Methods to Solve the PC Injection Molding Issues

Posted on : Aug 2 , 2022 By  GREFEE

There are many factors which affect the quality of PC products in different aspects. As long as you grasped the following factors, the quality issues can be improved significantly.  

 Raw material

The material used for injection is the transparent and colorless aggregate. Some of them are colored in mild yellow. The relative molecular weight is 2.3 × 10,000 · 3.2 × 10,000. Before the molding processing, the flowability of the PC should be determined, which is the molten flow rate (MFR). Determination conditions: the determination temperature is 300 ℃, and the load is 1200g. Determination conditions: the determination temperature is 300 ℃, and the load is 1200g. Although the water absorption rate of PC is low, the average water absorption is 0.3% in room temperature. However, the molding processing also has an impact on the products properties, which must be fully dried to decrease the water content to below 0.02%, or 0.015%, then the injection molding can be carried out.

There are many means to dry the PC resin, including the hot air circulation drying, direct bonded hopper drying, negative pressure boiling drying, vacuum drying, etc. The drying process also vary from different means.

The inspection of dry effect. It can be understood by means of moisture meter, tablet pressing method or slow air free injection method, of which the latter two methods are widely used.

One thing should be paid attention to is that if the dried particles are exposed in the air, they will be-re hygroscopic due to the influence of moisture in the air, and the PC after being re-hygroscopic will also affect the obtained products.

Process conditions and control

Molding temperature. For most of the PC products, the temperature of the barrel should be controlled within 250-310 degrees. Low temperature, high viscosity and insufficient feeding will cause the shrinkage or wrinkles, gloomy and dazzling silver threads. High temperature or higher than 310 degrees and long retention period will lead to serious degradation and burrs. Moreover, the color will turn to dark brown and there will be silver stripes, spots, and marks, and bubbles inside. The temperature of the barrel is determined by the wall thickness and the types of machines. The products with thin wall below 2mm should use higher temperature. The ideal time range should be 285-305 degrees. Products with wall thickness higher than 10mm, the temperature should be lower, which is about 250-280degrees. If the temperature exceeds 290 degrees, due to the long injection molding cycle, the tendency of overheating decomposition will increase, affecting the comprehensive properties of products.

The temperature of the barrel vary according to the types of injection molding machine. The temperature range for screw bar is about 260-285 degrees, and that of plunger type is 270 – 310 degrees. Thus, the screw bar injection molding machines are better because it can melt the plastic evenly under a low barrel temperature to avoid the degradation of material due to the high temperature inside the barrel.

The temperature of the hot nozzle should be equal to the front part of the barrel or lower by 5-10 degrees than that. High temperature will cause leaking, whereas too low temperature will lead to blockage and other defects due to the low temperature materials exist in products. The hot nozzles of the two kinds of injection molding machines should be heated and control the temperature within 260-310degrees. The barrel temperature of one side of the feeding port should be higher than the softening temperature of PC. Generally, this temperature is required to be greater than 230 degrees to reduce the resistance of the material plug and the loss of the injection pressure.

Mold temperature：The molding temperature of the PC molding temperature is high and it has a fast cooling speed, which is easy to cause the surface defects, and further cause a generation of the stress. An effective means to avoid it is to increase the mold temperate as high as possible. For most products, the mold temperature should be controlled within 80-120 degrees, of which the thin wall products is 80-100 degrees and thick wall products is 100-120 degrees. Low mold temperature will make the products hard to fill out or with high shrinkage, and other defects, like flow marks, hair spots, dark fringes, etc. It is worse that the higher residual stress will crack the mold as soon as it is released. However, high mold temperature (> 120 ℃) slows down the cooling and extends the molding cycle with poor surface gloss. What’s more, it is hard to release the mold due to sticking and causes warpage.

Injection pressure：The melting degree of the PC is high and a high injection pressure is required for molding products with long thin-wall processes or complex shapes with many intersections and products that are not sufficient to fill the mold cavity without high-pressure rapid injection. The others can carry out low pressure and speed injection to reduce the residual stress. When adopting rapid high pressure injection, the flow speed of the melt is fast and it goes forward in an irregular form, with eddy current, which may be mixed with air, resulting in melting cracks on the surface and large internal residual stress. However, too low injection pressure is not helpful for plunger and screw bar in overcoming the flow resistance of the melt, and it also will make the melt cooled in the mold cavity shrink because it cannot supply new material, and increase the deformation shrinkage of the product. The typical injection pressure is 98MPa·156MPa, and that for the screw injection machine is 78Mpa – 127Mpa.

The injection speed is 8m/s·10m/s. Too high injection speed is not beneficial for mold filling because it causes the defects such as silver lines, spiral lines, burns, etc. Pressure holding time also affects the products’ properties significantly. Short pressure holding time will cause shrinkage and the generation of the shrinkage cavities and vacuum holes.

Increase the time for pressure holding, especially for products with thick wall, increase its density to eliminate the vacuum bubbles to stabilize the dimension. Longer period will lead to the generation of internal stress and cracking due to the forced mold filling. Meanwhile, it extends the molding cycle, Thus, to reduce the internal stress to obtain good products, low pressure and short holding time will be adopted.

The plasticizing pressure (back pressure) is 10% — 15% of the injection pressure. A big back pressure will extend the pre-molding time and cause the degradation of the PC due to high temperature. Too low pressure is not beneficial to plastic exhaust, and will affect the melt density and temperature of the melts, and the evenness of the gloss.

