Saturday, January 31, 2009

Vacuum Forming

I've decided to scratch build all my tops, and bottoms in the case of the precipitators,  of any structure built using Rix water tank sections.    These include the stoves for both A and B furnaces, the precipitators (joint facility), and the B-furnace dust catcher.    Vacuum forming, for those not familiar, is basically taking heated plastic and forming it over a male mold, using a vacuum to draw the plastic into the mold while at the same time cooling/setting it.   It's good for larger parts, especially those with gradual curves/slopes.    You can buy a vacuum forming machine - they are slick but expensive, and most are designed to take large sheets of plastic.  For modeling HO scale this would be a waste since the scrap plastic from forming is really unusable as it has been stretched and the thickness at that point is variable and not consistent.    It's quite easy to make your own machine cheaply and easily.   The machine pictured on the right was built in literally 15 minutes.    I took a scrap piece of 3/4" birch plywood and ripped a 7" wide strip on the table saw.  Turn the strip sideways and cut 2 7" squares and then two 3" x 7" strips.  Cut two more 3" strips about 5.5" long.   Using glue and 2" brads (from a brad nailer) nail the four sides together and then nail on the bottom.  Take the top to the drill press and drill small holes about 1" apart or so and maybe 3/16 round.   No need to layout a grid or anything unless you need that perfection - just drill the holes - sand the top on both sides and clean out any plugged holes.  Take the bottom assembly and drill a 1 1/4" hole in one side - this is for a small Shop Vac.  Any vacuum should work so check your hole size first.  Glue and nail on the top.   Almost done - take some more scrap 3/4" plywood and cut two 9"x9" squares .  Then set the distance from the fence to the blade to about 1" or so and plunge cut the centers of the squares out.  If you are unfamiliar with this technique on the table saw it would be safer for you to use a jigsaw - remember it doesn't have to be perfect - the only requirement is that it clears your vacuum machine top and doesn't have an opening bigger than a 8"x8" sheet of styrene.    - You are done and ready to start vacuum forming - all you will need additionally, besides the vacuum is an electric stove or hot plate (heat gun is ok too) and four spring clamps, oh, and most importantly, you need to make your masters.  
In the picture you can see some of the masters I've made - one is for a McClure three-pass stove top, the other is a two-pass stove top, and then the third one is the precipitator top.  I also tried to form a conical reducer for the McClure flue pipe/stack, but the profile was too extreme and I couldn't get decent results.   All these pieces are turned on a small wood lathe from a 4x4 piece of cedar.  The cedar is soft and easy to turn, however, the grain can be a bit open on the ends - I will need to seal and fill these pieces, but before all that work I run a test just 
to check in the size is right,..etc.  

The vacuum forming process is simple - 

  • Cut a 8" square from .030 styrene
  • Clamp the styrene in the frames using a single spring clamp on each side - center the spring clamps as close as possible (this is so you can balance the assemble on two coffee mugs over the burner
  • Set the vacuum machine and vacuum up next to the stove (electric only)  
  • Place two coffee mugs on each side of the burner, turn it on to high
  • Place the mold master in the center of the vacuum form table
  • Take the frame/styrene and place centered over the burner, with two spring clamps resting on the coffee mugs - don't worry that the frame is wood, it won't be on long enough to catch fire
  • As soon as you place the styrene over the burner, turn on the vacuum so you won't be fumbling with this later, unless you have a helper, then have them turn it on just before setting the plastic.
  • Watch the styrene closely - DO NOT LEAVE THE ROOM, and always have a fire extinguisher close by.   Initially the styrene will begin to soften and the middle might first droop just a little and then it will rise a bit unevenly, as the piece begins to droop again, but this time consistently , get ready.  I usually let the plastic droop about 3/4" or so.  At this point pick up the frame and move over the machine and sort of just place the hot plastic squarely over the table top and - poof - the piece is instantly formed.  
  • Let the vacuum run for 15 seconds or so - this will cool the plastic and harden it
  • Turn off the vacuum
  • Unclamp the frame and remove the master.  
Now that I know I have the right size and know the process will work, I need to still clean up the masters.  I will fill the grain and any blemishes, add detail like rivets and panel seams, and then seal the piece and lightly wax it to make it easy to remove from the plastic.  

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