Why plastic cases?

Question

Early computers of the 'all in one' form factors, such as the Commodore PET, the early IBM microcomputers and later models of the TRS-80, as well as the 'box' form factors such as the Altair, used a metal case; this continued through the IBM PC and all the clones thereof to the present day.

On the other hand, most early computers of the 'CPU in keyboard' form factor, such as the Apple II, Sinclair Spectrum and Commodore machines from the Vic-20 on, used plastic cases. (The one exception I can recall being the Atari 800.)

A metal case is more robust, conducts heat better and acts as an RF shield. I'm given to understand it is also slightly cheaper. (I've seen a figure of $300,000 quoted for the injection molds for a plastic case in the late seventies, and that was a time when interest rates were much higher than they are today.)

Given all those advantages of a metal case, why did so many computers use a plastic case? I could put Apple down to Steve Jobs having strong aesthetic tastes that greatly differed from mine, but Jack Tramiel and Clive Sinclair were pragmatists; I would expect them to go with a plastic case only if it had cost or other practical advantages.

Answer 1

Cost, both in manufacturing and design. The basic metal cases are simply that -- basic. Even modern PC cases are pretty simplistic being a metal box, with a plastic facade.

For home computers of the time, they're essentially self contained unit with "no user serviceable parts inside". They were not designed to be opened by the consumer, instead having facilities to promote external expansion. Since they're not designed to be opened and closed, plastic is a perfect material.

Obviously, the Apple II was an exception to this, having a case designed to be opened. But it was in stark contrast to most everything else until the IBM PC showed up.

With plastic, they were able to get not just lower per unit manufacturing costs, but more interesting and consumer appealing case designs. With rounded shapes, acute angles, etc. Most certainly not a "box".

These more aesthetic designs are much easier to create in plastic compared to metal.

While injection molds are indeed expensive to make, the individual unit costs are vastly lower both from a material cost and a labor cost.

10 billions McDonalds Happy Meal toys can't be wrong. The C64, Atari, and Apple computers sold in the millions of units. Easily enough to absorb the start up tooling costs of crafting injection molds.

Answer 2

Injection molding has a costly tooling cost but very low per-unit costs. This makes it cheaper to build a large number of cases.

Metal cases have a lower tooling cost but higher per-unit cost. This makes it cheaper to build a small number of cases.

Answer 3

I was designing computer housings back in "the day".

Plastic cases were comparatively cheap in volume, but challenging to bring to market. The technology of choice was structural foam. It was strong but thick, up to about 1/4 inch (6 mm) thick. In the basic form, it required painting, but there was a co-extrusion process that would inject at the same time both the foam a skim-coat of ABS.

The thickness caused cooling problems and dimples at intersections, so the molds were complex to cool.

Even so, the molds for structural foam were relatively cheap because the injection pressures were much lower that injecting straight ABS. With lower pressures the mold could be made of aluminum rather than steel, although with lower tool life.

But, making the molds took a long time. CNC was not as advanced, nor was CAD. I was quoted over 26 weeks for a cheap and fast structural foam mold set. Instead, we went with hand laid-up fiberglass.

A molded plastic cabinet improved the appearance of the product, but it also spoke about the company. It showed that the company had been around long enough to make molds, and was capitalized well enough to pay the cost. The company was a grown up in a world where computer companies came and went weekly.

Answer 4

Another reason: Simple home computers used relatively slow (both in risetime and in clock frequency) circuitry which was also contained in a relatively small area (making the wiring a less effective antenna), so the chance of becoming an EMI nuisance due to a a partially-shielded/unshielded plastic case was smaller.

Also, some home computer systems were marketed as toys and/or educational appliances. There is a possibility that in some markets, customers will have (rationally or subconsciously) associated metal cased equipment with electric shock hazards. This isn't entirely unfounded: There were more TN-C type house wiring systems in the past than these days, and these have a worst-case failure mode of setting any metal cased equipment - even if it is perfectly intact - connected to a group of outlets LIVE.

Answer 5

The original Apple II case was designed before the FCC started enforcing Part-15 consumer product RFI emission rules.

Atari used a metal shell inside a plastic case for the 400/800 because they thought the FCC rules would be enforced more strongly. Apple instead switched to plastic with a conductive coating inside and lots of gaskets instead so the Apple II+/IIe could squeak by the US FCC RFI standards (just barely).

And, yes, Steve Jobs wanted a case with a certain specific type of chamfered corners, which was much more costly and difficult to do in metal (although Apple did get a metal transmission case vendor to make them for the Apple III).