The Troubleshooter: Essentials of the Injection Molding Process

Machine and process troubleshooting are the most important factors in controlling process and reducing defects. Poorly performing presses lead to process variance and uncontrollable scrap events, and underperforming machine components can result in validated process failures. This article outlines key factors that can lead to unplanned down time and process inconsistencies. 

Defining a validated process

Process validation entails two stages.

The first stage is developing a process that produces parts at efficiencies that meet the quoted cycle while producing 0 to 1.5% scrap — zero is always the true goal. Once this has been accomplished, the press must run eight hours with no adjustments made to maintain consistency. Then, all process setpoints are recorded and basic process monitoring variables are established. Mold setup variables also are recorded to ensure repeatability.

The second stage of process validation requires setting the mold exactly the same as in the initial setup. Input of press setpoints is identical to the initial press settings. True validation requires that second-stage startup produce good parts in three to five shots with minimal process adjustments. Once this is achieved, process monitoring data is recorded and process control limits are established. All key monitoring data is recorded for the purpose of having complex troubleshooting data for comparison.

With process validation defined, let’s look at some common defects that can come up when a validated process fails. Keep in mind that the first steps to take when a validated process is not working is mold cleaning and inspection followed by process verification.

Flash

A number of factors can lead to flash. The first step is to verify the process and inspect the mold for buildup. Drop hold and pack to zero to verify that fill-only is at 95 to 98%. Verify you are using the correct material and that there hasn’t been a lot change. Verify temperature actuals are the same as setpoint. Check to see if water is on, and water GPM is correct. Is tonnage correct, and has die height been set correctly? Are peak pressure and fill time within the process monitoring window? Are the platens parallel on all four corners? Inspect the mold, heat, and machine condition to find a solution.

Splay 

Is the dryer working properly? Is the dryer supply hose hot and the return hose warm? Have you checked the moisture content of the material? Is the material correct or has the lot changed? Are mold and barrel heater setpoints correct? Is there a water leak on top of the mold? Has fill time changed and is it consistent? Is your screw recovery time staying consistent? Remember to look at all moisture- and heat-related variables when troubleshooting this defect.

End-of-fill burns

Burns at end of fill generally are directly related to injection speed or venting. Is your tonnage set too high? Is the mold vented at end of fill? Are the vents worn out? Is your fill time too fast when compared to process? Is your melt temperature correct? Is your injection transfer into hold pressure too low? Have you checked the material moisture content? Is your mold clean?

Shorts and unfill

Shorts and unfill are usually related to unpacked parts or poor heating. Do your hot runner temperatures equal your setpoints? Are your barrel temperatures correct? With pack and hold to zero, does your fill-only shot equal 95 to 98%? Is your fill time correct? Is your press consistently maintaining proper pressures and speed? Is your mold dirty, causing gas trap shorts? Is your shot size correct?  Review all variables related to part weight, temperature, and pressure.

A plethora of causes can lead to the defects outlined in this article. First steps are always to verify the mold is clean and the process and equipment have been set up correctly. Troubleshooting beyond that has a direct relationship to the 5 M’s — man, mold, machine, material, and method (process). Correctly reviewing the data and processes simplifies identifying the cause of common defects.

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

How to Fix Injection Molding Inconsistencies

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