PC injection molding
Excellent performance, high transparency, good impact toughness, creep resistance, and wide injection temperature range.
Process characteristics of PC: melt viscosity has little sensitivity to shear rate, but high sensitivity to temperature, no obvious melting point, high viscosity of melt, easy hydrolysis of resin at high temperature, and PC injection easy cracking.
For these characteristics, it must pay special attention to the difference:
To increase the fluidity of the melt, it is not necessary to increase the injection pressure but to increase the injection temperature.
It is required that the runner path and gate of the mold are short and thick to reduce the pressure loss of the melt, and at the same time, with a higher injection pressure;
The resin needs to be sufficiently dried before the injection molding to control the water content below 0.02%;
In addition, the PC resin should be insulated during the processing to prevent re-absorption;
Not only reasonable product design is required, but also the molding process should be properly mastered, such as increasing the mold temperature and post-processing to reduce or eliminate internal stress. The process parameters should be adjusted in time according to the different conditions of the product.
Drying of raw materials
PC plastics, even in the case of very low moisture, can cause hydrolysis, breakage, molecular weight reduction and strength reduction. Therefore, before the molding process, the moisture content of the polycarbonate should be strictly controlled to be below 0.02% to avoid a decrease in the mechanical strength of the molded article or an abnormal appearance of bubbles and silver streaks on the surface.
PC is extremely sensitive to water, so it must be fully dried before injection to reduce its water content to below 0.02%.
General PC drying conditions: 100~120 °C for at least 4 hours;
The injection temperature must be based on the shape, size, and mold structure of the product.
Generally, the temperature is between 270 and 320 °C in the molding. If the material temperature is too high, over 340 °C, the PC will decompose, the color of the product will become darker, and defects such as silver, dark stripes, black spots and bubbles will appear on the surface. At the same time, physical and mechanical properties are also dropped significantly.
PC is also sensitive to temperature, melt viscosity decreases with increasing temperature, barrel temperature is 250 ~ 320 ° C. (preferably not more than 350 ° C), appropriate temperature barrel temperature is good for PC plasticization.
If necessary, the internal stress is annealed. The oven temperature is 125~135°C for 2 hours and naturally cooled to normal temperature.
It has a certain influence on the physical and mechanical properties, internal stress and molding shrinkage , and has a great influence on the appearance and release property of the products. Too low or too high injection pressure will cause some defects in the products. The injection pressure is controlled between 80-120 MPa.
For thin-walled, long-flowing, complex-shaped products with small gates, in order to overcome the resistance of the melt flow, in order to fill the cavity in time, a higher injection pressure is selected. -145 MPa). Thereby a complete and smooth surface product is obtained.
Poor fluidity, high pressure injection is required, but it is necessary to take into account the large internal stress of the rubber parts (may cause cracking). Injection speed: the wall thickness is taken at medium speed, and the wall is thin.
Holding pressure and holding time
The holding pressure and the holding time have a great influence on the internal stress of the PC product. If the holding pressure is too small, the feeding effect is small, and the vacuum bubble or the surface will be found, and if the holding pressure is too large,large internal stress is easy to be generated around the gate.
In actual processing, it is often solved by high material temperature and low pressure holding method. The choice of holding time should be determined according to the thickness of the product, the size of the gate, the temperature of the mold, etc.
Generally, the small and thin products do not require a long holding time. On the contrary, the thick products should have a longer holding time. The length of the holding time can be determined by the gate sealing time.
There is no obvious influence on the performance of PC products. Except for thin wall, small gate, deep hole and long products, it is generally processed at medium or slow speed, preferably multi-stage injection, generally adopting slow-fast-slow Multi-stage injection method.
Mold temperature control: 85~120 °C, generally controlled at 80-100 °C, for complex shapes, thinner, higher requirements, can also be increased to 100-120 ° C, but can not exceed the mold heat distortion temperature. The mold temperature is high to reduce the difference between the mold temperature and the PC material temperature, which can reduce the internal stress of the part.
Screw speed and back pressure
Due to the high viscosity of PC melt, it is beneficial to plasticize and facilitate venting. It is beneficial to the maintenance of the plastic machine and prevent the screw load from being too large. The rotation speed of the screw should not be too high. Generally, it is suitable for 30-60r/min. The back pressure control is preferably between 10-15% of the injection pressure.
PC injection should strictly control the use of release agent in the injection molding process, and the use of recycled materials should not exceed three times, and the usage should be about 20%.
1) Design the runner as thick and short as possible, with less bending, using circular cross-section runners and runner grinding to reduce the flow resistance of the melt.
2) The injection gate can be any type of gate, but the inlet water channel is not less than 15mm.
injection machine requirements
The maximum injection volume (including runners, gates, etc.) of the product is required to be no more than 70-80% of the nominal injection volume;
Clamping pressure: 0.47 to 0.78 tons per square centimeter of finished product projected area (or 3 to 5 tons per square inch);
Machine size: The weight of the finished product is about 40 to 60% of the capacity of the injection molding machine. If the machine is expressed in polystyrene (Aussie), it needs to be reduced by more 10%.
Screw: The length of the screw should be at least 15 times of diameters and the L/D is 20:1. The compression ratio is preferably from 1.5:1 to 30:1. The stop valve at the front end of the screw should adopt a sliding ring type.
Silver streaks from moisture refers to a silver streaks formed by moisture of plastic in a cylinder, due to the fact that the material particles are not sufficiently dried before molding.
Or the sufficiently dried material particles stay in the hopper for too long, and the hopper is WITHOUT effective insulation measure ,lead to re-absorb the moisture, and the water content exceeds the specified requirements again.
The moisture in particles is vaporized under the high temperature in the cylinder, which causes the resin to degrade during the melting process , which produce carbon dioxide gas.
The Co2 accumulates inside the molten resin, and when filling, it flows into the cavity with the Poly-carbonate to form silver streaks.
As a result, the strength of the plastic part is significantly reduced, and becomes brittle. This type of silver streak often occurs and relatively easy to identify.
It is usually distributed more evenly along the flow direction, and is densely found on the entire surface of the plastic part sometimes.
The fundamental solution for eliminating this silver streaks is to fully dry the pellets, and the moisture of the pellets must be controlled less than 0.03%.
For this reason, the process conditions for drying the raw materials must be strictly controlled, and the water content of the pellets should be measured before molding .
In the production, a simple inspection method can be used.
Two small glasses are used, and one detected pellet is sandwiched between the two glasses, and then placed on a 280-300 degree heating plate.
When the pellet is gradually melted by heat, and after smooth pressurization to crush the melt, observe whether there are bubbles in the material, if there is no bubble, it is dried to be used for next step, otherwise it needs to continue drying.
If the silver streaks is caused by moisture absorption in the hopper, the heat preservation device should be set or improved in the hopper.
And it needs control the amount of one feed , so as not to cause the dry pellets to stay in the hopper for too long and absorb moisture again.
In general, the maximum residence time should not exceed 30 to 60 minutes. At present, most of the heating and heat preservation devices of the hopper are provided with infrared bulbs on the upper part of the hopper, and the power can be determined according to the area of the hopper. Generally, a heating power of 25 to 30 W is required per square meter of area.
The decomposition of the silver streaks refers to the super-heating decomposition of the resin during the molding to generate a gas such as carbon dioxide producing silver streaks on the surface of the plastic part.
There are many specific reasons for the thermal decomposition of the melt :
- the temperature of the barrel is too high;
- dead space in the barrel or nozzle;
- the residence time of the melt in the barrel is too long;
- the molecular weight of the resin has been reduced. Therefore, the impact strength of the plastic part is inevitably lowered, the texture becomes brittle, and the use requirements cannot be met.
This type of silver streaks is generally recognizable from the appearance, and its distribution on the surface of the plastic part has no regularity,
In addition, the appearance of decomposed silver streaks is often accompanied by the darkening of the plastic parts or even the appearance of brown spots, which is especially evident in the hot runner, which can be used for discriminating the decomposition of silver streaks.
For the decomposition of silver streaks, the corresponding measures should be taken according to the reasons for decomposition.
