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How to select Plastic Injection Moulding Machine 7 |
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Written by Administrator
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Sunday, 30 September 2007 |
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f2.29 Electric heater rating Electric band heaters along the barrel provides the initial heat up to the resin at start up. It also supplements the heating by plastication (when the screw rotates) during the moulding cycle. A higher rating per heater has the advantage of shortening the initial heat up time. Usually, there are one to two band heaters per heating zone. As much as possible, the heaters are evenly distributed among the three phases. The maximum current drawn by the band heaters is ih (A) = electric heater rating (kW)*1000 / (3*single phase voltage(V)). Example 15: Tat Ming's ME175 has 6 band heaters each rated at 1.2 kW. The 6 heaters are distributed 2 to a phase in the three phase electrical system. Find the maximum current per phase it draws when the single phase power voltage is 220 V. ih = 6*1.2*1000 / (3*220) = 10.9 A. 2.30 Total power This equals the electric motor rating plus the electric heater rating. It is for planning the current in the electric power connection. However, motor overloading is not accounted for in total power as the motor rating is used. it = im + ih. Example 16: What is the total current per phase needed when installing Tat Ming's ME175? it = 42.3 + 10.9 = 53.2 A. 2.31 Number of heating zones The number of heating zones is defined by the number of thermocouples installed on the barrel. If discrete temperature controllers are used, it is the same as the number of temperature controllers. Usually, a temperature controller controls one to two electric band heaters. More heating zones provide better control of temperature along the barrel length. Since a bigger machine has a longer barrel, it also has more heating zones. 2.32 Oil tank capacity Oil tank capacity has significance in cooling and number of barrels of oil to purchase. More oil in a bigger tank reduces the temperature of the oil since the heat generated is spread out more. Furthermore, a bigger tank has a bigger cooling surface. Hydraulic oil comes in 200 litre barrels. An oil tank of 220 litre capacity requires the user to purchase two barrels. 2.33 Hopper capacity Once a hopper is filled to capacity, for how long could it be left alone before refilling? A bigger hopper capacity requires less attention by the operator. However, when moulding hygroscopic resin, a hopper must not be filled for the resin to remain in the hopper for more than an hour. The weight of resin (kg) to be fed into the hopper should be less than actual shot weight (g) * 3600 / (cycle time (s) * 1000). Example 14: The weights of each component and the runners are 14g and 12g respectively. The machine is producing 6 components per cycle with a cycle time of 24s. How much should a hopper be filled so that the resin does not stay in the hopper for more than an hour? The required weight = (6 * 14 + 12) * 3600 / (24 * 1000) = 14.4kg Since plastic materials comes in 25-kg bags, half a bag would sastify the requirement. 2.34 System pressure The most common hydraulic system pressure used in PIMMs is 140 bars, which approximately equals to 140 kg/cm2. This is limited by the vane pump. By its very design, vane pump has unbalanced pressure within, which limits it from reaching a higher pressure. A higher system pressure of 170 bars or even 200 bars are used with piston pump, which demands cleaner hydraulic oil to work with. At a high system pressure, either cylinder diameter could be reduced to get the same force or higher force could be obtained from the same cylinder diameter. With a higher force, response to the control signals is faster. 2.35 Machine dimensions The dimensions of a machine has significance in shipping in a container and in the floor space it takes up. Containers come in discrete sizes like 20-foot and 40-foot. If two machines could fit within one container, the shipping charge is almost halved. In places like Hong Kong and Singapore where rent is at a premium, smaller machines are always welcome. 2.36 Machine weight The weight of a machine has significance in hoisting, shipping by truck and in floor loading. Cranes and trucks are rated by the load they can carry. If a PIMM is not situated on the ground floor, the floor loading by the machine must be considered. 3. The on-off attributes A PIMM either has each of the following attributes or not. Hence they are termed on-off attributes. This section shows the readers how to read the features section of a PIMM specification. 3.1 Nitrided screw and barrel To protect the screw and barrel from wear and corrosion by the melt, especially acidic plastic materials like PVC and acetate, nitride treatment of the screw and barrel is common. Nitriding hardens the screw and barrel surface. 3.2 Bimetallic screw and barrel Glass fibre is getting popular as a material mixed with other resins. It is very abrasive. Bimetallic screw and barrel are used in this case. For the barrel, an inner tube of tungsten carbide (Xaloy 800) is used. For the screw, Colmony is sprayed onto the flight and tungsten carbide onto the land to protect the metal below from abrasion. Naturally, the non-return valve needs similar protection against abrasion. Bimetallic screw and barrel is about 3 times more expensive than nitrided screw and barrel.
