Energy Efficiency in the Plastics Industry

Energy Effic

iency

Power Quality & Utilization Guide

Rob Van Heur - Laborelec Mark Verheijen - GDF Suez

January 2009

Plastics Industry

Source: PackTech

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Energy Efficiency

www.leonardo-energy.org

1. Introduction to the industry

This application guide describes the energy consumption and opportunities for making

energy savings in the plastics industry based on examples from both theory and practice.

1.1 The industrial world

Two hundred and thirty million tons of plastic are produced annually worldwide.

Europe, including Switzerland and Norway, accounts for 25 per cent of total production,

this being approximately equal to the production of North America (24 per cent).

At 8 per cent, Germany is the largest producer of plastics in Europe, followed by the

Benelux. See Figure 2 for the production shares of the different countries.

Figure 1 World plastics production [7]

Source: PlasticsEurope, WG Market Research & Statistics

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Application Guide for Plastics Industry


The industry currently has to cope with price increases because of the high price of oil

and a temporary shortage of raw materials.

On average, prices have risen between 50 per cent (for polypropylene (PP)) and 100 per

cent (for polystyrene (PS)) since the start of 2004.

This increase has manifested itself in the increased price of plastic end products such as

packaging, building materials (insulation, pipes, window profiles, etc.), automobile

components, and a huge variety of other types of equipment. Further price increases are

expected in the coming months.

EuPC (European Plastics Converters) is also presently warning about the uncertain

supply of raw materials. Plastics processing companies are increasingly being confronted

by shortages of raw materials from suppliers, who in recent years have invested most

heavily in new production capacity in the Middle East and Asia.

Figure 2 World plastics production

Source: PlasticsEurope, WG Market Research & Statistics

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Energy Efficiency


The consistently strong growth in the consumption of plastics in China, India, Central

Europe, and Russia is certainly playing a role in this phenomenon. As a result, European

plastics processors are not always able to secure the quantities of materials they wish to

order. [1]

1.2 Type of products in the industry

The relatively low density of most plastic materials means the end products are usually

lightweight.

They also have excellent thermal and electrical insulating properties. However some

types can and are being used as electricity conductors.

Most plastics are resistant to corrosion. Some are transparent, and therefore have

applications in certain types of optical equipment.

They are also simple to mould into complex shapes, making the integration of different

materials and functions possible.

If the physical properties of a particular plastic do not adequately meet specific

requirements, the composition can be changed with the addition of reinforcing filler

substances, colours, expanding agents, flame retarders, softeners, etc. to meet the

requirements of the specific application.

The main applications of plastics are:

• Packaging (especially food packaging)

• Construction (insulation, piping, etc.)

• The automotive industry (fuel tanks, dashboards, airbags, etc.)

• Textiles

2. Description of the production processes

Many different production and sub production processes are used in the plastics industry.

The production processes are: [2]

• Extrusion

• Injection moulding

• Blow moulding

• Rotational moulding

• Thermoforming

• Composites

• Compression moulding

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The extrusion, injection moulding, and blow moulding production processes are

described in this section. These are the most common processes in the industry.

2.1 Process description

2.1.1 Extrusion

PRINCIPLE

Extrusion is a continuous process for the production of semi-manufactured products such

as pipes, profiles, cable sheaths, films, sheets, and plates. Although the design of the

mould and some extrusion components are different, each product has the same

production method.

Plastic pellets are added to the extruder. A screw pushes the grains through a heated

barrel. The grains are pressed together and melt. To heat the barrel, thermal oil is often

used, usually heated electrically. The plastic is pressed into the correct shape in the

mould at the end of the screw and cooled by water or air. When the product has reached

the desired length, it is cut to size.

Figure 3 Extruder unit, (1) Extruder, (2) Granulate (3) Extrusion mould (4) Extrusion

profile

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Energy Efficiency


ENERGY CONSUMERS

The biggest energy consumers in the extrusion process are the motors, heating units,

cooling processes, and compressors. Figure 4 shows a diagram of the extrusion process.

