Causes of Surface Defects in Injection molding Products

Waves or grooves are common defects in Router Injection Molding Products. Generally, the surface defects caused by the pause of the flow peak due to insufficient injection pressure or reduced injection speed are the result of stress induction of the product. Different surface defect forms have different causes. Exploring these causes and avoiding them is the only way to obtain high-quality products.
For injection molded products, surface defects are common quality problems. Generally, visible surface defects include cracks, silver streaks, grooves, ripples, ripple marks and embrittlement. These defects not only affect the appearance of the product, but more importantly, they also indicate that the molding process of the product has failed. Usually, these surface defects are caused by the internal and external stresses of the product exceeding the strength of the product itself. This stress-induced defect is related to the production environment, processing technology and polymer materials themselves, and sometimes involves the design of the mold or product. Therefore, a close look at the appearance of the product defects can help us find a solution to the problem. Common surface defects have their own characteristics. For example, grooves (or ripples, ripple marks) usually appear at the leading edge of the fluid. When the flow front pauses, pressure accumulates, then flows forward a short distance and then pauses again, it will form a groove. This defect is related to insufficient flow front pressure or slow injection speed. Embrittlement is caused by overfilling or underfilling. In addition, contamination or degradation of the polymer, or contact with environmental stress cracking media, can also cause embrittlement problems. Cracking can occur in parts of the product or on the entire part. Silver streaks are whitening caused by fine lines or small cracks, usually confined to a small area. Check the process and polymer. Usually, surface ripples may be caused by one of the following three processing problems, including: pressure or volume problems, position or transfer problems, and in a few cases, temperature problems. Generally, the limitation of the first stage filling pressure or the lack of speed control is the root cause of the ripples. Therefore, it is necessary to carefully check the maximum pressure of the first stage, which should be 200~400psi (14~28kg/CM2) lower than the limit pressure value of the first stage. In addition, if the holding pressure, speed or melt volume of the second stage is reduced, it will also cause ripples. At this time, the holding pressure and speed should be increased as much as possible. Incorrect positioning during the transition from the first injection stage to the second injection stage can also cause visible defects. For example, when the holding pressure of the second stage is reduced by 300psi (21kg/CM2) and converted to plasticizing pressure, or if the holding time of the second stage is reduced to 0 when the machine cannot complete this conversion, the product will be only 95%~99% filled. For thin-walled products, it manifests as a slight underfilling near the gate.
For cracking defects, especially cracking on thin-walled products, it may be caused by too fast injection speed. For this reason, it is necessary to try to change the injection speed or move the gate position. Obviously, in the transition from the first injection stage to the second injection stage, improper filling will cause visible defects. To compensate for this, the key is to improve the responsiveness of the hydraulic conversion. During the conversion, the pressure should be increased to the conversion point and then quickly dropped to the set pressure value of the second stage. If the pressure drops below the set point of the second stage, the flow front may stop and the viscosity will increase. When this happens, it means that the equipment needs to be repaired. Too low melt temperature or mold temperature is another source of defects. The temperature of the melt can be checked using thermal probe technology or suitable infrared sensors to ensure that the melt temperature is within the range recommended by the material supplier.