Figure 11. Bimetallic screw 3.3 Honed and chrome plated tiebars made of tensile steel The moving platen slides on the tiebars back and forth every cycle. Having a honed and chrome plated surface reduces wear. Held by the nuts at both ends of each tiebar, the tiebars provide the tensile force to the clamping cylinder to hold the mould halves together. Tiebars made of high tensile steel could provide the tons of force needed. It has a higher breaking stress than mild steel. 3.4 Cold start interlock 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. 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. 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. 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. |
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Last Updated ( Sunday, 30 September 2007 )
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How to select Plastic Injection Moulding Machine 6 |
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Written by Administrator
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Sunday, 30 September 2007 |
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2.20 Platen size The platens are thick steel plates to back up the moulds with. It is advisable that the moulds do not protrude beyond the platen limits to avoid bending the moulds during injection. Too small a mould would put undue bending stress on the platens, breaking them in the extreme case. Some manufacturers offer a choice of platen sizes for machine of a given clamping force. A car bumper is an example where a very wide platen is needed. 2.21 Platen thickness The moving platen and fixed platen must have sufficient stiffness to transmit the forces of the tiebars to the mould with minimum deflection. For a given geometry, a flat platen抯 deflection is proportional to the cube of its thickness. Especially for the moving platen, a compromise has to be struck between weight and thickness. Space between tiebars is related to platen size. If this space is increased without increasing the platen thickness, the platen under the same load deflects more . In short, one must not consider space between tiebars alone, but must consider it together with platen stiffness. Platen deflection causes the mould to deflect which in turn changes the shape and dimensions of the moulded article.
Figure 8. Platen deflection is affected by platen thickness and size Some machine makers put ribs on a platen to increase its stiffness while minimizing its weight. Since there is no standard rib patterns, comparison of platen stiffness across manufactures is not easy.
Figure 9 Ribbed stationary platen 2.22 Tiebar diameter Most PIMMs with tiebars have four of them, except small machines below about 20 tonnes, which have two. Together, their tension forces hold the mould halves together against cavity pressure during injection. If the tiebar tensions are even, the stress in each of them is given by stress = clamping force * 1000/(3.1416 * (d2/4) * 4) = clamping force * 1000/(3.1416 * d2), where stress is in kg/mm2 clamping force is in tonnes, diameter d in mm. High tensile steel has a breaking stress of more than 90kg/mm2. Mild steel has a breaking stress of 20kg/mm2. A tiebar breaks if its stress exceeds the breaking stress. More often then not, a tiebar breakage is due to uneven tensions among them. This is caused by a. non-parallel mould faces, b. non-symmetrical cavity with respect to the sprue, c. misadjustment of the mould height adjustment mechanism of a toggle clamp machine. When the mould expands due to higher temperature, it stretches the tiebars more than when the mould was set up when it was at room temperature. Example 13: Tat Ming's ME125 has four tiebars, each with diameter 75 mm. The clamping force is 125 tonnes. High tensile steel is used. What is the safety factor built into tiebars of this machine? Assuming even tension, each tiebar has
stress = 125 * 1000/(3.1416 * 752) = 7.07 kg/mm2. The safety factor is 90/7.07 = 12.7. Usually a safety factor of 10 or more is common in an industrial design. An example is the stress in the cables hauling a fully loaded lift up and down. Tiebar breakage occurs at the root of a thread where the radius is smaller and there is stress concentration. Please see section 3.11.8 on tiebar tension measurement. 2.23 Ejector stroke The ejector moves forward to eject the article from the mould. A long part requires a long ejector stroke. 2.24 Ejector force When a part cools, it shrinks around the mould and may need a big force to eject. This is especially so for a container with a small slanting angle. Sometimes, a (smaller) retraction force is also quoted. 