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Troubleshooting tips for plastic injection molding

Many companies apply scientific principles to their setup and validation of processes. After all, this methodology has proven its worth as a successful approach in process development. However, it is important to note that scientific molding (SM) goes beyond setup. The true test of processors is how they evaluate change in validated processes that have already been proven. This article will define criteria that identify root causes for change and provide information and approaches to evaluate and correct changes that occur.

The three steps of process development and validation

The key to successful troubleshooting when changes occur is understanding that the initial validated process serves as the foundation, and variances outside of established control limits are detrimental to established process control parameters.

A validated process is defined as 100%+ efficient based on quoted cycle and minimal (< 1% to zero) scrap that has run a minimum of eight hours at these efficiency/scrap levels. In addition, the goal for a validated process is packing acceptable parts by the third shot at startup.

It is important to note that these requirements may not always be achievable, but it is a processor’s goal to meet these expectations. There are three steps to process development and validation:

  • Process parameters are established using decoupled molding techniques.
  • Once validated, historical data are recorded. This information identifies normal operating conditions and can be used to identify changes within the molding process.
  • Process control limits are established to identify when change has occurred, and steps need to be taken for correcting the change to maintain process consistency.

These three steps are fundamental to processing strategies. Failure to complete the steps fully hampers a molder’s ability to quickly decipher changes that occur, identify root causes for the changes and implement corrections that return to the original molding condition. With these steps clarified, we are now able to consider potential changes and how to use historical data as a tool.

Garrett MacKenzie will host a devoted to process troubleshooting using scientific molding principles on Sept. 11, 2018. Produced by PlasticsToday, the webinar will provide attendees with specific scientific molding variables that can identify changes within an existing process, as well as provide insight into approaches that can be used to bring a validated process back into normal process parameters. The webinar is free, but you must .

Historical data

We must first define the process characteristics that should be recorded as key measurables. It is important to note that recording too much information is impossible. The more processing data we save when our process is in good running condition, the easier it becomes to identify changes and their root causes. The following measurables would be considered the minimum of measurements to be tracked:

  • Fill time
  • Peak pressure
  • Screw rotate time
  • Cushion
  • Cycle time
  • Part weight

At a minimum, these variables are the most common information recorded for comparison when molding conditions change. In most cases, they offer key data that can be used to identify change. Here is a list of other key measurables that can increase the amount of data available, and further define potential changes to the processing schematic when variance falls out of control:

  • Water pressure (GPM/LPM, supply and return)
  • Melt temperature (measured in running state)
  • Mold temperature (water temperature, setpoint vs. actual)
  • Mold temperature (steel temperature, measured at various points while the mold is in a running state)
  • Barrel temperature (actual vs. setpoint, steel temperature recorded between bands with press in running state)
  • Back pressure (setpoint vs. actual)
  • Hot-runner temperature (the data are recorded with hot runner exposed, measuring steel temperature between bands)
  • Moisture analysis (based on manufacturer’s recommendations)
  • Material let-down (measured upon receipt of each new material lot)
  • Dryer (setpoint vs. actual, manual indicator on both supply and return hoses)

» Publication Date: 06/08/2018

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