When starting up the heaters and before the barrel temperature reaches the set temperature, it is important that the screw does not turn to feed and to plasticize as the screw and barrel could be damaged by the pellets. Many machines have such an interlock to prevent the screw from turning before the set temperature is reached.
3.5 Low pressure mould protection
A part moulded in the previous cycle that has not been properly ejected could damage the mould when it closes again. Low pressure mould protection closes the mould at low pressure. Opposed by the jammed article, the mould mould not close completely in the preset time. This function would stop the closing and sound an alarm. It is not designed to protect the human body part, which is done by the interlocks at the safety gates.
3.6 Nozzle type
Simple nozzle, spring shut-off nozzle and hydraulic shut-off nozzle are the common types. Simple nozzle is suited to plastic materials that degenerates with heat, e.g. PVC. Being simple, it does not have stagnation points to accumulate stale plastic.
Figure 15 Nozzle
Spring shut-off nozzle is suited to plastics with low viscosity, e.g. nylon. The spring action closes the nozzle during feeding. Springs tends to lose its elasticity over time when strained at high temperature.
Hydraulic shut-off valve provides a positive shut-off through actuating a hydraulic cylinder.
3.7 Number of injection speeds
The more capable machines provide multiple injection speeds during injection. They are used to advance the melt front at a constant speed as the cavity is filled.
3.8 Number of injection pressures
The more capable machines provide multiple injection pressures during injection. They are used to overcome the varying resistance to melt flow as the cavity is filled. Sometimes, holding pressure is counted as one injection pressure.
3.9 Number of holding pressures
The more capable machines provide multiple holding pressures during the holding phase. As the melt cools and shrinks during the holding phase, its pressure is reduced. It is better that the holding pressure is also reduced in synchronism.
3.10 Temperature controllers
In order of increasing sophistication are on-off, proportional, PD, PID and PID fuzzy temperature controllers. The more sophisticated temperature controller gets the barrel temperature closer and faster to the desired temperature during startup and on the face of disturbances. Larger machines have longer barrels which require more heaters, thermocouples and temperature controllers.
Figure 16 Temperature controllers
3.11 Sophistication of closed loop control
Nowadays, PIMM barrel temperature control are always closed loop. Occasionally, nozzle temperature control uses a simpler temperature controller and may even be open loop.
Time control is considered open loop, e.g. the control of injection time.
In the control of PIMM, there are many variables that could be controlled in closed loop. By measuring the controlled variable and taking control action to correct any deviation from the set value, closed loop control guarantees good repeatability of the controlled variable despite changes in the uncontrolled variables, e.g. variation in the quality of reground materials, humidity of the plastic pellets.
3.11.1 Position control
In a PIMM, screw position, mould position, ejector position and mould height adjust position are measured, either by limit switches, proximity switches or potentiometers. Potentiometers offer position measurement throughout the whole stroke, while the former two only measure whether discrete positions have been reached. Depending on the stroke, a resolution of 0.1 mm is expected from potentiometer (the limit actually comes from the resolution of the A/D (analog to digital) converter).
Some manufacturers use a rotary encoder and rack and pinion to measure movement in order to avoid the cost of the A/D converter. In this case, the resolution of the encoder and the rotary to linear conversion factor determine the resolution of the movement.
Figure 17 Potentiometers
Screw position is measured to break down the injection stroke into stages each with a different speed/pressure. It is also used to measure shot size during feeding and decompression.
Mould position is measured to break down the mould movement into slow-fast-slow stages to reduce vibration in mould closing and mould opening. Mould position is also used in low pressure mould protection.
Ejector position is measured to short cycle the ejection stroke, especially in multiple ejection.
In a toggle clamped PIMM, the stroke of the mould height adjust mechanism could be measured by a potentiometer.
3.11.2 Injection speed control
It is important that injection speed is controlled to obtain a high quality part. This could be done in open loop, semi-closed loop or closed loop.
