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How an injection moulding machine works
by Tat Ming Technology Co. Ltd.,
September 1997
1. Introduction
Among all the methods to produce plastic parts, injection moulding takes center stage.This is a tutorial on the plastic injection moulding machine written for the manager and the purchaser whose company uses such machines to produce plastic products.
The article is divided into two sections: what makes up the machine and the injection moulding cycle.
2. Subsystems in a plastic injection moulding machine
A plastic injection moulding machine is made up of five subsystems. They are the injection unit, the clamping unit, the hydraulic system, the electrical system, and the control system. Four subsystems are visible in Figure 1. Could you identify them?
Figure 1 The plastic injection moulding machine
2.1 The injection unit
The injection unit is made up of the hopper, the screw and barrel, the non-return valve,the nozzle, the screw motor, the injection cylinders, the carriage and the carriage cylinder.
The plastic material in the form of pellets enters the barrel via the hopper. Once inside,it is fed,mixed and compressed, and metered by the three sections of the rotating screw before the melt reaches the nozzle, through which the melt is injected into the mould by the plunging screw. The band heaters around the barrel and the nozzle heater (which form part of the electrical system) help in melting the plastic. The heat generated due to compression by the rotating screw does the rest.
Figure 2 The injection unit
The three sections of the screw are shown in Figure 3.
Figure 3 The screw
The feed section has constant flight depth. Its function is to convey the plastic pellets from the hopper forward towards the transition section. This is done as the screw turns.
The flight depth is gradually reduced in the transition section where the plastic material is compressed into molten state. Some of the mechanical energy of screw rotation is converted into heat.
The flight depth is once again constant in the metering section which serves to pump the melt out of the transition section and through the nozzle.
The ratio of the flight depth in the feed section to that in the metering section is known as the compression ratio. For a standard screw, it is between 2.2:1 to 2.5:1. The ratio of the flighted length L to the screw external diameter D is known as the L/D ratio. It usually has a value from 18:1 to 22:1.
The screw rotates and is forced to the right during feeding, also called shot preparation. The effective length of the screw is reduced as the flighted length to the right of the hopper throat is irrelevant. The effective L/D ratio is reduced as a result. See Figure13a.
The barrel diameter is only marginally bigger than that of the screw, by about 4 X 10-3 of the screw diameter. For all practical purposes the two diameters could be equated.
At the front of the screw is a non-return valve. Its purpose is to allow plastic flow forward (in the direction of injection) during feeding and to block the flow backward during injection. The ring-type non-return valve is commonly used.
Figure 4 The ring-type non-return valve
The reciprocating screw is the predominant design of the injection unit. It is pushed backward as it turns during feeding (Figure 13b) and moves forward during injection (Figure 13a). The screw is respectively driven by a hydraulic motor (the screw motor) and a pair of hydraulic cylinders (the injection cylinders).
During feeding, the screw rotates, conveying the pellets forward from the hopper, compresses them to a melt and pumps it towards the nozzle. At this time the cavity is blocked by plastic from a previous shot so the pumping action of the screw forces the screw backward. The backward motion is opposed by hydraulic oil in the injection cylinders which is forced to flow out through a flow control valve (see section 2.3).
This creates a back pressure to the screw. The back pressure compacts and heats the melt. The non-return valve opens during feeding.
Figure 5 Cutaway view of the injection unit
At the end of the cooling cycle, the mould opens and the moulded part is ejected. The mould closes again and the melt in front of the screw, formed during feeding, is injected into the cavity through the nozzle. The non-return valve is closed during injection.
The nozzle at the end of the barrel is the interface between the injection unit and the mould in the clamping unit. There are three types of nozzle in common use: the simple nozzle, the spring shut-off nozzle and the hydraulic shut-off nozzle.
