The Troubleshooter: Standardization Is Key to Consistent Injection Molding Processes

Many companies struggle at replicating injection molding processes in between runs. Scrap events at startup can kill profits before a production run even starts. It is easy to point fingers in an attempt to escape blame, but in many cases it is the equipment itself that leads to process inconsistency. This article will outline many of the primary causes of poor startup events. It will also offer solutions to common failures that lead to large scrap amounts.

Setup

The most common culprit in poor startups is the machine setup itself. Molds must be watered the same way every time. Process setup must be verified. Barrel and mold temperatures must match prior setups, and actuals should be verified. The same applies to hot-runner temperatures and actuals. Are water gallons-per-minute (GPM) the same as the last run? Has the robot program been loaded correctly, and is the end-of-arm tool in the same state/condition as on the last run, when it performed well?

Machine

The state of your machine is also a key critical parameter in measuring process repeatability. Has clamp parallelism been checked and verified regularly? Has mold protect been set up properly? Even more important, did you review process monitoring variables as you started the press to ensure that variables matched the previous run? Fill time, screw recovery, and peak pressure and cushion are all important inspection points as the new run begins. Melt temperature should be verified to replicate the previous event. Normal preventive maintenance events on your machinery are essential to machine and process consistency.

Thermolator

Temperature control units should be standardized throughout your plant and repeatability thermolators should be married to the press. All thermolators do not perform in the same way, which makes it important to keep each unit stationary with a machine to ensure process repeatability. GPM, pressure to and from, and setpoint vs. actual performance are keys to process inspection.

Hot runner

It is also important to keep the same hot runner in place at each press. Hot runner setpoints and actuals should be within 2°F, with no more than a 2° swing. Record hot drop melt temperatures for comparison. These measurements can be taken after a press has run consistently for one hour to ensure proper heat soak. Examining heater performance is important. Manifold temperatures tend to rise slower than short leg and drop temperatures. Watch for slow rising zones because these can help to identify heater bands that are wearing out prior to failure.

Tooling

Mold care is a primary consideration when defining repeatability. Processors must take great care in shutting down a tool. Molds should be properly greased, cleaned, and inspected prior to startup. Parts should not be run until a mold has been meticulously inspected for cleanliness, function, and damage. Close and open speeds should be set for both cycle optimization and mold protection. Poor inspection leads to scrap and damage, which results in down time and missed production results.

Material

Material performance is crucial to a consistent startup and process. Material in hot runners must be fresh and clean. The press must be proficiently purged and free of cross contamination. If regrind is used, virgin-to-regrind ratios must be consistent and base-material friendly. Verify that materials are at the correct dryness, but not over dry. Consult the safety data sheet to ensure that temperatures and dry time meet the manufacturer’s recommendations.

Profitability in plastics processing demands repeatability and consistency. Understanding the variables that have the biggest effect on a plant’s ability to replicate production runs through consistent shut downs, setups, and startups is the difference between success and failure. Thorough inspections and procedures ensure repeatability and company profits.

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

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