The open loop approach uses the ordinary proportional flow valve. A voltage proportional to the desired flow rate is applied. Through the injection cylinder, the desired flow rate is mapped into the desired injection speed.
The semi-closed loop method uses the closed-loop proportional flow valve. The loop is closed as far as the spool position is concerned. The movement of the spool within the valve controls the rate of oil flow through it.
The closed loop method uses the linear screw speed to close the loop. Either a velocity transducer is used or the screw speed is derived from potentiometer readings in fixed intervals of time. The proportional flow valve is adjusted to nullify any deviation from the desired speed. Unless the control is done by dedicated electronics, closed loop speed control demands very much of the machine controller.
3.11.3 Screw rotary speed control
Screw rotary speed is monitored or controlled so as to control the screw surface speed to below a value appropriate for the resin. A speedometer, the kind used in a bicycle, is the usual analog measuring device. A chart converts screw rotary speed to screw surface speed which is a function of screw diameter. See section 2.13.
Figure 18 Screw rotary speed to screw surface speed chart
3.11.4 Hydraulic pressure control
Closed loop hydraulic pressure control provides more consistent injection pressure, holding pressure and back pressure from cycle to cycle. Note that hydraulic pressure control is not a good substitute for melt pressure control or cavity pressure control.
The signal from the pressure sensor adjusts the proportional pressure valve to nullify any deviation from the desired value.
Figure 19 Pressure transducer and display
3.11.5 Back pressure control
As the screw rotates, it is forced backward by the melt at the tip of the screw. This backward motion forces oil out of the injection cylinders through a flow control valve, which creates a back pressure on the screw.
The back pressure sensor is mounted at the back of the injection cylinder. The same sensor is used for hydraulic pressure control. See Figure 20.
Pressure and temperature are the two most important measurable process variables in injection moulding. It could be used to control the injection fill, pack and hold pressures.
Figure 21 Nozzle pressure sensor
3.11.7 Cavity pressure control
Located where the action is, cavity pressure control provides the most accurate injection fill, pack and hold pressures. In some cases, a temperature sensor is located within the same housing, providing temperature of the melt in the cavity as well.
Figure 22 Cavity pressure sensor location
The cavity pressure curve clearly shows the injection fill, pack, and hold phases. In Figure 23, 1-2-3 is the injection phase, 3-4 is the pack phase and 4-5-6 is the hold phase.
Point 3 is when the mould is completely filled. As the screw advances beyond 3, cavity pressure rises steeply as the melt is being compressed. At 4, injection pressure is reduced to holding pressure which keeps the mould filled as it cools and shrinks. At 5, the melt at the gate is frozen and the hold pressure could be removed.
Figure 23 Cavity pressure curve
3.11.8 Tiebar tension measurement
Tiebar tension measurement is used for clamping force control and for avoiding tiebar breakage.
Clamping force control is more appropriate for a toggle clamp than a hydraulic clamp as the toggle amplification of roughly 22 times makes adjustment of hydraulic pressure a poor gauge of clamping force. This is on top of the fact that the amplication is not known. It allows adjustment of the clamping force to a value that is needed (see section 2.2) instead of always at its maximum. The fatigue life of the mould, tiebars and toggles are increased. With the correct clamping force, flashing does not occur.
Clamping force adjustment is done during mould setup. As the mould heats up, it expands, increasing the clamping force. Therefore it may be necessary to readjust the clamping force during moulding.
For clamping force measurement, as few as one tiebar tension sensor is sufficient.
Tiebar tension control avoids tiebar breakage. An alarm is raised when a tiebar is over stressed which is usually caused by unparallel mould face, mould with an asymmetric cavity or an out-of-synchronization mould height adjustment mechanism.
To avoid tiebar breakage, as few as two sensors on diagonal tiebars could be used.
