processing | Moulding masterclass
The performance of even the best hot runner systems can be compromised by poor temperature setting. Moulding expert John Goff explains how to avoid the pitfalls
Setting up a hot runner
Main image: State-of
–the-art hot runner systems are designed to deliver outstanding results but care needs to be taken to ensure performance is not compro- mised by poor set-up
The reputation of any hot runner system is dictated by its trouble free performance and reliability in conjunc- tion with the mould tool and the application for which it has been specifi ed. Contrary to common belief, the temperature settings selected for any hot runner system only require to be at a value that maintains the actual melt temperature of the polymeric material (which has been derived during preparation of the melt within the screw and barrel assembly). The objective is neither to further liquefy the material nor signifi cantly increase its temperature to a situation where thermal degradation of the molten material will occur within the hot runner channels. Many processors seek advice or assistance in
overcoming thermal degradation issues, particularly when fl ame retardant grades of different polymeric materials have been incorrectly processed due to poor hot runner system temperature selection. It is impor- tant to understand that degradation can occur through the use of both too high or too low a temperature value being selected. Therefore, determination of the most effective and productive temperature setting is paramount when using hot runner mould tools. As a result, hot runner temperature selection should
be considered equally important as that for the screw and barrel assembly. In fact, many processors will argue that it is more important. Due to the inherent
18 INJECTION WORLD | July/August 2013
viscoelastic characteristics of the molten polymeric material, when it is compressed within the hot runner channels it can have a profound effect on the extent of screw movement that occurs during both the fi lling and packing stages of the moulding cycle. This is particu- larly the case with large impression mould tools using materials which are highly viscoelastic. Both fl ashed and short mouldings can prevail in the same shot when the extent of melt compressibility is non-uniform within the hot runner system. Hence the extent or percentage of volume imbalance between impressions is an important consideration. As most hot runner systems are designed and/or
tailored to suit the polymeric material being processed – in accordance with the throughput requirements and the size, length and confi guration of the melt channels and manifold – the correct temperature selection becomes extremely important with respect to its overall performance. Naturally, the tips (drops) play an important part but, on multi-impression mould tools, it is how equally the melt volume is distributed to each of the drops that provides the basis for consistency. Polymers in their molten condition will fl ow more in the path of least resistance. Therefore, by good design and temperature selection of the elements of the hot runner system, the uniformity of resistance allows consistency of component manufacture to take place. In
www.injectionworld.com
PHOTO: MÄNNER HOT RUNNER SYSTEMS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60