The hopper, the screw and barrel, the nozzle, the screw motor and the injection cylinders sit on a carriage which is moved by a carriage cylinder. The carriage moves forward so the nozzle presses against the mould's sprue gate before injection starts. The carriage moves backwards either to break the sprue or to avoid the much colder mould to cool down the melt at the nozzle, which would block the next injection. The shut-off nozzle prevents easy-flowing material such as nylon from drooling when the carriage is moved backward.
2.2 The clamping unit
The purpose of the clamping unit is to close the mould and to hold the mould halves together against cavity pressure. There are two types of clamping units in common use today: direct hydraulic clamp and toggle clamp.
Figure 6 The hydraulic clamp
Figure 7 The toggle clamp
Common to the two clamps are the fixed platen, the moving platen, the tailstock, the clamp cylinder and the tiebars. The two halves of the mould are attached to the fixed and moving platens.
Figure 8 The generalized clamp
The moving platen slides on four steel bars called tiebars. The clamping force is generated by the clamp cylinder fixed to the tailstock and pushing against the moving platen, stretching the tiebars. The tiebars, held between the tailstock and the fixed platen, provides the reaction to the clamp cylinder.
Since moulds vary in thickness, called height in the trade, the moving platen must be adjusted so that when the the toggles are fully extended, the mould halves come into contact with each other. The mechanism for doing so is called the mould height adjustment mechanism. In Figure 9a, the mould height adjustment hydraulic motor is shown near the lower left. In Figure 9b, it is shown near the upper left. Through the big gear wheel, the tailstock is moved in and out of the four tiebars during mould height adjustment. The moving platen is moved in step with the tailstock.
Figure 9a The mould height adjustment mechanism at the tail of a toggle machine
Figure 9b The mould height and mould height adjustment movement
Direct hydraulic clamp does not have this mechanism. Since the stroke is variable, mould height adjustment is simple: the piston moves back to accommodate a thicker mould, extends to take up the space left by a thinner mould.
Without the toggles, the direct hydraulic clamp has the advantage of simplicity and reduced maintenance. However, in order to provide the clamping force needed, the mould cylinder diameter is sizeable which has the side effect of lengthening the mould
closing/opening time. Speed booster cylinder is one way to reduce this side effect.
Hydraulic oil is compressible, so the hydraulic clamp is not as stiff as the toggle clamp. It is easier to cause flashing due to the opening of the mould by a high cavity pressure.
The toggle clamp is more widely used. The toggles provide a mechanical advantage so the clamp cylinder could be smaller than that of a direct hydraulic clamp having the same clamping force. It has the advantage of being self-locking: once the toggle is fully extended, the hydraulic pressure at the clamp cylinder could be removed while maintaining the clamping force. A lubricating system, either manual or automatic, is needed to keep the toggles well oiled.
In most machines nowadays, the low pressure mould protection feature is available. It prevents damage to the mould in case a plastic part is jammed between the mould halves. The mould closing would stop and an alarm will sound.
2.3 The hydraulic system
Hydraulic power is the predominant source of force and motion in injection moulding machines today. Emerging as a viable alternative is the electrical drive which is used to create an oil-free environment for moulding parts for medical use.
The hydraulic system consists of a hydraulic circuit and the hydraulic components that makes it up.
Figure 10 The hydraulic system
Hydraulic power is derived from the hydraulic pump which is driven by an electric motor. It is akin to the heart in a vascular system. Through it, hydraulic oil from the oil tank is pressurized. Hydraulic oil flows through various valves, cylinders and motors before returning to the tank at atmospheric pressure. In some injection moulding machines, an accumulator supplements the hydraulic pump to cater to high injection speed requirements.
The various types of valves in a hydraulic circuit includes
a. check valve,
b. pressure relief valve,
c. flow control valve,
d. and bi-directional valve.
A check valve allows hydraulic oil to flow in one direction but blocks it in the opposite direction. It is a one-way street through the check valve.