Figure 24 Piezoresistive gauge in tiebar
Figure 25 Strain ring on tiebar
3.11.9 Hydraulic oil temperature control
Hydraulic oil must be maintained at between 40 and 50oC. This is done by control of the cooling water flow.
Too high an oil temperature reduces the oil viscosity, and ages the rubber sealing rings faster. For consistent product quality and to improve the PIMM reliability, it is worth investing in the closed loop temperature control of hydraulic oil, if it is available as an option.
3.11.10 Hydraulic oil level control
In case the hydraulic system leaks, hydraulic oil level in the tank provides an indication. At its simplest, it is a visual level indicator. Alternatively, it could be a float, which activates a switch when the oil level is low. The switch sets off an alarm.
3.11.11 Hydraulic oil contamination control
Contamination and metal filings from cylinder/piston wear degrade the hydraulic oil. Hydraulic oil is filtered at the pump inlet and optionally filtered on return. A differential pressure sensor across the filter raises an alarm when the oil is too contaminated and must be replaced. Alternatively, an optical device immersed in the oil detects how dirty the oil is.
3.12 Energy efficiency
During the design of the machine, what considerations are made to save energy Some areas include insulation around the band heaters, using proportional valve instead of pressure relief valve, using a variable displacement pump, using a variable speed pump motor.
The simplest drive is made up of a constant speed motor and a constant displacement pump driving against a constant system pressure (set by the system pressure relief valve). The load to the electric motor is constant throughout the moulding cycle since flow rate and the pressure are constant. In phases of low flow demand, the excess pressurized oil flows back to the tank. When the pressure needed is below the system pressure, excess pressure is dropped at a relief valve or pressure reducing valve. In both cases, energy turns to heating up the oil.
An energy efficient design varies the load to the electric motor as the demand varies in the moulding cycle phases. The proportional valve sets a different system pressure at each phase. However, excessive flow still drains the pressurized oil to the tank. Variable displacement pump and variable speed drive/motor does better by varying the oil flow delivery, further reducing the load to the motor.
By itself, variable displacement pump is less efficient than fixed displacement pump. Similarly, variable speed motor is less efficient than fixed speed motor. However, by generating only the hydraulic oil flow that is needed, overall efficiency is increased.
Piston pump has a higher efficiency than vane pump but demands cleaner hydraulic oil to work well.
In short, an energy efficient design trades higher initial equipment cost for lower operating cost.
3.13 Safety features
The safety gate protects the human operator from mould closing. Once the safety gate is opened, a mechanical stop is lowered and/or electrical and/or hydraulic circuits are broken to prevent the mould from closing. The more methods of interlocking the safer is the machine. Some manufactures only provide mechanical and/or hydraulic locks as options.
Some machines provide the same safety features at the front as well as the back safety gates.
3.14 Metal detector option
When a resin is recycled, it may be contaminated with pieces of metal. A magnetic grating in the hopper prevents ferromagnetic metals from entering the barrel. Even better, a metal detector signals even when non-ferromagnetic metal passes through the hopper. A pump then removes the contaminated pellets.
3.15 Apple to apple comparison
While the discussion so far is centered on technology of the PIMM, it should not be overlooked that price is also an important consideration in machine selection. One must be cautioned of what options are included for that price.
There is almost a standard set of features most machine manufacturers would consider as options. This includes accumulator, core pull, pneumatic ejector, etc. However, there are deviations. Hydraulic safety interlock, cooling water flowmeters, automatic mould height adjustment could be standard in one machine but are options in another.
4. The non-quantifiable attributes
The following attributes of a PIMM is not easy to quantify. However, they should play an important part in the machine selection process.
4.1 Noise and vibration
Heavy masses are accelerated and decelerated during mould opening and closing. If speed control is not done well, they give rise to noise and vibration which affects the life of the machine and also the quality of the parts to be moulded. This is especially so in mould opening when the elastic energy stored in the tiebars, the toggles and the mould are released in a very short period. A good design absorbs the shock.