For cracking, silver streaks or embrittlement problems, you should look for stress causes related to processing, such as injection too fast or too slow. Injection too fast will lead to excessive molecular orientation, which is especially true for thin-walled products. Therefore, consider the rationality of the gate distribution to provide the appropriate molecular orientation and weld line distribution. You can try to inject the product quickly and slowly to observe the orientation results. If the product cracks or silver streaks just after demolding, it is best to check it before the product is ejected, and then completely slow down the ejection speed to see if the problem continues. If the problem is in the ejection, you should see if the mold demolding chamfer is reasonable. Typically, improper polishing in the ejection direction, too high an ejection speed, and an insufficient ejection area can cause this type of problem. Overfilling or underfilling can cause embrittlement of the part. This is because both situations can lead to excessive stress in the part, especially near the gate. Typically, overfilling at the gate causes the polymer chains to be compressed too tightly. At room temperature, the molecular chains of an overfilled part still have some freedom to move, but at low temperatures, the part shrinks and squeezes the molecular chains too tightly, causing cracks. Typically, the overpacked molecular chains will produce residual compressive stress, making the part brittle. In addition, when the mold is underfilled near the gate, it causes the polymer molecular chains to be too loose when cooling, resulting in tensile stress, which weakens the strength near the gate. To check whether the mold is overfilled or underfilled, a gate seal analysis can be performed to determine how long it takes for the part to cool or the gate to close, and to test whether the performance of the part with and without the gate sealed (determined by the application needs) is different. In addition, thermal cycle testing is very important to avoid warpage defects in products, because warpage defects are caused by the process of products changing from hot to cold and then hot again. Because molecules will try to eliminate stress under force, thermal cycles will tell you whether the molecules are in a state of stress or relaxation. Design defects Sometimes, cracking at the weld line may be caused by improper gate location. Usually, the appropriate gate location is to make the weld line in the lowest stress area. If possible, the gate should be designed at a certain distance from the intersection of the flow front, which can improve the strength of the weld line. In addition, local defects may also be related to the design of the mold or the product, such as sharp corners. Sharp corners will cause stress concentration, which is like a cut, generating stress and then spreading around, and the corner radius can spread the load out. Because some resins are very sensitive to the notch effect, for example, polycarbonate is more sensitive to the notch effect than ABS, so many products will choose to use PC/ABS blends. Degradation problems When cracking or embrittlement occurs throughout the product, it may be caused by certain processing conditions during the processing of the polymer. The most likely possibility is that the processing temperature is too high or hydrolysis occurs, causing the molecular chain to degrade. Generally, degradation shortens the molecular chain and improves the melt fluidity, but the material properties are significantly reduced. Using scientific molding theory and viscosity control methods, processors need to check the melt pressure when switching from the first injection stage to the second injection stage to see if it is lower than usual. Generally, too low melt viscosity may be a signal that degradation has occurred. If you want to know whether the degradation problem is caused by temperature, you can use a thermal probe or IR sensor to check the melt temperature and adjust the temperature if necessary. In addition, the heating condition and duty cycle of the entire barrel should be checked to see if the PID loop of the controller is normal? Does the heater need to be powered periodically? Does the heater need to be turned on or off continuously? At the same time, the residence time of the resin in the barrel is also very important. Generally, if the resin stays at high temperature for too long, it will also cause degradation problems. When the barrel and screw are damaged, it is easy to cause the resin to stay longer. Therefore, always check the condition of the barrel and screw, as well as the retaining ring or check valve to see if they are broken or notched. If degradation is caused by hydrolysis, check whether the polymer is hydrolysis-resistant and what is the minimum level of water that reacts with water in the barrel. Generally, water can cut long molecular chains into short chains (polyesters, polycarbonates, acetals, nylons and TPU are all susceptible to hydrolysis, but polystyrene, polyolefins and acrylates are not).
To avoid this kind of problem, always check whether the dryer is running well and whether the dry resin reabsorbs water before adding it to the injection machine. Recycling and coloring If the recycled material is degraded or contaminated, the product may crack or become brittle. Therefore, it is necessary to check the amount and quality of the recycled material and compare it with the product made of 100% raw material. Usually, the above problems occur due to poor local coloring or foreign matter, or the mismatch between the recycled material and the raw material. In addition, the melt index (MFR) of the polymer needs to be determined.
For this purpose, contact the pellet supplier to see if the polymer MFR matches the MFR provided by the supplier. When fillers (such as glass fiber) are added to the resin, there will be a big difference in MFR before and after processing because the screw will break the glass fiber. If the type or amount of colorant is used incorrectly, it will also cause cracking problems. Therefore, it is also necessary to detect the dilution ratio of the masterbatch and the type of masterbatch carrier resin. In addition, local cracking or overall cracking may be caused by solvents, surfactants or chemical additives. For this, the cleaning and handling procedures of the mold or product should be checked to find possible influencing factors, such as soaps, oils or surfactants.