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What are the thermal properties of plastic thermoforming materials

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there are many thermal properties of materials to be considered in the process of hot forming, including thermal deformation temperature, softening temperature range, thermal mechanical strength, specific heat capacity, thermal conductivity, thermal expansion coefficient, melting heat, thermal diffusion coefficient and thermal stability
the thermal deformation temperature is an important temperature condition to be considered in the hot forming process. Theoretically, the upper limit of the internal temperature of the material during the forming process cannot be higher than the unloaded thermal deformation temperature of the material itself, otherwise the gravity of the part itself will destroy the part during the forming process

for large parts, the thermal deformation temperature of the material under 1.82mpa is generally used as the processing temperature, while for small parts, the thermal deformation temperature of the material under 0.46mpa load is generally used as the upper limit temperature of thermoforming. In the actual molding, because the surface temperature of the plastic sheet is higher than the internal temperature, the process control temperature is often much higher than the thermal deformation temperature of the material

the temperature of the material drops rapidly in the hot forming process. The material needs to maintain appropriate flexibility, plasticity and elasticity in a wide temperature range in order to ensure the integrity of the edges and corners of the final product. The material also needs to have high thermal mechanical strength, otherwise the thickness will be seriously uneven after drawing in the thermal state, which requires that the molecular weight of thermoplastic used for thermoforming should not be too low. In addition, the thermoforming processability of materials can be improved by introducing strong polar groups or cross-linking structures into the molecular chains to limit the mutual slip of molecular chains

the specific heat capacity of blister packaging materials reflects the heat required by the unit mass of materials to increase the unit temperature. The effective heat required by the forming heater can be calculated according to the specific heat capacity and density of the sheet. The thermal conductivity of plastics is relatively low. In the preheating stage of the hot forming process of thick wall products, the surface may have melted, foamed or even decomposed, but the interior has not been softened. Therefore, the plastic varieties with high thermal conductivity should be considered in material selection, and the heating methods such as double-sided heating or far-infrared heating should also be considered in technology

in actual production, the thermal expansion in the heating stage can be ignored, but the dimensional shrinkage in the cooling stage can not be ignored. When cosmetic products contain metal inserts or plastic products need to be embedded in metal containers, it should be considered that the shrinkage rate of plastic is greater than that of metal. When the male mold is used for molding, the plastic is close to the mold surface due to shrinkage, and it is difficult to pull out the male mold. This should be paid special attention to when molding materials with large coefficient of thermal expansion, such as polyolefin and polyvinyl chloride (PVC)

the thermal diffusion coefficient is used to calculate the cooling time of thermoforming process, because the cooling time of hot sheet is directly proportional to the material thickness and inversely proportional to the thermal diffusion coefficient. The thermal diffusion coefficient can be expressed by the following formula: thermal diffusion coefficient = thermal conductivity /(density * specific heat capacity) the three parameters on the right of medium sign do not remain unchanged during thermoforming, and the crystallization heat release of crystalline or semi crystalline polymers during cooling is not taken into account. Therefore, in actual production, there is often a large deviation in calculating the cooling time by relying on the thermal diffusion coefficient. However, from the formula, we can at least know that the increase of sheet thickness, the increase of die temperature and the decrease of material density have the same influence trend on the required cooling time

compared with extrusion, injection molding and other molding processes, the processing temperature of thermoforming is much lower, and the thermal stability of materials is generally not considered. However, when making thick wall products from PVC and other materials with poor thermal stability, attention should also be paid to avoid overheating decomposition of the sheet surface

for plastics with strong hygroscopicity or strict requirements on moisture limit, such as ABS, polyamide and polycarbonate, they should be dried before heating, otherwise the surface of heated sheet will blister and the surface of molded products will be rough.

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