The Troubleshooter: Top 10 Injection Molding Mistakes, and How to Fix Them for Good

Twelve months of plastics production once again ends as 2024 comes to its close. The evolution of our manufacturing system depends on learning from our mistakes and developing approaches to reduce or, preferably, eliminate scrap from production. Scrap affects our bottom line. It is important to note that scrap is simply one of many problems that keep us from achieving molding success. To end the year, we review here the top 10 problems affecting production efficiencies, downtime, and scrap, along with some solutions to avoid those problems in 2025.

1. Scheduling

Poor planning leads to poor performance. This will result in scrap and downtime, which can lead to frequent mold changes, resulting in more startups and scrap. To improve your scheduling approach, line up jobs using the same base material whenever possible. This will result in better startups, less material lost to purging, and reduced downtime associated with barrel and hot runner temperature soak times. For best results with clear materials, marry these jobs to a press that only molds clear. Avoid color changes going from black to light whenever possible by molding mid-range colors first. Make sure all materials, tooling, and components are available prior to scheduling the changeover.

2. Material handling

Improper handling of materials can wreak havoc on your manufacturing system. Take great care to verify the proper material has been loaded for your next job. Track material lots and watch for changes that can occur from one lot to the next. Make sure colorant ratios are correct and blending equipment has been properly calibrated. Use the minimum amount of regrind to ensure process consistency. Make sure loading equipment, dryers, and hoppers have been cleaned properly to prevent contamination.

3. Mold changes

Be vigilant in setting up molds and auxiliary equipment in precisely the same manner every time. Verify water setup and temperatures on the barrel, hot runner, and mold temperature control unit. Verify that process setpoints match the setup sheet. Verify knock out lengths are correct and are tied in properly, when applicable. Document any conditions that cause longer downtime events and notify management to allow them to provide the proper tools, equipment, and materials to correct the issue.

4. Startup

Poor startups lead to excessive downtime and scrap events. To prevent this, a thorough audit of machine setup must be performed to ensure the press is ready to be started. Use the 5M formula — man (labor), mold, machine (robot), material, and method (process) — to verify that all needed equipment is available and set up correctly based on process setup information. Make sure the mold is clean and mold components are lubed and working properly. Verify that die height and mold pressure close have been set up properly. Verify that barrel and hot runner, if needed, are properly purged and ready for start. If material is dried, verify moisture content. On the first shot, remove pack/hold pressure to verify “fill only” shot is 95 to 98% full and at the proper weight. Step through robot at 30% speed to verify proper positioning and alignment. Start the press, and if good parts have not been produced by the fifth shot, stop to look for any problems that might be present.

5. Shutdown

Proper shutdown is just as critical as your setup and startup procedures. Leave the barrel dry, with the screw sucked back 1 inch to allow gas to escape. The material receptacle should be left empty if material is dried and the feed throat also should be left empty. Molds sitting idle for more than two hours must be sprayed with rust preventative, and the mold halves should be nearly touching not under tonnage. Barrel temperatures should be banked to 250°F if the press will be idle for more than an hour. Materials that degrade should be purged out with a suitable purge compound or polypropylene prior to banking barrel temperatures. Hot runner should be shut down one zone at a time, then master off once complete. The water temperature control unit should be powered down. Waters at main should then be turned off. 

6. Engineering

The engineering stage is the most critical step in establishing production efficiencies. Processes must be validated and verified to be robust and repeatable. This ensures successful startup with minimal scrap. Robot/mold and press setups must be standardized and streamlined to prevent human error and reduce downtime events. Cycle-time optimization is best performed during the engineering stage prior to production release to prevent scrap from reaching the customer.

7. Maintenance

Preventing unplanned maintenance events is key to production profits. A world class approach requires the development of a solid preventive maintenance program to inspect, identify, and resolve potential machine failures via planned downtime events. Monitor process monitoring data regularly to help identify machine problems in advance.

8. Quality

The first rule in your quality system should be that quality parts are best achieved by proper molding conditions, not sort events. Rework events are cumbersome, often resulting in poor labor rates, which affect machine time. They also do not guarantee that defective products won’t be overlooked and reach the customer. Quality alerts should be clearly posted, with sample defects tagged and available to the operator to make them aware of potential defects. Quality systems must be consistent from one shift to the next to prevent scrapping parts that meet molding criteria established by the customer. Jobs that consistently produce scrap rates above 1.5% should be analyzed for ways to reduce the scrap rate to zero. Your customer wants you to be successful in providing them with quality parts while making a profit. Make them a strong partner in resolving the problem by identifying the issues and telling them what they can do to help you find a solution.

9. Tooling

Mold design and function is key to production efficiencies and cycle times. Parts production free of scrap events is always the primary goal of manufacturing. The need for modification is best resolved during the engineering stage. When the mold hits production, zero scrap events during production while meeting the quoted cycle is imperative to company success. In addition, caring for the mold through proper cleaning, lubing, and inspection is necessary to prevent unwanted repair costs.

10. Training

Proper training is critical to the overall success of any molding operation. Operation guidelines should be clearly outlined and discussed within every department. It is also important to perform time studies to establish expectations in regard to each skillset. Faster cycle times aren’t better if operators do not have time to inspect each part thoroughly. Fast mold changes often lead to poor setups and potentially poor starts, causing scrap events. Evaluate the tasks and procedures to establish and streamline your guidelines. Then put great effort into monitoring all personnel involved to verify your system is consistent and repeatable.

Plastics companies face many challenges in their daily operations. There can be days when the challenges seem endless and impossible to overcome. In the end, you have to stop looking at the “mountain” ahead of you. Put one foot in front of the other and face each problem one at a time. By implementing some of the steps outlined in this article, you will find yourself much closer to molding profits and success.

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

Essentials of the Injection Molding Process

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