Rollet Project: Design for Prototype and Outline Costings
January 24th, 2002
Report II on the design of the Rollet
S F Bush
Introduction
The basic design given in our first report (3 August 2001) is unchanged, but we have decided to alter the initial manufacturing method for the superstructure (the side panels and shelves).
We now propose to form the superstructure side panels by rotomoulding a polythene powder instead of thermoforming (TF) and extruded sheet of either ABS or polypropylene SAFIRE sheet. The reason for the change is that on the initial quantities we are working on (10,000 per annum) thermoforming of extruded sheet would take the manufactured cost above £60 which we all agree is too high. (Note that the price of SAFIRE or ABS sheet is volume sensitive, so this manufacturing route will still be a candidate once volume has built up.)
Besides the cost consideration, rotomoulded panels have distinct advantages of their own:
- They will be essentially doubled-walled of low temperature impact resistant polyethylene. As a variant the cavity could be filled using our proprietary ROTOFOAM technology, although the present design doesn’t require this.
- Without foam, the side panels will be very abuse resistant: with foam, they will be super-abuse resistant. This manufacturing method can therefore be retained long-term for niche markets requiring this performance.
- A second variant is a SAFIRE reinforced roto-moulded skin where higher stiffnesses are required for certain applications. Again, this will NOT be necessary for our main target application – food distribution trolleys. However we are pursuing this variation as a CfM research project outside the Rollet project itself.
- We are aiming to clip the side panels together, so eliminating the cost of two corner posts for a three-sided TF superstructure.
The price we have been quoted for rotomoulded panels brings the overall manufactured cost of the Rollet down to around £54. Moulds are around £4,000 each so this is a very suitable approach for pilot full-scale Rollets – as we agreed we should aim for (rather than a scaled-down version).
We have a rotomoulder who is very keen to advance the project. He has provided some useful detailed design ideas. Since he has already quoted a competitive price to make the bases, he will have the strongest possible motive to ensure the whole thing fits together.