A pressure relief valve limits the maximum hydraulic pressure which is usually at 140 bars (~=140 kgf/cm2). This pressure could be set manually at a knob or electromechanically. In the latter case, the valve is called a proportional pressure valve:
the pressure setting is proportional to the voltage or current applied to a solenoid (by a PLC or a computer).
Figure 11 The proportional valve
A flow control valve restricts the flow of the hydraulic oil in order to control the speed of actuators. Once again, the flow could be set manually at a knob or electromechanically. In the latter case, the valve is called a proportional flow valve: the flow through the valve is proportional to the voltage or current applied.
A two-way bi-directional valve controls the motion of an actuator back and forth. It is in turn controlled by one solenoid against a spring or by two solenoids on either side in a push-pull fashion. A three-way bi-directional valve has a middle position, centered
by the springs on both sides when the two solenoids are not energized. It represents a stationary position of the actuator.
Cylinders and motors are actuators. A cylinder converts hydraulic power in the oil to force and linear motion. A motor converts hydraulic power to torque and rotary motion.
Cylinders are used in mould closing/opening, in injection and motion of the carriage. Motors are used in screw rotation and mould height adjustment.
The accumulator is an energy storing device. The most demanding phase of the moulding cycle is the injection phase which needs high speed and often at high pressure.
Without the accumulator, the electric motor must have sufficient horsepower to cater to the need of the injection phase. With the accumulator, hydraulic energy is stored in the form of compressed nitrogen in a phase of low flow requirement. This energy is let out during the injection phase, supplementing the flow from the hydraulic pump. As a result, the rating of the electric motor driving the hydraulic pump could be reduced.
2.4 The electrical system
The electrical system consists of the three phase electric motor that drives the hydraulic pump, the star-delta switch, the band heaters to raise and maintain the barrel and nozzle temperature at the desired values and their associated relays.
2.5 Energy efficient system
An injection moulding machine is run 24 hours a day and 7 days a week. An energy efficient system helps to reduce the direct cost to the moulders.
The simplest injection moulding machine has a constant speed electric motor driving a constant displacement hydraulic pump against a constant maximum pressure. Since the moulding cycle needs different speeds and different pressures at its various phases, excess flow is drained back to the tank through the pressure relief valve and excess pressure is dropped at pressure reducing valves. Not only is the energy in the excess flow and pressure lost, it also contributes to heating the hydraulic oil which needs more energy to be cooled down.
Recently, the proportional flow valve and the proportional pressure valve are in common use to tailor the pressure to demand. Still the excess flow returns to the oil tank and is wasteful. Variable speed motor drive, variable displacement pump, and multiple pumps are ways to vary the flow as it is needed, in order to save energy. They are more expensive vs. those in the simple injection moulding machines but would pay for themselves in the long run.
Insulating covers around the band heaters conserves energy which would otherwise be lost to the environment through convection and radiation.
2.6 The control system
The control system controls the sequence of operation of the moulding cycle (sequence control) and maintains the process temperature, time, pressure, and speed at the required values (process control). The three generations of control systems are a. electrical relays, b. programmable logic controllers (PLC), c. and computer control.
Figure 12 The control system
2.6.1 Electrical relays
In an electrical relay control system, an electrical relay starts the next operation based on the state of limit switches and timers. Limit switches are actuated by cylinder movement.
Pressure, speed and time are set manually at pressure relief valves, flow control valves and analog timers respectively. Temperature is controlled by individual analog temperature controllers. Repeatability of such settings, especially of the first two types, is not high.
2.6.2 PLC
In a PLC control system, limit switches are replaced by the more reliable proximity sensors which are magnetically activated as cylinders move. They feed into the inputs of the PLC, the outputs of which are controlled by a program. Such programs are usually written in a language called ladder diagram, a remnant of the electrical relayera.
Inputs regarding pressure, speed and time are set digitally, either through thumbwheel switches, or the more advanced (and simpler) membrane switches (like the keys of some calculators) with LED outputs (like the display of an old-style calculator).
