Re: Vacuum-formed plastic dash parts [long]

[Franklyn Berry <us009296@mindspring.com>]

At 06:50 PM 9/11/99 -0700, Ti wrote:

>... I sketched a rough drawing of
>the ABT unit just so you can have an idea what the it looks like
>by itself:
>
>	http://metalab.unc.edu/tkan/audi/audi-misc/abtdash.gif
>
>As you can see it's quite a complex piece. 

The unit has a fair amount of exterior detail, but making a mold can
sometimes be as simple? as 
1) making a wooden frame to support an existing piece, and sealing the "holes"
for things like gauges;
2) making a box to accept this "plug";
3) pouring epoxy in the box, and smashing the plug in while it sets up.

Various intermediate steps apply {-;

This process creates a cavity which, when perforated with appropriately
sized holes, can be used to pull a vacuum underneath a hot, semi-viscous sheet
of plastic. OR, one can add sheet wax to the 'mold' surface, make another
epoxy 'take-off' mold, and have a matched mold set into which urethane can
be cast. OR, one might make a net-size [no sheet wax] mold enabling a
mold that works from the interior 'male' side of the part.

This is the manner in which prototype parts are made for design studies on
motorcycles, cars, tractors, and garden shears, just to name a few. And
quite a few more production parts than one might imagine. Of course, there
are more $ophisticated tooling approaches that apply to higher quality
parts.

> What's not shown is that
>there are three protruding pegs on the rear side of this that plugs in
>to holes on the dashboard. 

In order to emulate this structural detail in a 'prototype' part, one
would generally either 
a) fiberglass-reinforce the interior, perhaps in selective places where
clearance could be gained for such a 'stiffener' [called FRP-ing,
or fiberglass-reinforced plastic];
b) cut a set of gussets out of ABS sheet [or rod] and chemically weld the
stiffeners [or bosses] into place.

> The front side of the unit is secured to
>the bottom of the cluster pod via two screws.  There is a vent grille
>on the top. 

Depending on vent complexity [of form, and to a lesser degree, opening shape]
one might hand trim [with scribe, knife and/or grinder] such detail, or
pour a fixture that mates to the interior surface, ventilate it to make
a vacuum jig, and program a CNC trimming machine called a router to poke 
the holes and periphery.

>The height is different front and back because it's mounted on the
>dashboard and the dashboard is sloped. 

In order to get the perimeter detail 'right', a fixture and some serious
hacking around in 5-axis programming is probably required. A talented, 
experienced person could probably trim one part in 8-12 hours, assuming 
they had a few to experiment with [and destroy in the process].
>
>> d) any guess as to average part thickness?
>About 2mm?
>

For a formed piece, that would translate to a starting material thickness
of about 4mm. Vacuum 'draws' the part in various places, sucessively thinning
the sheet as it is formed. The part will be thick in some places, but 
dramatically thinner in others; therefore its important to start with 
sufficiently thick sheet to accomodate finish-gauge objectives.

>I have no knowledge about plastic fabrication and manufacturing, but
>this looks like an expensive piece to make in small quantities.
>

>From your sketch, and having seen a gazillion similar parts, I would guess
that one would spend $3-600 on epoxy and lumber making a set of molds and
fixtures. A pattern-maker would charge another $1-2K to do the moldmaking
work, which would translate to 12-20 experienced shop hours; so the hobbyist
might manage to spend 150% more money on materials [at least] and 3-5
times the effort, for a total tooling budget of ~$1K and a pound of
sweat-equity {-;

The raw sheet, if we're talking color match, would cost some $40-80 per piece,
in an appropriate gauge. There are various schools and shops where one
might buy vacuuum-forming and trimming time to accomplish a set of prototypes
[including mine, but there is probably one in your state]. The forming and
trimming labor for the first part would run several hundred dollars, while
subsequent parts would be fairly affordable, at $40-50 dollars or so. Add
some fabrication cost and time for FRP'ing or gusseting.

This translate to a 'breakeven' market of maybe 8 or 10 parts for the
hobbyist,
and perhaps triple that for a commercial effort [marketing, labor cost, etc].

Hope this helps.
>
Franklyn Berry	
Maison de Pique-Assiette
Milwaukee, WI USA	
(414) 961-1641