Positive and negative forming

During thermoforming process only one side of the processed material contacts the mould. Its shape and texture are therefore reproduced on the surface of formed sheet or foil.
Depending on the side of material that contacts the mould we deal with positive or negative thermoforming.
During positive forming the inside of the blister becomes an exact mapping of the mould shape because this side contacts the mould. In case of negative forming it is the opposite and the outside of the blister is reproduced on the surface of the sheet – foil.
Following issues should be taken into consideration when positive thermoforming is taken into action:
• Creation of shrinking effects in case of rectangular moulds, especially if distance between clamping frame and the mould is significant
• Development of chill marks on the edges
• Uneven thickness of the walls on blister rims
• Uneasy detaching of the blister out of the mould caused by small angle of the wall
• Too shallow areas (not completely formed) between the areas of multi-cavity blisters, whose reduction is possible only through employment of additional clamping frame
• Positive forms are usually cheaper to make than the negative ones
Following issues should be taken into consideration when negative thermoforming is applied:
• Development of thick rims
• Eden thickness of the walls aroung the rim
• Development of thin edges
• Easy detaching of the moulds with single forming areas
• Łatwy demontaż form o pojedynczych obszarach formowania
• Negative forms are usually more expensive to make than the negative ones
Most of the setbacks can be reduced or eliminated through application of appropriate mould production techniques as well as control of the thermoforming process.

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