To gain a clearer understanding of energy management, it is advisable to look at energy

consumption in relation to time. It can then be seen if energy consumption is

commensurate with production. The large energy consumers in particular must be

carefully matched to the process to ensure they do not operate unnecessarily.

2.1.2 Injection moulding

PRINCIPLE

Injection moulding is a cyclical process mainly used for making plastic parts. Fluid plastic

is forced into a mould using an injection technique. It is a fast process and used for the

production of identical parts.

The injection unit used for injection moulding corresponds to that of the extrusion

process. The plastic is added, heated, and forced through the nozzle into a mould. The

difference between injection moulding and extrusion lies in the mould. With extrusion, the

mould is an opening where the plastic continuously flows out in a certain shape. With

injection moulding, the mould is a template into which the plastic is forced. The mould

consists of two parts that are electrically or hydraulically pressed against each other.

After the product has hardened, the mould is opened and the product is further

processed.

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Figure 5 Injection unit, 1 Extruder, 2 Granulate 3 Injection opening (nozzle) 4 Mould

(lower part) 5 Product 6 Mould (upper part)

ENERGY CONSUMERS

Energy consumption with injection moulding can be divided into two phases. In the first

phase, there is high energy consumption during the injection of the plastic and when the

parts are removed. The other phase has lower energy consumption over a longer period

when the plastic is cooled.

The majority of the energy is used by the heaters and motors. The remaining energy is

used by peripheral and other equipment.

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Energy Efficiency


2.1.3 Blow moulding

PRINCIPLE

Blow moulding is used to make hollow objects such as plastic bottles. Molten plastic is

blown by compressed air to form the desired shape.

The injection unit is similar to that for extrusion and injection moulding.

The most important blow moulding processes are:

• Extrusion blow moulding

• Injection blow moulding

With extrusion blow moulding, plastic is extruded in the form of a tube. Then a mould

closes around the tube and the plastic is blown by compressed air and pushed against

the mould wall. The object is cooled while under pressure. Figure 8 shows an example of

extrusion blow moulding.

Figure 7 Example of extrusion blow moulding

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In injection blow moulding, a preform mould is first filled by injection moulding. Then the

preform is placed in a blow mould where it is again heated and blown with compressed

air. Blowing can also take place at other locations, for example where the bottles are

filled. This reduces transport costs.

ENERGY CONSUMERS

The energy consumers for blow moulding are the same as extrusion and injection

moulding because the main process to produce the plastic is essentially the same.

2.2 Types of moulding machines

In this section, three different drive concepts for moulding machines are examined:

• Hydraulic

• Electrical

• Hybrid

As can be seen in Figure 9, the machines are the biggest energy consumers at the

factory.

If one looks at the purchase price and consumption of a moulding machine, the purchase

price of a moulding machine is lower than the energy costs of operating the machine over

its working life. Energy-saving machines will save money in the long term. It is therefore

not the purchase price that should be the top priority when purchasing a new machine,

but rather the consumption or ‘total cost of ownership’.

The following paragraphs describe the properties of the different drive concepts.

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Energy Efficiency


Hydraulic

The hydraulic drive is the oldest concept in terms of technical development.

The older systems have a single hydraulic pump with a high capacity and thus a high

energy consumption. Continuous pressure is supplied by a pump so that the system can

be used at any time. As a result, the electric motor is continually loaded. This results in a

high no-load.

In recent decades, multiple energy-saving pumps have been used in the hydraulic

systems in combination with pressure regulating valves. With no-load, the system is

provided with pressure by the smallest pump. Because multiple pumps are used, energy

savings can be achieved of up to approximately 30 per cent compared to the older

system with its single pump. These new, more economical hydraulic systems are

generally used in most modern machines. The conversion of an older existing machine

may be a worthwhile consideration.

Electrical

Fully electrically driven injection moulding machines have been on the market for some

time now. Servo motors are used to open and close the mould.

Greater energy efficiency is achieved by eliminating the no-load losses. These losses are

absent because continuous pressure does not have to be maintained anywhere within

the system. Direct transmission is used for fully electrical machines.

