What is PC or Polycarbonate – PC injection molding
PC is generally called polycarbonate. Because of its excellent mechanical properties, this is commonly known as bullet-proof plastic. PC injection molding has high mechanical strength, wide temperature range, good electrical insulation properties (anti-arc performance is unchanged), good dimensional stability and transparency, etc.
It is widely used in the electrical products, electric meter enclosure, and electronic products structural members. There are many modified products of PC usually added with glass fiber, mineral fillers, chemical fire retardants, and other plastics. PC has poor fluidity, and it needs high temperatures when working on the PC injection molding.
The PC injection molding process
PC injection molding has high water absorption. So before injection molding, it must be preheated dried. The pure PC is dried at 120 Celsius degree, and the modified PC should generally be dried for more than 4 hours with a temperature of 110 Celsius degree. Drying time should not exceed 10 hours. Generally, you can use the method of air extrusion to determine whether it’s dry enough. The utilization of recycled materials can reach 20%. In some cases, the utilization of recycled materials can be100%, the actual quantity is subject to the quality requirements of the products. Recycled materials can’t mix different masterbatch simultaneously, or it will seriously damage the nature of the finished product.
Selection of injection molding machine
Now, for the cost and other reasons, PC often uses modified materials, especially electrical products which should increase fireproof performance. When the flame-retardant PC and other plastic alloy products are molding, it should make sure the plasticizing system of the injection molding machine mix well and corrosion-resistant. It is unachievable for the conventional plasticizing screw. So when choosing, you should state in advance, usually, there are special PC screws for customers to choose from.
Design of mold and gate for PC injection molding
The common mold temperature is 80-100 Celsius degree, the temperature of mold adding glass fiber is 100-130 Celsius degree. Small products can use the pinpoint gate, Gate depth should be 70% of the thickest part, and other gates are circular and rectangular. If the gate is bigger, it’s better, which can reduce the defects caused by excessive shear. The depth of the vent should be less than 0.03-0.06mm; the runner should be short and round. The ejection gradient is generally about 30′-1.
Melt temperature for PC material
You can use a gas injection method to determine the processing temperature. Generally, the PC injection molding processing temperature is 270-320 Celsius degree. The temperature of the PC with some modification or low molecular weight is 230-270 Celsius degree.
When molding, relatively fast injection speed is usually adopted, such as electrical switch parts. Commonly, it is from slow to rapid injection molding.
Packing is 10bar around; it can be appropriately reduced without the gas lines and color mixing.
If staying in the high temperature for a long time, the materials will degrade, emit CO2 and turn yellow. You should use PS to clean the cylinder, not LDPE, POM, ABS or PA.
Some modified PC injection molding material, as being recycled many times (the molecular weight decreased) or an uneven of mixing ingredients, would easily produce the dark brown liquid bubble.
The key point to select injection molding machine FOR PC Plastic Injection molding parts
The finished projected area per square centimeter is multiplied by 0.47 to 0.78 tons (or per square inch by 3 to 5 tons).
The size of the machine: A finished weight is approximately 40-60% of the capacity of the injection molding machine for the best. If the machine puts polystyrene as the capacity (oz), which needs to be decreased by 10%. At the beginning, which is used as GUANG DA’s capacity. 1 ounce = 28.3 grams.
Screw: Screw length should be at least 15 diameters long, L / D of 20:1 is the best. The compression ratio is preferably from 1.5:1 to 30:1. The stop valve in front of the screw should be set into a sliding ring. The flowing gap of resin should be at least 3.2MM.
Nozzle: Tip opening should be at least 4.5MM (diameter) if the weight of the finished product is over 5.5KG, the diameter of the nozzle should be 9.5MM or above. Besides, the tip opening should be less 0.5-1MM than gate diameter, and segment road is shorter which is better, it’s about 5MM.
The key point on molding conditions for PC injection molding:
Melt temperature and mold temperature:
Best molding temperature setting is related to many factors, such as size presses, the screw configuration, the mold and the molded article in the design and the molding cycle time, etc. In general, to make plastics gradually melt, you should set a lower temperature in the rear section of the feed tube/feeding area. And set a higher temperature in front of the feed tube. However, if the screw is designed defectively or L / D value is too small, you can also set reverse-temperature.
For the mold temperature, the high mold temperature may provide a better surface appearance, and the residual stress will be smaller, for the thinner or longer finished PC plastic injection molding products, it’s easy to fill. However, low mold temperature can shorten the molding cycle.
The speed of rotation of the screw is suggested from 40 to 70rpm, but it’s subject to the design of the machine and screw.
Injection pressure: to fill the mold as soon as possible, the injection pressure should be bigger, which is better. Generally, it’s about 850 to 1,400 KG/CM2, up to 2,400 KG/CM2.
Packing: Generally, the lower the better. For the sake of uniformity in feeding, it is suggested to use 3 to 14KG/CM2
The speed of injection molding: firing rate has a great relation with gate design; when using a direct gate or edge gate, in order to avoid day sunshine phenomenon and the phenomenon of wave and current marks, you should use the slow firing rate. What’s more, if the thickness of finished products is above 5MM, you can use slow injection to avoid air bubbles or recesses.
In general, the principle of firing rate is faster for the thin and slower for the thick. From injection to holding pressure, holding pressure is as low as possible to avoid molding products appearing residual stress. However, you can use the annealing method to remove or alleviate the residual stress. The condition is 120 to 130 Celsius degree for approximately half to one hour.
Feed cylinder’s cleaning
1. Under the molding temperature of polycarbonate, add cleaning materials (general purpose polystyrene or transparent benzene), continuously inject for 20 to 30 times.
2. Put the machine back, and continue the cleaning material by air injection until the injected cleaning materials begin to swell blistering.
3. Reset the temperature of the feed cylinder at 200 to 230 Celsius degree.
4. Continue cleaning material by air injection until the melt temperature of the cleaning material reached 260 Celsius degree and feed cylinder looks very clean and transparent on the surface.
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