What is the thermal degradation risk to thick parts > 20mm Questions and Answers from ARMA Gold Coast Conference 2016

1. Rotomould Workshop
Problem 2
Thermal degradation risk to thick parts - > 20mm
Materials, by Ian Hansen, Rotoconsult

2. 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.

3. 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.

4. 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
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5. 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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6. 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.

7. 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.

8. 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

9. 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.

10. Ian Hansen Consult