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11 Golden Skills of PC Injection Molding Technology
Publisher : Wia Release time : 1/28/2019 2:51:18 PM
PC has excellent performance, high transparency, good impact toughness, creep resistance and wide temperature range.
PC process characteristics: the melt viscosity is less sensitive to shear rate, but more sensitive to temperature, no obvious melting point, higher melt viscosity, easy hydrolysis of resin at high temperature, easy cracking of products.
In view of these characteristics, we should pay special attention to the differential treatment:
To increase the fluidity of melt, the injection temperature should be increased instead of increasing the injection pressure.
It is required that the runner and gate of the die be short and thick in order to reduce the pressure loss of the fluid and at the same time to have a higher injection pressure.
Adequate drying treatment should be carried out before the resin is processed, so that the moisture content of the resin can be controlled below 0.02%.
In addition, thermal insulation measures should be taken to prevent the resin from re-absorbing moisture in the process of processing.
It is not only necessary to design the products reasonably, but also to grasp the forming process correctly, such as raising the temperature of the die and after-treatment of the products, which can reduce or eliminate the internal stress. Adjust the process parameters in time according to the different conditions of the products.
Next, we will talk about the moulding process.
1. Drying of raw materials
PC plastics, even when they encounter very low moisture content, will produce hydrolysis and breakage, molecular weight reduction and physical strength reduction. Therefore, the moisture content of polycarbonate should be strictly controlled below 0.02% before forming, in order to avoid the decrease of mechanical strength or the abnormal appearance of foam and craze on the surface of the product.
PC is extremely sensitive to water, so it must be fully dried before injection to reduce its water content to less than 0.02%. The general drying conditions of PC are: 100-120 C, at least 4 hours;
2. Injection temperature
The injection temperature must be integrated with the shape, size and die structure of the product. The product performance, requirements and other aspects of the situation can be considered before making.
Generally, when the temperature is between 270 and 320 C, if the material temperature is too high, the PC will decompose, the color of the product will become darker, the surface will appear defects such as silver wire, dark strip, black spot, bubble, and so on. At the same time, the physical and mechanical properties will also be significantly reduced. PC is also sensitive to temperature. The melt viscosity decreases obviously with the increase of temperature. The barrel temperature is 250-320 C. (It is better not to exceed 350 C). It is good for PC plasticization to increase the temperature of barrel appropriately. Internal stress annealing is necessary. The oven temperature is 125 - 135 C, and the time is 2 hours. It is cooled to normal temperature naturally.
3. The physical and mechanical properties, internal stress and shrinkage of PC products have a certain influence on the appearance and demoulding performance of the products. Lower or higher injection pressure will cause some defects of the products. Generally, the injection pressure is controlled between 80 and 120 MPa, which has a complex shape and a smaller gate. In order to overcome the resistance of melt flow and fill the cavity in time, a higher injection pressure (120-145 MPa) is selected. Thus a complete and smooth product can be obtained.
For poor fluidity, high pressure injection is needed, but the large residual internal stress (which may lead to cracking) of the rubber parts should be taken into account. The injection speed is medium wall thickness and high wall thickness.
4. Pressure holding pressure and time
Pressure holding pressure and holding time have great influence on the internal stress of PC products. Pressure holding pressure is too small, shrinkage bubble or surface is easy to appear, pressure holding pressure is too large, and internal stress around gate is easy to produce. In actual processing, it is often solved by means of high material temperature and low pressure holding. The choice of holding time should depend on the thickness of the product, gate size, die temperature and so on. Generally, small and thin products do not need a long holding time, on the contrary, large and thick products should have a longer holding time. The holding time can be determined by the test of gate sealing time.
5. Injection speed
There is no obvious influence on the performance of PC products. In addition to thin wall, small gate, deep hole and long process products, medium or slow processing is generally adopted, preferably multi-stage injection, and slow-fast-slow multi-stage injection is generally adopted.
6. Mould temperature
Mold temperature control: 85-120 C, generally controlled at 80-100 C, for complex shape, thinner, higher requirements of products, can also be raised to 100-120 C, but can not exceed the hot deformation temperature of the die. High die temperature can reduce the difference between die temperature and PC material temperature, and reduce the internal stress of the parts.
7. Screw Speed and Back Pressure
Due to the high viscosity of PC melt, it is advisable to control the back pressure between 30-60 r/min and 10-15% of injection pressure, which is beneficial to plasticization, exhaust gas, maintenance of plastic machine, prevention of excessive screw load, and the requirement of screw rotation should not be too high. It is advisable to control the back pressure between 30-60 r/min and 10-15% of injection pressure.
8. Use of Auxiliaries
In the process of injection moulding, PC should strictly control the use of dropping agent. At the same time, the use of recycled materials should not exceed three times, and the use should be about 20%.
9. PC injection moulding, higher requirements for the mold
1) Design the runner as thick and short as possible with less bending position, and use circular cross-section shunt and runner grinding and polishing to reduce the flow resistance of the melt.
2) Injection gate can be used in any form, but the diameter of water level is not less than 1.5mm.
10. Plastic Machine Requirements for PC Products
The maximum injection volume (including runner, gate, etc.) of the product should be no more than 70-80% of the nominal injection volume.
Lock die pressure: 0.47 to 0.78 tons per square centimeter of projected area of finished product (or 3 to 5 tons per square inch);
PC process characteristics: the melt viscosity is less sensitive to shear rate, but more sensitive to temperature, no obvious melting point, higher melt viscosity, easy hydrolysis of resin at high temperature, easy cracking of products.
