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Polyurethane Properties - The Basics

An Overview - Rigid Polyurethane

Thermosetting resins are generally in a liquid form at room temperature and harden irreversibly with heat or chemical addition - hence the term reaction injection moulding or RIM. The definition of thermosetting:-

'denoting substances (especially synthetic resins) which set permanently when heated'

During solidification, typically referred to as the curing phase, certain molecules along the polymer chain become chemically activated and bond with their neighbouring molecules. This phenomenon is called cross-linking and is the cornerstone as to why thermosets cannot be heated and turned back into a liquid phase. If the material is heated excessively, it will simply just degrade.

When you compare this to thermoplastics, both amorphous and semi-crystalline can return to their liquid state to be re-formed many times over.

This curing phase and cross-linking is one of the reasons why 'sink', the bain of many a designer, is not really an issue in RIM. Whilst it would be incorrect to state sink never happens with reaction injection moulding, sensible design, DfM input, combined with good mould tool design and operation, means you can benefit from a greater wall section variation and significantly lower your risks of imperfections. In addition, most RIM mouldings are painted so any surface imperfections can be removed during this stage.

There is no ‘better’ material choice when considering thermoplastics or thermosets, it simply depends on your project and what is important to you. These decisions can be complex and will be based on many factors.

Factors such as mechanical properties, aesthetics, costs for both tooling and final parts, ease of manufacture, volumes or batch sizes required, the lifetime of a project, and ease and flexibility of design.

Although not an exhaustive list, below you will find how thermoplastics compare to thermosets.

What is Sink?

Sink refers to the unsightly defects that you see on some plastic mouldings, typically where wall sections are too thick or processing has not been optimised. Ribbing, boss features, large corner radii all increase the risk of potential defects.

Image Credit SPE.org - Plastics Engineering 2016

These thicker areas create unequal cooling ratios which manifest themselves as a sink or unintentional recessed or concave features on the part surface. Typically, these features show up on the visible customer surface which is not great!!

Image Credit SPE.org - Plastics Engineering 2016

There is a tonne of resources on the net that advise on how to avoid sink with injection moulding, however, less so for RIM. I recommend starting with these three very basic principles.

  1. Where possible, keep your wall sections even - just because the process can accommodate variable wall sections, doesn't mean you should go mad. Even wall sections help reduce mass, avoid excessive heat spots and possible weak areas with thin sections, it also helps with tolerance control. Constant wall sections help to maintain a consistent cure rate and when parameters are consistent, sizes are more predictable so batch to batch variance is much lower.
  2. Maintain the same wall section for ribbing as the general wall thickness but remember, RIM mouldings with a 5mm GWT are pretty strong. Less ribbing may be possible and the bonus is that you free up room inside your chassis.
  3. Boss features can be attached to the sidewalls, making them stronger without affecting internal space too much.
  4. Optimised designed for manufacture offers many benefits - this applies to both injection moulding and RIM moulding. If you are not sure ask and ask early in the design phase. Working closely with the manufacturing expert early should save you time, effort, and quite possibly, money.
  • Improved consistency
  • Better tolerance control,
  • Weight optimisation,
  • Lower part and tooling costs, or certainly cost drivers identified
  • Robust design with lower failure rate in service

This image shown above is an excellent, albeit simple part that shows how to design well for RIM moulding. Even wall sections, rib thickness corresponding with the general wall thickness, and boss features merged with the side wall.  You will also notice the small radii around all internal features which help the material flow and prevent sharp corners in the tooling or part that could act as stress raisers.

If you would like to chat in more detail about RIM moulding and how it may support your next project, get in touch 

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Polyurethane Properties - The Basics