2.25 Carriage stroke The carriage moves back to allow servicing of the nozzle, nozzle heater and the front end of the barrel. Sufficient space behind the screw motor must be allowed for. 2.26 Carriage force The carriage moves forward so the nozzle press against the sprue bush. Carriage force seals the interface from melt dripping. It is also called nozzle contact force. 2.27 Dry cycle time plasticizing. Dry cycle time is the mould closing time plus mould opening time plus idle time. It is defined by EUROMAP 6 recommendation. Dry cycle time is the ultimate cycle time as there is no cooling period. An alternative expression is cycle rate, the number of cycles per minute. Running a machine at the maximum possible cycle rate is not desirable if the machine is not running smooth and stable. This is another example why an attribute should not be evaluated by itself alone. 2.28 Electric motor rating The hydraulic system is driven by an electric motor. It converts electrical energy to mechanical energy at a certain efficiency. An electrical motor is rated in terms of kW or hp which denotes its maximum power delivery under the specified conditions like temperature of its windings. Some manufacturers offer a bigger pump size as an alternative. The motor size is also increased. It is important not to confuse the power rating of the electric motor to energy efficiency. A lower power does not by itself mean a PIMM is more energy efficient than another with a higher rating. It means it is overloaded more during the moulding cycle. A three-phase motor is about 90% efficient over a wide range of power rating. The moulding cycle demands widely varying hydraulic power in its different phases. At the electric motor, this translates to a similar demand in electrical power. Usually, the injection phase is the most demanding phase of the cycle. An electric motor is rated at below that power, requiring it to run above its rating in the injection phase. For a PIMM without an accumulator, the injection phase presents an overload to the electric motor. Most motors could be overloaded to two times its rated torque for short periods. Since a three phase motor runs at a relatively constant speed, even at overload, the extra power comes from increased torque. Because power = rotary speed * torque, the extra power comes from increased torque. Since motor current is proportional to torque, an overloaded motor heats up (proportional to the square of current) more than it is rated at, reducing its long-term reliability. A motor with a higher power is overloaded less. The story is different if the PIMM has an accumulator which does allow the electric motor to have a lower rating. Hydraulic energy is stored into the accumulator in phases of low demand to be used in the injection phase. In short, it evens out the motor loading during the cycle and reduces its overloading. A motor with a high rating does not use up more energy. How much energy is used depends on the load (the work to be done) which in turn depends on the electric drive, hydraulic drive and hydraulic circuit design. See section 3.12. The current per phase drawn by a three phase motor at its rated power is im (A)
= motor power rating (kW)*1000 / (3*single phase power voltage(V)
*efficiency*power factor)
= motor power rating (hp)*746 / (3*single phase power voltage(V)
*efficiency*power factor) For most three phase motors,
efficiency = 0.88 - 0.91,
power factor = 0.84 - 0.88. Example 14: Tat Ming's ME175 is driven by a 30 hp three phase motor. Find the current per phase it draws when the single phase power voltage is 220 V. Assume an efficiency of 0.91 and a power factor of 0.88. The current drawn per phase at the rated power of 30 hp is
im = 30*746/(3*220*0.91*0.88) = 42.3 A.
Figure 10 Power demand during the moulding cycle |
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Last Updated ( Sunday, 30 September 2007 )
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How to select Plastic Injection Moulding Machine 5 |
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Written by Administrator
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Sunday, 30 September 2007 |
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2.11 Accumulator Some PIMMs have an accumulator as an option to boost injection speed. An accumulator is an energy storing device that stores up pressurized hydraulic oil in a phase of low demand to be used in the injection (high demand) phase. It evens out the load on the electric motor and reduces its overloading. While increasing the electric motor and hydraulic pump sizes (available as an alternative by some manufacturers) does increase injection speed by about 25%, an accumulator does so with about three times increase.
