Building Bathtubs

you appear to be inquiring about several different types of products in general - some 'top view' information -

a) bathtubs - generally made from a thinner gage acrylic capped thermoformable sheet that is dual or tri extruded - the back side will generally accept a post formed added rigid backing material - usually fiberglas (less expensive) or urethane (more expensive) - a few larger players dominate most of the 'commodity' market - quite a few regulations and certifications that you should make yourself aware of before offereing these products to the construction industry -

For high volume production and accurate shrinkages, you would use a water cooled aluminum mold where cooling lines would be molded into a sand cast aluminum or drilled through a machined aluminum mold. Best place to start would be with someone with plastics expertise who could help you from a consulting basis design the part in 3D to produce a pattern to be used with the casting house -

b) truck bed liners - usually made from Polyethylene, again for high rate on on Aluminum water cooled molds - few new people can enter this highly competitive market since the big players command so much volume in the market - good luck here

shrinkages on the materials in a) and b) are quite different

c) Truck boxes - can be rigidized with fiberglas like a tub, but are also often made from Polyethylene. These are easy to machine apart to get mating boxes and lids are often rotationally molded - totally different process

Jim Wilson