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Die-casting is a fascinating process. The design and engineering that went into these molds was tremendous. In most cases, the tolerances are less than 1/1000th of an inch. Some tool designs simply performed better than others. The better the tool runs, the better the finished product was (and is).
One of the issues with forcing molten ZAMAC (zinc, aluminum, magnesium and copper alloy) into a mold is having the proper venting for the gas to escape. If the tool is designed well (which is sometimes a function of what is being cast) the gas will exit quickly and allow the molten material to flow quickly and freely to all corners of the mold cavity.
It's important to note that the material is forced into the cavity under pressure. Die-casting is not a gravity feed process. That's why all these stovetop repro parts you see have bubbles in them. No pressure to force out the gas.
Even that is not enough, however. If there are any impurities in the ZAMAC (as there always are at the front load of the shot), there has to be a way for those impurities to pass though the cavity. When tools incorporate this sort of pass-though ability for the material, the cavity is ultimately filled with pure, gas-free ZAMAC. By the way, achieving all this in a tiny thin-wall die cast mold with up to 4 slides was very difficult.
If the mold was polished correctly, the end result would have a wonderfully smooth finish (what many think is polished). If not, there will be "splay" and micro-gas bubbles along the surface. It may still look ok and pass quality control, but the cars with the shiniest finish out of the mold have the shiniest finish out of the BP!
I know there are a bunch of you out there that believe the cars were polished before they were plated. They were trimmed and deburred, and maybe they were slightly polished during that process, but the tool design and cavity polish ultimately determined the smoothness and reflective quality of the finished, painted product.
So one could say that the AMX2 mold was one that had less than 4 slides (it only had two) and that the ZAMAC was allowed to pass through the well-polished cavity, leaving behind a stellar part for plating and painting.
Or look at the side-loading Beach Bomb. Ever notice how smooth the slab sides are on these cars? Very nice indeed. For kicks, go and study the rear loaders. Not one of them has a smooth finish on any side. Why? The original tool did not allow the gas to evacuate well, and the impurities were trapped in the cavity. How did the side loader get so improved? The enlarged sunroof became a vent point for the gas, and a reservoir for the front load of the ZAMAC (with the impurities) to reside. (Technical point: As a note to anyone that thinks the huge sunroof was a "shutoff", that is true, but it also had vents built into it to allow the ZAMAC to pass into a reservoir area).
That said, there is a little problem after the zinc cars pop out of the mold. They have what is called "flash". The flash is a small amount of zinc that slips though the meeting points of the molds (for the technically inclined- where the slides meet each other and the shut-offs meet the core). This flash is sharp, brittle and must be removed before anything else can happen to the cars.
So how did they remove the flash? Did they spent endless man-hours hand stripping the little diamonds in the rough? Not a chance. Not even with Hong Kong wages would our good friends at Mattel have done that. The cars were placed in tumblers (barrels) and were gravity rolled with a small media to remove the flash. While it may not sound reasonable, this is how it is done. In today's world the media is plastic (very small bits indeed). The key in this process is for the media to be LESS HARD THAN THE PARTS THAT ARE BEING TUMBLED. This is key because if the media is harder than the zinc, the fine details are blown out, not to mention all the sharp, defined edges on the cars. If the cars were turned with "stones" (which they weren't) they would have to be mighty soft stones. It is also possible that this was done with fine sand or other organic media back then. If you believe the cars were polished with little rocks, you are entitled to your opinion. But then how did the cars retain their beautiful sharp details through that process? And how did the rocks polish areas like the recessed grill of the Custom Mustang? By default, the process knocked down any sharp edges. It is my contention that none of the edges it smoothed out were part of the design of the car. In other words, if the tumbling process affected the design details, the cars would not look as crisp as they do.
