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The Troubleshooter: How to Fix Part and Runner Sticking in Injection Molding Processes

The causes of — and solutions to — some sticky situations commonly encountered by plastics processors are outlined.

Garrett MacKenzie

May 21, 2024

4 Min Read
injection molding machine
TongTa/iStock via Getty Images

Parts or runners sticking can cause frustration for any injection molder. Runners sticking can lead to dry shots or cause the press to go down frequently when the robot detects missing parts. Parts sticking can lead to flash when the mold closes with a part or parts still in the mold. Worse yet, the mold may close on parts out of position, which can cause parting line or mold damage. This article will outline potential causes for these sticky situations and suggest solutions to correct the problem.

Sprue sticking in cover. 

If the runner is sticking in the stationary side, check the sprue tip and verify that the nozzle tip is 1/16 in. smaller than the bushing orifice. In some cases you might want to take the tip down two sizes smaller than the bushing. 

Too-short cool time can also lead to the runner sticking. If the runner is still soft, the sprue can separate from the runner causing the runner to separate from the sprue. Running a longer cycle time might fix the issue. A soft runner can also separate if the breakaway speed is too fast. Slow down the mold break speed to see if it resolves the issue.

Also, inspect the shank of the sprue for signs of wear and galling. Polish the bushing — you can even use a Q-Tip. A burr on the bushing lip could also be the culprit. Check the area where the sprue attaches to the runner for signs of damage or wear and have tooling make repairs, if needed.

Polish can be used on cover-side details to improve separation at mold break as long as there is no texture in the area where sticking is occurring. When polishing, be sure the polish is being completed in the direction of part separation.

Sprue break can be used as a last resort. It should be set to retract after charging is complete as long as screw rotation finishes 1.5 to 2 seconds before the mold opens. Keep in mind that when sprue break is used it leads to premature damage of nozzle tips and can cause drooling, as well.

Parts sticking in cover.

This condition is common when working with thin-walled parts or parts with heavy detail on the stationary side of the mold. Breakaway speeds can lead to this condition. Slower open speeds might fix the condition. Too-short cool time can also cause this issue. Run a longer cycle to give the parts more time to shrink to the core on the movable side. Divots or pull marks can be applied with a punch or Dremel on the movable side as long as the core is Class C. Be sure to verify with your customer any tooling modifications before making changes to tooling,

Parts sticking on lifters.

Remove parts by hand to determine where the parts are sticking to lifters. Polish the affected areas to improve separation. Also inspect the lifters for potential damage that might be leading to part hang ups. Whenever polish is used, take care to polish in the direction of part removal. Polishing against the tool grain can actually worsen the problem. Discuss with tooling potential long-term fixes to correct the condition, such as coatings or high-grade polish techniques.

Parts or runners sticking to ejectors.

Do a manual inspection to identify the specific areas of part hang up. Inspect pins for galling or burrs. Polish the pins in the direction of part separation to improve part removal. A radius might also need to be added to improve part removal. Work with tooling to determine the best method of improving part extraction.

Sticking parts or runners can definitely cause a lot of frustration. Frequent restarts not only add to machine time, but also increase scrap and shrink profit margins. By using the tools and methods outlined in this article, sticking conditions can be corrected, improving overall production efficiencies.

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

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

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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