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The Troubleshooter: A Primer on Plastics Processing Fundamentals

An efficient, profitable plastics processing operation depends on these key factors.

Garrett MacKenzie

April 7, 2024

6 Min Read
injection molding machine
yoh4nn/iStock via Getty Images

At a Glance

  • Maximizing machine availability is a key success factor
  • Any machine downtime saps revenue, but planned downtime for maintenance will pay off in the long run
  • To minimize scrap (and lost profits), apply the 5M method

Maintaining a profitable plastics injection molding business requires an understanding of key metrics related to production capacity. After all, if you do not understand the fundamentals of a successful plastics production process, you won’t know where to begin to improve and streamline your operation. This article not only will identify the key criteria that define the success or failure of a plastics processing operation, it will provide insights into gathering production data and using that information to drive continuous improvement.

Machine time

Machine availability is a key factor in the success or failure of any plastics plant. Key negatives that inhibit or reduce machine time availability are:

  • Scrap;

  • unplanned down time;

  • poor cycle efficiencies;

  • shortage of available manpower to perform the work.

These primary factors determine how lean or ample your machine’s production capabilities are. Poor handling of any of them will reduce the amount of available machine time. Misuse of machine time leads to scheduling less work than the machine is capable of handling.

In addition, these variables fluctuate and are inconsistent, making it difficult to schedule work. This can affect a company’s ability to meet customer deadlines and to follow the just-in-time protocols expected of any lean manufacturing organization.

Machine utilization

Utilization is a measurement of how much total time a machine or cell needs to produce products. Total hours are compared with how many actual hours the machine(s) were in a state of production. For instance, one plastic injection press is capable of running 24 hours a day, 168 hours in a seven-day work week. Ten presses have a total utilization time of 1,680 hours. If the actual utilization of these machines comes to 1,400 hours, machine utilization would be 83%. Factors that might affect utilization include a lack of work (sales), unplanned down time (maintenance, a lack of materials or components), part-to-part mold change times, and poor scheduling.

Machine efficiency

Machine efficiency has a direct relationship to cycle times. As molders, we quote work to potential customers based on how much time it will take us to mold a requested quantity of parts. If a job is quoted to run at 35 seconds per shot and we are running at 50 seconds, our efficiency is 70%. In this scenario, we are losing a potential 30% production and we may even be losing money based on lost machine time. On the other hand, if the work was quoted at 50 seconds and we are able to run quality parts in 35 seconds, we are running at an efficiency of 140%. Factors that may adversely affect the ability to achieve quoted cycle times include:

  • Poorly designed tooling;

  • inadequate or an absence of automation;

  • incorrect setup of robot/end-of-arm tooling;

  • incorrect setup of process;

  • water/cooling performance. 

It is important to note that running fast is not always ideal. Operators need adequate time for part inspection and some materials need to be cooled in the mold to reduce shrinking. Running at or slightly above the quoted cycle is necessary to control profits and keep machine time available for potential new work.

Labor usage

How effectively we maintain and utilize our labor pool is critical to our overall production success. Operators should be kept busy while providing time to effectively perform the duties of their job while inspecting the product they are handling. Overwhelming your work force not only promotes poor morale, it tends to lead to high turnover rates and having to train new people. New employees create a potential for poor performance that could lead to bad parts reaching your customers. For best results, conduct time studies of the molding operation to determine how much time is needed to perform the work and then establish your process to meet that requirement. If labor time is longer than what the quoted cycle demands, automation should be considered to streamline the operation.


There are many ways that scrap adversely affects our ability to effectively optimize our production system. From a plastic injection standpoint, our goal is to run 0 to 1.5% scrap, with 0 scrap being the goal. Any scrap that moves through the inspection system has the potential of reaching your customers. Thus, zero scrap is the best defense against losing the confidence of your customer base. In addition, scrap events can lead to sort and rework, which has a negative effect on labor usage. In addition, by scrapping parts you are either losing material or requiring the parts to be recycled or ground to re-produce parts. Again, that is a negative labor investment. Scrap parts also must be re-produced, which adversely affects available machine time. The best way to troubleshoot a scrap event is to apply the 5M method: Scrap is caused by failures that can be directly attributed to man (operator), mold, material, machine (maintenance), or method (process).

Down time, planned or unplanned

Many of you have heard the saying, “If it ain’t broke, don’t fix it.” While this is true to some extent, there are many times that unplanned maintenance can adversely affect the production system. If press parts must be ordered, you may need to pay expedite fees and leave a press down for long lengths of time, grossly affecting machine time availability. In addition, it creates the need for scheduling changes, extra mold changes, rushed decision-making to address the immediate need, and so forth. Planned maintenance clearly is more desirable and puts less stress on the production system. Taking the time to create an extensive preventive maintenance task list and using historical data to establish the frequency of inspections will greatly reduce unplanned downtime.

By understanding the scope of the production system and how to track data, we are able to make fundamental changes that can dramatically improve your production system. Multiple factors can weaken the production environment and lead to reduced profit or even complete system failure. By analyzing the information outlined above, you can continuously improve your manufacturing processes and expand your profit margins.

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

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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