A die-cast product is one which was produced by the injection of molten metal into steel molds under high pressure. The process is practically identical in concept to plastic injection molding, but uses molten metal. Many non-ferrous metals can be die-cast. The process is quick and precise, and is used to produce a wide variety of parts in many sizes. Some toys, including certain Transformers toys, are made partly or wholly from die-cast parts.
The metal used in die-cast toys is one of a family of alloys known as ZAMAK or ZAMAC. This peculiar name comes from the alloys' composition, which includes Zinc, Aluminum, MAgnesium, and Copper (Kupfer to the alloy's German originators), in varying amounts.
...did I mention it's a lost art?
Use in Transformers toys
The amount of die-cast in old Transformers toys varies wildly, though it almost never makes up the majority of the toy. In most cases it was used merely for car hoods and occasionally side-panels for the larger toys. In some examples, like Tracks or the Inferno/Grapple mold, only a very small amount of the toy uses die-cast metal.
Die cast metal vs. injection molded plastic
The similar processes used allow practically the same parts to be produced in die-cast metal or injection-molded plastic. Both processes are capable of reproducing very fine detail, though in either case, higher pressures must be used as the detail becomes finer.
Advantages of die-cast metal include strength and stability. ZAMAC alloys have a breaking strength of about 41,000 psi; the high-impact polystyrene and PVC plastics used in Transformers can only withstand 6000 psi. Suppose two copies of Generation One Bluestreak's roof were made: one from die-cast ZAMAC; the other from molded, unfilled (clear) polystyrene. The die-cast roof would withstand a force roughly 6.8 times greater than the plastic part before breaking.
It should be borne in mind, however, that the die-cast roof would also weigh 6.2 times as much as the plastic part. This is sometimes an advantage: G1 Grapple and Inferno have die-cast feet, which lowers their centers of gravity, allowing them to stand more stably. In most cases, however, the added weight increases loads on joints, adds greater potential for damage when dropped, and raises shipping costs, so the strength advantage of die-cast parts is usually offset by the thinner sections and smaller stressed areas that are used to save weight.
Furthermore, high-impact polystyrene can deform about three times as far as an equivalent ZAMAC part before breaking. Frequently, this makes the plastic parts more forgiving than the die-cast would be of slight misalignments and unexpected loads, though they do not resist them as well. The resilience of molded plastic can also be increased by the addition of various fillers; many fans are well aware of the greater durability of opaque (filled) plastic parts over clear (unfilled) parts, though the resins are usually the same.
Another advantage of molded plastic, when used in Transformers toys, is that it can be easily molded in color. Neon, Playskool, or subdued, none are possible for ZAMAC, which must be its natural color or be coated with paint. Paint can chip with impact, and toys live a life of many impacts, be they a small nick from a spring-fired projectile, or a high-speed re-entry from a second-floor landing. The need for paint can make it difficult to make die-cast parts with very fine detail, which a thick coat of paint would obscure.
Stability is seldom a major consideration in toy design. While plastics do deteriorate with age far faster than properly mixed ZAMAC, they both last far longer than necessary for any child's plaything. There still remain other reasons besides strength (and sometimes weight distribution) to use die-cast parts. Plastic is derived from fossil fuels, and when these are expensive, it may well cost less to substitute equivalent die-cast parts (keeping in mind that the volumes and areas of sections can usually be greatly reduced, due to the greater strength of die-cast metal). This seems to have been the case in early Generation One, when most Transformers toys included some die-cast parts (some of which were later redesigned in plastic when that material became cheaper). Finally, the "heft" of a heavy die-cast toy is equated by some fans, and perhaps some parents, with high quality. While this may not be a reason Swerve the metallurgist would give, it is nevertheless a valid marketing consideration.
Perhaps this perception of high quality also comes from their memories of the early toys with die-cast parts. Unfortunately, the Generation One toys with die-cast metal are among the most breakable of Transformer toys. Examples include Bluestreak's and Jazz's roofs, Mirage's waist, and Swoop's wings and beak. This is not all the fault of the die-cast parts - the plastics used were more brittle, and the methods of joining and construction less forgiving of abuse - but they certainly must share the blame.
For its real advantages of strength and perhaps weight, and for its perceived advantage in that nebulous quantity known as "quality", die-cast construction still commands respect from a good many fans, casual and avid. As normally used in Transformers, however, the material is less forgiving of abuse, and must usually be coated in chip-prone paint. These real disadvantages, coupled maybe with a dislike of nebulous quantities, cause a good many other fans to dislike die-cast construction.
Die-cast construction today
Nowadays, die-cast is only used extensively in more "adult-aimed" toy lines, such as the now-defunct Japanese Binaltech and Hasbro's Titanium Series toy lines (both of which still used much plastic, including the Titanium 3-inch figurines).