Figure 3. Accumulator 2.12 Injection rate As an alternative to injection speed, some PIMM specifications use injection rate. Injection rate is the maximum volume swept out by the screw per second during injection. It is expressed in cm3/s. Injection rate = injection speed * 3.1416 * (d/2)2,
where d = screw diameter in cm. Note that injection speed is independent of screw diameter, but injection rate is. 2.13 Screw rotary speed Screw rotary speed is specified as a range in rpm. Screw rotary speed by itself is not as critical as screw surface speed. The two are related by the screw diameter. Screw surface speed (mm/s)
= 3.1416 * screw diameter (mm) * screw rotary speed (rpm) / 60 Each plastic material has a recommended maximum screw surface speed which must not be exceeded. For example, UPVC should not experience a screw surface speed of higher than 200 mm/s. Abbreviation | Optimum surface speed (mm/s) | Maximum surface speed (mm/s) | GPPS (PS) | 800 | 950 | HIPS | 850 | 900 | ABS | 550 | 650 | AS (SAN) | 400 | 450 | LDPE | 700 | 750 | HDPE | 750 | 800 | PP | 750 | 850 | PPVC | 150 | 200 | UPVC | 150 | 200 | PA-6 | 400 | 500 | PA-66 | 400 | 500 | PMMA | 350 | 400 | PC | 400 | 500 | POM (Copolymer) | 200 | 500 | POM (Homopolymer) | 100 | 300 | PET | 300 | 400 | PBT | 300 | 350 | CA | 400 | 500 | PPO-M | 400 | 500 | PPS | 200 | 300 |
Table 7. Optimum and maximum surface speed of resins Example 12: What is the maximum rpm for a 60 mm diameter screw injecting UPVC?
Maximum rpm = 60 * 200 / (3.1416 * 60) = 64. 2.14 Screw motor torque The hydraulic motor that turns the screw has a rated torque, expressed in Newton-meter (Nm) in SI unit. It represents the maximum amount of turning moment the motor can produce at the specified hydraulic pressure. A viscous material needs a high torque and a low rotary speed, vice versa for an easy-flowing material. A higher torque is needed for screw C (large diameter) than screw A (small diameter). The proportional pressure valve is used to adjust the motor torque to the needed value during feeding. 2.15 Plasticizing capacity Plasticizing capacity is the amount of PS that a PIMM can uniformly plasticize, or raise to a uniform moulding temperature, in one hour at maximum screw rotary speed and zero back pressure. Since it is rated in PS, an amorphous material, a higher plasticizing capacity is needed for semi-crystalline materials. Although the barrel heaters also contribute to melt the plastic, their capacities are not counted in plasticizing capacity. To check if the plasticizing capacity of a PIMM is not being exceeded, calculate the weight of component and sprue per shot W (g) divided by screw rotation time t (s), and convert the quotient to kg/hour: W * 3600/(t * 1000). This must be less than the plasticizing capacity of the machine. Since cycle time is longer than screw rotation time, the shot weight S (g) of a machine and its plasticizing capacity G (kg/hr) set a lower limit on cycle time Tmin (s) as follows. Tmin = S * 3600/(G * 1000). It is particularly important to match shot weight and plasticizing capacity in the case of fast cycling machines producing thin walled or closed tolerance components. Plasticizing capacity could be increased by a larger electric motor and hydraulic pump. Section 2.16 to 2.27 describe other attributes of the clamping unit. The next five attributes relates to the dimensions of the mould the machine could accommodate. They indirectly relate to the maximum dimension of the moulded part. 2.16 Mould opening stroke Mould opening stroke is the displacement of the moving platen from mould close to mould open. Mould opening stroke determines the maximum height H of the moulded part the machine is capable of. The relationship is mould opening stroke >= 2H + sprue length L In a hot runner system, L = 0. The inequality allows for a clearance for gravity, the robot arm or human hand to remove the part.
Figure 4. Mould opening stroke 2.17 Mould height (thickness) Mould height is left over from the days when presses are vertical. In a horizontal press, a more appropriate description is mould thickness.
Figure 5. Mould height, width and length In a toggle clamp PIMM specification, mould height is expressed as a range, from the minimum to the maximum mould height the machine could accommodate. The difference is the mould height adjustment the machine is capable of. In a direct hydraulic clamp PIMM specification, mould height is expressed as a number, the minimum mould height the machine could accommodate. The actual mould height must be bigger than the machine minimum mould height for the mould to be closed and clamped. Otherwise, a smaller machine (to be exact, a smaller clamping unit) is called for. The actual mould height must be less than the machine maximum mould height for the mould to fit in. Otherwise, a bigger machine is called for.