The initial cost of an electrically driven machine is generally higher than a hydraulic

machine. However, the energy savings can make an electrically driven machine

financially attractive in the long term.

Besides energy savings there are also the following advantages:

• No hydraulic oil present that can soil the product

• Lower maintenance costs. No storage, replenishment, drainage, etc. necessary for

hydraulic oil

• Less susceptible to failure

• No start-up delay

• Less noise

• Lower water consumption

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Hybrid

The advantages of both techniques are applied in hybrid machines.

Most hybrid machines have a hydraulic pump for clamping the mould. Servo motors are

used to drive the screw.

Hybrid machines are generally less expensive to buy than fully electrical machines. They

are, however, not as energy efficient. This may change in the future because this

technique is still seeing significant advances in applied technology.

3. Energy data from the plastics industry

Energy consumption in the plastics processing industry is mainly electrical. Energy

consumption is attributable to the following actions:

• Melting of raw materials

• Cooling (mould, gauges, oil, etc.)

• Driving peripheral equipment such as grinders, compressors, pumps, pre-driers,

mixers, etc.

• Vacuum formation of semi-manufactured products

Machinery, including peripheral equipment such as grinders, hopper fillers, dosing

systems, and conveyor belts account for the majority of the electricity costs. The

remainder is attributable to space heating, cooling, compressors, and lighting.

The energy consumption is specific to the processing technique. Energy consumption for

the processor is an important factor because it represents a considerable operating cost.

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Energy Efficiency


Figure 9 Energy balance of a plastics processing plant

4. Energy measures

4.1 Specific consumption of plastics processing processes

The energy consumption per kilogram of end product can be calculated for each plant.

The literature provides the relevant data in Figure 10 for the examined production

process in relation to energy consumption per kilogram.

There is no major difference between injection and extrusion moulding. The average

consumption per kWh/kg barely differs.

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Specific energy consumption however can differ quite significantly from plant to plant.

Consumption can even be higher than the values given in the figures below, but savings

are usually then possible.

Energy consumption depends on a variety of different factors:

• Type and characteristics of the plastic (for instance, each material has a different

melting temperature)

• Design, complexity, and size of the end product. The greater the pressure on the

mould, the more energy is consumed

• Each technique used for the shaping of the product has its own specific energy

consumption, depending on heating, moulding, and cooling

• The higher the quantity of production, the lower the specific energy consumption

• The cycle time determines how long the pump or electrical motor is switched on

during the moulding process

• Size of the plant

• Frequency of use of the mould

• Outside temperature (there is a 10 per cent higher consumption in the summer)

Figure 10 Specific energy consumption for some plastic processes from the

literature

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Energy Efficiency


4.2 Energy-saving measures

Extrusion

The following list can be taken into account in order to reduce energy consumption in

extrusion processes:

• Choose the right extruder. A poor choice of screw/mould combination results in

higher energy consumption.

• Optimize the speed of the extruder.

• Switch off as many energy consumers as possible when there is no production.

This is mainly the extruder’s heating and cooling. Stand-by power consumption is

thus limited.

• Make sure the housing of the extruder is well insulated.

• Keep the melting temperature of the plastic pellets as low as possible.

• Try to minimize the use of compressed air. Use fans for cooling instead of

compressed air.

• Thermal heating is often mostly electrical. Compare this with using gas.

• Use free cooling. Free cooling can be used when the outside temperature is lower

than the cooling water that goes back to the chiller. This cools the water before it

goes to the chiller. The lower the outside temperature, the greater the effect of free

cooling. This means the compressors use less energy for cooling.

Injection moulding

The following list can be taken into account in order to reduce energy consumption in

injection moulding processes:

• Make sure the parameters for the object being produced are optimal

• Is the process stable?

• Is the mould periodically cleaned?

• Optimize the cycle time. Determine if the cycle time can be reduced.

• Is the current installation still suitable for the product being manufactured?

• Are procedures in place for switching off energy consumers during longer duration

production stops?

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• Is the pressure set correctly for the product being formed?

• Is the mould being used to its maximum capacity? (For instance, do not use a four-

part mould to manufacture one product per cycle.)

