Buzz: a question came in from someone wanting to explore the following: “What is the best technology to fabricate the large scale molds from aluminum alloys for making bathtubs & liners for small truck boxes, by vacuum thermoforming plastic sheet?”
Skippy: this person appears to be inquiring about several different types of products in general –
a) Bathtubs – are generally started using a thinner gage, often acrylic capped vacuum or ‘thermoformable’ sheet that is dual or tri extruded - the back side will generally accept (be adhered to) a post formed added rigid backing material - often fiberglas (less expensive) or urethane (more expensive) - a few larger players dominate most of this 'commodity' market - quite a few regulations and certifications that you should make yourself aware of before offering these products to the construction industry.
b) truck bed liners - usually made from Polyethylene, again for high rate, usually formed on Aluminum water cooled molds - few new people enter this highly competitive market since the big players command so much volume in the market, the product is 'mature' and the cost to enter with a better product would be relatively prohibitive - good luck here.
Buzz: What about the similarity processing questions on the forming?
Skippy: For high(er) volume production and accurate shrinkages on construction related products, you would probably want to use a water cooled aluminum mold where cooling lines would be molded or embedded into a sand cast aluminum (or drilled through a machined aluminum mold.) Best place to start would be with someone with plastics expertise who can help you design the part in 3D to produce a pattern to be used with the casting house. Note - shrinkages on the materials in a) and b) are quite different, and it is important to work with your designer and sheet extruder on the ‘orientation’ of the sheet.
c) Truck boxes - could be made more rigid with fiberglas like a tub if constructed from the right materials, but are also often made from Polyethylene. When designed into the mold, these are easy to machine apart to get mating boxes and lids. They are often ‘rotationally molded’- totally different process.