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Vacuum Forming
 Moderated by: Herb Kephart
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 Posted: Tue Oct 15th, 2013 03:07 am
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thtroll
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My first attempt at vacuum forming.  I have built 2 vacuum tables, one @12" x 12" and the other 6" x 6".  My goal is to create the upper hull of a Ram Kangaroo for a kitbash of an early Sherman Kit by Tamiya.  I hope to use the skills learned to create light rock moldings, tarp covered loads, ore loads, roofs, forms for casting, etc..  The possibilities ... hmm.

The table was built with 1/2" ply that was kicking around, and some research from the internet.  1/8" holes drilled 1/2" apart, the a grove was cut connecting all the holes.  This maximizes the efficiency of the vacuum.  The box is created from 2 pieces of 1/2" ply.  The bottom is 8" x 8", the sides of the box are 2" wide, glued and screwed to the base.  The lid is 6" x 6", glued ans screwed to the sides.  The smaller the area inside the box, the better the efficacy of the vacuum.  (High School physics coming back to you yet?) Drill and appropriate size hole in the base for your vacuum hose.



The frame for the styrene is created from one piece of 1" ply and 1/4" ply.  It measures 8" x 8" with a 6 1/8" x 6 1/8" center cutout.  There are 'guides' on the sides to help align the frame during the forming process.  They also are 'legs' for the frame, so the drooping plastic doesn't touch the oven rack.  As you see the 1/8" ply is on the bottom of the frame, this allows the plastic to seal onto the table.  I recommend this style of table for ease of build and its high efficiency.





I created the mold by filling the Sherman hull with air dry clay.  After it dried and shrank, I built up the shrinkage with more clay and carved the mold to shape.  I corrected the width but not the length, I will have to fix that with styrene. 

I cut 60 thou styrene to size and installed in the frame.  Heated in the over at 350 F for about 3 min.  First attempt was OK, but the edges where rounded at the base of the mold.  Increased heat in oven for second attempt.  Total FAIL, too hot and the plastic 'sagged' too much and creased terribly.  Reduced heat back to 350 F and placed frame in for 4 1/2 min.  Better result, but the base still rounded too much.  Final attempt was with the first form.  This time I raised the form up off the table about 1/16" , placed the first attempt over the form and heated with a heat gun.  Starting from the outside and worked toward the mold.  heated for about 2 min and the started the vacuum.  Heated the piece until the shape was perfect and is what you see above.

Thanks for looking.



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 Posted: Tue Oct 15th, 2013 04:50 am
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W C Greene
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This is just the kind of information that makes this site the best. I don't really need to do any vacu forming but the tips shown here are great. Thank you Heath for posting this.

Woodie



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 Posted: Tue Oct 15th, 2013 01:12 pm
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Herb Kephart
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We had a commercial vacuum table at the industrial model building shop that I ran years ago. It had a large tank that would "store" the vacuum, which was pumped up (down!) while the frame with the sheet was heating under a set of nichrome heater elements. On some things (no rhyme or reason that I could see) better results were attained by opening the valve between the tank and the platen quickly to "dump" the whole tank onto the platen. Some times it was the only way to get a draw that conformed to the pattern, other times it just tore the plastic. Had to play around with the time under the heaters, and the speed of the vacuum application. One thing that we found however, was that letting the pump draw the platen vacuum down - instead of using the tank approach- usually resulted in poorer results.

Remember that you are never going to get crisp detail--any feature on the pattern is "rounded" by the thickness of the plastic sheet.

Herb

 



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 Posted: Tue Oct 15th, 2013 01:23 pm
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thtroll
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I had read about sheet thickness affecting the mold. The right side was carved 60 thou deeper and a hole was drilled to help the plastic conform to the curve. I raised the final attempt twice the thickness of the sheet to counter the curve. Definitely a quick vacuum is best from what I have seen on line.



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 Posted: Tue Oct 15th, 2013 01:30 pm
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Bernd
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I'd like to add a bit more information to Heath's vacuum forming.

A long while back a friend of mine that is into Gauge 1 set up a vacuum forming rig to do styrene sides for a British passenger train. Being interested in how that's done and thinking that it can be applied to the smaller scales I set out to find more info. I found it at Lindsays Technical Books. They have gone out of business but some of their books are still available through http://www.youroldtimrbookstore.com I checked and can't seem to find the two books I have.

I've made a copy of the covers.



I believe Douglas Walsh published his own book. There is no library number in it, however there is an address: Vacuum Form, 272 Morganhill Drive, Lake Orion, MI. 48360.

The other book by Vincent Gingery is also self published. David J. Gingery Publishing, P.O. Box 318, Rogersville, MO 65742

If you are in the larger scales these books would be a great asset if you want to try vacuum forming. I'd recommend hunting them down and getting a copy.

While on the subject of vacuum forming I'd like to add a bit about casting to this thread if Heath doesn't mind.

I needed a vacuum chamber to get the bubbles out of my first attempt at resin casting using Alumilit Casting kit. What I used was a refrigerator compressor. Yup, compressor. It functions both as a compressor and a vacuum. It's a great combo for a compressor for painting and for vacuum forming. Here's a pic of the one I removed from a fridge from the transfer station. I have collected several compressors.



I made a base from MDF and then laminated counter top material for a nice smooth surface. I routed a trough and epoxied a piece of 1/4" brass tubing. A hole was drill into the counter top in the exact center.



Next I bought a 3 bowl set of Pyrex Glass and made up a rubber ring using a latex sealant.



And here's is the whole works assembled, ready to degas a casting or rubber mold.



The fridge compressor can be used for vacuum forming in conjunction with a 20lb propane cylinder. Since the compressor would not be able to evacuate the fixture fast enough you would use the propane cylinder as a vacuum storage tank. Evacuate the tank first and then open the valve of the tank to evacuate the air from the form in one fast motion. Gingery's book explains how to build the whole vacuum forming fixture. Great for production work.

Hope this has added more info for those that want to try vacuum forming or for those looking for better castings. Questions? ask away. I'll see if I can answer them.

Bernd 



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 Posted: Sun Jan 21st, 2018 04:50 pm
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William M
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Anyone having bought one of those small dental vacuum forming machines such as Mictro Mark have, may wonder where to obtain spare heating elements...At $14,80 a throw as opposed to a USA dental firm selling the element at $50+ you might want to try:-

https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20180121025046&SearchText=Dental+lab+Heating+tube+for+Vacuum+Forming+Molding

Shipping to the UK was free...May be the same for the USA....

I put this on GR magazine (but not everyone reads it.......)

EDIT  1/FEB /2018  SPARE PART arrived 31 Jan 5 days from mailing from China.  Not bad!

[toast]

Last edited on Thu Feb 1st, 2018 12:52 pm by William M



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