• Variable Speed Drive on motor (case)

BLOW MOULDING

The energy-saving measures described above are applicable to blow moulding. Blow

moulding is after all a derivative of extrusion or injection moulding.

5. Practical examples

Case 1: Frequency controller on injection moulding machine ]6[

Introduction

There are various manufacturers supplying frequency controllers on the market to limit

the power consumption of moulding machines.

The plastics plant discussed in this case has two frequency controllers assembled on a

moulding machine.

Current situation

The moulding machines are set up with a 90kW motor not provided with frequency

controller.

Proposal

The proposal is to fit the machine with a frequency controller to reduce power

consumption.

Figure 11 shows the difference between the power consumption of a moulding machine

with and without frequency controller. During the whole moulding cycle, there is better

energy efficiency when the moulding machine is fitted with a frequency converter.

In total the cycle for manufacturing the product takes 71 seconds. The screwing process

and cooling stage are not shown.

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Energy Efficiency


The blue line shows the motor power consumption without frequency controller. The

magenta line shows the motor power consumption with frequency controller.

Figure 11 Motor power consumption moulding machine

When the set point varies from 100 per cent, it means that the oil pressure of the pumps

is excessive in relation to the required pressure in the current situation. Surplus oil will go

back to the tank without being used. A frequency controller will control the oil pressure of

the pumps according to the given set point. Hence, no excess oil pressure is produced

and energy will be saved.

During the closing of the mould, the full power of the pumps is required to generate

sufficient pressure in a short time. The pumps now operate at full power. The graph

shows that the power consumption is just about equal for both driving mechanisms. No

energy saving is possible here.

The biggest energy saving can be achieved during the holding and ejection processes.

During the mould’s holding process, no machine activity is required. Pump pressure is

only needed to keep the mould closed. Hence, the motor speed can be decreased which

results in lower energy consumption.

In the current situation, the pumps are switched on too early in the ejection process.

The ejection of the product only takes place when the robot has given a command for

this. This means the power consumption of the pumps is greater than needed. A

frequency controller keeps the pumps at idling speed until they receive a command. The

speed of the motor is then increased to the speed required for the removal of the product.

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Estimated savings

When a frequency controller is used, savings can rise to 15.6 kW over the whole cycle.

This halves the power consumption.

Estimated investment

The investment and installation cost of a 90 kW Variable Speed Drive is around EUR

20,000. Depending on the number of running hours, the payback time on this investment

can be as low as two years.

Case 2: Energy efficiency of production machines

Introduction

The energy consumption of production machines in the plastics industry amounts to 60

per cent of total energy consumption. The energy consumption of different machines was

measured during an energy scan at an injection moulding plant. The most important

parameters are the power consumption (in kW) and the product produced (in kg/hour)

per machine. Measurements were recorded for a total of 16 machines of different sizes.

See Figure 12.

Measurement results

The following table shows the measurements

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Energy Efficiency


Figure 12 Measurement results

Specific consumption

Figure 13 shows the specific consumption of the machines measured. It can be seen by

the measurement values that the smaller machines have higher specific consumption.

The large machines have average specific consumption lower than 1.2 kWh/kg. If this

value is compared with the literature shown in Figure 10, this value falls within the range

of the literature.

However, consumption when producing smaller parts is inefficient. We advised the use of

smaller machines or applying multi-component injection moulding. The company will take

this advice into account when ordering new moulds but decided not to replace the

existing ones. It must be mentioned that a new mould is expensive and does not pay for

itself with the reduction of energy costs.

Machine Power consump-tion

Production / hour Specific con-sumption

(kW) (kg/h) (kWh/kg)

M1 5.5 0.65 8.44

M2 6.8 1.2 5.68

M3 12.4 3.23 3.84

M4 11.9 4.99 2.38

M5 9.2 4.21 2.18

M6 28 16.84 1.66

M7 12.1 9.4 1.29

M8 25.5 19.88 1.28

M9 10.1 8.54 1.18

M10 18.8 16.24 1.16

M11 15.6 14.31 1.09

M12 22.4 22.35 1

M13 27 27.65 0.98

M14 37.8 40.78 0.93

M15 17 27.36 0.62

M16 18.7 30.4 0.62

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Case 3: Frequency controller on 22kW cold water pump

Introduction

The plastics plant discussed in this case mainly produces plastic components for the

automobile industry and housings for electronic parts. These parts are produced by

injection moulding. Parts produced range from a few grams to 1.5 kg.

