John Kliene of Francis Ward discusses the latest techniques in rotational moulding including in-mould pressurisation and in-mould cooling.

1. Latest techniques in rotomoulding
As leaders in the field of rotational moulding, Francis Ward keep themselves up to
date regarding process developments. Understanding the latest techniques enables
them to improve their service, and ensure they continue to provide the very best
rotomouldings for their clients.
Recent discussions in this arena include in-mould cooling
and in-mould pressurisation, and here the company’s Technical
Director John Kliene (pictured) reviews both.
“Both of these process technologies have been under
development for a number of years. Dr. Paul Nugent discusses
mould pressurisation to remove bubbles from parts in the second
edition of Crawford et al, Rotational Moulding of Plastics -
Wiley-Blackwell, 1996. Internal air cooling of parts has been an
accepted technique for many years. Recently there has been
interest in internal water cooling of parts.
In-mould cooling
The ability to cool parts evenly from both the outside and inside simultaneously provides parts
that have very balanced, homogenous properties throughout the part wall. This enhances the
product’s performance and provides greater impact strength. There is also less chance of
warpage and distortion if the moulding has homogenous properties.
Using this method can shorten cycle times meaning products may be produced faster. When
moulding thick walled parts there could be a significant benefit achieved by reducing the long
cooling cycles experienced, and reducing or eliminating the tendency to oxidise or burn the
inside of the mouldings. Tougher thick walled parts could be produced using shorter cooling
cycles.
It is important to point out that the rate of cooling is critical and considerable control of the
cooling is needed to avoid parts with reduced performance. Rotational moulding machines
need to be designed to enable in mould cooling as do moulds. These investments need to be
considered against the product performance gains. For most thin to medium walled parts
modern polyethylene grades provide intrinsically good mechanical properties if processed well.

2. We look forward to learning more about internal water cooling, but I am sure the same
control considerations apply as those required when using internal air cooling.
In-mould pressurisation
During the rotomoulding process there is a tendency for air to be trapped between sintered
powder particles. The trapped air appears as small bubbles in the wall of the moulded part.
This bubble formation process has been well researched and it has been proven that during
the heating cycle the bubbles disperse in the molten polymer.
As the heating process progresses the bubbles diminish in size and number. It can be difficult
to remove bubbles in some materials especially those of higher density and low melt index.
Included bubbles in parts can reduce the mechanical properties of the part and affect the
impact strength. Bubble free parts have higher mechanical strength.
Various techniques have been tried to remove bubbles. The most effective is the application of
a small pressure within the mould whilst the polymer is in its melt state. This encourages
rapid dissolving of the bubbles.
If it is felt desirable to remove bubbles using internal mould pressurisation moulds have to be
designed which can withstand the expected pressures. Particular attention is needed with
flange design and clamp arrangements. Rotational moulding machine designs must also
accommodate the plumbing required.
Adopting the technologies
Both of these technologies will ultimately create tougher products in shorter cycle times, but
the question is whether the product requires them. Most products created by traditional
rotomoulding techniques are tough enough for normal operating conditions.
Francis Ward has built in the ability to provide both of these technologies in its new POD
machine. We continually assess whether these and other process technologies provide benefits
to our customers, and where such technologies are thought to be beneficial we will discuss
them with the customer accordingly."
Francis Ward
http://contractmoulding.francisward.com/
+ 44 (0) 1274 707030