So, the cars now have the flash removed (de-burred), and are ready for hand polishing? NOT! How much would that cost? Not enough money or time in the world. It's no secret that the cars were zinc plated, and this was the next part of the process. Here's where it gets interesting. For the most part, zinc plating comes in two flavors- barrel and rack. Barrel plating is where thousands of parts (at minimum) are placed into a tumbler and all plated simultaneously. Rack plating is where each part is attached to a dip rack (usually with a small piece of wire) and then dipped into the various solutions to produce the plating. What's the difference between the two different processes? Well, the rack method is about 10 times the cost of barrel plating, and rack plating leaves a smooth, "mirror" shine while barrel leaves more of a "grainy" effect. This is the difference between US and Hong Kong approaches. The labor economics allowed the HK plants to rack and the US labor economics forced the barrel method.
Now before I move on, just one more point on plating. The final zinc plating step is the application of a chromate finish to protect the zinc plating from all of our Cheetos-covered fingers. That comes in a number of flavors. The common ones are blue chromate, white chromate and yellow chromate. Guess what, Mattel did them all. I'm not talking about stark colors in the plating. I'm talking about subtle stuff; only a slight tint. It's so subtle, that you can't even notice it until you get to see a batch of them straight out of the tanks or barrels. Some flash blue, some flash yellow, and you can't notice the difference until they are side by side. These slight variations in the shade of the zinc plating wreaked all sorts of havoc on the spectraflame rainbow. Get out your color wheels and work through the possibilities. It increased the number of "colors" (as some color guide writers would call them) or color hues exponentially. To make a small guess, I'll bet money Mattel was not fazed one bit with the variations. So where did the purple "cast" come from on true Ice Blue cars? You guessed it. It came from the blue chromate zinc plating on light blue cars. While it's true that Ice Blue cars have some purple mixed into the color, the strange deep hue that is typical of Ice Blue cars is driven by more than paint. For those of you that believe any light blue car is Ice Blue, pay no attention to this paragraph.
So now we're at the point were the spray pots are getting loaded with whatever color is flowing well today, and the cars on the line are randomly hitting the lines. How much thought went into this? Well, at General Motors in Detroit this kept over 1000 analysts busy (during the sixties with no personal computers). At Mattel, this was probably a bit less calculated. It was what it was. From their point of view, the more random, the better. So while we're trying to cover the historical perspective of the colors, we should be very careful to not overcomplicate the reality of the whole thing.
The funniest part about this, is no matter what the source people will always theorize their own "bent". It gets even worse when we pay attention to resources that claim to have been there, and were not.
Toning is an interesting thing. As we all know, many cars had toning right from the factory. Other cars have toned right before our very eyes while in our collections. In order to better understand toning, I think it's important to know what it is.
"Toning" is the word we use for dark blemishes in the paint finish on the SpectraFlame redlines. Some have used the terms "hot spots", "mottling", or "blotching" for the same issue. The process that led to the great looking paint jobs was a multi-step affair that began with the die-cast part. Mattel was looking to replicate the popular "candy color" finishes of the era. The classic way to achieve this (on real cars) was to undercoat with a bright silver base coat. Then, a transparent top coat would simply add a color tint to the base coat and its reflective properties. Ultimately, what they were after was a cheap way to get that "Christmas tree ornament" effect on the little cars, and that's exactly where they got their cues. The key to the bright, reflective paint jobs on the little cars is the undercoat, or in this case, the zinc plating. This plating is quite thin, and lies on top of the zinc die-cast part.
Anyway, back to toning. Ever noticed a car in the blister with a completely toned hood? Guess what, it's probably not toned. More than likely, that hood made it through QA without being zinc plated, or with bad zinc plating. As a sidebar topic, the cars were painted with the hoods and movable parts assembled (hoods, hatches, etc.) but these separate parts were plated separately. So when this nice little die-cast body hit the paint lines, the hoods came out dark when the paint hit them. Why? Because the bright zinc plating was absent as the base coat on the hood, and the paint just laid there. You might ask why there are not more cars in the reverse orientation (completely toned bodies and nice bright hoods). Well, I think the bodies were probably caught more easily by QA either before they were painted or at the very least, after they were painted. So there's an answer to the "dark hood" mystery: Probably not toning at all!
Now, on to the good stuff. If you look at the US and HK cars independently, you will notice that toning shows itself differently between the two castings (particularly in the early stuff). Why so? Because the two types were plated differently, and I think the ZAMAC (what the cars were cast out of- there is no such thing as pot metal) composition was slightly different between the two countries. Another thing to note: it's not easy to zinc plate non-ferrous metals like zinc! The more conductive the metal is to electricity, the better the plate! Look at all those pretty steel nuts and bolts at Home Depot! Those are all zinc plated, and they are steel. Not much toning going on there!
Also, the plating process was slightly different between the two countries. Everything in HK was cheaper and faster than the US. That may not be completely true, but hang with me for a minute. The thicker the plating, the better the protection (that's why things are generally zinc plated, to keep the underlying material from oxidizing). The faster the process, the thinner the plating. Keep in mind, Mattel had no intention of these little things lasting for 30-plus years! Just get enough plating on the casts to make them shine, get some paint on them, and bada-bing! Packaged and sold!
Done correctly, the zinc plating can be put right over the raw castings, and it was, in both the US and HK. The reason it flakes off on the HK cars is due to poor plating techniques and the bodies being exposed to the environment before they were plated. The longer they sat around, the more the oxygen got to the zinc. The more it oxidized, the less the plating took to the bodies. That's why some are still perfect, and others are toast!
Early US cars were copper plated first (like you would have done chrome plating on zinc back then, and possibly even now, depending on the system you use). I don't have the particulars, but the HK plating process was never possible in the US due to environmental concerns. It did accomplish exactly what they wanted, however: brilliantly shiny bodies for the transparent enamel paints. The fact that it had a shorter shelf life was not even a consideration back then. The HK cars were plated differently, and probably not as thickly. The paint is probably not as thick either, and that's where the trouble comes in. If you allow any oxygen to interact with the plating, it will begin to oxidize (that's the way things go when your planet has atmosphere. Put your collection in space if you want to halt the degradation of the cars!). Even if the paint was as thick, the zinc alloys were probably slightly different on the HK cars, and the plating solutions were different. I'd bet money on this. Cheap labor ain't the only reason they did so much production in HK. Materials were less expensive and lower quality. As always, I think you get what you pay for. BTW- keep the following in mind. The longer the plated parts sat laying around waiting for paint, the more they would have oxidized. Is it humid in Hong Kong?
How careful do you think they were with all of this? I think they were not thinking about anything but the immediate buck (or 69 cents as it were) to be made; end of story. With apologies to Neil Young, once the oxidation starts, it never sleeps. So you end up with strange toning characteristics between the two countries, and you get some cars that were blemished before they were packaged. In some cases, the cars just had plating blemishes (little spots) that did not reflect very well. Those spots would have been there as soon as the paint was sprayed.
So you can watch some cars tone in your displays. You can speed it up or you can slow it down, but it will never stop. That's just the way it is. Keep the cars dry, cool and out of direct sunlight (and all light, if you want the colors to last in the paint and plastic parts). Other than that, stay away from restoration jobs that polish the old zinc plating off of the bodies (to make them shine). That bare zinc will not go very far into the future (assuming they were left open to air before they were painted). And stay away from polished bases, unless you want to keep polishing them through the years like that silverware in the drawer.
The plating process has changed a great deal in the last 35 years. In California (where the US cars were done) the environmentalists have pretty much tied that industry into a knot in the last 20 years. The truth is, there were more than 5 different plating process used on the US cars alone. It's an interesting study with a bunch of good clues.
The guys that were close to the project back then can only remember now what they knew from their limited exposure. It's hard to find a resource that had any project management experience from that time. Only they would be able to help answer all the questions about the various efforts and when they were done.