What material are those molds made out of? Positioned on the pallet makes it look heavy

The pallets underneath can lead to a misleading assumption as to weight as in actuality the underside is completely hollow so they are relatively lightweight. I have attached a photo of a bunch of them stacked vertically in a cardboard box for comparison - hardly ideal for stacking half a dozen supposedly very heavy molds in and vertically no less! Note the two primary feeder tubes protruding from the foreground mold used to vacuum the air from the mold itself during the process.

The manufacturing process works on vacuforming a raw plastic substrate which is heated and then a series of tubes on the underside essentially draws the air out and forms the plastic to the shape of the mold itself so there is no need for a second mold half.

I have also attached a pic of the underside of another original BW ramp mold showing the hollow underside and the series of tubes I mentioned as part of the vacuforming process (again all these pics are courtesy of PinballInc.)

I have many more photos of original BW ramp molds if you are interested.