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Calculating runners is a big deal. Use our new Runner Calculator tool to get it right!

By Team PPI on Jan 27, 2020 4:06:21 PM

With the launch of our new website, Pleasant Precision is proud to introduce a FREE Runner Calculator tool! Find it on the Resources drop-down at the top of every page. Click here to check it out now!

Most of us know that it is important to calculate runners properly when designing molds.

Get it right, and you have:

  • Higher quality, more consistent parts
  • Reduced part costs
  • More accurate mold quotes

Get it wrong, and you could see:

  • Higher (sometimes excessive) part cost
  • Reduced material properties & parts with cosmetic defects
  • Narrower processing windows


The job of the feed system is to transport molten plastic from the machine nozzle to the mold cavity. The runners in the mold are the flow channels between the injection unit or nozzle and the injection gate. In a properly calculated runner, the molten plastic can increase in temperature to improve flow into the mold, while not requiring excessive pressures to fill it. When the runner system is optimized, a balanced fill pattern can be achieved for all cavities in the mold. This is important because it is critical that all the cavities are filled at the same time. Using a balanced runner layout, where the flow distance is the same to each cavity, helps to ensure even filling for part consistency.

Make sure your runner system is properly sized - it’s critical not only for reducing part cost, but also for manufacturing high quality parts.


There are some rules of thumb that are good to remember when sizing runners for a mold. Gather the information below before beginning calculations.

  1. What is the wall thickness?
    Look at the part’s thickest section. In general, the runner diameter should be 1.5 times the thickness of that section.
  2. What type of material is being run?
    Amorphous materials are more sensitive to shear than crystalline materials. To avoid issues when using amorphous materials, use diameters 30-50% greater than with crystalline materials.
  3. How many cavities in the mold?
    Start with the smallest diameter at the runner feeding the gate and finish with the sprue O-diameter, using runner size increments of .025 inch.

To easily calculate your runner volume using our free tool, choose the style of runner, enter your runner size along with the length of the runner. You will need to enter the data for each branch of the runner between the sprue and the gate. After entering the data, the calculator will display the total runner volume.

Our easy-to-use Runner Calculator includes all of these tools:

  • Runner Volume Calculator (full round or trapezoid)
  • Sprue Calculator
  • Total Runner Calculator

Make your runner calculations easier and more accurate

Access our FREE TOOL here.



Topics: processes

Author: Team PPI

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