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The Troubleshooter: Fundamentals of Injection Molding Press Startups

Follow these simple steps to achieve quick, efficient press startups and reduce scrap and downtime.

Garrett MacKenzie

December 18, 2023

5 Min Read
injection molding plant
yoh4nn/iStock via Getty Images

At a Glance

  • Setup audits, in most cases, will eliminate scrap and downtime after a mold change
  • Screw preparation is the most important part of the initial startup stage
  • When opening the mold, verify core positions are correct prior to separating the mold halves

The key drivers of an injection molding plant’s profits are downtime, efficiency, and scrap. A company’s success relies heavily on quick and efficient startups. Poor startups not only lead to significant amounts of scrap, they also affect machine time availability. Preventing unplanned downtime plays a critical part in profitability.

Startup procedures greatly affect a processor’s ability to achieve successful production events and press restarts. Poor attention to detail quite often can lead to processing failures, downtime, and scrap.

This article will outline potential failures that have a direct relationship with poorly performed press start procedures. It will also provide an approach based on performing a solid setup audit that will help to reduce and, in most cases, eliminate scrap and downtime associated with new starts after a mold change.

Screw audit

Screw preparation is the most important part of the initial startup stage. Some materials, such as nylon, acetyl, and PVC, may require purging the screw with an inert material such as polypropylene, styrene, or a purge compound to prevent degradation. Material degradation can cause screw/tip breakage, even screw/barrel lock up requiring screw barrel replacement. The cost of repairs and downtime associated with these types of service can be astronomical!

It is important to verify that temperatures are correct for the material you are running. You should also check that the proper material is being used. Purge material through the nozzle and verify the material is clean with no bubbles. Also, verify that you are using the correct color for the job.

Hot-runner audit

Hot-runner care is crucial to prevent mold damage. Hot runners that are left on for extended periods of time can perform poorly because of overbaked and/or degraded materials. This can lead to runners and parts breaking at start, causing unneeded mold cleaning and repair.

In the case of clear polycarbonates, long heat exposure can cause contamination issues that result in scrap. High fiberglass content left in the hot runner can lead to plugged drops. Any one of these situations can require extra mold changes and unplanned mold maintenance.

Make sure the temperatures for each zone match the setup sheet. It is also good practice to purge the correct material and color through the mold just before running your first shot. Watch for burns, spitting, and equal distribution through each drop. This will ensure the hot runner is up to temperature and there is no degradation in a drop.

Mold inspection

Any time a mold has been changed, it is time for a thorough mold inspection and maintenance. Slide slots and ejector/guide pins should be inspected for damage/galling/lubrication. The mold should be cleaned thoroughly, verifying that all mold saver residue has been removed. In addition, do not spray the moveable half with mold cleaner! This breaks down ejector lubrication, which causes oil to get on the parts at restart. Spray mold cleaner on a rag prior to cleaning the ejector half.

Verify that clamp speeds are fast but safe for mold open and close and that low-pressure close has been set properly. Verify also that ejector pins are ejecting properly — not bottoming out the ejector plate — and at a reasonable speed that is quick but without the potential of pin push defects. Inspect texture for damage and gas buildup.

Tonnage should be high enough to prevent flashing but not so strong as to prevent vents from performing properly.

Water inspection

Water setup is crucial to mold performance and damage prevention. Verify that water temperature matches the setup requirements, and that setpoints are equal to actuals. It is good practice to feel lines by hand individually to verify turbulent flow is sufficient. In addition, gallons-per-minute (GPM) measurements should be compared to historical data. This will verify that water setup has been repeated. Supply, return, and jumper lines should be consistent from one change to the next to ensure mold temperatures are repeated.

Core review

Core operation is critical to the prevention of mold damage and proper part formation. When opening the mold, verify core positions are correct prior to separating the mold halves. With the mold in the open position, verify that core set is set, core pull is pull, and, if sequencing is to occur, that sequencing has been set up properly. Cores should always be set up to perform by position and limits, never by time. Time setups endanger the mold and can lead to crashes.

Valve gates

Poor valve gate setup or performance can lead to high scrap or mold damage. It is good practice to verify that the valve gate unit is on and sequencing is correct. Valve gates should be fired manually to make sure that they have been hooked up correctly and are firing properly based on sequence setup. Visually inspect the valve gate open and close positions.

Final inspection

Once you have completed all of the audits above, it is time to run your initial shots. All setpoints and actuals should be verified against the setup sheet prior to running your first shot in semi. First shot should always be run in semi-automatic to verify part ejection and removal. Prior to the first shot, set your hold and pack pressures at zero. Your fill only — no hold or pack — shot should be 95 to 98% full. This means the parts should exhibit a small short, or have a sinky appearance.

Press startup is the clear pass or fail test for production success or failure. Auditing the mold change prior to startup is critical to production performance. Poor inspection leads to poor startup results, which cause high scrap, increased down time, and poor efficiency. By taking the time to meticulously review all setup conditions, you will be able to improve your success rates and revenue.

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

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

The Troubleshooter: How to Prevent Mold-Change Failures

About the Author(s)

Garrett MacKenzie

Garrett MacKenzie is the owner/editor of plastic411.com and a consultant/trainer in plastic injection molding. He has provided process-engineering expertise to many top companies, including Glock, Honda, Johnson Controls, and Rubbermaid. MacKenzie also owns Plastic411 Services, which provides maintenance and training support to Yanfeng Automotive Interior Systems, IAC, Flex-N-Gate, and other top automotive suppliers. He was inducted into the Plastics Pioneers Association (PPA) in 2019, where he serves on the Education Committee evaluating applications from college students seeking PPA scholarships. You can reach him via e-mail at [email protected].

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