by Michael L. Stowe, P.E., senior energy engineer, Advanced Energy
” title=”Efficient energy consumption can greatly increase profitability.”>
Efficient energy consumption can greatly increase profitability.
” title=”Figure 1. Typical single screw plastic extruder “>
Figure 1. Typical single screw plastic extruder
” title=”Figure 2. Energy-saving impact of insulating the extruder barrel”>
Figure 2. Energy-saving impact of insulating the extruder barrel
” title=”COURTESY/RUTLAND PLASTICS – Figure 3. Typical plastic injection molding machine”>
COURTESY/RUTLAND PLASTICS
Figure 3. Typical plastic injection molding machine
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Energy consumption is critical to molding operations, but efficient energy consumption can greatly increase profitability. Following the basic steps in transforming a raw material into a final product and then looking at these steps in terms of required energy input is helpful in determining where potential process energy savings can be realized.
In the following sections, each process step will be examined for possible energy savings and potential process improvements.
1. Start with the Raw Material
As with any recipe, the final product is only as good as the ingredients. The consistency of the plastic resins and their properties is absolutely critical to the successful downstream processes of extrusion or injection molding. Depending on the resin supplier, there may be variability between batches of resins and having a small lab to check such things as resin density, melt index, shear rate vs. viscosity and tensile strength can help ensure consistent, high-quality resins.
Remember that scrap costs both money and energy to replace. Ensuring good raw material quality on the resins will help to ensure good quality extruded and injection molded parts downstream.
Plastic Resin Storage: Moisture can be the enemy of plastic processing and can cause all kinds of quality issues with both extrusion and injection molding. Needless to say, the plastic resins and other ingredients must be stored in a dry location free from contamination and extreme ambient temperature variation.
Compounding: Along with raw materials and storage, the mixing and compounding of the final recipes for extrusion or injection molding is critical for the stability and consistency of the final products. Again, good, consistent compounding helps reduce scrap.
Drying:
At all steps in the storage, mixing, compounding and transport of the plastic resins, prevention of moisture must be considered. Drying requires energy input in some form to drive the moisture out of the plastic resins. Controlling the drying and reducing the energy needed for drying can be a good source of cost savings.
2. Be Aware during Resin Transport
At various points in the plastic resin delivery, storage, compounding, drying and delivery to the extruder or injection molding machine, the material must be transported. The methods and timing of this transport can offer energy saving opportunities.
If the plastic resin, warm from drying, is directly delivered to the molding machine, less energy is required at the machine to raise the plastic temperature to the melting point. For example, if the plastic is fed into the equipment at room temperature, the specific energy to raise its temperature will be much more than if the plastic is delivered to the machine at say 150ºF. If operations allow, keeping the plastic warm from upstream processing will allow for better energy efficiency.
If possible and practical, it may be an energy benefit to insulate hoppers and transport piping, especially if the transport piping runs outside the building.
3. Look at the Molding Process
Extrusion Opportunities
A lot of things need to go right to obtain efficient and consistent extrusion. Three key process parameters for extrusion are melt pressure, melt temperature and motor load.
These three items need to be measured and monitored to ensure stable, consistent and steady-state extrusion, thereby minimizing scrap. Related to these three items are the main energy users for the pipe extruder and the extrusion die, which include the following:
- extrusion screw main drive motor (typically the largest load)
- extrusion screw barrel heaters
- extrusion die heaters
- extrusion screw barrel cooling water and/or air
Figure 1 shows the typical equipment arrangement for a single screw plastic extruder. The larger energy loads are circled in red.
Ideally, there will be just enough energy input to the barrel and screw to melt and extrude the plastic. If the screw is running too fast, it can overheat and require excessive cooling water as counterbalance. If too slow, it may not melt the plastic properly.
Each resin has a specific energy consumption (SEC) for heating and melting. Ideally, the screw supplies 80 to 90 percent of the melt energy, with the barrel heaters supplying the balance of 10 to 20 percent. If the screw provides more than the SEC, then cooling is required to take away the excess heat, which is inefficient and potentially degrades the plastic. The optimum extrusion point is where the screw provides precisely 100 percent of the SEC for the specific plastic resin currently in use. Anything above that indicates a hyperactive screw and requires extra energy for extra cooling.
Also, overheating can lead to degradation of the plastic resins, reduced melt strengths at the die exit and can make the extrusion more difficult to cool. Running an extruder barrel too hot and requiring cooling is like driving your car with your foot riding the brake. Careful optimization of barrel temperature is critical for efficient extrusion.
It is a good practice to install insulation blankets around the barrel and die to hold in as much heat as possible. Also, having an enclosure or hood over the barrel and die can help prevent convective heat losses caused by air currents carrying heat away from the extrusion equipment. Figure 2 shows a graph of steady state (SS) power versus barrel temperature with and without insulation.
As seen in Figure 2, energy savings with insulation on the barrel and the die can be significant. Also, the cost for this insulation is fairly low so the payback is fast.
