Ask Dr. Nick: How is molding nylon different from molding polyethylene?

Q: How do I establish the correct molding conditions for nylon, compared to my usual grade of polyethylene?

ARM Technical Director
Dr. Nick Henwood

Dr. Nick: “Nylon” is a generic term, commonly used to identify a family of polymers known as polyamides.  These polymers exhibit different physical properties to polyethylene, most notable of which are:

  • Increased stiffness, hardness and temperature stability
  • Improved paintability
  • Significantly better barrier properties to hydrocarbons (eg gasoline)
  • Reduced cold impact strength and increased brittleness

There are many different versions of polyamide (PA) offered for sale.  In North America, the main options available as rotomoldable powders are PA6 and PA11.  In Europe, PA12 is more readily available than PA11.  When I sold these products, many years ago, I used to joke that the numbers referred to their price relative to PE; significantly increased cost is something you’ll notice immediately!  In reality, the numbers designate differences in molecular structure.

You will also see polyamides with two trailing numbers (eg polyamide 6,6), but these are not generally suitable for rotomolding.

Over the years, I’ve tried to rotomold a number of non-PE polymers, with success and failure in roughly equal measure.  My common experience is that polyethylene is much more tolerant to process variations than most other polymers.

Setting up your process to mold nylon involves a similar set of principles to polyethylene.  Most importantly, you need to establish the molding window for the product.

The “molding window” or “processing window” has traditionally been defined as the set of process conditions where the material is neither under-cured nor over-cured. 

Polyethylene has the big advantage for rotomolding that this window can be relatively wide, especially when compared to other materials.  In practice, rotomolders can get away with molding at conditions representing a degree of under- to a degree of over-cure.  In my experience many molders do this, even if they are not aware of it!

The simplest way of establishing a set of processing conditions (in terms of oven temperature and cook time, for a conventional machine) is to use a device that can measure Internal Air Temperature.  If you’re new to this, ARM ran a webinar recently on choosing and using these devices. (Login required.)

With a limited amount of trial and error, you can link oven temperature and cook time to the achievement of a Peak Internal Air Temperature (PIAT).  Your material supplier should be able to guide you towards a suitable PIAT for the product they are supplying.  You should also ask them to provide a suitable range of acceptable PIATs, because, in a normal industrial environment, it can be hard to hit an exact figure.

If you don’t have PIAT information, you need to establish a molding window yourself.

For PE, the boundaries of the window can be identified by impact testing, using the ARM Protocol.  Remember that this involves conditioning test plaques at -40º before they are impacted.

Beyond the lower boundary of the window, the failure mode of plaques will be predominantly brittle.  Beyond the upper boundary of the window, the failure mode of plaques will also be predominantly brittle.  For well-formulated roto grades, the failure mode of plaques will be predominantly ductile (with <20% of failures being brittle) within the molding window.  The picture below shows the differences between brittle failure (left) and ductile failure (right).

The transition from predominantly brittle to predominantly ductile will usually be quite marked (see below).  In the graph, “BF%” means the percentage of failures that are brittle.  In under-cooked and over-cooked conditions, BF% normally reaches 100%.  So the boundaries of the molding window are quite sharply defined.

I originally suggested this way of defining molding window in a paper presented to ARM in 1996 and, in the following years, almost all PE suppliers have adopted it.  If you’re interested in further study of this, see some of my more recent published work.

For other polymers, this “PE approach” may not be as helpful.

For example, nylons will not exhibit the same low temperature (ie -40º) impact behavior as polyethylene and failures are likely to all be brittle, whether inside or outside the molding window.  If you substitute -40º conditioning for something less aggressive (eg Room Temperature conditioning), you may find that all failures are ductile.  So, for non-PE polymers, the “PE approach” will need to be modified.

The lower boundary of the PE molding window coincides with the point where entrapped air bubbles (created as the powder particles sinter together) have mostly disappeared.  Below this boundary, the bubbles act as stress concentrators and cause brittleness in the part.

For non-PE polymers, you can define the lower boundary of the molding window in the same way.  Have the entrapped air bubbles disappeared?

In the early days, we cut shavings across the wall thickness of samples and examined them under a microscope.  A much easier way is to track bubble presence and effect using a densitometer, which is a modified chemical scale and measures Part Density (aka “As Is” density).  Phil Dodge, one of our industry’s original giants, introduced this concept in 2001.  Densitometers are relatively low-cost pieces of equipment and can be used as a handy quality check.

In the graph below, you can see how the Part Density registers as significantly lower than the expected Material Density (in this case 0.935 g/cm3), because of the buoyancy effect of entrapped air bubbles.

Having established the lower boundary of the processing window, what about the upper boundary?

The upper boundary of the molding window is associated with the tendency of the material being molded to degrade.  Polymer on the inside surface of the part degrades and forms a thin layer that will act as a crack propagator during impact. 

You can see evidence of this degradation simply by comparing the outside (the “mold side”) surface of the part to the inside (“air side”) surface; the airside will normally start to yellow and will attain a glossy appearance.  See below for a comparison of appearance of the mold side (left) and the airside (right).  You will also notice an acrid smell.

The attached paper describes some work I did with polymer scientists at Manchester Metropolitan University in the UK, that provided new insights into this effect.  Using infra-red spectroscopy, we were able to link loss of impact with the sudden formation of degradation products such as aldehyde species (that’s mainly what you smell).

With PE, the degradation is thermo-oxidative; caused by a combination of heat and oxygen from air.  In the case of heavily unsaturated polymers (e.g. the polybutadiene component of ABS), the material will degrade, even in an inert atmosphere (ie with no oxygen present).

Polyamides are hygroscopic polymers; nylon 6 is especially hygroscopic, but PA11 and PA12 also suffer from this property.  This means that atmospheric water will chemically bind on to the polymer surface and this will catalyze (ie speed up) degradation.

So, for a non-PE polymer, your molding window is the gap between:

  • the point at which bubbles disappear
  • the point at which the material starts to degrade

If you’re lucky, the gap will be wide enough to work with.  Unfortunately, with some polymers (eg ABS), this gap is pretty much non-existent!

My bottom line is as follows:

Defining the molding window for PE is relatively easy.  Other polymers will need to have their own definitions developed; preferably something that makes practical sense.

Happy rotomolding!

Dr. Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30 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.

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