Energy is the largest variable cost item in plastic injection molding after raw material. Even so, in most plants it is seen as a single total bill — nobody knows how much each piece of equipment consumes.
Consumption that is not measured cannot be managed. In this article we look at where the energy goes and at which points there are real gains.
Where is the energy going?
In a typical injection molding plant the rough breakdown of consumption is as follows:
| Area | Approximate share |
|---|---|
| Injection molding machines | 50 – 60% |
| Drying | 15 – 25% |
| Cooling (chiller, tower) | 10 – 20% |
| Compressed air and conveying | 5 – 10% |
| Lighting, heating, other | 5 – 10% |
The most surprising item in this table is usually drying. Because it runs quietly in the background, it is overlooked; yet because it heats continuously, its share is high.
1. The machine side
Drive type
Three machines producing the same part consume markedly different amounts depending on the drive system:
| Drive | Typical consumption | Why |
|---|---|---|
| Conventional hydraulic | Highest | The pump runs continuously throughout the cycle, and unused energy turns into heat |
| Servo hydraulic | Medium | The pump runs only when needed and at the required speed |
| All-electric | Lowest | Each movement by a separate servo motor, only while moving |
The difference grows as operating hours increase. On a machine running three shifts, the drive difference can close the investment gap within a few years.
Details of the comparison: Servo, hydraulic and all-electric machine differences · Products: FE all-electric series · SV servo series
Idle consumption
A machine whose heaters stay on during a stoppage continues to consume without producing a single part. The temperature reduction function should be used during long breaks and at the end of shifts — this feature is standard on hot runner control units: Hot runner control systems
Heater insulation
An uninsulated barrel gives off a significant part of the heat it produces to the surroundings. An insulation jacket both lowers consumption and prevents the production area from overheating — the latter also reduces the air conditioning load in summer.
2. Drying — the area with the most waste
Most drying energy is spent unnecessarily. The reasons:
Drying longer than necessary. Drying for 8 hours when the material needs 3 does not improve quality; it leads to thermal degradation and wasted energy. Times by material: material processing values
A higher temperature than necessary. Every extra degree goes straight onto consumption.
Wrong sizing. A dryer far above your consumption holds the material longer than necessary and heats it for nothing. A small dryer, on the other hand, cannot keep up, and production runs with damp material.
Uninsulated hopper and line. If hot dried material cools on its way to the machine, part of the energy spent is lost en route. Closed, insulated feeding prevents this: central material conveying
A dryer running continuously. When production stops, the dryer must stop too or switch to standby mode.
A correctly sized dryer running under control saves both energy and quality: Raw material drying systems · More on the subject: Raw material drying and hygroscopic plastics
3. The cooling side
The cooling system removes the heat drawn from the mold — and consumes electricity doing so.
Chiller water temperature is the most effective setting. Keeping the cooling water colder than necessary costs a great deal of energy; a mold that is too cold also produces quality problems. It is essential to run at the highest water temperature the process allows.
Tower water and chiller water should be separated: tower water is sufficient for jobs such as hydraulic oil cooling, and if the same line is supplied with chiller water, an unnecessary cost arises.
Stability of the mold temperature also gives an indirect gain: a fluctuating mold temperature lengthens the cycle time, and a longer cycle increases the energy per part. Mold temperature control units
4. Compressed air and vacuum
Compressed air is one of the most expensive forms of energy to produce. Two typical kinds of waste:
- Leaks — small leaks along the line, added together, make up a significant part of the compressor's load. When production stops, a leak survey can be done by listening along the line.
- A higher pressure than necessary — raising the system pressure unnecessarily increases both consumption and the leak flow.
On the central conveying side, vacuum pump sizing is similarly important: an undersized pump runs continuously at full load, an oversized pump wastes energy. Vacuum pumps and filter units
5. The relationship between cycle time and energy
When the energy bill is divided by the number of parts, cycle time is directly an energy parameter. The machine consumes even while it is standing ready; producing more parts in the same time lowers the energy per part.
That is why cycle optimization is also an energy project: How to shorten cycle time
The same logic applies to scrap: a scrap part is a part that has consumed all its energy but cannot be sold.
Measurement: kWh/kg
The metric used for comparison between plants and between machines is kilowatt-hours per kilogram.
How it is measured: 1. A separate energy analyzer is fitted to the machine (or line) 2. The kWh consumed in a given period is recorded 3. The kilograms of raw material processed in the same period are determined 4. The ratio is calculated: kWh / kg
Once this value has been measured, the effect of every subsequent improvement can be seen concretely. Rather than a single measurement, the trend should be monitored: if the value rises over time, a loss has started somewhere — a hydraulic leak, a worn screw, a contaminated cooling channel or an air leak.
Checklist
1. Measure — at least put energy analyzers on machine groups 2. Review drying — are the time, temperature and sizing correct? 3. Raise the chiller temperature to the highest the process allows 4. Survey compressed air leaks 5. Reduce or switch off heaters during stoppages 6. Consider barrel insulation 7. Improve cycle time — treat it as an energy project too 8. Reduce scrap — every defective part produced is energy spent
For equipment selection and consumption assessment, you can consult our technical support team.
The biggest determinant of energy consumption is the drive system. For a comparison of the servo hydraulic, hybrid and all-electric options, see the injection molding machine series page.