Making Vacuum Forming Patterns
The quality of any vacuum-formed part starts with the quality of the pattern. To withstand the temperatures and pressures involved in the forming process, patterns must be produced from solid materials with no internal cavities.
Choosing the Right Pattern Material
For clear canopies, we strongly recommend Crystacast plaster patterns, as clear plastics such as PETG faithfully reproduce every detail of the pattern surface, including imperfections that may not be visible to the naked eye.
While MDF can provide a smooth surface for many applications, even the slightest wood grain, joint, or sanding marks can become visible in a clear canopy after forming. Crystacast provides a much denser and smoother surface, producing the highest levels of optical clarity and the best possible finished appearance.
This may require you to first make a wooden pattern to create a mould, then cast Crystacast into it, before repairing minor defects by filling imperfections with automotive body filler, sanding smooth with progressively finer abrasive paper, and finishing by polishing the surface with a metal polish such as Autosol.
The final pattern should have a smooth, gloss-like finish, as any surface defect will be transferred into the finished canopy.
Patterns for Cowlings and Other Components
For non-transparent components such as engine cowlings or other components solid MDF, hardwood or dense balsa patterns are generally suitable. As these parts are usually produced from white HIPS and subsequently painted, achieving optical perfection is less critical than it is for clear canopies.
Regardless of the material used, all patterns should be properly sealed, filled and sanded to ensure a smooth formed finish.
Avoiding Undercuts
One of the most important considerations when designing a vacuum-forming pattern is ensuring that it contains no undercuts. An undercut will mean that where the formed plastic wraps around a feature, it will prevents the moulding from being lifted off the pattern once it has cooled. If undercuts are present, the finished part may become permanently locked onto the pattern or be damaged during removal.
If a component cannot be designed without undercuts, the solution is often to divide the part into multiple sections.
The individual mouldings can then be trimmed and joined together after forming, allowing far more complex shapes to be produced while still ensuring reliable release from the pattern.
In Summary
Successful vacuum forming relies on the use of properly designed solid patterns. For clear canopies, Crystacast plaster patterns provide the best surface finish and optical clarity, while MDF or dense balsa patterns are perfectly suitable for cowlings and other painted components. Whatever the application, patterns must be smooth, durable and free from undercuts to ensure clean release and high-quality mouldings every time.