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:

1. 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.
2. 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.
3. 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.
4. 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.

Report for Richard Grant Accessories Ltd.

S F Bush

Summary

A large proportion, in the order of 90%, of front bumpers made from one lot (about 15 tons) of ABS sheet supplied by Telcon, have been found to fail by cracking in a particular region of the bumper. The vast majority (95%+) of the failed bumpers were found to have failed after they were installed on the vehicles, but before they entered service (i.e. at the pre-delivery checks). The ABS sheets supplied by Telcon were made in turn from Montsanto Lustran QE 1110, and were thermoformed into bumpers by Richard Grant during May and June 1980. This particular batch was the second in a sequence of three using Telcon sheet, the first batch being made in late 1979. Reported failures from batches one and three were very few and generally arose for a variety of reasons not connected with the cracking problem.

The investigation reported below is one of several commissioned by the interested parties to identify reasons for the cracking. Fundamentally the issue is to decide between the ABS granules supplied by Montsanto, the ABS sheet as delivered by Telcon, the processing into bumpers by Richard Grant, the installation by Skoda, and a combination of two or more of those. In arriving at a decision on this it is important to recognise that a fault in one area can show up as a fault in another. Thus processing can significantly alter the microstructure of the ABS material; conversely subtle unperceived changes in the ABS can cause difficulties in the processing.

Taken with the earlier investigations made by others, the new results reported below represent about as comprehensive a range of tests as can now be done, two years after the event. It is believed that the results obtained by all investigators are fundamentally consistent. The broad conclusion of this report is that the ABS material in the batch in question was significantly more crack-prone than that in the first batch.