Mould designs--for AK

Colin · 11-20-2002, 05:11 PM

Here's a question for AK:

When designing moulds, why do some models have the wing as a seperate unit connected to the fuselage (like your Caravelle, 737 and 767) while others, like the BAC-111 and the Connie, have the wings moulded into part of the lower fuselage? The latter does make for a nasty seam.

Is it an ease of manufacturing issue? Does it make it easier to put the colours on?

PH-LKK · 11-20-2002, 06:27 PM

Or is it a balance issue? I suspect the rear fuselage is hollow.

David

Sgt Caribou · 11-20-2002, 06:53 PM

Recall reading something about AC using "spin casting" for their recent moulds which reduces the number of seams. Having no knowledge of the process, will not say anything more, except that I remember them addressing the issue of seams in this way. Anyone care to elaborate?

Gordon Werner · 11-21-2002, 12:21 AM

no ... the AeroClassics DC-6, L-188, L-1049, BAC 1-11, VC-10, Vanguard, and viscount are spin cast ...

the 707, Tu-154, Caravelle are injection moulded.

as for why and how the wing is attached ... it really depends upon the size of the model and where the model's center of gravity is. If it is too far aft then the fuselage needs to be hollowed out.

Older AeroClassics moulds were also made by hand which made them cruder.

Gordon

Colin · 11-21-2002, 07:03 AM

OK, for the uninitiated, what's "spin casting" and how is "injection moulding" done? I have a vague idea of the former and a decent appreciation of the latter (I think) but we might as well hear from those in the know.

PH-LKK · 11-21-2002, 10:54 AM

I seem to recall a thread on this subject a while back. Another candidate for the Hall of Fame? Maybe we just need a good FAQ.

A search for "spin casting" on google yields some pretty interesting info. But AK, Brian, the folks of ADI, Gordon, or even the Doc is probably better informed regarding the application of these technologies to the manufacture of die-cast airplanes.

How about it guys? Someone want to give us a re-cap?

Sentinel Chicken · 11-21-2002, 03:33 PM

As I understand it, injection moulding is just what is sounds like- molten metal (or plastic in the model kit world) is forced under pressure into a mould. The pressure of the injection process forces out any air trapped in the mould which would otherwise ruin the part in question.

Any of you scale modellers out there who've tried to make your own resin castings know what I'm talking about with regards to the problems with trapped air in a mould.

In spin casting, the mould rotates and it's the centrifugal force that pushes the molten metal into all the nooks and crannies of the mould, forcing the air out.

In the injection moulding process, usually from what I remember there's some sort of cooling apparatus that allows the molten metal to set quickly. As a result, cycle times in injection moulding are very short as a combination of the injection pressures and the cooling of the mould itself.

In spin casting, cycle times are typically measured in minutes rather than seconds because the mould has to spin up to the right speed and for the proper amount of time to force any trapped air out.

Because of the lower pressures involved, spin casting moulds are more inexpensive than injection moulds, and are ideal for short production runs with lower-temperature melting metals. Using such metals means a spin cast mould can be made out of vulcanized rubber as opposed to the steel (I think) of an injection mould. Most vulcanized rubber moulds I've heard last only in the neighborhood of 200-300 shots. But, they're easily modified after the first test shots whereas metal moulds aren't.

Injection moulding is a bigger investment of capital and resources and is more suited to higher production runs as the moulds are more expensive to create and modify following the first test shots.

If anyone here has experience in diecast metals production, I'd be open to any corrections.