Wuhan HanDern Molding Technology
日期：2020/11/30 9:06:26 / 阅读： / 来源：本站
Wuhan HanDern Molding Technology
The thermoforming operation process includes: (1) clamping of the sheet; (2) heating (3) forming (4) cooling; (5) demolding.
The clamping frame usually consists of two upper and lower frames. The upper frame is controlled by compressed air, which can often evenly and forcefully press the sheet on the lower frame. Balanced clamping is a necessary condition to ensure uniform distribution of materials. The clamped sheet should have reliable air tightness, otherwise air leakage will occur during molding, causing the sheet to slide and bend and twist. www.handern.net Hollow Profile Sheet Extrusion Machine
The time required to heat the sheet to the forming temperature is generally about 50-80% of the entire forming cycle. Therefore, the consideration of how to shorten the heating time is of great significance. The time for heating (or cooling) the sheet increases with the increase of the thickness and specific heat capacity of the sheet, but decreases with the increase of the thermal conductivity and heat transfer coefficient of the sheet.
Based on the above reasons, the thickness of the used sheet should be uniform, otherwise the temperature will be uneven, which will make the product
With internal stress. The thickness tolerance of the profile sheet should generally not be greater than 48%, otherwise the heating time should be extended to allow the heat to be transferred to the thick wall. Due to the low thermal conductivity of plastics, when heating thick sheets, if you use a heater with a larger heating power or the heater is too close to the sheet, it is easy to burn. In this case, it is best to use double-sided heating, preheating or Use high heating to shorten the heating time. The lower limit of the molding temperature should be the maximum temperature at which the sheet does not turn white or show obvious defects in the largest area, while the upper limit is the highest temperature at which the sheet does not degrade and excessively sag on the holder. Taking ABS as an example, its low limit molding temperature can be as low as 127℃, and its high limit can reach 180℃. When using rapid vacuum forming with low pinch crystal, the molding temperature is about 140℃, and the temperature of deep-drawn products is about 150℃. , Only more complex products use high molding temperature, about 170 ℃.
The most prone fault during molding is uneven thickness of the product, which is mainly due to the different stretches of each part. In addition, the speed of drafting or dragging the sheet will also affect the uneven thickness of the product. Generally speaking, the speed should be as fast as possible, which is beneficial to the molding itself and shorten the molding cycle, so sometimes the holes are even changed to long and narrow slits. For example, when a refrigerator cover is formed, it takes 2 to 5 seconds to extract air with a normal vacuum hole, but after switching to a long seam, the air extraction time can be reduced to 0.5 seconds. The rate of drafting also depends on the temperature of the sheet. Thin sheets should generally be drawn faster than thick ones. www.handern.net Hollow Profile Sheet Extrusion Machine
4. Cooling and stripping
In sheet thermoforming, in order to shorten the molding cycle, manual cooling is generally used. There are two types of cooling: internal cooling and external cooling. The molded product must be cooled below the deformation temperature before it can be demolded. For example, the temperature of polyvinyl chloride products is 40~50℃, the temperature of cellulose acetate is 50~60℃, and the temperature of plexiglass is 60~70℃. Insufficient cooling, the product will deform after demoulding. www.handern.net Hollow Profile Sheet Extrusion Machine