Cutting Energy Use in Injection Molding Plants

Electricity is the second largest cost in most molding shops after resin, and unlike resin it is controllable at the level of a single valve, a single set point or a single hour of idle time. The plants that run at 0.45 kWh per kilogram and the plants that run at 1.4 kWh per kilogram are often molding the same part on similar tonnage. The difference is the machine drive, the auxiliary equipment and the discipline of turning things off.

Figure 1. Cutting Energy Use in Injection Molding Plants.

Where the energy goes

A useful first step is to stop thinking of the plant as a single meter. Sub-metering a typical cell usually produces a split close to the following.

Two numbers should be tracked monthly: kWh per kilogram of good parts shipped, and kWh per machine hour. The first catches process waste, the second catches idle time. A press that runs at the correct specific energy but sits powered up for two hours between jobs will still look bad on the second metric, and that is exactly where the cheap savings sit.

The machine drive is the largest single lever

Hydraulic presses with fixed-displacement pumps run the pump at full flow whether the machine is injecting or holding. The pump relief valve dumps the surplus as heat, which then has to be removed by the chiller, so the waste is paid for twice.

• Servo-driven hydraulic pumps match flow to demand and typically cut machine energy by 30 to 50 percent compared with a fixed pump. Retrofit kits exist for common tonnages and usually pay back in 12 to 24 months on a two-shift operation.
• All-electric machines remove the hydraulic circuit entirely. They are most advantageous on thin-wall, high-cycle packaging work where the machine runs continuously, and less advantageous on thick parts dominated by cooling time.
• Variable-frequency drives on existing pumps are the low-cost version of the same idea, with a smaller saving.
• Barrel heating is a fixed load but not an unchangeable one. Insulated heater bands, well-fitted nozzles and avoiding oversized barrels all reduce it. An oversized barrel on a small shot also degrades resin through long residence time, so the energy argument and the quality argument point the same way.

Idle time, startup and shutdown

It is common to find that 10 to 20 percent of machine hours are powered but not producing. The fixes are procedural rather than technical.

• Standby modes: most modern controllers have a standby or setback mode that drops barrel temperatures and stops the pump after a set idle period. Enable it and set the delay to match real interruptions.
• Grouped scheduling: running one press for 20 hours rather than two presses for 10 hours each halves the fixed load of barrel heating, dryers and chillers.
• Weekend shutdown: chillers, dryers, compressors and dryers left running over a non-production weekend can account for several percent of a monthly bill.
• Startup discipline: a press that is heated for 90 minutes before it is needed is heating for 60 minutes for nothing. Align heat-up with the actual start of production.

Drying is the most over-used auxiliary

Not every resin needs a dryer, and almost no dryer needs to run at full regeneration frequency all week.

A desiccant dryer that is regenerating continuously on a non-hygroscopic material is a pure loss. Where drying is needed, insulate the hopper, seal the lid, route return air properly, and size the dryer to the throughput rather than to the machine tonnage.

Cooling systems and chillers

Cooling is the plant's second hidden cost. Mold temperature control units and chillers are usually specified for the worst case and then run at that setting all year.

• Raise the chilled water set point where the process allows. Every degree of colder water costs roughly 2 to 3 percent more compressor energy.
• Use free cooling in cool months with an air-blast or dry cooler instead of the compressor.
• Variable-speed pump control on the main cooling loop, driven by differential pressure rather than a fixed valve position.
• Check for short cycling: a chiller that starts and stops frequently wastes energy and wears the compressor. Add buffer tank volume.
• Separate circuits: mold cooling, hydraulic oil cooling and machine cooling have different temperature requirements. Running everything on one cold loop wastes energy and can cause condensation on the mold.

Condensation matters here. Pushing mold temperature too low to save chiller energy causes sweating on the mold face, water marks on parts and corrosion, so the energy target has to respect the dew point in the shop.

Compressed air: the most expensive utility per unit of work

Compressed air is roughly ten times more expensive than electricity per unit of energy delivered, because most of the input energy leaves as heat. Leaks of 20 to 30 percent are normal in plants that have never been surveyed.

• Survey with an ultrasonic leak detector during a non-production shift. A 3 mm leak at 6 bar can cost more than 1,000 EUR a year in continuous operation.
• Replace open blow-offs with engineered nozzles and air amplifiers.
• Shut off air to idle machines with solenoid valves rather than relying on operators.
• Check the compressor set pressure. Every bar of unnecessary header pressure adds about 7 percent to compressor energy.

Process settings that pay twice

Several process changes reduce energy and improve quality at the same time, which makes them easy to justify.

• Lower melt temperature within the validated window reduces both barrel energy and cooling load. Many parts run 20 to 30 °C hotter than they need to.
• Shorter cycle through better cooling, not through cutting cooling time below the point where parts still eject cleanly.
• Reduce scrap and startup shots. Regrinding is not free: it costs grinding energy, drying energy and often a virgin top-up.
• Right-size the shot. A machine running at 30 percent of its shot capacity is wasting barrel heating and residence time.
• Hot runner versus cold runner: a hot runner consumes manifold heat but eliminates runner regrind. On long runs the energy balance usually favors the hot runner; on short runs it does not.

Measuring and sustaining the gain

Energy programs fail when they are one-off projects. A simple monthly routine keeps them alive.

• Read sub-meters weekly and publish kWh per kilogram by cell.
• Log machine states: producing, idle, down. Idle time is the first target.
• Set a target for specific energy per part family, not for the plant as a whole.
• Verify savings after each measure with a two-week baseline, because seasonal cooling load changes the numbers.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist for an energy review

• Sub-meter the press hall, auxiliaries and compressed air for one month before changing anything.
• Identify presses with fixed-displacement pumps and evaluate servo retrofit on the highest-run-hour machines.
• Audit dryer assignment: which materials are actually hygroscopic, and which dryers run unloaded.
• Raise chilled water set points stepwise and watch part quality and mold condensation.
• Survey compressed air for leaks on a non-production shift and fix everything above 1 mm.
• Enable standby modes and set idle delays on every controller.
• Align heat-up times with the production plan and shut down idle auxiliaries at weekends.
• Track kWh per kilogram by cell and review it in the monthly operations meeting.

Energy work in a molding plant is unglamorous, but it is one of the few cost lines where a 20 percent reduction can be reached without a capital program, purely by measuring, matching supply to demand and switching things off.