There are two types of plastics: thermoplastics and thermosetting polymers. Thermoplastics will soften and melt if enough heat is applied; examples are polyethylene, polystyrene, polyvinyl chloride and polytetrafluoroethylene (PTFE)[4]. Thermosets can melt and take shape once; after they have solidified, they stay solid.
Most your plastic bottles or other recyclable plastics include this:
1. PET (PETE), polyethylene terephthalate: Commonly found on 2-liter soft drink bottles, water bottles, cooking oil bottles, peanut butter jars.
2. HDPE, high-density polyethylene: Commonly found on detergent bottles, milk jugs.
3. PVC, polyvinyl chloride: Commonly found on plastic pipes, outdoor furniture, siding, floor tiles, shower curtains, clamshell packaging.
4. LDPE, low-density polyethylene: Commonly found on dry-cleaning bags, produce bags, trash can liners, and food storage containers.
5. PP, polypropylene: Commonly found on bottle caps, drinking straws, yogurt containers.
6. PS, polystyrene: Commonly found on "packing peanuts", cups, plastic tableware, meat trays, take-away food clamshell containers
7. OTHER, other: This plastic category, as its name of "other" implies, is any plastic other than the named #1–#6, Commonly found on certain kinds of food containers, Tupperware, and Nalgene bottles.
The issue is you can't just melt them into a liquid and poor. They have to be maintained at a correct temperature during the molding phase first off. Without the proper venting (not for fumes) and heating, your plastic in just burn. Your looking at melting points of 250 to 350 or more. In order to achieve the best results you have to maintain that temperature, which you have to know in the first place. Most plastic bottles are done by extrusion blow molding. The plastic have a viscosity of peanut butter than a liquid so there is no way you can slush cast it. They have to be injected by #-Ton press. Also you can't just remelt and get the same properties of the old plastic. New, unused and to be combined with recycled, about a 5 to 10 percent.
So my advice: Just forget the idea. Sounds like a good idea, but trust me it is not.