Hydraulic pressure and cylinder speed are controlled by proportional pressure and flow valves, which in turn are controlled by the PLC through amplifiers. Time control is integral to the PLC. Temperature is controlled by individual digital temperature controllers. Digital settings make for good repeatability.
2.6.3 Computer control
A computer control system contains a CPU, which is usually based on the open design compatible with that found in a personal computer. Like a personal computer, the display is either a cathode ray tube (CRT) or a liquid crystal display (LCD). Display of all input settings and outputs are done by flipping through screens. Magnetic card and/or diskette are used to retain the settings of a particular job, facilitating repeat of the same job at a later time.
In a computer control system, potentiometers or encoders are used to measure cylinder movements. This eliminates the multitude of proximity switches and their inherent manual settings on sliders. Settings are now done through input at the keyboard.
Temperature, time, pressure and speed control are done by the computer.
2.6.4 Temperature control
Temperature controllers are classified by the complexity of their control algorithm, which ranges from the simplest on-off control, through proportional control, PD (proportional plus derivative) control, PID (proportional plus integral plus derivative) control and more recently the PID fuzzy logic control. The more sophisticated the algorithm, the faster and the closer the actual temperature to the desired value will be attained.
Common to all types of controllers is the measurement of temperature by thermocouples. Thermocouple of type K is widely used since it measures a temperature range of up to 400oC which covers most of the thermoplastic melting temperatures.
Temperature controllers are either analog or digital. In an analog temperature controller, the set temperature is input via a dial, and the temperature deviation is shown via a meter. In a digital temperature controller, both inputs and outputs are done digitally.
An A/D (analog to digital) converter converts the amplified analog thermocouple signal
before it is processed.
2.6.5 Cooling control
The mould, the barrel throat and the hydraulic oil need to be cooled. The simple method is by running water through coils and controlling the flow rate using flow control valves. Flow meters are available for monitoring. The running water is in turn cooled at a water tower and is recirculated.
The mould is cooled so the melt in the cavity solidifies in a reasonable amount of time. In applications where cycle time is critical, the cooling period could be shortened by running chilled water through the mould. The temperature controller for the chiller is in a separate unit from the injection moulding machine.
The barrel throat (near the hopper) is cooled so plastic pellets remains solid to be conveyed forward in the feeding section of the screw. Molten plastic sticks to the screw and does not move forward.
Hydraulic oil needs to be kept below 46oC to reduce leakage at the rings, and to avoid deterioration of the oil and the rings.
Injection moulding machines capable of handling thermosetting plastics need to have active cooling control on top of active heating control at the barrel. The cooling avoids the plastic from reaching thermosetting temperature within the barrel, destroying it from further functioning.
2.6.6 Closed loop control
Nowadays, temperature control in injection moulding machines is closed loop. It means the actual temperature is measured, compared to the desired temperature and the appropriate control action is taken to remove any deviation in an optimal fashion.
Closed loop control removes the effects of all disturbances to the target variable attaining the desired value. As a result, the moulded part is repeatable from one shot to the next.
Mould position and screw position are measured by limit/proximity switches or by potentiometers or encoders. As a result, position measurement is also closed loop.
However, more often than not, pressure and speed control are open loop for cost reasons.
Three types of pressures could be measured. In increasing order of difficulties are hydraulic pressure, melt pressure at the nozzle and cavity pressure.
If hydraulic pressure is measured, the pressure sensor is located close to the injection
cylinders as accurate injection pressure is to be controlled during injection. As hydraulic oil is about 40oC and the pressure 140 bars, the environment is not demanding.
Nozzle pressure sensor works in a more demanding environment. The melt is from 200 to 400oC and pressure is often at 1400 bars. Furthermore the sensor surface must be flush with the nozzle inner surface to avoid stagnant materials from degrading through prolonged heating. Sometimes, a temperature sensor is also incorporated in the same housing. Together, temperature and pressure give the state of the melt. (Conventional nozzle temperature measurement measures the temperature of the nozzle, not the melt
at the nozzle).
