Latest techniques in rotomoulding

As leaders in the field of rotational moulding, Francis Ward keep themselves up todate regarding process developments. Understanding the latest techniques enablesthem to improve their service, and ensure they continue to provide the very bestrotomouldings for their clients.

Recent discussions in this arena include in-mould coolingand in-mould pressurisation, and here the company’s TechnicalDirector John Kliene (pictured) reviews both.

“Both of these process technologies have been underdevelopment for a number of years. Dr. Paul Nugent discussesmould pressurisation to remove bubbles from parts in the secondedition of Crawford et al, Rotational Moulding of Plastics -Wiley-Blackwell, 1996. Internal air cooling of parts has been anaccepted technique for many years. Recently there has beeninterest in internal water cooling of parts.

In-mould coolingThe ability to cool parts evenly from both the outside and inside simultaneously provides partsthat have very balanced, homogenous properties throughout the part wall. This enhances theproduct’s performance and provides greater impact strength. There is also less chance ofwarpage and distortion if the moulding has homogenous properties.

Using this method can shorten cycle times meaning products may be produced faster. Whenmoulding thick walled parts there could be a significant benefit achieved by reducing the longcooling cycles experienced, and reducing or eliminating the tendency to oxidise or burn theinside of the mouldings. Tougher thick walled parts could be produced using shorter coolingcycles.

It is important to point out that the rate of cooling is critical and considerable control of thecooling is needed to avoid parts with reduced performance. Rotational moulding machinesneed to be designed to enable in mould cooling as do moulds. These investments need to beconsidered against the product performance gains. For most thin to medium walled partsmodern polyethylene grades provide intrinsically good mechanical properties if processed well.

We look forward to learning more about internal water cooling, but I am sure the samecontrol considerations apply as those required when using internal air cooling.

In-mould pressurisationDuring the rotomoulding process there is a tendency for air to be trapped between sinteredpowder 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 duringthe heating cycle the bubbles disperse in the molten polymer.

As the heating process progresses the bubbles diminish in size and number. It can be difficultto 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 theimpact strength. Bubble free parts have higher mechanical strength.

Various techniques have been tried to remove bubbles. The most effective is the application ofa small pressure within the mould whilst the polymer is in its melt state. This encouragesrapid dissolving of the bubbles.

If it is felt desirable to remove bubbles using internal mould pressurisation moulds have to bedesigned which can withstand the expected pressures. Particular attention is needed withflange design and clamp arrangements. Rotational moulding machine designs must alsoaccommodate the plumbing required.

Adopting the technologiesBoth of these technologies will ultimately create tougher products in shorter cycle times, butthe question is whether the product requires them. Most products created by traditionalrotomoulding techniques are tough enough for normal operating conditions.

Francis Ward has built in the ability to provide both of these technologies in its new PODmachine. We continually assess whether these and other process technologies provide benefitsto our customers, and where such technologies are thought to be beneficial we will discussthem with the customer accordingly."

Francis Wardhttp://contractmoulding.francisward.com/+ 44 (0) 1274 707030