High Pressure Forming - continued ...
Pressure Forming uses sheet plastic which is stretched into the mould. It is not possible, therefore, to have any section of the part thicker than the original sheet thickness. Nor is it possible to mould ribs or bosses, although these can be added afterwards. But there are no sink marks with Pressure Forming - crucial for large flat panels.
Normally the integral strength of Pressure Formed parts is greater than Injection moulded parts because the plastic resins are superior. The process also operates at lower temperatures so the product has less residual moulding stress. Pressure Formed parts are lighter and less brittle than foam mouldings and do not require painting.
The kind of parts which have been successfully produced include:
Normally the integral strength of Pressure Formed parts is greater than Injection moulded parts because the plastic resins are superior. The process also operates at lower temperatures so the product has less residual moulding stress. Pressure Formed parts are lighter and less brittle than foam mouldings and do not require painting.
The kind of parts which have been successfully produced include:
- Automotive Trim Panels
- Bezels and Instrument Panels
- Scientific and Medical Instrument Casings
- Machine Covers
- Signs
- Precision packaging
- Machine Guards
- Aircraft Components
- Terminals
Undercuts & Styling
Undercuts
Most Pressure Formed parts are made from cavity tools which lends them nicely to the use of undercuts around the periphery of the part.
Undercuts can, in principle, be located at any point on the walls of the part and can take the form of ribs in the walls or side cores. These options, however, involve much more expensive moulds since they require retractable cores or more complicated mould design.
The controlling factor in determining the size of undercut is the gap below the undercut. If the gap below the undercut is large the material will easily flow into the gap, stretching will be minimised and it will be possible to mould a good undercut. But, if the gap below the undercut is small the level of stretching will be greater and thinning will occur unless the undercut is very small.
The same thinning problem occurs in corners where the plastic is subjected to a three-dimensional stretch. As the undercut moves further away from a corner it is easier to mould an undercut.
Increasing the starting thickness of the sheet can reduce the thinning issue but it increases the cost of the part. In certain circumstances no amount of increase in thickness will reduce the thinning problem because the flow requirements are too severe.
'Witness Lines' always occur when undercuts are used but these are disguised by tool texturing.
Styling
Normal styling features are:
Most Pressure Formed parts are made from cavity tools which lends them nicely to the use of undercuts around the periphery of the part.
Undercuts can, in principle, be located at any point on the walls of the part and can take the form of ribs in the walls or side cores. These options, however, involve much more expensive moulds since they require retractable cores or more complicated mould design.
The controlling factor in determining the size of undercut is the gap below the undercut. If the gap below the undercut is large the material will easily flow into the gap, stretching will be minimised and it will be possible to mould a good undercut. But, if the gap below the undercut is small the level of stretching will be greater and thinning will occur unless the undercut is very small.
The same thinning problem occurs in corners where the plastic is subjected to a three-dimensional stretch. As the undercut moves further away from a corner it is easier to mould an undercut.
Increasing the starting thickness of the sheet can reduce the thinning issue but it increases the cost of the part. In certain circumstances no amount of increase in thickness will reduce the thinning problem because the flow requirements are too severe.
'Witness Lines' always occur when undercuts are used but these are disguised by tool texturing.
Styling
Normal styling features are:
- Controlling flat/curved surfaces, ribs, flanges, crowning or internal stiffening,
- How to achieve Strong Edges,
- How to achieve Strong corners,
- Stiffening (styling) lines - raised or recessed,
- Protrusions,
- Textures, letters, logos - texture essential to expel air and give uniform finish
David Russell Associates Ltd.
1 Westhall Gardens, Uphall
BROXBURN - West Lothian - EH52 5DL
Tel/Fax: +44 (0) 1506 854 986
Mobile: 07762 296771
Contact us: Please click HERE
1 Westhall Gardens, Uphall
BROXBURN - West Lothian - EH52 5DL
Tel/Fax: +44 (0) 1506 854 986
Mobile: 07762 296771
Contact us: Please click HERE