4.2 Cycle time
Cycle time is the sum of mould closing time, injection time, cooling time and mould opening time. Cooling time is not so much a PIMM attribute as a mould and moulded part attribute. It could be a substantial part of the cycle time. Cycle time is to be as short as possible without affecting the rejection rate of the moulded parts and the long-term reliability of the machine.
4.3 Availability of spare parts
Off-the self electrical relays, timers, temperature controllers that the user could purchase locally helps to reduce the duration and expense of machine down time. PLC controller and computer controller goes against such convenience as they are proprietary, at least the programs in them are.
4.4 Reliability
Tiebar breakage, platen breakage or toggle failure are catastrophic as their replacement is usually beyond the means of a moulder. Reliability could be measured by mean time between failure (MTBF). It could also be measured by availability which is the percentage of the machine up time. Both could only be measured by the user over a number of years of use. Nevertheless, it could be the most important non-quantifiable attributes of all.
5. Unit conversion
When comparing PIMMs from Europe, Japan and the USA, one needs to convert among the various systems used in the specifications. The SI system is the preferred one.
5.1 The SI system
The SI system is used by European manufacturers. It is a metric system, distinguished by the use of Newton for force, and bar for pressure.
5.2 The Metric system
The Metric system is used by Japan and Far Eastern manufacturers. It is a metric system using gravitational kg (kgf to be exact) for force and kg/cm2 (kgf/cm2 to be exact) for pressure. One tonne is one thousand kg.
In the Far East, oz for shot weight and hp for electric motor power are still commonly used.
5.3 The Imperial system
The Imperial system is used by USA manufacturers. It is not a metric system. It is characterized by the use of oz (ounce) for shot weight, in (inch) for dimension and stroke, in3 for injection volume, gal (gallon) for oil tank capacity, lb. (pound) for force and hopper capacity, psi (pound per square inch) for pressure, kW for heating capacity, hp (horse power) for electric motor power and oF (Fahrenheit) for temperature. One oz is one sixteenth of a pound, one (short) ton is 2000 lb.
5.4 Unit conversion
The conversion between SI system and Metric system is related to the gravitational constant. The approximation listed below are commonly used.
1 N = 1/9.807 kg = 0.102 kg ~= 1/10 kg
1 kN = 1/9.807 tonne = 0.102 tonne ~= 1/10 tonne
1 Nm = 1/9.807 kg-m = 0.102 kg-m ~= 1/10 kg-m
1 bar = 1.020 kg/cm2 ~= 1 kg/cm2
1 kg = 9.807 N ~= 10 N
1 tonne = 9.807 kN ~= 10 kN
1 Mp = 1/10 kN
1 kg-m = 9.807 Nm ~= 10 Nm
1 kg/cm2 = 0.9807 bar ~= 1 bar
Occasionally, Pa (Pascal) or MPa is used. 1 MPa = 10 bar.
1 in = 25.4 mm
1 in3 = 16.4 cm3 = 0.0164 litre
1 gal = 3.785 litre
1 oz = 28.4 g
1 lb. = 0.454 kg = 4.448 N
1 (short) ton = 0.908 tonne
1 in-lb. = 0.01153 kg-m = 0.1131 N-m
1 psi = 0.07031 kg/cm2 = 0.06895 bar
1 hp = 0.7457 kW
1 g = 0.0352 oz
1 kW = 1.341 hp
1 kg/cm2 = 14.22 psi
1 bar = 14.5 psi
oF = oC * 9/5 + 32
oC = (oF - 32 ) * 5/9
6. Some mistakes
Some mistakes moulders have made selecting PIMMs are listed below.
6.1 Incorrect shot weight
Is an ounce of gold heavier than an ounce of cotton The answer to this trick question is very often incorrect. (The correct answer is no.)
Many a moulder who selects PIMM by shot weight alone often thinks an ounce of PP is the same as an ounce of PS, which is not the case.
