[sci.electronics] Fabrication of cases

bobm@anasaz.UUCP (Bob Maccione) (09/25/90)

Does anyone out there have any hints/pointers to making custom cases
for electronic projects. The case should be plastic ( or fiberglass )
and I would like to make it not a simple box shape ( ie curves, etc ).
Although it will be small it should be rugged.

thanks for any help
bob maccione

bwhite@oucsace.cs.OHIOU.EDU (Bill White) (09/25/90)

In article <3015@anasaz.UUCP> bobm@anasaz.UUCP (Bob Maccione) writes:

>Does anyone out there have any hints/pointers to making custom cases
>for electronic projects. The case should be plastic ( or fiberglass )
>and I would like to make it not a simple box shape ( ie curves, etc ).
>Although it will be small it should be rugged.

	There is a good article about this in the April 1983 Radio
Electronics -- check your local library's microfilm or bound section.  
Among the things discussed: choice of plastics, cutting, heat forming,
solvent welding, embossing, drilling, polishing, matte and other finishes,
and so on.
	Some pointers:  First off, buy your plastic in sheet form -- it's
a lot cheaper.  DON'T use a scribing tool to try to break the plastic as
opposed to cutting it -- those tools never seem to work.  Use a table saw
with a blade designed for cutting plastics.  Cut and drill at high speed,
with water as a lubricant if the machine will permit -- if not, use a slow
feed rate to avoid fusing material.  Incidentally, this is all for PMMA
plastic (Plexiglas (tm) or Lucite (tm)) which is the best value for most
uses.
	Keep the "chips" -- they will be handy later.  Do not use Super
Glues to join the plastics -- use solvent cements instead.  Solvent cement
makes a weld joint.  Ask at your plastic supplier which kind of solvent
cement to use for a given job (there are lots of them, with various
viscosities, drying times, etc.)  The "chips" can be mixed with solvent
cement to make a filler putty which works wonderful to fill holes.
	Heat bending works best with a linear heat element.  Buy two or
three 1000W elements, connect them with switches to allow variable
wattages (3000W, 2000W, 1000W, 300W, etc), mounted as you wish on a
heat-resistant material.  Space each element between two "ledges" of
material so you can lay the plastic piece over the gap.
	Let the plastic heat up, periodically turning the plastic piece
over.  Do not heat to the point that bubbles form.  If you want a wide
radius of curvature, heat a larger area.  You can either hand-form the
resulting limp pieces of plastic, or you can use a wooden form.  Avoid
metal as it absorbs heat too quickly and can cause localized cooling,
leading to cracking.
	If you make a sharp bend, or work with thick plastic, be very
careful to insure the plastic is limp enough when bending, and/or heat
temper it afterwards.  If you do not do this, cracks will form at the
edges and move across the bend.  Also, avoid the use of open flame,
but keep in mind a soldering iron or a light bulb won't cut it.  A heat
gun is probably OK, but be sure to wear gloves.
	Any type of joint may be made (butt, lap, etc).  Plastic welds,
when made properly, are of the same strength as the plastic itself (or
nearly so).  Do not try to heat-fuse the plastic.
	One note about drilling -- be careful not to chip the plastic.
Use a drill press, high speeds, and a bit made for cutting plastic.
Water lubrication helps.  Also, try to use bolts more than screws, which
can crack the plastic.  If you must use screws, drill a pilot hole at
least as large as minor diameter of the screw.
	If you really want a precise hole, use an abrasive material slurry
and a solid rod (similar to the way you drill glass).
	A matte surface is best in terms of practicality because it won't
show scratches and is easiest to make -- just use fine grained sandpaper.
If desired, abrasives and rouge may be used to give a gloss surface.

	Other plastics may be used.  Lexan (tm) is exceptionally resistant
to shock (you can hit a Lexan sheet with a sledgehammer and it won't break
easily) and is suggested for rugged equipment.  PVC is commonly available
in tubular shapes (ie plumbing!), plus joins easily with solvent cements.
Lexan cannot be heat-formed as far as I know (at least I've never been able
to do it!)  Nylon materials are rugged and flexible, but Nylon glues are
hard to find and expensive (contact me if you are interested in Nylon glues).
Polystyrene (solid, not foam) is durable, thermoplastic, and can be solvent
welded.
	Polyethylene is flexible and cheap, but not as tough as most plastics.
It comes in low density and high density varieties -- low density (I believe)
is not suitable for most uses except packaging; high density is more solid
and durable (I may have these backwards).
	There are special-purpose plastics for other uses.  If interested,
contact your local industrial plastics supplier.

	PMMA comes in two forms, sheet and solid (block, rod, etc).  In
general, the sheet form is "Plexiglas (tm)" and the solid is "Lucite (tm)".
Colored forms are more expensive than the transparent.  I usually buy the
sheet form in 1/8 or 1/4 thickness (inches); 1/16 is not suitable for
anything structural except in small cases; it is, however, good for a front
panel.

	Hope this helps.  Contact me if you want further information -- I've
done several plastics working projects (including a 10 gallon water bath
complete with heating elements and circulation system, made with 1/8" PMMA
with supports).
	

>thanks for any help
>bob maccione


-- 
|   Bill White			Internet: bwhite@oucsace.cs.ohiou.edu	|
|	LALICH'S LAW OF POLITICS:					|
|		Be good.  But if you can't be good, be careful.		|

mmm@cup.portal.com (Mark Robert Thorson) (09/26/90)

Sheet tin or copper is easy to shape, easy to cut (with tin snips),
cheap, and can be soldered.  I made some nice templates using sheet tin
cut from some galvanized serving trays which cost only $0.50 each.
In the old days, you could use a tin can, but that was a long
time ago.

A local dealer in scrap metals would be a better source if you need larger
quantity.

dmt@PacBell.COM (Dave Turner) (09/29/90)

In article <1991@oucsace.cs.OHIOU.EDU> bwhite@oucsace.cs.OHIOU.EDU (Bill White) writes:
>In article <3015@anasaz.UUCP> bobm@anasaz.UUCP (Bob Maccione) writes:
>
>>Does anyone out there have any hints/pointers to making custom cases
>>for electronic projects. The case should be plastic ( or fiberglass )
>>and I would like to make it not a simple box shape ( ie curves, etc ).
>>Although it will be small it should be rugged.
>
Another recent article is in the August, 1988 issue of QST:

	Build it yourself with plastic	pages 30-34


-- 
Dave Turner	415/823-2001	{att,bellcore,sun,ames,decwrl}!pacbell!dmt