Ask Dr. Nick: How Does Wall Thickness Affect Impact Strength and MFE Results?

Question: We’re molding briefcases at 3 mm and 6 mm thickness using the same resin and oven temperature (600°F), testing impact per ARM -40 using the macro. With the same PIAT, the thicker part—tested with a 20 lb dart—has 3 to 3.5 times the MFE (Mean Failure Energy) of the thinner part, tested with a 10 lb dart.

We’ve observed this pattern consistently and are wondering:

1. Is there a way to calculate MFE at other thicknesses for a given resin and temperature?
   
2. In cases where our molds produce top walls 10–13% thicker than the bottoms, would you expect the MFE to scale proportionally with thickness, even though the bottom may experience slightly more heat?

Dr. Nick: I think that there may be two competing mechanisms happening, one at a macro level (ie significant differences between samples) and another at a micro level (ie small thickness differences).

Macro Level

No question that a 6mm part will have a significantly higher MFE than a 3mm part, all other factors being equal.

I once heard a supplier say that the relationship between MFE and sample thickness was roughly linear.  I would question this, from my own experience, certainly at the macro level.  I suspect more of a power relationship:

MFE  =  K (thickness)^α

Based on your current experience (MFE @ 6mm is 3 – 3.5x MFE @ 3mm), the term α would appear to be in the range 1.5 – 1.8.  We’d need some actual MFE data to calculate the term K.

Some other aspects to bear in mind:

1. It’s not strictly valid to compare MFE values obtained with different dart weights.  For a scientifically valid experiment, you should measure all MFE’s with the same dart (would have to be the 20# dart).
2. I have found that MFE values measured with a heavier dart come out slightly higher than MFE values measured on identical samples measured with a lighter dart.  Different weight darts providing the same MFE will strike the sample at different terminal velocities.
3. The incidence of Brittle Failure increases with thicker samples, so the predominant failure mechanism (brittle vs ductile) may change.  This will affect the MFE value.

Micro Level

In my latest internal test protocol, I label individual sample plaques cut from my lab parts with a position identifier.  Despite all my current best efforts, I know that some positions are still routinely thinner or thicker than the mean thickness.  Overall, my thickness variation is within the ±10% suggested (but actually not proscribed) in the ARM Procedure.  So, for example, I know that a sample plaque marked with a “BB” position identifier may actually be 20% thicker than one marked “FF”; this is an artifact of my current protocol which I am still trying to improve.  You will be experiencing something similar, between top and bottom surfaces of your “briefcase” parts.

Anyway, I’ve noticed that the “FF” sample plaques (relatively thin) can often survive a higher drop height increment than the “BB” sample plaques (relatively thick).  This is because the thinner sample plaques can deflect on impact, which absorbs some of the impact energy.  However, this only happens if the material failure mechanism is predominantly ductile.  If the failure mode is brittle, thinner sample plaques fail easier than thicker ones.

Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.