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

5. The internal stress of transparent products

In the production of PC transparent products such as sunglasses, windshields, eye masks, etc., product deformation, astigmatism, poor transparency and cracking are often found, which are mainly caused by internal stress in the product. In fact, there are internal stresses inside opaque products, but they are not obvious. Internal stress refers to the stress generated inside the plastic due to improper molding and temperature changes in the absence of external force. The essence is that plastic molecules with high elastic deformation are frozen in the product. The internal stress of plastic products can affect the mechanical properties and use performance of the products, such as warping, deformation and even small cracks, optical properties of the products being deteriorated and the products becoming turbid. The internal stress will also cause the injection molded product to show higher mechanical properties in the flow direction, while the strength being perpendicular to the flow direction is lower, making the product performance uneven and affecting the use of the product. The cracking of the product will be accelerated, especially when the product is heated or in contact with organic solvents.
 
The internal stress of PC products is mainly caused by orientation stress and temperature stress, and sometimes it is also related to improper demolding. After the macromolecules inside the orientation stress injection molded product are oriented, internal stress is easily generated, causing stress concentration. During injection molding, the melt is rapidly cooled, and the viscosity of the melt is higher at lower temperatures. The oriented molecules cannot be fully relaxed. The internal stress generated in this way has an impact on the mechanical properties and dimensional stability of the product. Therefore, the melt temperature has the greatest influence on the orientation stress. When the melt temperature is increased, the melt viscosity decreases, resulting in the shear stress and orientation decreasing. In addition, the degree of relaxation of the orientation stress is greater at high melt temperatures, but when the viscosity decreases, the pressure transmitted by the injection molding machine screw to the cavity increases, which may increase the shear rate and cause the orientation stress to increase. If the holding time is too long, the orientation stress will increase; increasing the injection pressure will also cause an increase in the orientation stress due to the increase in shear stress and shear rate. The thickness of the product also affects the internal stress. The orientation stress decreases with increases of the thickness of the product, because the thick walled product cools slowly; the melt cools and relaxes in the cavity for a long time and the oriented molecules have sufficient time to return to a random state. If the mold temperature is high, the melt will cool slowly, which can reduce the orientation stress.
 
Temperature stress plastics have a great temperature difference between the melt temperature and the mold temperature during injection molding, which makes the melt being close to the wall of the mold cool more quickly, resulting in uneven stress distribution in the product volume. Because of the big specific heat capacity and low thermal conductivity of PC, the surface layer of the product cools much faster than the inner layer. The solidified shell layer formed on the surface of the product will hinder the free shrinkage of the interior when it continues to cool. As a result, the tensile stress will be generated inside the product and compressive stress outside the product. The greater the stress generated by the shrinkage of thermoplastics is, the lower the stress generated by the compaction of the material in the mold becomes, that is, the holding time is short and the holding pressure is low, which can greatly reduce the internal stress. The shape and size of the product also have a great influence on the internal stress. The bigger the superficial area to volume ratio of the product is, the faster the surface cooling and the greater the orientation stress and temperature stress become. The orientation stress is mainly generated in the thin surface layer of the product. Therefore, it can be considered that the orientation stress should increase as the ratio of the superficial surface of the product to its volume increases. If the thickness of the product is uneven or the product has metal inserts, it is easy to produce orientation stress. Therefore, the inserts and gates should be set at the thick wall of the product.
 
Through the above analysis, it can be seen that it is impossible to completely avoid internal stress due to the structural characteristics of plastics and the limitations of injection molding process conditions. Internal stress can only be minimized or make internal stress evenly distributed in the product as much as possible. The methods for reducing internal stress are as the following:
 
The injection temperature has a great influence on the internal stress of the product, so it is necessary to increase the barrel temperature appropriately to ensure good plasticization of the material, make the components uniform to reduce the shrinkage rate and internal stress; increase the mold temperature to make the product cool slowly to relax oriented molecules and to reduce internal stress.
 
Excessive injection pressure can increase the orientation of plastic molecules and generate greater shearing force, so that the plastic molecules can be arranged in an orderly manner, and the orientation stress of the product increases. Therefore, try to use a lower injection pressure; if the holding pressure time is too long, the pressure in the mold will increase due to pressure compensation, and the melt will have a higher extrusion effect. The degree of molecular orientation will increase, which will increase the internal stress of the product. Therefore, the pressure holding time should not be too long.
 
The influence of injection rates on the internal stress of injection molded parts is much smaller than temperature, pressure and other factors. However, it is best to use variable speed injection, namely fast filling. When the cavity is full, switch to low speeds. On the one hand, variable speed injection has a fast filling process and can reduce weld marks. On the other hand, low speed holding pressure can reduce molecular orientation.
   
Design the gate position rationally, and it is best to use slot or fan shaped gates for flat products; the ejector device should be designed with a large area ejection; the ejection angle should be big. Use better materials which have less impurities and big molecular mass as much as possible; the gate materials should not be used.
 
When the product has a metal insert, the insert material needs to be preheated (generally required to reach about 200°C) to avoid internal stress caused by the inconsistency of the linear expansion coefficient of metal and plastic materials. A circular arc is required for the transition.
 
Heat treatment can be used to eliminate internal stress after mold release. The heat treatment temperature is about 120°C, and the time is about two hours. The essence is to make the chain segments and chain links in the plastic molecules have certain mobility, the frozen elastic deformation relaxed and the oriented molecules return to a random state. It is best not to use a release agent, otherwise it is easy to cause internal stress, resulting in opacity, streaks or cracks of the product.