[Coco] make-do ROM Paks

[Joel Ewy]

Robert Gault wrote:
> Roger Taylor wrote:
>> <snip>
>> What exactly is a vacuum forming box and is that the method of making
>> it, or what it does after you seal it shut?
Robert's description is right.  Just a little more detail.  The "box"
I'm talking about is a tool for doing vacuum forming at home.  It is a
shallow wooden box, sealed up with duct tape at the seams to make it
more-or-less air tight.  There's a hole in one end to which is affixed a
vacuum cleaner attachment so you can plug in a mini shop vac.  The top
of the box is made of peg board, so it is covered with holes.  On top of
that you place a form that has the shape and size (minus the thickness
of the plastic you are going to use) of the part you want to make.  The
plastic you start with is a flat sheet of a thermoplastic (one that
becomes plastic, or soft and pliable, with the application of heat, as
opposed to a thermo-setting plastic, which hardens when heated, like
casting resins) such as styrene.

You affix the plastic sheet in a frame and heat it until it is soft and
moldable.  Then you set the frame down on the form and turn on the
vacuum.  The air is sucked out and that pulls the soft plastic down and
conforms it to the shape of the form.  When the plastic cools it will
retain that shape, but it will still all be one big sheet of plastic. 
It is as if Wile E. Coyote has slammed into a brick wall from behind,
propelled by Acme Rocket Skates.  To get usable parts out of this, you
need to cut away the excess material with a sharp hobby knife (or a
motorized tool with some kind of appropriate rotary saw blade) and you
may need to do a little sanding.

Most mass-produced plastic parts are injection molded, meaning that the
plastic is completely melted to a liquid state and forced into a hollow
mold.  Most plastic model kits are done this way, and the individual
pieces are shipped still stuck to a framework that corresponds to the
channels the plastic flowed through to get to the mold cavities.  The
equipment, set-up, and tooling for injection molding is more costly than
for vacuum forming however, so certain model kits that aren't expected
to sell in sufficient quantities to justify an injection molded kit are
made available as vacuum formed kits.  These are a little more
challenging to assemble, because almost every piece consists of two
halves you have to cut out, sand to fit, and glue together, and you
don't get the convenient registration tabs and holes of an injection
molded model.  Just the basic shapes in two halves.  I once built a
vacuum formed 1/72 scale MiG 3, and it was certainly more challenging
than the injection molded models.

But for something as simple as a CoCo project box, the cutting out and
sanding would be less critical, and the shapes would be less daunting. 
It won't involve cutting 1/72 scale propeller blades out of a sheet of
styrene, that's for sure.

What would be cool is if somebody could come up with a font that looked
like the lettering Tandy used on the CoCo's case.  If that could be
transferred to some kind of 3D lettering that could be applied to the
form for the lid, resulting in a raised logo molded into the plastic.

JCE
> ><snip>
>
> Picture a negative relief mold of something, a face for example. You
> drill very small holes all over the mold so that the surface becomes a
> screen. You place a piece of plastic with thermal properties such that
> is becomes flexible at temperatures significantly below the melt point
> on the inside of the mold. Heat is applied to the plastic and a
> partial vacuum is applied to the opposite side of the form. Within
> seconds you can reshape the flat sheet into the shape of the face.
>
> With the correct mold and plastic, the plastics industry can vacuum
> form just about any shape.
>
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