The following points should be paid attention to during PC injection:

Mold release agent：PC is transparent plastic and it is suggested to use mold release agent in the molding process to avoid affecting the transparency of the products and cause defects like the patterns and cracking. For products that have difficulty in mold releasing, some stearic acid or silicone oil derivatives can be used as mold release agent, but the amount must be strictly controlled.

Mold temperature：The molding temperature for PC is high and it cools fast, but is quite likely to cause surface defects and a generation of the internal stress. The mold temperature should be as high as possible to avoid these issues. For most of the products, the mold temperature should be controlled within 80-120 degrees and that for thin wall product is lower, which is 80-100 degrees, but higher in thick wall products, which is 100-120 degrees. Low mold temperature will make the mold hard to fill and crack as soon as it is released. However, high mold temperature (> 120 ℃) will slow the cooling and extend the molding cycle, as well as a gloomy surface. Besides, the mold is hard to be released due to the sticking, and further leads to warpage.

Injection pressure：The melt viscosity of PC is high and the residual stress of products with thin-wall and long process, complex shape and many intersections, and products that are not enough to fill the mold cavity without high-pressure rapid injection. When adopting high pressure and rapid injection, the flow speed of the melt is fast and moves forward by an irregular modes. The eddy current indicates a mixture of the air and it is likely to generate the crack on the surface and internal stress. While, too low injection pressure will make it hard for the plunger or screw to overcome the flow resistance. Moreover, it will also makes the cooled molten inside the cavity shrinkage due to the insufficient feeding, and increase the deformation and shrinkage rate of products. The typical injection pressure is 98MPa —156MPa, whereas that for the screw injection molding machine 78MPa—127MPa.

The injection speed is 8m/s·10m/s. Too high injection speed is not beneficial for mold filling because it causes the defects such as silver lines, spiral lines, burns, etc. Pressure holding time also affects the products’ properties significantly since short pressure holding time will lead to the generation of the shrinkage cavities and vacuum holes. This also appear in PC + ABS injection molding products. Increase the time for pressure holding, especially for products with thick wall, increase its density to eliminate the vacuum bubbles to stabilize the dimension. Longer period will lead to the generation of internal stress and cracking due to the forced mold filling. Meanwhile, it extends the molding cycle, Thus, to reduce the internal stress to obtain good products, low pressure and short holding time will be adopted.

The plasticizing pressure (back pressure) is 10% — 15% of the injection pressure. A big back pressure will extend the pre-molding time and cause the degradation of the PC due to high temperature. Too low pressure is not beneficial to plastic exhaust, and will affect the melt density and temperature of the melts, and the evenness of the gloss.

 The following points should be paid attention to during PC injection:

Mold release agent：PC is transparent plastic and it is suggested to use mold release agent in the molding process to avoid affecting the transparency of the products and cause defects like the patterns and cracking. For products that have difficulty in mold releasing, some stearic acid or silicone oil derivatives can be used as mold release agent, but the amount must be strictly controlled.

The use of the inserts：Due to the difference of the thermal inflation of the metals and plastics is big and their connection surface will generate a large internal stress, which is especially evident in the PC martial. The products are very like to crack during the use and thus, reduce the number of used inserts will carry out the necessary post-treatment for products can ease this phenomenon. The inserts should not have any sharp corners or loopholes. Preheat the insert during the molding process (generally is over 200℃) to lower the temperature difference between the inserts and melts. Ensure the temperature is at least over 120 degrees when the inserts get contact with the resin molten. For insert products, it had better to select resin of which the grade with larger relative molecular weight.

The use of the recycled material：Due to the great stability of the PC, gates, runners or unqualified products during the processing, they can be reused after the cracking. Indeed, the impact to the property of the products should be considered when using recycled material. The increase of the amount the recycled material or the times of recycled, both will impact the PC quality. To ensure the quality and a stable property of the products, it is required that the times of the recycling should not exceed three, and the use amount should be approximately 20%.

The recycled PC should be dried throughly before the molding processing in addition to the prevention of the pollution.

 Refueling barrel and shutdown:

(1). The cleaning of the barrel.

When starting up, if the material in the barrel is PC, push out the old material at the molding temperature by using the new material (generally 2 times · 3 times of air injection process). If the material in the barrel is PVC, POM and other resins with low molding temperature and slightly poor thermal stability, PC is not used to push the barrel, and the no need to increase the barrel temperature. Instead, the barrel should be cleaned with polyethylene, polystyrene and other resins with good thermal stability at low temperature. After that, increase the barrel temperature to the PC processing temperature, clean the barrel with PC recycled material and empty other resins, then use new materials for processing.

(2). Shutdown

When it comes to temporary shut down or suspend the production, it is required to lower the barrel temperature below 160 degrees to prevent the color of the material from degradation and color changing due to the long shut down time. When the processing is completed, clean the material cylinder with polystyrene, polyethylene and other resins, and then the temperature is reduced before the machine is shut down.

Post treatment of products： Measures that should be taken to eliminate the internal stress of PC products during heat treatment. The mediums of heat treatment include the air, glycerin, liquid paraffin, etc. Among these, the hot air is most commonly used since it is clean and simple. The heat treatment temperature of PC products is required to be controlled within its glass transition temperature shape. Its use requirements and heat treatment medium can range from tens of minutes to tens of hours.

As a matter of fact, it is quite difficult to eliminate the internal stress completely by heating treatment. What can do is to decrease the stress value to the allowed range. Thus, there must be a certain internal stress existed. The situations related to the internal stress can be measured and understood by the polarimeter or polar solvent. Polar solvent method is a commonly used for PC injection molding products in processing plants in different places, like America, China, and Europe. It words by putting the PC products into some solutions and to determine the value of the stress and position of the distribution according to the time and states of the cracking.

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