Analysis of Common Defects in PC Injection Molding and Its Solutions (Part Two)

2. Silver streaks, bubbles and vacuum bubbles occurring on the product
The appearance of silver streaks, bubbles and vacuum bubbles on products is one of the common product defects of PC materials. The causes for these defects are many and complex, so it is more difficult to judge and eliminate them. Silver streaks or gas streaks refer to defects that appear on the surface of the product due to the interference of gas during the filling process concerning melt. The composition of gas mainly include water vapor, air, decomposition gas and solvent gas, among which water vapor, decomposition gas and air are more common. When these gases exceed a certain limit, the cavity loses pressure after injection molding, and the gas close to the surface of the product will emerge and etch along the direction of the material flow, forming a series of large and small sparkling bubbles under the light. It is called a silver streak or gas streak. In fact, the presence of gas in the injection molding process is inevitable, and a considerable part of it still remains inside the plastic. When the pressure in the mold is large enough and the gas content does not exceed a certain limit, the gas is dissolved into the plastic in a dispersed state. However, when the pressure in the mold is not great enough and the gas content exceeds a certain limit, these gases will be released from the molten plastic one after another and reaches the surface of the product to form silver streaks, being cased in the thick wall and forming bubbles. Whether it is the silver streak on the surface of the product or the bubbles in the wall of the product, it may be the result of one of the four gases or the joint action of several gases. These gases are related to raw materials, molds, plasticizing systems, adjustment of process parameters, even changes in weather, especially changes in humidity and other factors, so this question is more complicated. The focus of the problem and the solution should be focused on the gas, that is to say how to control the gas content no matter what.
      
(1) Generally speaking, if bubbles are irregularly dispersed on the surface of the product, it is mostly caused by water vapor.
The hot melt of PC is very sensitive to moisture, and the moisture content is required to be below 0.02%. Therefore, it is necessary to fully dry the material to control the moisture content. Generally, the drying temperature of PC material is about 120℃, and the drying time is about 4 hours. The time should not be too long. If it exceeds 10 hours, the material will easily deteriorate, especially the material with flame retardant added. Speaking of the drying method, the dehumidifying dryer has the best effect and has no effect on the material. To check whether the drying effect is good, use the air injection method to see whether the injected material is continuous, smooth and does not emit white gas.
 
(2) If bubbles are extremely fine and dense and they are mainly distributed around the gate of the product, forming ray or fan shaped patterns, which are mostly caused by air.
The air is from the air which is cased in the material. When there are a lot of gate materials with different particle sizes, the air is easily cased. Therefore, it is best to filter out the powder when using the gate material. If the back pressure is too low during melting and the screw speed is too high, the screw will retreat too fast and the air will be easily pushed to the front of the barrel along with the material. Therefore, it is generally recommended that the melting time should be extended as much as possible during the cooling time, which is helpful to improve the quality of plasticization. If the temperature of the feeding section is not well controlled, a part of the material will melt prematurely and block the passage of air from the feeding port due to too high temperature. If the temperature is too low, the preheating will be insufficient, causing a part of the particle material to enter the homogenization section and be wrapped in air. In addition, excessive loosening will also entrain air. In the case of the above situation, generally adjusting the screw speed, back pressure and loosening amount can generally solve exhaust during mold filling.
 
In order to make the PC material with high melt viscosity fill the mold smoothly, the melt temperature and the injection pressure are generally increased. Under high temperature and high pressure, if the injection rate is fast, the melt will suddenly pass through the narrow runner and gate to the cavity with large free space. The gas released from the melt will entrain the air in the runner and mold cavity, forming a high-speed jet state, and there will be traces of dispersed air appearing on the surface of the condensed plastic, which is called a gas streak. In addition, if there are many corners, great thickness difference, many inserts or improper gate positions in the mold cavity, the melt will flow into the mold cavity, agitating the air in the mold to form a vortex and forming gas streaks on certain parts. For example, this situation often occur on switch socket panels of molded electrical products, because the plug in position, connecting position and switches are concentrated in one place. On the one hand, the solution to this defect is to modify the mold, improve the mold exhaust and optimize the gate position. On the other hand, reduce the mold filling rate, especially the injection rate of the part containing gas streaks.
 
(3) Decomposition gas caused by the PC material molding at high temperatures. Some decomposition is inevitable, but how to avoid a large amount of decomposition and how to remove the gas is worth discussing.
Like the above discoloration, the main cause for decomposition gas is that the melt temperature is too high. For example, the barrel temperature is set too high, or the heating ring of the barrel is out of control. Start with the nozzle and check the heating ring step by step to reduce the barrel temperature. The melt stays in the barrel for too long, when large equipment is used for manufacturing small products. The molding cycle is too long; the excess material in the barrel or the material in the dead corner is decomposed due to long-term heating; or the melt is subjected to strong shear in the barrel. If the compression ratio of the screw is too great; the screw speed is too high and the back pressure is too great, it will also cause decomposition. In addition, too small nozzle aperture, mold gates and runners and great resistance of the cavity can make the passing melt decompose due to partial overheating caused by friction. Therefore, when PC materials are processed, sizes of the nozzle aperture, gates and runners should be big and the size of the exhaust groove should be deep. Also, it is not suitable to manufacture products with thin walls. Another important reason is that the PC itself is of poor quality and is easy to decompose. This is often overlooked by users, and the problem is pushed to the mold and processing equipment, failing to find the correct solution to the problem.
 
(4) Solvent gas is mainly related to quality of the operation in production, such as the machine barrel being not cleaned and the additives being added too much. Most of the solvent gas can be removed by adequate drying, and it has little effect on the gas streaks.
It is sometimes difficult to distinguish between bubbles and vacuum bubbles inside transparent products. It is generally believed that if a bubble is found at the moment of mold opening and there is no change in volume after a period of time, it is a bubble which is caused by gas interference. If it appears and becomes bigger during demolding and cooling, it is a vacuum bubble. The formation of the vacuum bubble is caused by insufficient material or low pressure during mold filling. Under the action of rapid cooling of the mold, the surface of the melt which is in contact with the mold wall solidifies first, and then the melt in the center part cools and shrinks, causing the volume to shrink and form a hollow, that is, a bubble.
 
The solutions to bubbles are as the following: