The amount of expansion and depression
Heating thermoplastic on the thermoforming machine, in order to determine its depression, we must know the linear thermal expansion coefficient of plastic ι , the thermal expansion of plastic is basically linear, can be calculated through the linear thermal expansion coefficient ι :
The amount of linear expansion produced by heating △ι=ι₁·λ·(T₂-T₁)
△ι--The amount of thermal expansion produced,mm;
ι₁--The length or size of a material at temperature T₁,mm;
λ--Coefficient of linear expansion
T₁--Initial temperature, usually temperature
T₂--The temperature at the end of heating
The above formula is only applicable to the operating temperature and low temperature during the thermoforming process.
Examples of expansion in the continuous operating temperature range:
ABS material, ι₁=800mm,λ =90×10⁻⁶× K/1, T₁=20℃, T₂=60℃
Linear expansion ▲ι=800mm×90×10⁻⁶K/1×(60－20)K=2.88mm
Shrinkage ∫ can be simply calculated by following the formula:
∫=0.62×√b₁×√△b₂
∫--Shrinkage,mm
√b₁--The width of the sheet in the machine holding frame or the width of the conveying sheet;
△b₂--When the temperature difference is T₂-T₁, the linear expansion of the sheet is calculated according to the △ι of the above formula.
Note: When calculating the shrinkage amount according to the above formula, the internal stress of the forming material is not taken into account.
When a thermoplastic material is heated on a thermoforming machine and the material is heated above the forming temperature, the following conditions will occur.
① Thermoplastic sheet expands until softening temperature.
When the softening temperature (glass transition temperature) is exceeded, the freezing stress will be released; Highly oriented sheets, such as OPS or LDPE, where the main chain is extended in the die frame and at the softening point; Other materials, such as PP or PVC, continue to expand until they reach the forming temperature.
③ When the contact plate is heated, the expansion will change the contact pressure, and the bonding wave (adhesion strechs) will be generated on the surface of the formed sheet.
When radiative heating is performed, two problems arise.
a. If the sheet is not supported and dented, the expansion caused by its own weight must be added to the amount of thermal expansion.
Radiant heating may result in underheating, or the sheet may be damaged by heat. When producing thin PP materials, the method of freezing stress must be used when heating on the hot forming machine, otherwise, it is very difficult for the sheet not to melt down.
b. Where the molding material is supported by air to prevent sagging, wrinkles will occur in order to maintain its level, and if there is a significant temperature difference between the raised and depressed parts, it will negatively affect the quality of the molded product.
It is impossible to calculate the actual amount of dent for the forming material with severe dent.
In terms of thermal expansion and depressions, the operator should know the following.
(1) Depression is a function of the type of plastic material and the molding temperature, for almost all sheets, there will be different degrees of depression; When placing an order, ask for relevant information if necessary.
(2) As far as polypropylene is concerned, its blend with 10%PE or filled PP sheet will almost not produce depression, and the high-shrinkage sheet will show less depression when heated.
(3) In the heating process of sheets with air support, PP sheets should be appropriately dented so as to expand and wrinkle as much as possible; If the heated sheet cannot be supported by air during or after heating, in addition to several expansion, the sag becomes the operating factor of the plastic material, and the sag is related to the strength of the material under the forming humidity, and also related to the internal stress caused by the sheet during the production process.
When a thermoplastic material is heated on a thermoforming machine and the material is heated above the forming temperature, the following conditions will occur.
① Thermoplastic sheet expands until softening temperature.
When the softening temperature (glass transition temperature) is exceeded, the freezing stress will be released; Highly oriented sheets, such as OPS or LDPE, where the main chain is extended in the die frame and at the softening point; Other materials, such as PP or PVC, continue to expand until they reach the forming temperature.
③ When the contact plate is heated, the expansion will change the contact pressure, and the bonding wave (adhesion strechs) will be generated on the surface of the formed sheet.
When radiative heating is performed, two problems arise.
a. If the sheet is not supported and dented, the expansion caused by its own weight must be added to the amount of thermal expansion.
Radiant heating may result in underheating, or the sheet may be damaged by heat. When producing thin PP materials, the method of freezing stress must be used when heating on the hot forming machine, otherwise, it is very difficult for the sheet not to melt down.
b. Where the molding material is supported by air to prevent sagging, wrinkles will occur in order to maintain its level, and if there is a significant temperature difference between the raised and depressed parts, it will negatively affect the quality of the molded product.
It is impossible to calculate the actual amount of dent for the forming material with severe dent.
In terms of thermal expansion and depressions, the operator should know the following.
(1) Depression is a function of the type of plastic material and the molding temperature, for almost all sheets, there will be different degrees of depression; When placing an order, ask for relevant information if necessary.
(2) As far as polypropylene is concerned, its blend with 10%PE or filled PP sheet will almost not produce depression, and the high-shrinkage sheet will show less depression when heated.
(3) In the heating process of sheets with air support, PP sheets should be appropriately dented so as to expand and wrinkle as much as possible; If the heated sheet cannot be supported by air during or after heating, in addition to several expansion, the sag becomes the operating factor of the plastic material, and the sag is related to the strength of the material under the forming humidity, and also related to the internal stress caused by the sheet during the production process.