A scientific troubleshooter is not someone who pushes buttons faster or has more tricks up his or her sleeve. Rather, a scientific troubleshooter knows the history of a properly documented process, makes each adjustment based on knowledge, and verifies and documents the result of each change.
Good troubleshooting does not begin when a non-conformance occurs. It starts prior to first-piece-approval. If you are more of a traditional troubleshooter—one who has little use for process documentation, makes adjustments based on learned behavior, and does not verify the effectiveness of each change—you might want to adjust your approach. Here are seven steps to get you going:
STEP 1—SCIENTIFIC MOLDING
Merriam-Webster defines science as “the state of knowing.” The purpose of establishing a scientific process is to create the most stable and reliable process possible, based on good fundamental knowledge of the mold, machine, material, and process.
You are likely familiar by now with terms such as “Decoupled Molding,” “Intelligent Molding,” or “Scientific Molding.” To demystify these terms: Each methodology applies to a process that separates first-stage filling from second-stage packing by using a short-shot during injection. In each case, the intent is to build a strong, reliable process that best compensates for the inherent variability of plastics and thus reduces the overall need for troubleshooting. Although there are many factors that contribute to a good process, these attributes are most common:
Filling: First-stage fill uses velocity control to inject with as few velocity steps as possible. There must be adequate maximum pressure to avoid a pressure-limited process. First-to-second stage transfer should take place using screw position. The part should have a visible short shot at this time.
Packing: Second-stage packing pressure should be high enough to finish filling the mold cavity and compensate for material shrinkage during cooling. This pressure is traditionally 50% to 75% of the pressure used during the first stage, though it can drop as low as 25% for some thick parts and may exceed 100% for many thin-walled parts. Second-stage packing time should be determined by graphing part weight versus second-stage time. The optimal second stage time is the time at which the part weight does not increase with an increase in second-stage time.
Recovery: Screw delay or decompression must be used prior to screw recovery to relieve the packing pressure. Screw recovery should take approximately 80% of the cooling time. Screw decompression must be used after recovery whenever a check ring is used. For most screws, the optimal amount of decompression is equal to the check-ring travel.