Figure 6. Mould height 2.18 Maximum daylight The maximum opening between the fixed and moving platens when the clamp is wide open. It is related to mould opening stroke and minimum/maximum mould height as follows. For a toggle clamp machine,
maximum daylight = mould opening stroke + maximum mould height. For a direct hydraulic clamp machine,
maximum daylight = mould opening stroke + minimum mould height. 2.19 Space between tiebars The mould must fit within the space between tiebars. This space is expressed in horizontal and vertical dimensions. Refer to Figures 5 and 7. The mould width must fit within the horizontal space between tiebars if the mould is lowered from above. The mould length must fit within the vertical space between tiebars if the mould is slit in from the side. It is advised that there is a clearance of 25 mm on each side for a small mould, and 50 mm for a big mould. This is to avoid banging of the heavy mould against the tiebars during loading, denting them and subsequently affecting the bearing in the moving platen which travels over them.
Figure 7. Space between tiebars Tiebarless PIMMs do not have this restriction. |
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Last Updated ( Sunday, 30 September 2007 )
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How to select Plastic Injection Moulding Machine 4 |
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Written by Administrator
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Sunday, 30 September 2007 |
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f2.3 EUROMAP size rating EUROMAP size rating is a standard way for specifying the size of the clamping unit and the injection unit of a machine. EUROMAP is a the European Committee of Machinery Manufacturers for Plastics and Rubber Industries. It publishes a number of recommendations. The rating is made up of two numbers: xxx-yyy. xxx is the clamping force of the clamping unit in kN. yyy is the product of injection pressure (in kbar) and injection volume (in cm3). Hence, xxx is the rating of the clamping unit, yyy is that of the injection unit. For a given injection unit, yyy is constant with respect to the choice of screw diameter. Some manufacturers provides several injection units for a machine of a certain clamping force. The different injection units are specified by their yyy rating. The higher is yyy, the more powerful is the injection unit. Example 9: Tat Ming's ME75 has the following specifications.
Clamping force 75 tonnes,
Injection pressure (screw B) 1264 kg/cm2,
Injection volume (screw B) 215 cm3. xxx = 75 * 9.807 = 736,
yyy = 1264/(1.02*1000) * 215 = 266. The ME75's EUROMAP size rating is 736-266. Using the approximation that 1 tonne = 10 kN, and 1 kg/cm2 = 1 bar, the EUROMAP size rating is 750-272. See section 5.3 for unit conversion. 2.4 International size rating In the Far East where kN and kbar are less well-known than tonne and kg/cm2, an alternative size rating is used instead of that by EUROMAP. It is made up of two numbers: aaa/bbb. aaa is the product of injection pressure (in kg/cm2) and injection volume (in cm3) divided by 1000. bbb is the clamping force of the clamping unit in tonnes. Note the order of the two numbers are reversed from those in the EUROMAP counterpart. Example 10: Find the International size rating of ME75. aaa = 1264 * 215 /1000 = 272,
bbb = 75. ME75's International size rating is 272/75. 2.5 Screw diameter For a given injection unit, most manufacturers offer a choice of screw diameters. The screw diameter directly affects the L/D ratio, and the injection volume (and hence the shot weight.) 2.6 Screw L/D ratio For machines that provide a choice of screws, the screw diameter and hence the L/D ratio is an important attribute in the selection process. A high L/D ratio of 22:1 or above provides better mixing and more uniform heating due to compression in the transition section of the screw. It is selected for moulding parts with high requirement, e.g. moulding engineering thermoplastics, or high precision, e.g. within 0.01mm dimension tolerance. For a given L, a higher L/D ratio translates to a smaller screw diameter. The injection pressure is increased, the injection volume and the shot weight are reduced. A medium L/D ratio of 20:1 is used for general applications with medium requirement. A low L/D ratio of 18:1 or lower is used for low requirement where shot weight is the more dominant selection criterion. The injection pressure is low. 2.7 Injection pressure As stated in a PIMM specification, injection pressure means the maximum pressure in the barrel during injection, not the maximum hydraulic pressure. The two are related by the ratio of the screw cross section area to the injection cylinders area. Usually, injection pressure is higher than the maximum hydraulic pressure by about 10 times. Where there is a choice of screws for a given injection unit, the smaller diameter screw produces the higher injection pressure. A high injection pressure helps in moulding engineering thermoplastics. Material manufacturers publish minimum and maximum injection pressures in the specification of the materials. 