve a vacum gauge (auto parts store) teed in between the forming machine and the vacum pump. This would also enable you to test for leaks. If this is not enough info, I have a book on the subject and will dig it out for you. We only live about 40 minute away from each other, and I would also be willing to lend you a vacum pump until you find your own. Quote Link to comment Share on other sites More sharing options...
BIGGUNDOCTOR Posted September 20, 2009 Share Posted September 20, 2009 Small sheets need to be worked quickly. In your case probably in a couple of seconds. For a vacuum pump you can use an air compressor, and hook onto the intake side. The form needs to have a lot of fine holes in it to allow all of the air to be evacuated evenly, and quickly. Some larger units have a tank that is drawn down, then when the part is sealed down a valve is opened allowing the tank to suck in a large volume of air quickly. Have you thought about blow molding? Make a reverse of your pattern, and pressurize the plastic. This would require another chamber, but may work for you. These methods of forming canopies will produce some distortion, and may not be very clear. To get very clear parts they are usually cast/molded/ Quote Link to comment Share on other sites More sharing options...
Ian Posted September 20, 2009 Share Posted September 20, 2009 If you can't get hold of a proper vacuum pump then raid the local dumps for old refridgerators. The pumps on them draw a decent vaccum, especially if you link two together. I use such a set up for vacuum assisted casting of metals. I'd also go with the idea of a seperate tank to draw down. You can build up a large vacuum and then when your ready hit it all at once. Quote Link to comment Share on other sites More sharing options...
matt87 Posted September 20, 2009 Share Posted September 20, 2009 I've used 2 or 3 vac-forming machines in the past, on HIPS and acrylic up to abou 3mm thick. They all used a rack of electric heat elements to soften the sheet, which was clamped in place cold, the rack being swung or slid away when ready so that the form had sufficient clearance when the table was raised. I wonder if some sort of hot-air gun or hairdrier could be substituted for the heaters? Easier to get hold of, less scary to play around with, and you might be bale to keep teh heat going as you are forming. If you made the device in two parts I think you'd avoid a lot of awkwardness. The first is a box consisting the pump, (fixed) table and guides for the second part. This second part has a rigid frame for clamping the sheet and the hot-air gun mounted. To use you would clamp the sheet to the frame and switch on the heat. When it's softened you switch on the pump and lower the upper assembly (with sheet) onto the lower assembly, with the form on the table. Quote Link to comment Share on other sites More sharing options...
Avadon Posted September 21, 2009 Author Share Posted September 21, 2009 Are you familiar with the Vacuform Machine that was sold in the late 50's or early 60's ? It was a toy but made fantastic parts. All the plastic sheets had holes around the periphery and the machine had a hinged frame. The plastic sheet would lay in the frame sandwiched within it and locked in place by small pins aligning with the holes. The frame would swing 180 degrees. At 0 degrees it would lay over an electric hot plate. When it go saggy you would swing it to 180 which would place it over the form and create a air tight seal. You would pump a hand pump right away creating a vacuum and forming the part. The short time to transition from hot plate to forming position and positive alignment really made it work well. Hope this makes sense . Dick I am famaliar with that machine, at least a little because I actually set my master parts to a guy who had one and he was unable to draw parts that deep with it no matter what he tried. So that little machine is out of the question. Quote Link to comment Share on other sites More sharing options...
Avadon Posted September 21, 2009 Author Share Posted September 21, 2009 Have not seen your setup, but can throw a couple guesses based on experience. 1. Air leaks. How tight is the seal between the forming machine and the plastic to be shaped? 2. Hole size and placement. 3. Improper vacum pump. While it is called a "vacum", a shop vac is unlikely to create a strong enough vacum. This is just a guess, I could be wrong. Most of the setups I have seen use an actual vacum pump. A quick test would involve a vacum gauge (auto parts store) teed in between the forming machine and the vacum pump. This would also enable you to test for leaks. If this is not enough info, I have a book on the subject and will dig it out for you. We only live about 40 minute away from each other, and I would also be willing to lend you a vacum pump until you find your own. This is not a bad idea, maybe next time I build my drawdown frame I should build a dense rubber perimeter so the second I place the frame with the plastic sheet in it i'm getting heavy as much suction from my shop vac or compressor as possible. I have a fairly big shop vac. I mean it will defintiely lift things off the ground with the nozzle. I'm not even sure my compressor moves more air on the intake, however what air it does move it probably moves with serious power. Still I don't want to choke the intake of my compressor that could be bad and then injure the piston. I think probably the bigger part of my problem is keeping the heat on while drawing. I've tried a heatgun but it was simply not hot enough to keep it at the heat level that the oven did. What kind of heat element could I readily use that is hot and on a rheostat? Only problem I could see here is it is radiant heat and not atmospheric like the household oven. Quote Link to comment Share on other sites More sharing options...
Avadon Posted September 21, 2009 Author Share Posted September 21, 2009 Small sheets need to be worked quickly. In your case probably in a couple of seconds. For a vacuum pump you can use an air compressor, and hook onto the intake side. The form needs to have a lot of fine holes in it to allow all of the air to be evacuated evenly, and quickly. Some larger units have a tank that is drawn down, then when the part is sealed down a valve is opened allowing the tank to suck in a large volume of air quickly. Have you thought about blow molding? Make a reverse of your pattern, and pressurize the plastic. This would require another chamber, but may work for you. These methods of forming canopies will produce some distortion, and may not be very clear. To get very clear parts they are usually cast/molded/ I also thought about casting as well. I have smooth-on products and i'm famaliar with casting. Here again though i don't have a pressure pot/vessel to remove the bubbles. But worse then that sanding a master canopy is very difficult. It's like sanding a lens. You have top and bottom to worry about and then trying to get the resin in and the air out during pouring of such a thin item. Yikes! I suppose I could fake it with just a large brick of clear, but there goes all the cockpit internals, pilot, etc. Quote Link to comment Share on other sites More sharing options...
Dick L. Posted September 21, 2009 Share Posted September 21, 2009 I've done some forming , and laminating both vacuum bagging and with sheet . The most important thing I've found is an airtight seal between your frame and source of vacuum. Also having a volume pulled down to draw from.A rotary vane pump could do a pretty good job for you.A pump from a dairy barn milking set up would be a great candidate too. That first 5-7 seconds is going to be the most important I think. Dick Quote Link to comment Share on other sites More sharing options...
Avadon Posted September 21, 2009 Author Share Posted September 21, 2009 The other question I have is should i use Polystyrene or Acetate Butyrate? Which is easier to draw with? Quote Link to comment Share on other sites More sharing options...
K. Bryan Morgan Posted September 22, 2009 Share Posted September 22, 2009 Avadon, my personal experience with making model parts is a little dated, however this is what I did. I made a 12" sq box 3" deep. sealed the joints by putting a 1/4" sq piece of wood with glued and brads. I used peg board for the top plate and a frame that folded over that, that was hinged. I always used polystyrene, got it saggy warm, placed it over the form, then turned on the shop vac. I always got great results. If you look at the modeling sites, like Scale Modeler, I'm sure they will have some great information for you. Quote Link to comment Share on other sites More sharing options...
Dragons lair Posted September 22, 2009 Share Posted September 22, 2009 Built 3 dies for a refridgeration company(door panels) Steel dies about 50lbs per half. Needed perfect seals, strong vacume,hot sheets. About 40 sec per double door sheet. Was a royal PIB to get them right. Ken Quote Link to comment Share on other sites More sharing options...
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