How to sovle the Water Grain problem of PVC INJECTION MOLDING.
The water grain is a kind of crazing, the crazing of injection molding products includes the surface air bubble and the inner air hole. The main cause of defects is the interference of gas (mainly Water Vapor, water vapor, decomposition gas, solvent gas, air) . The specific reasons are as follows:
1. Machines and equipment
(1) the cylinder, screw wear or over rubber head, over rubber ring there is a dead angle of the flow, long-term heating and decomposition.
(2) heating system out of control, resulting in too high temperature and decomposition, should check the thermocouple, heating ring and other heating elements are not problem. Improper screw design, resulting in a solution or easy to bring into the air.
(1) poor exhaust.
(2) in the mold runner, gate, cavity friction resistance, resulting in local overheating and decomposition.
(3) imbalanced distribution of gate and mold cavity and unreasonable cooling system will lead to local overheating and Air blocking passage.
(4) The cooling passage leaks water into the mold cavity.
(1) the humidity of the plastic, add recycled material proportion is too much or contains harmful chip material (chip material is easy to decompose) , should fully dry the plastic and eliminate the chip material.
(2) absorbing moisture from the atmosphere or from colorants, colorants should also be dry, it is best to install a dryer on the machine.
(3) the amount of Lubricant, stabilizer, etc. added in the plastic is too much or mixed unevenly, or the plastic itself has volatile solvent. Mixed plastic can also decompose when it is difficult to balance the degree of heating.
(4) contaminated plastic mixed with other plastics.
(1) setting temperature, pressure, speed, back pressure, melt glue motor speed too high to cause decomposition, or pressure, speed too low, injection time, inadequate pressure, back pressure too low, due to the failure to obtain high pressure and insufficient density to melt the gas resulting in craze, appropriate temperature, pressure, speed and time should be set and multi-stage injection speed should be used.
(2) with low back pressure and fast rotation speed, the air can easily enter the cylinder, with the molten material into the mold, the melting material will decompose when the cycle is too long and the heating is too long.
(3) insufficient material, too large cushion, too low material temperature or mold temperature all affect the material flow and forming pressure, promote the formation of bubbles.