Trimming and Polishing the Hardened Plastic Block

Block trimming is virtually unnecessary when moulds such as those described above are used. But should trimming be required, a hacksaw with a fine toothed blade can be used.

The procedure generally advocated for producing a high gloss surface to the block is to hand polish first on a graded series of emery paper, then jeweller's rouge and finally a metal polish such as Brasso. We found hand polishing so time consuming even for a small block (2 × 3 × 4 cm) that the prospect of using this method on blocked material of dogfish skeleton sufficient for a class of 150 students became very unattractive both from the point of view of economy and the sheer physical effort involved.

The answer to both problems came with the installation of a Linisher belt sanding machine. Various small modifications to this sander (Figure 5) allow the polishing process to go on virtually unattended. Also the addition of a vacuum suction tube to the apparatus permits the removal of troublesome plastic fluff and dust from the atmosphere. We used a coarse belt (No. 50), medium belts (No. 60 and 80) and a fine belt (No. 100). The blocks were then polished on plywood discs (Figure 5) to which hat felt had been glued. We soaked the felt on one disc with Brasso and the second with Silvo. The first polish was with the Brasso followed by the Silvo. A vigorous rub on a Dacron surface gives a high gloss finish, or the surface can be spray-coated with plastic.