The Troubleshooter: How to Fix Injection Molding InconsistenciesThe Troubleshooter: How to Fix Injection Molding Inconsistencies

The key to being a good plastics processor requires more than setting up validated injection molding processes — you also need to be able analyze those processes when they stop working. Process validation ensures consistent efficiency with minimal to zero scrap. The first, and most important, question to ask when processes fail is: What changed? Changing process setpoints should always be the last resort. This article will define common reasons for molding inconsistencies and recommend how to return a process back to a state of validation.

Setup

The first troubleshooting step is to evaluate whether you are working with a fresh start or a restart. Has the press been running for days and suddenly shifted into a “fail” state? The most common cause of failure in a fresh start is poor setup. Has the mold been cleaned? Is there damage? Has the die height been set correctly? Are thermolators, hot runners, and valve gate units set up properly and performing as intended? Has the correct nozzle tip been installed? Review the entire setup thoroughly to ensure process repeatability. Even robotics and automation setups can lead to defects.

Process setpoints

If setup is confirmed as correct, the next step is to review process setpoints. Are clamp speeds, injection speeds, hold pressure, and back pressure setpoints correct? Are the shot size and cut-off setpoints correct? What about barrel and mold temperature setpoints? Do all actuals read the same as setpoints? With hold and pack and pack set at zero, does your fill-only shot equal 95 to 98% full? 

Historical data

Historical data is one of the most effective weapons in a troubleshooting arsenal. Review fill time, peak pressure, screw rotate time, and cushion. Verify melt temperature and mold and barrel steel temps as they relate to data recorded during successful runs. Fill time can point toward viscosity shifts. Screw rotate time can identify poor loading conditions. Inconsistent cushions can pinpoint screw/check ring failures.

Mold watering

It is important to remember that changes in a mold watering setup can lead to significant fluctuations in tooling hot spots and performance. Review your watering schematic to ensure that the tool has been watered properly. Remember to compare new historical data with successful runs. Variations in gallons per minute (GPM) or water pressure to and from your mold might point to dead-headed circuits or scaling in the water circuits.

Human error

Changes that occur in the middle of a run can often be caused by human interaction or mistakes. Is the machine operator doing something causing the defect? Is the material still the same, or has there been a material or lot change? Have changes been made to process or procedures that are leading to a negative result? Review all potential scenarios where human error might be leading to an unsuccessful molding condition.

Material handling

Poor material handling processes can often lead to defects. Have the proper materials been loaded? Is contamination caused by poor cleaning of hoppers or loaders? Are color additives being dispensed at the proper ratio? Correct material should be verified prior to startup. Color swirls on a fresh start can be caused by poor purging and should be quickly addressed. It is important to note that the first and last three gaylords of any particular lot should always be considered suspect. Changing to a different material lot can sometimes correct poor molding conditions.

Machine and auxiliaries

Middle-of-run changes often are the result of changes in the machine or auxiliary equipment. Compare actuals to setpoints on the injection molding machine, thermolators, hot runners, and valve gate units. Don’t forget to review historical data to identify equipment changes, as well.

Follow the rules

Remember: Once a process has been validated, changes to that process should not be necessary. The first step in troubleshooting is to always review mold and process setup and historical data changes. By following the basic rules of process evaluation, changes in the molding process can be quickly identified and corrected. This will lead to improved consistency and greater profits.

Got a problem with this, that, or the other thing? You might find answers in some previous "Troubleshooter" columns:

How to Become a World-Class Injection Molder

Standardization Is Key to Consistent Injection Molding Processes

Separating Fact from Fiction in Color Molding

Developing Robust Maintenance Procedures in Plastic Injection Molding Operations

Setting Up Your Injection Molding Process for Success

A Primer on Plastics Processing Fundamentals

How to Interpret Plastic Injection Molding Data

How to Identify and Correct Shear-Related Splay Defects

The Role of Moisture in Injection Molding Splay Defects

Preventing Flash in Injection Molded Parts

How to Prevent Common Failure Modes in Injection Molding

Building a Validated Plastic Injection Molding Process

Fixing Color Defects in Injection Molded Parts

Key Steps for a Stable Injection Molding Process

Fundamentals of an Injection Molding Plant or Cell Startup

What to Consider When Buying an Injection Molding Machine

How to Prevent Mold-Change Failures

Fundamentals of Injection Molding Press Startups

Preventing Speed- and Time-Related Defects in Injection Molded Parts