Injection Molding and Mold Extraction Opportunities
The main energy consumers for injection molding machines are typically:
- hydraulic pump motors (approximately 80 percent of the total energy usage)
- process cooling water (PCW) systems for cooling the hydraulic oil
- chillers to supply chill water to the mold
- barrel heating bands
Figure 3 shows the typical equipment arrangement for a plastic injection molding machine. The larger energy loads are circled in red.
Hydraulic pump motors represent a large source of energy consumption in plastic injection molding machines. The injection molding process is essentially a batch system and has a varying hydraulic load demand during the cycle. The actual time of peak hydraulic demand is low, as the peak is only needed for the plastic injection. There are several opportunities for saving energy in the hydraulic system. Each is shown with some factors for consideration.
1) All electric injection molding machines, which use electric servo motors for both injection and clamping
- Energy savings from 25 to 60 percent.
- Repeatable, consistent and accurate performance.
- No hydraulic oil to leak or clean up.
- No hydraulic oil to cool, reducing PCW system load.
- Quieter operations.
- Higher initial equipment cost.
- Not good for retrofitting with existing molds; difficult to adapt.
- Better for new installations with new molds.
- Torque for long-time holds, as with PVC, may be an issue.
- Reliability of core pulls may be an issue.
2) Hybrid injection molding machines, which use electric servo motors for injection and traditional hydraulics for clamping:
- Offer an energy advantage over straight hydraulic machines.
- Repeatable, consistent and accurate performance.
- Fast clamp open and close speeds.
- Less hydraulic oil to leak or clean up.
- Less hydraulic oil to cool.
- Somewhat quieter.
- Initial cost is between all-electric and all-hydraulic equipment.
- Good for use with existing molds.
3) Variable speed drive (VSD) hydraulics for both injection and clamping, optimizing hydraulic motor run time and speed:
- Distinct energy improvement over straight hydraulic machines, with savings ranging from 25 to 55 percent, depending on machine size/cycle.
- Good option for retrofit on existing machines.
- Less costly than buying a new all-electric machine.
- Reduced hydraulic heat load that must be cooled.
As mentioned, the energy load on the PCW system is reduced with the reduction of hydraulic load on the injection molding machines. Going all-electric or hybrid, or adding a VSD to the hydraulic motor, will all reduce the PCW heat load. Additionally, for process cooling towers, the addition of a VSD to the cooling tower fan can also achieve good energy savings. Try to run the mold chill water temperature set point as high as possible without compromising mold operations. Just because the chillers are capable of producing 40ºF chill water does not mean that is where the set point needs to be. If the molds are sweating from condensate on a humid summer day, like a glass of tea on the back deck, chances are the chill water set point is too low. Overcooling the chill water is just spending energy money unnecessarily.
In opposition to the need for mold chill water, the barrel for the injection screw must stay hot. Consistency and stability in barrel temperature lead to better quality and more accurate, repeatable weight and dimensions on injected parts. Typically, electric resistance heater bands are used to accomplish this. Barrel insulation wraps are available specifically for plastic injection molding machines. This is certainly a good option, but two other energy-efficient technologies are available for barrel heating:
1) Induction Heaters
- Faster heat-up time.
- Use up to 75 percent less power versus electric resistance heater bands.
- An average 50 percent reduction in part weight variability.
- An average 25 percent reduction in part dimensional variability.
- Induction technology uses a copper coil wrapped around the barrel and high-frequency AC to induce eddy currents to heat the barrel.
- Frequency can be optimized to ensure the proper depth of heating in the barrel wall thickness.
- With heat induced in the barrel, the barrel itself effectively becomes the heating element.
2) Infrared (IR) Radiant Heaters
- Faster heat-up times with nearly instant on and off.
- Use up to 50 percent less power than electric resistant heater bands.
- IR heating elements are built into the insulation, creating a very tight and efficient thermal barrier.
- More consistent and stable heating along the barrel length.
- Barrel temperature stability of +/- 1ºF.
- Estimated service life of five years.
- Estimated payback of 12 to 18 months.
Once the injection molding cycle is completed, the part is extracted from the mold and through conveyors or other material handling equipment sent downstream for final processing.
Conclusion
A variety of opportunities exist in any plastics molding or extrusion facility to reduce energy consumption and costs, including those discussed here. Others that could not be addressed include demand-side power management, motor management, lighting and compressed air systems. Reducing scrap also reduces energy usage: (1) energy has already been consumed to create the part that is scrapped, and (2) more energy must be consumed to create a replacement part.
Mike Stowe is a senior energy engineer with Advanced Energy in Raleigh, North Carolina. He has more than 28 years of experience in manufacturing plants, including roles as production manager, maintenance manager and plant engineer. Currently, Stowe works with utilities, industrial equipment vendors and manufacturing plant teams to find the best technical and most energy-efficient solutions for industrial processes. For more information, call 919.857.9043 or visit www.advancedenergy.org.
