DC Plastics uses vacuum molding, like many other aftermarket plastic manufacturers. It is lower cost and doesn't involve the expensive machines and molds that injection formed plastic
parts do.
I've got one of their front fenders on a motorcycle of mine. Vacuum molding doesn't exactly replicate stock
parts, but gets them close. The size is generally off, fine details absent, finish not as smooth, and plastic thinner. For a rider, they are THE cost effective solution. Not fit for a true restoration. The overwhelming percentage of machines are riders, and the aftermarket plastic companies have consistently stepped up to the plate for real requests, including DC.
Injection molding is for high volume production with fine detail and intricate designs. I've worked as a mold machine operator in a injection plastic factory. Very expensive, large machines with equally large and expensive molds, that have to function within tight tolerances, tightly controlled temperatures, repeatedly, at high production rates, 24 hours a day. We produced
parts for several brand name companies and all types of devices.
For a seat pan which structural, in that the fenders attach to it, I'd think it'd have to at least be a reinforced plastic. Better yet, carbon fiber. I've also worked in industrial fiberglass construction, inside of multi story chemical tanks and large diameter fiberglass industrial piping. Hand laying composites yourself, for a small one-off part, isn't that complicated. With the internet buying, shipping to your door and all the information available at your fingertips, it's easier than ever. Having a complete, good condition original part is what the aftermarket plastic companies need anyway. If you've got one, go for hand laying one yourself.