In short. Plastics and rubber processing combine heating, hydraulics, drives, cooling and material movement, and an outage affects the thermal state of the process as much as the electrical state of the machine. Backup power should be scoped around what must happen when supply fails: keep barrels hot and screws turning, open moulds, keep chillers alive, or carry full production. Size from the operating cycle, treat heater banks as stepped loads, and allow for servo and inverter drives.
The thermal problem behind the electrical one
An injection moulding machine that loses power mid-cycle leaves melt in the barrel and a part in the mould. A single-screw extruder running PVC or an engineering polymer that stops for more than a few minutes risks degraded material that must be purged, sometimes with the screw pulled. A blown film tower collapses its bubble. A rubber mixer stopped with a batch inside may have to be dug out by hand once the compound cools and stiffens, and a curing press that loses heat and pressure scraps the charge. None of these losses appears in a kW figure, which is why the plant’s own process engineers must state what each machine needs during and after an outage before the generator is sized.
Processors around Hyderabad and across Andhra Pradesh see this most clearly in the monsoon, when outages are frequent but short. For many, the right answer is not a generator that runs the plant but one that holds the process in a recoverable state until the mains returns.
Define the backup scope before the kVA
There are three defensible scopes, and the plant should choose one in writing.
| Scope | Loads on generator | What it achieves | Rating against plant running load |
|---|---|---|---|
| Essential auxiliaries | Barrel and die heaters on hold, hydraulic power to open moulds, extruder screw at low speed, chiller or cooling tower pumps, compressed air, controls, lighting | No material degradation, no stuck batches, fast restart | Roughly 25 to 40 percent (illustrative) |
| Selected machines | Essential auxiliaries plus one or two revenue-critical lines at full cycle | Export or continuous orders continue | Depends on the machines chosen |
| Complete production | All machines, utilities, material handling and building services | No production loss | Full running load plus starting and heater steps |
The scope should also list what is deliberately excluded, such as granulators, conveying blowers or office air conditioning, so that nobody later assumes the generator protects the whole factory. If auxiliaries and machines share feeders, splitting the essential loads onto their own bus is often the first and cheapest piece of work. A fuller treatment is on load shedding and priority control.
Load characteristics that shape the generator
Heater banks
Barrel heaters, hot runner systems, die heaters and mould temperature controllers are resistive, but they are rarely steady. During heat-up every zone is on and the demand is at its highest; in production the zones cycle, and if the controllers use contactors rather than solid-state relays the generator sees frequent steps of a few kW to tens of kW. A plant restarting from cold on generator will draw the heat-up load for 20 to 60 minutes before the running load settles, and the sizing must cover that period, not the average.
Hydraulic and servo machines
Conventional hydraulic injection machines have a large pump motor that starts unloaded and then takes load in pulses with each injection and clamp movement. Several machines together produce a fluctuating load that the engine governor and alternator voltage regulator must follow. All-electric and servo-hydraulic machines are gentler on average demand but are nonlinear loads with high crest factor, and they regenerate briefly on deceleration. Ask the machine manufacturer for generator operation guidance, and read alternators and nonlinear loads before fixing the alternator size.
Mixers, mills and calenders
Internal rubber mixers are the heaviest single loads in the sector, with motors from 150 kW to over 1,000 kW and a torque demand that peaks sharply when the ram drops. A mixer on a generator needs the generator to be sized for the mixer’s peak electrical demand, not its average, and the start must be planned, usually through an inverter drive or a slip-ring starter rather than direct-on-line. Two-roll mills and calenders add steady but substantial load.
S_required = (P_heaters,heat-up + P_motors,running) / PF + S_largest-start
In this expression S_required is the indicative generator kVA, P_heaters,heat-up is the heater demand with all zones on, P_motors,running is the measured motor load of the protected machines, PF is the power factor and S_largest-start is the transient kVA of the biggest motor start after the base load is connected. For an essential-auxiliary scope with 60 kW of heaters on hold, 70 kW of pumps, compressors and screws running at 0.85 PF, and a 30 kW chiller compressor starting direct-on-line at six times full-load current, S_required is about 153 kVA plus about 235 kVA of transient, pointing to a 250 kVA class set if a moderate dip is acceptable (illustrative). A soft starter on the chiller would bring that down by a frame size, which is often cheaper than the larger engine. The sizing method is described in full on generator sizing.
Cooling and compressed air are part of the process
Moulds do not open cleanly and extruded profiles do not hold dimension without cooling water, and most machines will not cycle without compressed air for ejectors, robots and core pulls. Chillers, cooling tower fans and pumps, and compressors therefore sit in every scope. Chillers are also among the largest starting loads on the site, so old direct-on-line units should be upgraded to soft start as part of the backup project. Starting behaviour is covered on starting kVA.
Operating cycle, growth and the record you should keep
Provide the operating pattern rather than the installed capacity: how many machines run at once, the heater and motor ratings of each, how long a cold start takes, and whether night shifts run fewer machines. Record the assumptions used for sizing and revisit them when machines are added, because plastics plants expand machine by machine and a set sized for eight machines will be quietly overloaded at twelve. If growth is likely, plan the generator room, cable route and switchboard so that a second set can be paralleled later; see generator paralleling.
Common mistakes
- Sizing on the nameplate heater rating plus every motor in the shop. The set is oversized, runs below 30 percent load and develops wet stacking and injector fouling.
- Sizing on average running load and forgetting cold heat-up. The first restart from a long outage overloads the set when every heater zone switches on together.
- Putting servo and all-electric machines on an alternator sized for linear load. Voltage distortion causes drive faults and the alternator runs hot.
- Leaving the chiller and compressor out of the essential scope. Machines have power but cannot cycle, and moulds and dies are damaged by uncontrolled cooling.
- Expecting the generator to “protect production” without a written scope. Everyone discovers the boundary during the first real outage.
What to send us
Send a machine list with heater kW and motor kW for each machine, the number of machines that run together on a typical shift, the ratings and starter types of chillers, compressors and any mixers, and a statement of which scope you want: essential auxiliaries, selected machines or full production. A single-line diagram and photographs of the main switchboard and any existing generator nameplate complete the picture. Send your plastics or rubber plant details.
Questions engineers ask
What size generator do I need for an injection moulding shop?
It depends on the scope. Holding heaters, cooling and controls for a shop of ten mid-sized machines may need 150 to 250 kVA, while running all ten at full cycle from cold could need four or five times that (illustrative). Measure the running load, add the heat-up demand and the largest start, and decide the scope first.
Can an extruder stay running on a generator during an outage?
Yes, if the screw drive, barrel heaters, downstream cooling and haul-off are all on the generator and the transfer gap is short enough for the drive to restart without a purge. Many plants run the screw at minimum speed on generator to keep material moving rather than running full output.
Do all-electric machines need a bigger generator than hydraulic ones?
Their average demand is lower, but they are nonlinear and regenerative loads. The alternator usually needs to be selected for nonlinear duty, and the machine manufacturer should confirm generator operation and any limits on source impedance.
How should a rubber mixer be started on generator power?
Through an inverter drive or slip-ring starter, after the base load is already on the bus, and with the generator sized for the mixer’s peak demand during the mixing cycle. Direct-on-line starting of a large mixer on a generator is rarely practical.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.