Choose A Reliable Pet Preform Manufacturers
Posted by preform nicole on January 24th, 2019
(1) the significance of pressure preservation
After filling the cavity, a holding pressure is applied to compensate for resin shrinkage caused by cooling. The proper holding pressure and holding time will be maintained to push the mold screw and continue to Pet Preform Mould resin into the mold cavity. The holding pressure has a great influence on the shrinkage rate of the product.
(2) Keep pressure/time confirmation
The amount of holding pressure affects the shrinkage rate during plastic injection molding. Keeping the pressure increased can make the product larger, which helps shrink compensation.
Instead, the product size will decrease, causing shrinkage. However, if the pressure is kept too high, mechanical forces and residual stresses will be generated, resulting in deformation of the product.
Usually, the proper holding pressure is 70 - 80% of the maximum value. Injection pressure.
Hold time: Hold time is related to cavity thickness because thicker products require longer time to cool. During this time, the resin needs to be continuously filled, otherwise product shrinkage will occur. However, after the gate solidifies, the molten resin cannot enter the cavity. After all, the hold time is equal to the solidification time of the gate, so the gate should be properly designed according to the thickness of the product. The gate solidification time depends on the gate thickness and length.
(3) Optimal holding pressure setting
Generally, if pressure is applied at a constant rate, the gate area will experience greater pressure, resulting in lower shrinkage; and the most distal end will not be subjected to sufficient pressure resulting in a higher but uneven shrinkage. This uneven increase in residual stress is considered to be an important cause of deformation. Therefore, the holding pressure should not be set in a uniform manner, but rather gradually reduced to minimize the pressure difference within the cavity by maintaining the pressure setting for several rounds.
In addition, the gate should be located in the thicker part of the product. Generally, the shrinkage rate depends on the pressure and the rate of solidification. Positioning the gate at a location with low solidification speed and high shrinkage provides sufficient pressure to ensure uniform shrinkage. The holding time should be longer than the gate solidification time. If not, the screw will be plasticized backwards (usually fed after holding), the resin material compressed at the front end of the gate will reflow, and then the shrinkage around the gate will rise.
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