The injection unit of a PIMM has an injection volume which is constant irrespective of the type of material used. The shot weight of a PIMM is roughly the weight of PS in this injection volume, which is different than the weight of PP in the same volume. Eight ounces of PS in the shot volume is only 6.6 ounces of PP in the same volume. Selecting an 8-ounce machine would not be adequate for moulding 8 ounces of PP. See Example 2 in Section 2.1.2.
‘Experienced’ moulders take care of such discrepancies by oversizing a PIMM, which is inaccurate and could be wasteful in investment and in the energy cost running it.
6.2 Wrong screw selected
A moulder is moulding ABS articles with a total weight of 4.5 oz. In order to be sure, he specified a ‘more powerful’ machine with shot weight of 9 oz. During moulding, it was discovered that there was excessive shrinkage.
A portion of the machine specification is shown in Table 9.
What the moulder should have selected is screw A with a shot weight of 5.5 oz since injection pressure is high. To be ‘safe’, screw C was selected which has a shot weight of 9 oz, but has a lower injection pressure. The low injection pressure caused the excessive shrinkage.
The moulder was correct in that a more powerful selection puts him on the safe side. However, a higher shot weight is not more powerful; a smaller screw diameter giving an adequate shot weight but higher injection pressure is what should be considered more powerful.
6.3 Mould height neglected
A moulder has only considered the space between tiebars and has found a certain model of PIMM could accommodate his mould. When installing the mould, it was then discovered that the mould was too high for the machine. It often helps to send the mould to the manufacturer to mount it before a decision is made.
6.4 Mould mounting holes too far apart
A moulder sent his mould to the manufacturer to do test shots. Then it was discovered the mould mounting holes are too far apart for the mould. A smaller machine was selected, which did the job and saved the moulder a bunch.
6.5 Wrong interpretation of electric motor rating
A moulder finds the higher wattage electric motor in a PIMM of one brand ‘uses up more energy’ than a lower wattage one of another brand. A higher wattage by itself does not use up more energy. Rather, the overload to the motor is reduced. A misunderstanding on electric motor rating turned a good attribute into a bad one. See Sections 2.28, 3.12.
6.6 Misinformation in the machine specification
Due to typographic error or otherwise, the data in a machine specification may not reflect the capability of the machine. Some errors could be discovered by cross checking redundancy data. Examples are
injection unit size rating = injection volume * injection pressure, (see Sections 2.2, 2.3)
injection pressure inversely proportional to the square of screw diameter,
shot weight (g) numerically less than injection volume (cm3),
clamping unit size rating = clamping force, (see Section 2.2, 2.3).
Data for which there is no redundancy could not be checked easily. An example is if clamping unit size rating is not specified, maximum clamping force could not be checked. In this case, the following checks could be done.
In a toggle clamped machine, assuming toggle magnification is 22,
maximum clamping force = system pressure * clamping cylinder area * 22.
In a direct hydraulic machine,
maximum clamping force = system pressure * clamping cylinder(s) area(s).
Measurement of maximum clamping force by load cell or measurement of tiebar tension could be done on a PIMM, although a moulder may not necessarily want to spend the effort.
7. Selection example
Assume a moulder is making PET preforms each at 33.5 mm diameter and 103.5 mm long. Each preform weighs 15 g. There are four cavities per mould.
His selection is narrowed down to three machines from three different manufacturers. The machine specification is shown in Table 10. Based on quantifiable attributes alone, which one would you have chosen
Table 10 Choosing from three 50-ton models
It is quite common that not all parameters are listed. For example, brand C does not list screw L/D ratio, brand B does not list plasticizing capacity. Unless one calls the manufacturer for further information, comparison is done based on partial information.