Cavity pressure sensor works in a similarly harsh environment. It is located inside a mould to detect when the cavity is filled so the injection speed could switch to holding pressure. The sensor actually touches the melt but must not leave a noticeable mark on
the article. Such sensor is often the very expensive quartz type which needs an expensive charge amplifier for conditioning before the signal could be read by a computer. Furthermore, being part of the mould, such sensor is usually not removed for reuse in another mould.
With a sensor on each tiebar, tiebar tension measurement can indicate the tiebars are not evenly strained due to unparallel mould faces and the non-symmetrical cavity. A more economical setup has sensors on two diagonal tiebars. Tiebar tension measurement prevents tiebar and platen breakage which is costly in terms of downtime.
In a toggle clamp machine, tiebar tension control sets and maintains the clamping force of the machine when increase in mould temperature expands the mould and increase the clamping force. Mould height adjustment is called upon to reduce the clamping force to its set value. With tiebar tension control, the clamping tonnage could be set to below its maximum. Assuming the tiebars are evenly stressed, this could be accomplished using a sensor on only one tiebar.
The speed of the screw could be measured and fedback during injection. This is usually done by measuring the displacement of the screw in fixed intervals of time.
The back pressure could be measured by a hydraulic oil pressure sensor at the back of the injection cylinder.
Screw rotational speed is measured by a rotary encoder and converted to screw surface speed, which is what needs to be controlled.
Closed loop control requires proportional valves with fast response to take the control action. Such valves are called servovalves. Not only are they much more expensive, they also need a much cleaner oil to work with. This translates to higher filtration requirements.
3. The injection moulding cycle
To mould a plastic part, the injection moulding machine goes through a series of steps that together forms the injection moulding cycle.
a. The carriage is moved forward to press the nozzle against the mould’s sprue gate.
b. The safety gate is closed.
c. The mould is closed.
d. The injection cylinders bring the screw forward (without turning) to inject the plastic melt into the mould cavity. The screw now acts as a plunger: the non-return valve at the tip of the screw is closed. In most machines nowadays, multiple speeds are available during the injection phase. The end of the injection is determined by either by time, screw position, hydraulic pressure or cavity pressure. At this point, there is still a melt cushion in front of the screw for use in the next phase.
e. The screw is held at holding pressure while the mould cools. The holding pressure fills the mould from the melt in the cushion to make up for shrinkage. The cooling period is controlled by a timer.
f. The screw is turned to feed the plastic material from the hopper to the transition section where it is plasticized. The pressure generated by the screw on the material forces the screw, the screw drive, and the injection cylinder pistons back, leaving a reservoir of plasticized material in front of the screw. The screw will continue to turn until the rearward motion of the injection assembly hits a limit/proximity switch, which stops the rotation. The position of this switch is adjustable, and it determines the amount of material in front of the screw (the size of the shot + melt cushion). In a
machine with a potentiometer, this position is set at the keyboard.
g. In machines with the decompression feature, the screw is retracted (without turning) slightly at this point so the material does not drool out of the nozzle during feeding.
This is also called suck back.
h. At the end of the cooling period, the mould is opened.
i. The article is ejected from the mould by the ejector mechanism. In automatic photosensor mode, a light sensor at the chute detects the article has left the mould so the next cycle could begin.
j. If needed, the carriage is moved backward and the cycle repeats at a.
k. Otherwise, the cycle repeats at b.
Figure 13 The movement of the screw
Copyrighted by Tat Ming Technology Co. Ltd., September 1997
Unit 919, Tower A, Regent Centre,
63 Wo Yi Hop Road,
Kwai Chung, Hong Kong.
Tel: (852) 2790-4633, Fax: (852) 2797-8774
Email: tatming@netvigator.com
Web site: http://www.tatming.com.cn
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