Our final idea will be made of polypropylene. This plastic is very strong and resilient; this is due to the ‘inherent hinge property’, which means the material will withstand repeated flexing and movement. Polypropylene is an incredibly strong material, which makes it suitable for use in a protection context, like in our project.
This strength makes it easier to form and bend the plastic without breaking, making it suitable for the thermo dome blowing manufacturing process.
This is a thermoplastic so it can be moulded when heated. Blow moulding is used in industry to produce hollow plastic parts such as our spherical casing. In industry, extrusion and injection moulding are more commonly used, but for applications in our design a more simplistic type of thermoforming could be used. Our method involves air being pressured evenly across a heated sheet of plastic.
To thermoform Polypropylene correctly it must be heated uniformly. Standard heating ovens are suitable for this on small scale productions but alternative methods may be used in industrial manufacturing such as Infra-red heated vacuum forming machines these can be used when quality and appearance are important, but this method isn’t required in our situation as appearance isn’t as important due to the fact they will be underground.
Specific heating times and temperatures will depend upon a number of factors, including thickness of the sheet, the type of mould being used and the degree of stretching required. For polypropylene to be thermoformed a temperature just below the melting point typically 165°C is used.
When Polypropylene sheet is heated to 160°C – 170°C it becomes flexible and can be formed into shapes by the application of force such as air pressure when dome blowing. If held to that shape and cooled below 90°C it will retain the shape and if reheated will return to its original flat condition. This property of being able to heat and reshape plastic is known as a thermoplastic in comparison to a thermosetting plastic which, when heated and shaped stays this shape and size permanently and so can’t be changed without breaking the plastic.
As an alternative to oven heating, certain infra-red heaters can be used such as those with quartz or ceramic elements. However, these do heat the surface of Polypropylene very quickly, so heaters and heated plates must be designed to give uniform heating under controlled conditions to prevent overheating and degradation of the sheet.
When Perspex cast sheet is heated it will shrink, such that on cooling again it will be approximately 2% smaller in both length and breadth with a perceptible increase in thickness. No further shrinkage will occur on reheating but this initial shrinkage should be taken into account when cutting sheets into blanks prior to thermoforming. After thermoforming, the plastic can be lifted off the at a temperature of around 90° - 100°C with the plastic to be kept untouched until the temperature has reached around 60°C. Uniformity of cooling is important to prevent warping and stress fractures across the moulded dome.
As the sheet is stretched during thermoforming there will be an inevitable thinning at the top of the dome, so a larger than first expected plastic thickness will need to be used to account for this thinning. Ideally our model will be a thickness of 20mm which will thin at the top to roughly to 17mm.
For the casing to be effective we have decided that an outer layer of thickness 20mm will be suitable. Due to this thickness the thermo forming process by Blow Moulding may not be as easily carried out in a manufacturing industry. It also proved to be problematic as it caused a thinning to our material at the top of the Dome, which would weaken the device a little. Therefore we have explored Rotational Moulding as a possible alternative manufacturing method.
Rotational moulding is a low pressure process