7.1 Shot weight
PET has an S.G. of 1.35 vs PS’s 1.05. The shot weight (in PS) equivalent to 4 x 15 g = 60 g of PET is 60 * 1.05/1.35 = 46.7 g. Using the 80% rule, a shot weight (in PS) of 58.3 g should be chosen. All screws except the 25 mm screw of Brand A give sufficient shot weight.
7.2 Clamping force
Neglecting the unknown runner projected area, the projected cavity area is 3.1416 * 3.352 = 35.3cm2. Using the high estimate from table 3, the clamping force needed is 35.3 * 0.93 = 33 tonnes. All three brands are adequate.
7.3 International size rating
The first figure in the International size rating is the power of the injection unit. In increasing order of power are Brand A, B and C.
7.4 Screw L/D ratio
Since PET is an engineering thermoplastic, a high L/D ratio is needed. Screw diameter 31 mm of Brand A (L/D ratio 17) and screw diameter 36 mm of Brand B (L/D ratio 18) were eliminated. Since the screw L/D ratio of Brand C screws is not stated, no screws could be excluded on this count.
Table 11 Elimination based on screw L/D ratio
7.5 Injection pressure
PET needs a high first stage injection pressure of 1600 bars. The screws eliminated for low L/D ratio were confirmed to have insufficient injection pressure. Screw diameter 33 mm of Brand B is further eliminated.
7.6 Injection stroke
Brand C does not specify injection stroke in its specification. However, it could be deduced from injection volume and screw diameter since
The injection stroke of the three brands are 130, 120 and 150 mm. Brand A and especially Brand C designers have opted for longer injection stroke instead of bigger screw diameter to increase injection volume (and hence shot weight) as is explicit from the injection stroke/screw diameter ratios. A big ratio takes away the advantages offered by a high L/D ratio at the start of injection as the screw is so much retracted as to reduce its effective length and reduces its effective L/D ratio.
Table 12 Further elimination based on injection pressure
Table 13 Further elimination based on injection stroke/screw diameter ratio
7.7 Injection rate
Despite the more powerful injection unit of Brand C, its injection rates for the three screws are less than those of Brand B, diameter for diameter. The designer of Brand C has traded off higher injection pressure for lower injection rate. Given that injection pressure is satisfied, one opts for a higher injection rate.
7.8 Mould opening stroke
The PET preforms are each 103.5 mm long. Assuming there is no sprue, the mould opening stroke should be at least 207 mm. The machines have at least 220 mm maximum opening stroke and all would qualify.
7.9 Maximum mould height
Unlike Brand A and Brand C which are toggle clamped, Brand B is a direct hydraulic clamp machine. For such machines, the maximum mould height is usually not specified, but it always equals the maximum platen daylight (= maximum opening stroke + minimum mould height) which is 550 mm. For machines of similar clamping force, a direct hydraulic clamp machine has a much bigger maximum mould height than a toggle clamp machine.
No mould dimensions are provided but they could be estimated from those of the moulded article. Since the PET preform is 103.5 mm long, all three machines should have sufficient maximum mould height.
7.10 Minimum mould height
Since the PET preform is 103.5 mm long, the minimum mould height (which must be bigger than 103.5 mm) exceeds those of Brand A and C which are 80 mm and 75 mm respectively. The minimum mould height is expected to exceed the 150 mm specification of Brand B.
7.11 Space between tiebars
The four cavities for the preforms are expected to be arranged in a 2 by 2 fashion. The mould length and width are expected to be accommodated in the space between tiebars of all three machines.
7.12 Electric motor rating
At 7.5 kW, the electric motor of Brand A and C is less powerful than that of Brand B which is at 11 kW. This is so despite the fact that the injection unit of Brand C is more powerful. On this count, Brand B is preferred to the other two brands.
7.13 The final selection
Every attribute considered, screw diameter 30 of Brand B is the clear choice. It has a high L/D ratio, a high injection pressure, a high injection rate, a low injection stroke/screw diameter ratio, and a powerful electric motor.