In view of these characteristics, we should pay special attention to the differential treatment:
To increase the fluidity of melt, the injection temperature should be increased instead of increasing the injection pressure.
It is required that the runner and gate of the die be short and thick in order to reduce the pressure loss of the fluid and at the same time to have a higher injection pressure.
Adequate drying treatment should be carried out before the resin is processed, so that the moisture content of the resin can be controlled below 0.02%.
In addition, thermal insulation measures should be taken to prevent the resin from re-absorbing moisture in the process of processing.
It is not only necessary to design the products reasonably, but also to grasp the forming process correctly, such as raising the temperature of the die and after-treatment of the products, which can reduce or eliminate the internal stress. Adjust the process parameters in time according to the different conditions of the products.
Next, we will talk about the moulding process.
1. Drying of raw materials
PC plastics, even when they encounter very low moisture content, will produce hydrolysis and breakage, molecular weight reduction and physical strength reduction. Therefore, the moisture content of polycarbonate should be strictly controlled below 0.02% before forming, in order to avoid the decrease of mechanical strength or the abnormal appearance of foam and craze on the surface of the product.
PC is extremely sensitive to water, so it must be fully dried before injection to reduce its water content to less than 0.02%. The general drying conditions of PC are: 100-120 C, at least 4 hours;
2. Injection temperature
The injection temperature must be integrated with the shape, size and die structure of the product. The product performance, requirements and other aspects of the situation can be considered before making.
Generally, when the temperature is between 270 and 320 C, if the material temperature is too high, the PC will decompose, the color of the product will become darker, the surface will appear defects such as silver wire, dark strip, black spot, bubble, and so on. At the same time, the physical and mechanical properties will also be significantly reduced. PC is also sensitive to temperature. The melt viscosity decreases obviously with the increase of temperature. The barrel temperature is 250-320 C. (It is better not to exceed 350 C). It is good for PC plasticization to increase the temperature of barrel appropriately. Internal stress annealing is necessary. The oven temperature is 125 - 135 C, and the time is 2 hours. It is cooled to normal temperature naturally.
3. The physical and mechanical properties, internal stress and shrinkage of PC products have a certain influence on the appearance and demoulding performance of the products. Lower or higher injection pressure will cause some defects of the products. Generally, the injection pressure is controlled between 80 and 120 MPa, which has a complex shape and a smaller gate. In order to overcome the resistance of melt flow and fill the cavity in time, a higher injection pressure (120-145 MPa) is selected. Thus a complete and smooth product can be obtained.
For poor fluidity, high pressure injection is needed, but the large residual internal stress (which may lead to cracking) of the rubber parts should be taken into account. The injection speed is medium wall thickness and high wall thickness.
4. Pressure holding pressure and time
Pressure holding pressure and holding time have great influence on the internal stress of PC products. Pressure holding pressure is too small, shrinkage bubble or surface is easy to appear, pressure holding pressure is too large, and internal stress around gate is easy to produce. In actual processing, it is often solved by means of high material temperature and low pressure holding. The choice of holding time should depend on the thickness of the product, gate size, die temperature and so on. Generally, small and thin products do not need a long holding time, on the contrary, large and thick products should have a longer holding time. The holding time can be determined by the test of gate sealing time.
5. Injection speed
There is no obvious influence on the performance of PC products. In addition to thin wall, small gate, deep hole and long process products, medium or slow processing is generally adopted, preferably multi-stage injection, and slow-fast-slow multi-stage injection is generally adopted.
6. Mould temperature
Mold temperature control: 85-120 C, generally controlled at 80-100 C, for complex shape, thinner, higher requirements of products, can also be raised to 100-120 C, but can not exceed the hot deformation temperature of the die. High die temperature can reduce the difference between die temperature and PC material temperature, and reduce the internal stress of the parts.
7. Screw Speed and Back Pressure
Due to the high viscosity of PC melt, it is advisable to control the back pressure between 30-60 r/min and 10-15% of injection pressure, which is beneficial to plasticization, exhaust gas, maintenance of plastic machine, prevention of excessive screw load, and the requirement of screw rotation should not be too high. It is advisable to control the back pressure between 30-60 r/min and 10-15% of injection pressure.
8. Use of Auxiliaries
In the process of injection moulding, PC should strictly control the use of dropping agent. At the same time, the use of recycled materials should not exceed three times, and the use should be about 20%.
9. PC injection moulding, higher requirements for the mold
1) Design the runner as thick and short as possible with less bending position, and use circular cross-section shunt and runner grinding and polishing to reduce the flow resistance of the melt.
2) Injection gate can be used in any form, but the diameter of water level is not less than 1.5mm.
10. Plastic Machine Requirements for PC Products
The maximum injection volume (including runner, gate, etc.) of the product should be no more than 70-80% of the nominal injection volume.
Lock die pressure: 0.47 to 0.78 tons per square centimeter of projected area of finished product (or 3 to 5 tons per square inch);
Machine Size: 40-60% of the weight of the finished product is easy for the injection moulding machine. For example, when polystyrene is used to represent its capacity (oz), it needs to be reduced by 10%, starting with the capacity of GUANG DA. 1 ounce = 28.3 kg.
Screw: The length of the screw should be at least 15 diameter lengths, and its L/D is 20:1 best. The compression ratio should be 1.5:1 to 30:1. The stop valve at the front end of screw should be sliding ring type, and the resin flowable clearance should be at least 3.2MM.
11. Post-processing of PC injection molding: PE material is used to pass through the machine.
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