Current situation

The factory has a 22 kW cold water pump. This cold water pump is designed for the total

production capacity. Normally some 65 per cent of the production capacity is utilized. The

pump always operates at its full flow rate and maximum speed irrespective of the

production capacity and cooling demand. The pressure and flow rate are manually

regulated using a valve.

Proposal

The proposal is to fit the pump with a frequency controller and control the speed based

on the pressure in the pipe.

When more production machines are switched on, the pressure in the pipe will decrease.

The pressure decrease in the pipe causes the frequency controller to increase the speed

of the pump until the set point is reached. This system means the power consumption of

the pump depends on the cooling demand.

Estimated savings

To calculate the saving, it is assumed that during normal operation 65 per cent of the

production machines are running, and as a result 65 per cent of the cooling demand is

needed.

The savings are calculated as follows:

Cold water pump without frequency controller

Power cold water pump 22kW

Production hours a year 6,800 h/y

Annual consumption 134.6 MWh/y

Price of electricity EUR 90/MWh

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Energy Efficiency


Cold water pump with frequency controller

Machine capacity normal operation 65 per cent

Electrical consumption at 65 per cent normal operation 47 per cent

(Calculated with simulation tool) 10.3kW

New annual energy consumption 70 MWh/y

Annual energy saving 64.3 MWh/y

Annual cost savings EUR 5,787

Estimated investment

The investment sum includes the costs of a frequency controller, a pressure sensor, and

installation. The costs are subdivided as follows:

Frequency controller EUR 1,600

Material costs switch box EUR 500

Pressure measurement EUR 200

Unforeseen costs EUR 1,000

Labour costs EUR 2,000

Total EUR 5,300

Cost recovery time: 0.9 years

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6. Conclusion

Different production processes are used in the plastics industry. The most

common are extrusion, injection moulding, and blow moulding. These

processes are very similar to each other. The extrusion and injection moulding

processes consist mainly of the melting of plastic pellets and the pressing of

this mass into a certain shape using a mould. With blow moulding, an already

produced shape is blown by compressed air in a mould to form the final desired

shape.

Specific energy consumption can be associated with these processes. This is

the average consumption per kWh/kg. The specific consumption per process is

shown in Figure 10. The specific consumption can differ from plant to plant.

This depends on various factors.

The biggest consumers at a plastics processing plant are the machines.

Moulding machines are used to produce the plastic products. The following

three concepts are available on the market to drive these machines: hydraulic,

electrical, and hybrid. The most efficient machine is electrically driven. This has

different advantages compared to hydraulic machines. The disadvantage of the

electric machine is the higher purchase price. The hybrid machines have the

advantages of both techniques. Hybrid machines are generally less expensive

to purchase than fully electrical ones, but they are not as energy efficient. This

may change in the future because the maturity of this technique is still

emerging.

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Energy Efficiency


References [1] Fedichem press release — 1 October 2007

[2] Recipe, 2006; Low Energy Plastic Processing; project financed by the

European Commission, www.eurecipe.com

[3] British Plastics Federation et al, 1999; Energy in Plastic processing, a

practical guide, UK good practice guide 292

(http://www.tangram.co.uk/TI-Energy_in_Plastics_Processing_

(GPG292).pdf)

[4] Department of Science Technology and Environment of Ho Chi Minh

City, 2002; Energy and Environment Management in the plastic forming

industry of Ho Chi Minh; study financed by ADEME, France

[5] Tangram, 2001; Energy efficiency in plastics processing, practical

worksheets for industry; www.tangram.co.uk

[6] NRG Control. SyncroSpeed Saves at Southend

[7] PlasticsEurope, WG Market Research & Statistics

Documents

Energy Efficiency in the Plastics Industry