Professional manufacturer of thermoforming machines.
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Professional manufacturer of thermoforming machines.
The forming operation of various forming methods is mainly to bend and stretch the preheated sheet according to predetermined requirements by applying force. The most basic requirement for molding is to make the wall thickness of the product as uniform as possible. The main reasons for the uneven wall thickness of the products are: first, the degree of stretching of each part of the formed sheet is different; second, the size of the stretching speed, that is, the gas flow rate of air extraction and inflation or the moving speed of the mold, clamping frame and pre stretching plunger. Forming is another important process after plate (sheet) heating, including the control of important technical parameters such as forming temperature, forming pressure and forming speed.
①Plastic Thermoforming Machines Forming Temperature
After the material, process type and equipment are determined, the forming temperature is the main factor affecting the product quality, which directly affects the minimum thickness, thickness distribution and dimensional error of the product, and also affects the elongation and tensile strength of the product, and even affects the forming speed. Therefore, it should be taken seriously. When the thermoforming sheet is heated, ensure that the whole forming surface is heated evenly and the temperature setting is reasonable.
According to practice, the best molding temperature is the temperature at which the elongation of plastic is maximum. If the stress caused by the molding pressure is greater than the tensile strength of the plastic at this temperature, the sheet will be excessively deformed and even damaged. At this time, the molding temperature or the molding pressure should be reduced. Lower molding temperature can shorten the cooling time and save energy, but the shape and dimensional stability of the product will be poor, and the outline definition will be bad. At a high molding temperature, the reversibility of the product becomes small, and the shape and size are stable. However, too high a temperature will cause resin degradation and material discoloration. In the actual thermoforming production process, there is a certain time interval between heating and forming of the sheet, and some heat will be lost, especially for the thin sheet with small specific heat capacity. The actual heating temperature of the sheet is relatively high, and the actual optimal forming temperature is generally determined through experiments and production.
The stretching speed is closely related to the temperature when the sheet is formed. If the temperature is low and the sheet deformation capacity is small, the sheet should be stretched slowly. If a high stretching speed is adopted, the temperature during stretching must be increased. Since the sheet still radiates heat and cools down during molding, the stretching speed of thin sheet is generally higher than that of thick sheet.
②Plastic Thermoforming Machines Forming Pressure
The effect of pressure makes the sheet deform, but the material has the ability to resist deformation, and its elastic modulus decreases with the increase of temperature. At the molding temperature, only when the stress caused by the pressure in the material is greater than the elastic modulus of the material at this temperature can the material be deformed. If the pressure applied at a certain temperature is insufficient to produce sufficient elongation of the material, the molding can be smoothly formed only by increasing the molding pressure or increasing the molding temperature. Because the elastic modulus of various materials is different and has different dependence on temperature, the molding pressure varies with the polymer type (including relative molecular weight), sheet thickness and molding temperature. Generally speaking, plastics with high molecular chain rigidity, high molecular weight and polar groups need high molding pressure.
In addition to the influence of molding temperature, mold temperature and drawing effect, the finished product accuracy of thermoformed parts mainly depends on the effective molding pressure between the thermoformed parts and the mold.
The general molding pressure for molding (male mold): 0.2-0.3mpa for large-area molded parts; Small parts up to 0.7MPa. For vacuum molding, the molding pressure is low and mainly depends on the atmospheric pressure. At an altitude of 0, when a high-quality vacuum pump is used, the molding pressure can reach about 1 MPa.
Since the pressure generated by the vacuum is equal to the pressure difference between the atmospheric pressure on one side of the molding material and the vacuum generated on the other side, the molding pressure depends on the air pressure and the sealing degree. Therefore, even if the best vacuum pump is used, the molding pressure will continue to decrease with the increase of altitude.
③Plastic Thermoforming Machines Forming Speed
The main reasons for the uneven wall thickness of the products are: first, the degree of stretching of each part of the formed sheet is different; second, the size of the stretching speed, that is, the gas flow rate of air extraction and inflation or the moving speed of the mold, clamping frame and pre stretching plunger. The forming speed refers to the drawing speed of the plate (sheet). Increasing the forming speed can shorten the forming cycle and is beneficial to improving the productivity. However, the excessive forming speed will affect the quality of the product. Generally speaking, high drawing speed is beneficial to the forming itself and shortening the cycle time, but rapid drawing often causes the wall thickness of concave and convex parts of the product to be too thin due to insufficient flow; However, if the stretching is too slow, the deformation capacity of the sheet will decrease due to excessive cooling, and the product will crack.
The mold action is driven by hydraulic pressure, air pressure or motor. During hot forming, the plastic plate (sheet) will be stretched and deformed under the pressure or plunger. The stretching speed of the material is different with the forming speed. The running speed of the mold can be controlled by levels, and the mode of fast first and slow later is generally selected. The action speed of the mold must match with the pre stretching speed. If the action is too slow, the temperature of the plate will decrease, which is not conducive to forming, and if the action is too fast, the plate may be torn. For the sheet with a certain thickness, the heating temperature shall be properly increased and the faster forming speed shall be adopted.
Professional manufacturer of thermoforming machines.
