(v) Stress cracking and melt fracture
1. Some plastics are sensitive to stress, easy to produce internal stress when forming, and brittle and brittle. Plastic parts crack under the action of external force or under the action of solvents.
For this reason, in addition to adding additives in the raw materials to improve the crack resistance, the raw materials should be dried and the molding conditions should be reasonably selected to reduce the internal stress and increase the crack resistance. And should choose a reasonable plastic parts shape, should not install inserts, etc. to minimize stress concentration.
When the mold design should increase the stripping slope, choose a reasonable feed inlet and ejection mechanism, forming should be appropriate to adjust the material temperature, mold temperature, injection pressure and cooling time, try to avoid the plastic parts too cold and brittle release After molding, plastic parts should also be post-treated to improve crack resistance, eliminate internal stress and prohibit contact with solvents.
2. When the polymer melt with a certain melting index passes through the nozzle hole at a constant temperature and its flow rate exceeds a certain value, a significant transverse crack occurs on the surface of the melt and is called melt fracture, which may damage the appearance and physical properties of the plastic part. Therefore, in the use of high melt index polymer, etc., should increase the nozzle, runner, inlet section, reduce the injection speed, increase the material temperature.
(6) Thermal performance and cooling rate
1, a variety of plastics have different specific heat, thermal conductivity, heat distortion temperature and other thermal properties. When the heat of high specific heat requires a large amount of heat, an injection machine with a large plasticizing capacity should be used. The cooling time with high heat distortion temperature can be shortened and the mold release can be made early, but the cooling deformation should be prevented after demoulding. Low cooling rate with slow thermal conductivity (such as ion polymer, etc., cooling rate is very slow) must be fully cooled, to strengthen the mold cooling effect. Hot runner molds are suitable for plastics with low specific heat and high thermal conductivity.
The plastics with large specific heat and low thermal conductivity, low heat distortion temperature and slow cooling speed are not conducive to high-speed forming, and must be cooled with an appropriate injection machine and reinforced mold.
2. All kinds of plastics must maintain proper cooling rate according to their variety characteristics and plastic parts shape. Therefore, the mold must be set according to the requirements of the heating and cooling system to maintain a certain mold temperature. When the material temperature makes the mold temperature rise, it should be cooled to prevent the plastic parts from deforming after demoulding, shorten the molding cycle and reduce the crystallinity.
When the plastic waste heat is insufficient to maintain the mold at a certain temperature, the mold should be provided with a heating system to maintain the mold at a certain temperature to control the cooling rate, ensure fluidity, improve the filling conditions or to control the plastic parts to slow cooling, To prevent the uneven cooling inside and outside the thick-walled plastic parts and improve the degree of crystallinity.
For good fluidity, large forming area, non-uniform material temperature, according to the molding of plastic parts sometimes need heating or cooling alternate use or local heating and cooling and use. For this purpose the moulds shall be provided with a corresponding cooling or heating system.
(7) Hygroscopicity
Because of various additives in the plastic, it makes the water have different degrees of affinity, so the plastic can be roughly divided into two kinds of moisture absorption, adhesion of moisture and non-absorbent and difficult to adhere moisture, the water content in the material must be controlled to allow In the range, the moisture becomes gas or hydrolysis occurs under high temperature and high pressure, which causes foaming of the resin, deterioration of fluidity, and poor appearance and electromechanical performance. Therefore, hygroscopic plastics must be preheated with appropriate heating methods and specifications as required, and infrared radiation must be used to prevent reabsorption. (Finish)
1. Some plastics are sensitive to stress, easy to produce internal stress when forming, and brittle and brittle. Plastic parts crack under the action of external force or under the action of solvents.
For this reason, in addition to adding additives in the raw materials to improve the crack resistance, the raw materials should be dried and the molding conditions should be reasonably selected to reduce the internal stress and increase the crack resistance. And should choose a reasonable plastic parts shape, should not install inserts, etc. to minimize stress concentration.
When the mold design should increase the stripping slope, choose a reasonable feed inlet and ejection mechanism, forming should be appropriate to adjust the material temperature, mold temperature, injection pressure and cooling time, try to avoid the plastic parts too cold and brittle release After molding, plastic parts should also be post-treated to improve crack resistance, eliminate internal stress and prohibit contact with solvents.
2. When the polymer melt with a certain melting index passes through the nozzle hole at a constant temperature and its flow rate exceeds a certain value, a significant transverse crack occurs on the surface of the melt and is called melt fracture, which may damage the appearance and physical properties of the plastic part. Therefore, in the use of high melt index polymer, etc., should increase the nozzle, runner, inlet section, reduce the injection speed, increase the material temperature.
(6) Thermal performance and cooling rate
1, a variety of plastics have different specific heat, thermal conductivity, heat distortion temperature and other thermal properties. When the heat of high specific heat requires a large amount of heat, an injection machine with a large plasticizing capacity should be used. The cooling time with high heat distortion temperature can be shortened and the mold release can be made early, but the cooling deformation should be prevented after demoulding. Low cooling rate with slow thermal conductivity (such as ion polymer, etc., cooling rate is very slow) must be fully cooled, to strengthen the mold cooling effect. Hot runner molds are suitable for plastics with low specific heat and high thermal conductivity.
The plastics with large specific heat and low thermal conductivity, low heat distortion temperature and slow cooling speed are not conducive to high-speed forming, and must be cooled with an appropriate injection machine and reinforced mold.
2. All kinds of plastics must maintain proper cooling rate according to their variety characteristics and plastic parts shape. Therefore, the mold must be set according to the requirements of the heating and cooling system to maintain a certain mold temperature. When the material temperature makes the mold temperature rise, it should be cooled to prevent the plastic parts from deforming after demoulding, shorten the molding cycle and reduce the crystallinity.
When the plastic waste heat is insufficient to maintain the mold at a certain temperature, the mold should be provided with a heating system to maintain the mold at a certain temperature to control the cooling rate, ensure fluidity, improve the filling conditions or to control the plastic parts to slow cooling, To prevent the uneven cooling inside and outside the thick-walled plastic parts and improve the degree of crystallinity.
For good fluidity, large forming area, non-uniform material temperature, according to the molding of plastic parts sometimes need heating or cooling alternate use or local heating and cooling and use. For this purpose the moulds shall be provided with a corresponding cooling or heating system.
(7) Hygroscopicity
Because of various additives in the plastic, it makes the water have different degrees of affinity, so the plastic can be roughly divided into two kinds of moisture absorption, adhesion of moisture and non-absorbent and difficult to adhere moisture, the water content in the material must be controlled to allow In the range, the moisture becomes gas or hydrolysis occurs under high temperature and high pressure, which causes foaming of the resin, deterioration of fluidity, and poor appearance and electromechanical performance. Therefore, hygroscopic plastics must be preheated with appropriate heating methods and specifications as required, and infrared radiation must be used to prevent reabsorption. (Finish)
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