Ask Dr. Nick: Warpage in a polypropylene tank

Question: In a cylindrical tank made of PP powder, we have experienced a problem of warpage (internal and external waves). I wonder if you could give me your technical opinion. The inside part of the mold is welded with an additional metal stripe and in this part of the mold we are facing warpage in the molded part. The warpage area is focused in the middle part of the welded metal stripe. In the warped area, the wall thickness is between 7.5 – 8.5 mm. In order to eliminate the warpage problem, our customer has added externally a metal plaque to prevent overheating. The part is cooled up to 80-85 degC (176 – 185 degF) inside the mold. Then, the part is moved from the mold and is left for cooling in the environmental temperature.

Dr: Nick:  I’ll try to give an opinion on this problem. 

1.       I’ve previously seen warpage problems when molding a particular grade of PP.  The shape I was molding was a simple cylinder. The material supplier told me that PP has “natural lubricity”, by which I understood that something in the polymer migrates to the mold surface and provides what amounts to an internal release agent.  However, I have successfully molded many other grades of PP, without seeing the problem.
2.       Generally, PP shrinks less than PE, so you would expect that warpage problems (which are caused by unequal shrinkage rates in different sections of the molded part) would be less.
3.       Warpage effects tend to occur more often with thick parts; at 7.5 – 8.5 mm, I would consider your part to be pretty thick.
4.       The area of the mold containing the welded metal stripe may result in a different heating condition compared to the rest of the mold surface.  This may result in a lesser or greater wall thickness building up at the stripe. It’s not clear from your description which it is, although the fact that the problem was fixed by reducing the heat to the stripe area (by adding the external metal plaque) indicates that the stripe area was previously heating up more than the rest.  Did you measure the wall thickness of the part in this area, compared to the rest of the part?  In any case, thickness variation around the part is another cause of warpage.
5.       You’ve not mentioned anything about mold release agent (mra); your choice and level of application may be a factor.  If the PP grade you’re using has this natural lubricity (see note 1 above), then reduce the level of mra applied. You can immediately reduce the release properties of an existing surface (ie one which already has mra applied) by gently abrading with a scotch pad or similar non-metallic product.
6.       Slower cooling can reduce warpage; you don’t specify how you cooled or the cooling rate.  In extremis, don’t apply any external cooling and allow the mold to cool naturally in ambient conditions.  Worth trying, just to see if it helps, even if this is not practical in production.

I hope the above list gives you some pointers to the problem.  Whilst the root causes of warpage are similar across production, the way these causes come together to manifest a particular warpage problem can be complicated.

Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 25 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.