R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

6.49 Plaster fi lled SL mould.
Case studies 179
point of the wax precluded its use as a vacuum-forming mould. Instead,
reversing the shelling operation used previously resulted in a negative
pattern. The intention was to cast a plaster vacuum-forming mould from
the wax pattern. However, in practice the pattern was so fragile that it was
destroyed during transport to the laboratory.
Other centres have successfully employed Computer Numerically
Controlled (CNC) machining in this and similar applications (8, 9). In
comparison to RP processes, CNC is a viable option for this application as
it is unlikely to encounter undercuts or re-entrant features. To investigate
the approach, a trial mould (case 1) was machined from a medium-density
board, typically used in industrial model making. Unlike reported techniques
that utilise soft foams for ease and speed of machining, this mould
proved to be perfectly adequate for direct use as a vacuum-forming mould,
requiring no surface treatment or modifi cation (9).
Mask manufacture
The masks were manufactured in the usual manner by vacuum forming
sheet PETG over the RP mould. It is common practice to drill holes
through moulds to provide even spread of the vacuum. However, holes
were not drilled in these cases and the masks were formed perfectly well
without them.