Influence of Heating Effect on Thermoforming Process

The appropriate heating conditions of thermoforming machines should also keep all parts of the entire sheet rising evenly during the heating process. The heating effect affects the uniformity of the wall thickness distribution to a great extent, which is related to the thickness of the plate (sheet), the heating speed, and the regional control of the heating unit.

To this end, the thickness of the selected sheet is first required to be as equal as possible. The thickness of the sheet should be as uniform as possible, and the tolerance should generally not be greater than 4% to 8%, otherwise it is easy to produce internal stress due to uneven temperature. In that way, the heating time should be extended, so that the heat is transferred to the inside of the thick wall, so that the board (sheet) is "heat permeable" and has a uniform temperature to ensure the quality of the product.

Due to the poor thermal conductivity of plastics, when heating thick sheets, such as single-sided heating, if a high-power heater is used in order to rapidly heat up or the heater is too close to the plate (sheet), there will be a large temperature difference between the two sides of the sheet, and even the side close to the heater will be burned. When the top surface of the plate has reached the required temperature, the bottom surface temperature is still very low; When the bottom surface temperature meets the requirements, the top surface temperature has exceeded the requirements, and even has been burned. The plate (sheet) with insufficient bottom surface temperature and too high top surface temperature should not be used for molding. In order to change this unfavorable heating situation, use a double-sided heater that can heat two surfaces of the sheet at the same time, and also use preheating or high frequency heating or far infrared heating to shorten the heating time.

In the process of heating, the plate (sheet) is prone to melting. The reason of sheet melting is caused by thermal expansion and melt flow. Thermal expansion is a common phenomenon in the heating process of all non-oriented sheets, usually increasing the size of about 1% to 2%, and the melt flow mainly depends on the melt viscosity of the plastic. Any overheated sheet will produce a melting phenomenon, the use of oriented sheets can overcome the melting phenomenon to a certain extent, because the oriented sheet (sheet) has a thermal shrinkage behavior. However, it must also be noted that excessively oriented plates (sheets) often have too much stress and make molding difficult. The most common way to control the melting is to control the heating time and shorten the residence time of the sheet in the oven.

Due to the different shapes of the parts, the drafting of each part of the sheet is also different. In order to prevent the thickness of each part from being uneven due to different drafts, the flower plate cut from paper can cover the area with a large draft ratio when forming, and make this part less heated when heating, so that the temperature in the area is slightly lower. At present, the uneven wall thickness caused by different deformation degree can also be solved by using programmed heating and other methods to make the heating temperature of the sheet different.

In addition, the mold and auxiliary plunger should also use the appropriate temperature according to different plastics in order to obtain a good quality of the product.