Ask Dr. Nick: Are There Guidelines for Vent Sizes?

During a recent “What’s Your Problem?” session, we were asked whether any guidelines existed relating to the sizing vents for rotomolding tools.

This subject is covered in the ARM webinar series on “Operator Training.” I have reproduced the most relevant slides below:

The first slide shows a “rule of thumb” that has been cited by several sources:

0.01in² of ID / ft³ mold volume

If you have an approximate number for the volume of your mold, you can use this to calculate the cross-sectional area required to be provided by your vent, or vents.

If you have a CAD file for the mold, the volume should be available directly.  Otherwise you can calculate an estimated volume by splitting the mold up into a series of standard shapes like rectangles, triangles and trapezoids.  Or you could fill a molded part with water and weigh it; the density of water is approx. 62 lb/ft³.

The second slide shows the kind of results you will obtain from this type of calculation.  For large molds, it’s generally safer to provide several vents, if you can.  That way, if one blocks, you still have some venting available.

It’s important to stress that I’m not aware of any research that has gone into proving this rule, so apply it with discretion and use your own experience to complement the calculations.  Actually, it’s not even clear whether the rule applies to vents that are open or vents that have packing.

Using these calculations will give you vent size recommendations that are larger than what I’ve generally observed in the field.  In light of the frequent incidence of vent-related problems that I see (especially blow-holes in parting lines), I suspect that many vents are significantly under-sized.  When sizing vents, I would advise to err on the generous side, if the layout permits.

Another important point is that, ultimately, the rotomolder is responsible for vent sizing.  Your moldmaker will, understandably, be reluctant to size the vents for you.

I would be very interested to get feedback from any of my readers about whether you consider these recommendations to be realistic and practically useful.  I hope that this is, at the very least, a useful starting point.

Happy rotomolding!

Dr. Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30+ years experience in rotomolding, specializing in the fields of materials development and process control. In 2022 he was inducted in the Rotational Molding Hall of Fame. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.