- If it is caused by high temperature of the barrel, the temperature of the section should be lowered;
- if there is a dead angle in the barrel or nozzle, the dead part should be cleaned and the dead angle should be removed;
- if it is due to the melt residence time in the cylinder too long, the molding cycle should be shortened as much as possible .
If still failed, it should be considered to replace the injection molding machine with a smaller capacity for processing.
This kind of silver streak refers to the silver streaks produced by mixing air in the mold cavity with molten material ,due to unreasonable structural design of plastic parts, severe uneven wall thickness or sudden change of cross section, which causes the molten material to expand or contract rapidly during mold filling.
This kind of default mainly affects the appearance, but with little influence on the strength and impact toughness of plastic parts.
The structural silver streak is characterized in that when the process conditions are fixed, the distribution shape and position of the silver wire are also fixed, and the silver wire is generally always distributed along the injection direction, and the positions where the silver streaks occurs are mostly behind the positions with abrupt cross sections.
Because the structural silver streaks are caused by sudden change of cross-section and serious uneven wall thickness . As a result, the surface of the plastic parts often has faults such as depression and shrinkage, and sometimes even generates bubbles inside the plastic parts somewhat.
Structural silver streaks can generally be eliminated by changing the injection speed. When the cross section of the plastic part changes little, the injection speed can be slowed down. When the slow injection is adopted, the flow material can be smoothly filled into the mold cavity, so that the flow material cannot be mixed with air when passing through the abrupt cross section, and silver streaks can be avoided.
However, short shot may occur when the injection speed is reduced, which needs to be solved by adjusting other process conditions such as mold temperature and nozzle temperature. When the cross-section varies greatly, the injection can be carried out at a higher injection speed and is suitable for increasing the injection pressure so that the gas can be forcibly discharged from the parting surface.
If the failure cannot be eliminated by improving the injection speed or increasing the injection pressure, improving the plastic part structure and the mold venting system could be solutions.
Sprue and runner
The silver streaks due to runner is caused by unreasonable design of the gating system or partial blockage.
The causes and solutions are as follows:
(1) Too large sprue draft . If the sprue draft is too large, the flow material will leave the sprue wall at the initial stage of injection, resulting in gaps. As the mold cavity is gradually filled, air will be mixed and trapped in the flow material to enter the mold cavity, forming silver streaks. The main feature of the silver streaks is that its distribution is completely arranged along the injection direction.
In order to eliminate the streaks, the possibility of decomposition of resin should be eliminated by injecting into the air first, and then check whether the sprue is appropriate. If the angle is greater than 1 0 degrees, the above phenomenon may occur. An angle of 4~6 degrees is appropriate. If the angle of A is too small, demoulding will be difficult and material flow will not be smooth. The silver streaks due to sprue are sometimes eliminated by changing the injection speed, but the fundamental method is to trim or replace the sprue to reduce the cone angle.
(2) The unreasonable gate design. If the cross-sectional area of the gate is too small, turbulence will occur when the flow passes through the gate, which will mix the melt with air, thus generating silver streaks near the gate. This kind of streaks is mainly characterized by radial distribution along the injection direction with the gate as the center. The elimination method is to enlarge the gate or change the cross-sectional shape of the gate to eliminate the silver streaks. If the injection speed is appropriately reduced while reworking the gate, it will be more effective.
(3) The cold material at the nozzle mouth causes the shunt channel or local blockage of the gating system during the injection process. The mechanism of partial blockage of the runner or gate is similar to that of silver wire produced by too small a gate. When identifying such silver wire, traces of cold material should be found in the pouring and injection system. The way to eliminate this kind of failure is to increase the cold material cavity in the mold and to increase the nozzle temperature.
Venting silver streaks refers to the silver streaks formed when air cannot be expelled out of the flow material during the mold filling process. The main feature is that it is accompanied by obvious weld lines, and silver streaks often appears near the weld lines, while there is no silver streaks in other parts.
The basic method to eliminate this kind of silver streaks is to change the gate position,cut effective exhaust slots or modify the plastic part’s main structure.
In actual injection molding production or mold testing, changing injection molding machine conditions are often adopted, such as adjusting injection pressure and injection speed, changing temperature difference between fixed mold and moving mold, etc.