A closer look at scientific molding theory: Page 3 of 4

A primer on the science of process development, recording, standardization and repeatability
September 28, 2016

The key to any successful molding operation is to record all data that is available when the process is producing minimal scrap and is at optimum efficiency. By replicating these variables at machine start-up, you ensure that you are repeating your previous run. Here are some other factors that can help to determine the success of your operation:

Process variable


Fill time

Fill time is the amount of time taken from the beginning of shooting material to the point of reaching cut-off.

Peak pressure

Peak pressure is the maximum pressure achieved at the point of velocity cut-off prior to dropping off into hold pressure.

Running mold temperature

Mold temperature should be measured at various points in the mold in a running state. Measure individual cavities, runner system, bushing area etc.

Screw rotate time

Screw rotate time is the amount of time it takes for the screw to recover.

Melt temperature

Melt temperature is the actual temperature of material as it exits the nozzle tip. This measurement should be taken while barrel is in running state.

Cycle time

The amount of time taken for each shot to be produced.


Cushion should hold steady between 0.15 and 0.35, depending on part size.

Water pressure (to process)

Gallons per minute measurement taken prior to mold entry.

Water pressure (from process)

Compared with "to process" pressure for calculation of pressure drop.


Barrel temp actuals

Actual running temperatures of barrel zones. Comparisons should be made between barrel temp actual and setpoints to ensure barrel temperature is in control.

Mold open time

Actual time mold is open between shots.

Back pressure (actual)

Actual pressure held during recovery stage (PSI).

Material moisture

Material moisture is a critical control that should be measured regularly to ensure that material has been properly dried.

Regrind percentage

Regrind usage should be controlled to ensure that process variance is minimal and consistent. It is important to maintain consistent regrind usage through proper blending or reextrusion.

It is important to note that there are several steps that are part of establishing a robust process. These studies include:

  • Viscosity curve;
  • cavity balance;
  • pressure drop study;
  • gate seal;
  • cool time.


  • Melt temperature is a key variable that is often overlooked! Once processes are validated, it is imperative that melt temp be recorded. When problems occur, melt temp should be one of the primary checks made prior to process adjustments. It is one of the focuses that can identify problems with your machine, or changes within the molding environment.
  • Measure and record the GPM of your tool's circuitry. By measuring this variable on every circuit, you are able to test your mold in the future when molding conditions show signs of overheating. It can help you to determine if the tool requires descaling. Each circuit should have a unique identification number. This is done by repeating your watering procedure. Keep the same supply and return pattern either by hard plumbing your loops, or by establishing a watering diagram that maps out the watering layout. It is also recommended that your circuits be uniquely identified in or out and that color identification (green, yellow, white etc.) be used. This not only improves your watering time, but will reduce the potential of miswatering.
  • A validated process should not require change. Process parameters should be capable of being repeated each time a mold is set and started. Before changing your process, it is important to look at your monitoring variables first! What changed? For instance, if fill time is slower, look at your temperature actuals. Check your thermolator: Is the actual temperature the same as your setpoint? Yes, there will be times when you will need to change your process to correct a molding condition. First, check for mechanical changes. After confirming that

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