Latest Techniques in Rotomoulding
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Francis Ward remains at the forefront of rotational moulding by exploring the latest techniques, specifically in-mould cooling and in-mould pressurisation, to enhance product performance. In-mould cooling allows for uniform cooling, reducing cycle times and improving part strength, while in-mould pressurisation helps eliminate air bubbles that can compromise mechanical integrity. The company evaluates these technologies to determine their benefit to customers and integrates them into their new pod machine as needed.
Latest Techniques in Rotomoulding
- 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