Troubleshooting Injection Molding Defects
Inside Look at Jim Johnson | Entec - Director, Commercial Plastics Technology, Approach to Troubleshooting
Anyone involved in the injection molding process will, at some point, have parts exhibiting some type of molding defect. The defect may be related to part design, mold design, processing parameters, material issues or other causes. Understanding how to troubleshoot the defect to identify the root cause and to take corrective action is critical. In addition, being able to quickly and effectively troubleshoot these problems results in minimum lost time, material, and production.
While there are a number of good troubleshooting guides available that can help to guide you to possible causes and corrections of molding defects, these guides generally only address the typical causes of defects and cannot delve deeper into the less obvious causes. This document will present one approach to the troubleshooting process to help solve these less obvious, more challenging issues.
My Approach to Troubleshooting
Jim Johnson | Director, Commercial Plastics Technology
Whenever I have a part defect or molding issue that I need to resolve, my first step is to literally take-a-step-back and really try to understand exactly what the problem is. Once I have identified the problem, I brainstorm what could be the possible causes of the problem. This includes, but is not limited to:
- Could it be a material issue?
- Could it be a moisture issue?
- Could it be a material handling issue?
- Could it be a process parameter issue?
- Could it be a machine issue?
- Could it be a mold issue?
- Have there been any recent changes to the dryer, machine, mold or other equipment?
- Could it be an operator issue?
- Could it be some unique, less obvious issue, less apparent issue?
Taking a little time up front to try and answer these questions, can not only save some time during the troubleshooting process, but can also help determine what things need to be looked at and in what order you may conduct these steps.
Once these items have been reviewed you can then start the actual troubleshooting process. I first try to address the items that are simple or easy to check or evaluate. For example, I would confirm that the correct material and grade is being used. I would make sure that this is indeed the material being fed into the machine. I would check the dryer settings to ensure the correct drying temperature and time are being used. I would also confirm that the dryer is actually working properly by checking the dew point and, if possible, measure the moisture content of the material to ensure it is within the suggested range.
Next, I might review the process set up to ensure that the barrel temperature settings and mold temperature settings are correct for the material being processed. This would include making a measurement of the actual melt and mold temperatures and not just relying on the machine temperature set points. I would then adjust the temperatures if necessary.
Next, I would look deeper into the other process settings, such as injection speed, cushion, screw speed, screw decompress, switchover position, etc. and adjust as necessary.
For more typical molding defects such as gate blush, knit lines, flash etc. the troubleshooting process is often simpler and more straightforward to resolve. However, some processing problems or part defects may not be as simple to diagnose or solve and often require a lot more effort and creativity and some thinking outside the box. These are the ones where having a robust troubleshooting methodology becomes invaluable.
Below are a few examples of troubleshooting difficult processing or part defects that highlight the need to be more creative in the thought process to ultimately be able to resolve the problem.
A customer was molding a large container and lid out of a polypropylene resin. They were adding a yellow color concentrate and a POE elastomer at the molding machine to improve the drop impact performance of the molded parts. They noticed white streaks appearing on the lid radiating outward from the sprue gate. These white streaks would appear anywhere from a few hours to a few days later. After reviewing all of the background information and processing parameters, it was initially suspected that there was poor mixing of the color concentrate and/or POE elastomer in the screw and barrel causing the streaks to appear.
In order to eliminate the color concentrate as a possible cause, parts were molded with only the natural polypropylene resin and POE elastomer. The white streaks still appeared in the parts, and it was apparent that the color concentrate was not the cause. In addition, the customer suspected that their screw and barrel may have been worn so they were replaced, but this still did not resolve the streaking issue. The customer then tried using an alternate impact modifier that eliminated the white streaks, but the parts did not pass the impact test requirements. While it didn’t make sense that the alternate impact modifier eliminated the white streaks, it did offer us some clues on what could be happening. Because this impact modifier was a harder pellet, we focused our attention on how the color concentrate and impact modifier were being blended and added to the polypropylene. What we discovered is that the soft impact modifier pellets were sticking together, forming clumps of several pellets (the harder impact modifier pellets did not have this problem). These softer pellets were then blended with colorant and polypropylene resin, transferred to the machine hopper and when they were fed into the machine barrel, they stayed clumped together during melting and did not disperse. This resulted in the impact modifier forming the white streaks. By eliminating the typical causes and focusing on the ‘big picture’ of the entire handling, mixing, feeding and processing operation we were able to ultimately solve this somewhat puzzling problem and produce parts with no white streaks and that passed all impact testing requirements.
In this troubleshooting example, the customer was molding a very large part out of PC/ABS and was experiencing severe splay marks on the entire part surface. We had several discussions with the customer regarding possible causes of the splay marks, which included drying, processing temperatures and shear. We reviewed the customer's drying and processing conditions and did not see anything unusual or suspicious. The customer confirmed that they checked their dryer and it was working properly. They also measured their actual melt and mold temperatures, and these too were within acceptable ranges. They had tried varying the injection speed in an effort to vary the shear on the material, but this did not solve the issue. We then mentioned one other possible cause of splay marks: screw decompression (suck-back). While the amount (distance) of suck-back is generally not an issue, we have discovered that how fast the screw is retracted can be more of an issue. A faster suck-back speed can pull in more moisture laden air than with a slower speed. This is even true in low humidity climates like Arizona. Once the customer modified their suck-back parameters the splay defect immediately disappeared. This problem was a little surprising in that splay issues with suck-back do not normally show splay over the entire part surface. Rather, it's normally located nearer the gate or shows as just a few random splay marks. But in this case, the very large part surface was entirely covered by the splay, which initially led us to believe that it was not due to suck-back on the screw.
A customer was injection molding a small cylindrical part about 6 inches long and 3 inches in diameter out of unfilled nylon 6. The part utilized a small gate in the center of the long dimension. The customer was experiencing two defects: burn marks at the end of the fill and excessive splay. The splay marks were the primary defect and the one the customer needed assistance in resolving. The customer assumed that the splay marks were caused by excessive moisture in the nylon and they tried a number of things related to drying the material. They tried drying for a longer time; they tried using a higher drying temperature; they even resorted to utilizing two dryers in series to dry the material even more. None of these things helped to eliminate the splay defects. The only thing that seemed to help was increasing the injection speed, but this caused the burn marks to get significantly worse. Upon further investigation, it was discovered that the normal injection speed that was used, and resulted in excessive splay, was exceptionally slow, about 0.4 inches/second. Because this injection speed was so slow, the material was building up a thicker frozen layer in the part which resulted in excessive shear during the part filling. This caused a shear splay to form. The excessive drying of the material also caused an increase in the material viscosity (poorer flow) which contributed to making the shear splay worse. Increasing the injection speed eliminated the shear effect but resulted in burn marks. The molder was reluctant to increase the venting in the mold to help eliminate burning, so they were forced to balance the injection speed to try and minimize both the shear splay and burn marks and produce acceptable quality parts.
Every molder will be faced with part defects and the need to troubleshoot the molding process. Basic defects are relatively simple to solve, but more complex problems require more thought to be put into the troubleshooting process and may require a more in-depth and thorough review of the entire process from start to finish, including less obvious potential causes. While these types of issues can be frustrating, they are also very rewarding once solved and provide for an excellent learning opportunity.