moulding masterclass | Screw design – part 3 shot volume of 0.7086cm3
is obtained (calculated by
dividing the total shot volume by the screw stroke). So it follows that for the recommended screw stroke value range of 30mm to 90mm, the respective shot volume range will be 21.26cm3
to 63.77cm3 (calculated by
multiplying the volume per mm of screw by the screw stroke limits of 30 and 90mm). We can easily convert these shot volumes into
percentage shot capacities (calculated by dividing the recommended upper and lower shot volume limits by the total shot volume) to obtain the values of 17.1% and 51.42%. It can be seen that the recommended values hold up very well for good process capability. If the ratio of 4D is used, then the shot capacity is 120 mm of stroke (4 x 30mm) which is equal to 85.032cm3 0.7086cm3 (85.032cm3
(120 x
) and subsequently corresponds to 68.57% /124cm3
).
Looking at the data for the 110mm diameter screw, the maximum available stroke is 460mm and the total shot volume (cylinder head volume) is 4,372cm3
.
Therefore, the available shot volume per mm of screw stroke is 9.504 cm3
(4,372cm3 /460mm). Given the
recommended ratio of 1D to 3D (and using the same calculations as previously) then the equivalent shot volume ranges from 1,045.44cm3
to 3,136.32cm3 , or 24%
to 72% of the barrel. It can be seen from this result that for the larger screw diameters the 1D to 3D ‘rule of thumb’ approach becomes a less compliant tool for assessing good process capability regarding effective component manufacture. It is, therefore, recommended that a simple calculation be carried out. It is also important to consider that, for moulding
Figure 1: Key technical data for 300mm and 110mm diameter screws Machine clamp capacity Injection unit classification Screw diameter L/D ratio
1,500 320 30 20
Injection pressure
Cylinder head volume Shot weight
Injection rate (without accumulator) Injection rate (with accumulator) Plasticising rate, Motor I (120 bar) Plasticising rate, Motor II (120 bar) Electric screw drive
Maximum screw stroke Maximum nozzle retraction Nozzle dipping depth Nozzle sealing force
Data courtesy of Sumitomo SHI Demag 54 INJECTION WORLD | January/February 2015
2,420 124 110
120/170 460
20/29 16/23 19
175 350 20 80
13,000 8,000 110 20
1,815 4,372 3,900 850
2,260 140 95
141 460 860 45
110 kN mm
bar cm3
g (PS)
g/s (PS) g/s (PS) g/s (PS) g/s (PS) g/s (PS) mm mm mm kN
applications using screw diameters of 80mm and above, the typical cycle times are likely to be in the region of 45 to 120 seconds. At such cycle durations, the residence time is sufficient to allow the material to absorb the required heat energy and to fully homogenise so a more effective melt conversion is likely to be achieved than with a smaller screw operating at the same shot capacity. A shot value of 4D is often regarded as too high for good moulding capability due to over utilisation of the effective screw length, meaning that melt inhomogeneity is likely to be present within the injected shot volume. This problem is likely to be particularly evident when moulding polypropylene. Many moulders are likely to disagree and say that they regularly produce mouldings using shot capacities of 69% or even greater with no problems. That may be the case in some circumstances as the cycle time used for component manufacture can also have an important influence. In general, the longer the cycle time, the greater the likelihood of producing components of the required quality standard. Within our own moulding shop at G&A Moulding
Technology, however, we have carried out exhaustive trials with respect to process capability analysis over a range of shot capacities. Our finding is that at shot capacities of 60% and above it is likely that reductions in shot-to-shot consistency and part quality will be encountered. As the shot capacity value approaches 70%, increased variability prevails and process capabil- ity is significantly reduced. This does not mean that mouldings cannot be effectively produced, but rather that there will be a greater number of defective parts. When moulding components using relatively short
cycle times and high shot capacities, particularly when processing semi-crystalline polymers, it is often found that the residence time for the material in the screw and barrel assembly is too short. This prevents the polymer material from fully absorbing the available conductive heat energy and results in temperature non-uniformity and the presence of solid particles within the semi-crystalline material. Such inhomogeneity can result in spasmodic
production of short mouldings as a result of solid particles becoming trapped between the pressure back ring and the sliding sleeve within the screw tip assem- bly, causing the polymer melt to flow back up the screw rather than into the mould cavity. These partially plasticised particles will eventually melt due to the additional absorption time and the screw tip will then continue to operate normally, producing fully packed mouldings to the required dimensional and visual requirements. Short mouldings can also be caused by solid particles causing gate blockages in thermal or fixed probe hot runner systems .
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