Ask Dr. Nick: What issues should I expect with mold pressurization?

Question: We have a question about molding with pressure inside the mold.  We have rock & roll machine making medium sized parts.  We have been experiencing significant warpage, especially on flat surfaces and it has been suggested that we should apply air pressure inside the mold, to improve our parts.  Please could you provide some guidance?  Specifically, we are looking for recommendations on hardwiring air pressure during cooling to 1 PSI in a safe way.

ARM Technical Director Dr. Nick Henwood

Dr. Nick: The ability to deliver compressed air to the mold, even during the process (ie when the mold is in motion), is a very useful feature.  Modern machines, of all styles, usually have at least one air supply coming into the machine.  In the case of a rock-and-roll machine, you only require one rotating coupling to enable the air to pass from its fixed supply piping to the mold while the mold is rotating.  For a biaxial machine, you will require two couplings and at least one of these will see high temperatures when the arm is in the oven.

Rotating couplings that are subjected to repeated heat/cool cycles will need A LOT of maintenance and tend to be subject to regular breakdowns.  Specifically, the sealing materials will have a relatively short life in these arduous conditions.

I’m thinking that the coupling on your machine, which I believe is rock-and-roll, will probably be located outside the oven, which will certainly make life easier for you.

For many modern machines, two air passages are supplied to the mold.  The idea is that one can be maintained at near shop pressure, to operate devices such as air amplifiers and drop boxes.  These items usually need a significant pressure (at least 50psi) to function correctly.

The other supply is available for duties inside the mold and must be maintained at a MUCH lower pressure.  Rotational molds are generally not designed to withstand ANY internal pressure and may be severely damaged if excessively pressurized.

A regular mold, without any pressurization but with a vent that’s totally blocked will, in theory, develop approx. 9psi of extra pressure if it is heated from, say, 70ºF (ambient shop temperature) to 400ºF (typical Peak Internal Air Temperature at the end of the cook cycle).  If you want to check my calculations, remind yourself about the Universal Gas Laws (by Boyle and Charles) that they taught you in your High School Physics class!

The fact that most molds that experience blocked vents don’t immediately blow open is an indication that, in reality, most molds can withstand a small extra pressure.  However, BE CAREFUL and don’t push the envelope too far.

As you mentioned in your question, there are two main reasons for attempting to pressurize a mold.

Firstly, a small amount of pressure (usually less than 1.5 psi), applied in the later stages of cooking, will speed up consolidation of the melt and improve defects like surface pinholing.

Be aware that you may be able to achieve a similar effect by a combination of slightly longer cooking and/or a slightly higher oven set point.  If you take this route, make sure that you’re not degrading your material by this extra cooking.

Secondly, pressurizing the mold during cooling will ensure that the plastic stays in contact with the inside wall of the mold for a little longer than if you operate at atmospheric pressure (ie your vent is working properly).  Eventually, the forces created by polymer shrinkage (especially for polyethylene – PE) will delaminate polymer from the mold wall.  However, even a small delay may help you with a difficult warpage problem (eg a large flat wall) in a part.  For this purpose, I’m not aware of any precise recommendations for pressure.  However, it can’t be too much, for all the reasons explained above.

Be aware that you may be able to achieve a similar effect by altering your mold release surface conditions.  If you’re using a semi-permanent mold release agent, consider using a less powerful one on areas of the mold prone to warpage.  Or carefully degrade that area by gentle rubbing with a scotch pad.  If your mold is Teflon coated, you have less immediate options in this regard.

Despite the above caveats, you may decide that mold pressurization is necessary.  So… how can you safely and reliably deliver low pressure to the mold during cooking and cooling?

Given the potentially serious consequences of over-pressurization, you certainly need a system that is well designed, constructed and maintained.

Top priority: look carefully at the pressure regulator supplied on the low pressure line.  If it’s a standard unit (typically with a range up to 120psi), I would suggest that this is not ideal.  Replace it with a regulator that’s designed to operate at MUCH lower pressures.

Consider incorporating an additional safety feature.  In the oil and gas industry, devices such as bursting / rupture discs are employed as a back-up to PRV’s (pressure relief valves).  At some rotomoulders, I’ve seen home-made devices featuring a water column; if the air pressure exceeds a certain value (which can be accurately set by the height of water in the column), air leaks into the column as a stream of bubbles.  Talk to your local compressed air specialist and do some research online.

I went to one rotomolder who was using an air flow from a small centrifugal fan to provide a suitable low pressure supply, rather than using compressed air.  Such a fan can be designed to only deliver a low operating pressure, so you can’t over-pressure your devices.

Check that your low air pressure line is adequately sized.  You may need a bigger diameter for this line than for the high pressure line.  Also, examine the layout and fixing of any piping that sees hot temperatures: when it experiences thermal expansion, will it become over-stressed and bend?  If so, incorporate a simple pipe loop to accommodate this.

Finally, you may have a single rotating coupling that delivers to both high and low pressure air supplies.  Make sure that there is no possibility that the separation between high and low pressure can be compromised, eg by failure of seals.  I’ve seen this become a serious problem in some designs of biaxial machine.

When I was a much younger person, I worked in the oil and gas industry as a Process Engineer.  Our leaders always encouraged us to consider the unlikely, and even the near-impossible, especially if the consequences of failure could be catastrophic.  I suggest that delivering air to the inside of a mold warrants the same approach.

Hopefully, some of this will help to get you going.  ARM runs regular “What’s Your Problem?” sessions, where members meet online and discuss common issues that face them.  I would strongly encourage you to join in and ask questions; we’re a friendly and helpful bunch!

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.

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