Rotomould WorkshopProblem 2 Thermal degradation risk to thick parts - > 20mm

Materials, by Ian Hansen, Rotoconsult

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What is the risk of moulding large parts with wall thickness > 20mm?In particular the risk of thermal degradation to outer wall and air bubbles towards internal wall.

Thermal degradation - Overcure This is usually the result of oven cycle time being

too long or the oven temperature being too high. Over-cure will also occur if the mould had

insufficient powder charge (underweight). Impact strength is affected significantly, as the

weakened inner layer acts as a crack initiation zone. Flexural (or bending) strength and Tensile

(elongation) strength can appear unaffected because only the thin inner layer is degraded.

What can thermal degradation look like?

Internal surface often is :- Very glossy Sometimes ‘pitting’ Orange discolouration Usually has a ‘burnt’ smell No air bubbles X-section

EXAMPLE OF THERMAL DEGRADATION

OVER CURE

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Moulding thick parts – Thermal Degradation

The moulder question could also be : Thick parts require long oven cycles, can this degrade the polymer? Polymer requires good short term antioxidant,

as well as good long term antioxidant. With good thermal stabilization thick parts

can withstand long oven cycles required to remove air bubbles and cure the material.

60min @ 300C

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M20

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QUESTION: Thick parts require long oven cycles, can this degrade the polymer?

The bubbles disappear from the mould surface first Bubbles disappear from internal wall last.

Polymer material will show thermal degradation during processing from the inner surface first, as this is in contact with oxygen longer.

Thermal degradation is unlikely to occur in the polymer near the mould surface because there is no oxygen.

It is important to be aware that the polymer material you use to make thick parts must be designed for long oven cycles.

Large tank grades like Qenos LL711UV or Chevron Phillips TR942 or Nova RMs245 are formulated with AntiOxident packages that provide high levels of protection for thick parts with long oven cycles

WARNING - many polymers do not have AntiOxident protection for thick parts that require long cycles.

Recommendation Contact your supplier of polymer and ask them to

confirm that their material is suitable for ‘long’ oven cycles.

Technical Data Sheets do not provide specific information on heat protection capability – you need to ask your supplier

Recommendation Be specific and ask your supplier your moulded thick

part Your supplier can test the outer layer of polymer that

has been in contact with the mould surface and they can test the inner layer of polymer.

Your supplier can measure the amount of antioxidant consumed in each layer, and they can test for any thermal degradation of the polymer.

Ian Hansen Consult

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