processing feature | Moulding masterclass
Figure 1 (above left) shows a looped mould set-up against Figure 2 (right) which uses a more effective independent solution
This scenario will very often result in moulding
quality issues such as dimensional inconsistency across different impressions within a multi-cavity mould tool, surface finish variations, and poor shape retention of moulded parts. These issues will differ according to the polymeric material being used and the wall thickness distribution within the component. In the case of processing a semi-crystalline material,
the presence of looped circuits will often dictate the extent of crystallinity created within sections of the moulding where there is a non-conformity or sharp change in wall section, resulting in warpage or change in shape or dimension of the part after it has been produced. Where an amorphous material is being processed,
however, the use of looped circuits may result in an extension of the time required to cool the part to below its heat distortion temperature (HDT), particularly within thicker sections of the moulding. This will result in an unnecessary extension of the cooling time in order to overcome possible shape and dimensional changes on removal from the mould. Such a shape change is termed “distortion”. Very often, the cooling circuits within a mould tool
Figure 3 (right) shows a typical flow regulator, in this case by Wittmann
are machined to accommodate a manifold that is permanently secured to one of its faces. Such tech- niques have evolved to accommodate the need to increase the surface temperature in certain parts of the mould tool, such as to improve polymer flow when producing larger components with long flow length to wall thickness ratios. As stated earlier, for
effective heat removal to take place the use of independent circuits is preferred. However, the choice of whether flow regulators or distributor manifolds are employed
54 InjeCTIOn WOrLD | April 2012
can often also influence the ultimate outcome. Flow regulators are sometimes supplied with the
moulding machine and therefore tend to be more popular (Figure 3). Aside from the availability, they are often preferred by users as they readily display the flow rate value and temperature of the cooling medium. Distribu- tion manifolds are supplied in pairs or zones, where all the respective inlet/outlet circuits are connected to a designated manifold or zoned section. Traditionally, distribution manifolds relied upon part quality indicatiors to assess the flow rate - where a lack of coolant flow rate caused the mould surface tempera- ture to increase this would often be reflected in changes in part quality. Today, the use of electronic or ultrasonic flow meters, in association with a distribution manifold, is becoming more popular and in such situations output values can be accurately measured without flow rate restriction/reduction. Good moulding practice entails the use of distribu- tion manifolds with the appropriate flow meters or suitably sized flow regulators. In addition, the position of flow regulators on the moulding machine is also important. Common problems that can occur include: l Over lengthy pipe runs from the mould tool to the regulator
l Flow restrictions within the area around the control valve
l Poor visibility of flow rate values due to discoloration of flow tubes l Lack of resolution of the scale on the flow tube necessitating the use of flow markers l Leakages between flow tube and control valves l Staining of flow tubes due to dirty or untreated water l Blockages due to the presence of sediment or silt in the cooling medium l Incorrect tube and float sizing restricting flow rate
www.injectionworld.com
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