Injection moulding machines The moulding cycle
Monitoring the power drawn by an injection machine presents a picture of the moulding cycle (Figure 2) and can be divided into two elements: base load and process load.
For standard hydraulic machines, the base load (energy consumed when the machine is idling such as during cooling phases) can amount to 75% or more of total energy consumption. A high base load can indicate that the machine is too large for the job. In the example in Figure 2, the base load is 64%.
The high base load of standard hydraulic machines means it is essential that the main motor is switched on as late as possible during start-up and off when production is interrupted or shut-down. The latter can be automated using controls linked to platen movement.
Figure 2: Injection moulding machine power draw plotted against time showing a base load of 64%
120 100 80 60 40 20 0
Process load Base load 11:04 11:05 11:06 11:07 11:08 11:09 11:10 11:11 11:12 11:13
120 100 80 60 40 20 0
11:05:30
For all-electric and hybrid machines, the base load is much lower as motors are only running when required. Typical base loads are in the region of 10-20% and are largely accounted for by barrel heating.
Process load can also be explored through examination of a machine’s power draw. Figure 3 shows an expanded view of the 38.6s cycle in Figure 2 (the large peak shows the injection phase and the smaller peaks platen movement and cooling). Using graphs such as this, energy use can be optimised by adjustment of process settings such as barrel temperatures and profile, injection speed, back pressure, clamp force, hold pressure, hold time, cooling time and screw back speed while preserving process consistency, production rate and production quality.
Figure 3: This expanded view of injection machine power draw shows detailed cycle stages
1 cycle = 38.6s Process Load
Base Load 11:06:00 11:06:30 11:07:00
An energy saving guide for injection moulders 7
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