2.8 Injection stroke For a given screw diameter, injection volume (see next section) could be increased by injection stroke. Increasing injection stroke, however, lengthens the injection time and hence the cycle time. It also reduces the effective screw length and hence the effective L/D ratio. Hence, the advantages of a high L/D ratio is lost. From the statistics of machine specifications for L/D ratio of 18:1, injection stroke is about 4 diameters. Example 11: The data for screw C in the three injection units of Tat Ming's ME series are tabulated below. Injection unit rating | 272 | 860 | 1603 | Screw C diameter (mm) | 43 | 60 | 70 | Screw C L/D ratio | 18:1 | 18:1 | 18:1 | Injection stroke (mm) | 180 | 250 | 300 | Injection stroke/diameter | 4.19 | 4.17 | 4.29 |
Table 5. Injection stroke/diameter ratios of Tat Ming's ME series One should watch out for excessive injection stroke for the purpose of increasing injection volume and hence shot weight, at the expense of injection time and L/D ratio. 2.9 Injection volume Injection volume is theoretical. It equals the cross section area of the screw times the injection stroke. Injection volume (cm3) = 3.1416 * (d2 / 4) * i where d = diameter of screw , in cm (~= diameter of barrel) i = injection stroke, in cm Due to leakage pass the screw tip and the backward movement of the non-return valve, the actual injection volume is about 90% of the theoretical injection volume. To convert the actual injection volume to shot weight, the resin S.G. at plasticizing temperature is used. See Table 6. Resin | Abbreviation | S.G. at plast. temperature | General Purpose Polystyrene | GPPS (PS) | 0.886 - 0.901 | High Impact Polystyrene | HIPS | 0.895 - 0.917 | Acrylonitrile Butadiene Styrene | ABS | 0.895 - 0.908 | Acrylonitrile Styrene | AS (SAN) | 0.907 - 0.917 | Low Density Polyethylene | LDPE | 0.730 - 0.740 | High Density Polyethylene | HDPE | 0.752 - 0.772 | Polypropylene | PP | 0.712 - 0.737 | Plasticized Polyvinyl Chloride (soft) | PPVC | 1.050 - 1.389 | Unplasticized Polyvinyl Chloride (rigid) | UPVC | 1.134 - 1.219 | Polyamide-6 | PA-6 | 0.958 - 0.995 | Polyamide-66 | PA-66 | 0.958 - 0.995 | Polymethyl Methacrylate | PMMA | 0.996 - 1.012 | Polycarbonate | PC | 1.018 - 1.037 | Polyoxymethylene (Polyacetal) | POM | 1.187 - 1.214 | Polyethylene Terephthalate | PET | 1.129 - 1.172 | Polybutylene Terephthalate | PBT | 1.102 - 1.113 | Cellulose Acetate | CA | 1.074 - 1.104 | Polyphenylene Oxide, modified | PPO-M | 0.873 - 0.890 | Polyphenylene Sulfide | PPS | 1.075 - 1.109 |
Table 6. Specific gravity of resins at plasticizing temperature Instead of using shot weight and the 35% to 85% rule in selecting a PIMM, some manufacturers recommend using injection volume and the following rule. For low requirement moulding, use between 20% to 80% of the injection unit injection volume. For high requirement, use between 40% to 60%. 2.10 Injection speed As stated in a PIMM specification, injection speed is the maximum speed of the screw the machine is capable of during injection. It is expressed in cm/s. Injection speed affects the injection time. Moulding thin-walled articles requires high injection speed so that the melt does not solidify before the cavity is completely filled. Through controlling hydraulic oil flow, some machines have multiple injection speeds available during injection. The constant melt front theory stipulates the best moulding occurs when the leading edge of the melt (the melt front) moves in the cavity at constant speed. Since the mould cavity varies in cross sectional area, this requires multiple injection speeds during injection. Some machines have as many as ten. |
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Last Updated ( Sunday, 30 September 2007 )
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How to select Plastic Injection Moulding Machine 3 |
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Written by Administrator
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Sunday, 30 September 2007 |
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2.1.4 Selecting a machine with sufficient shot weight Shot weight should not be equal to the combined weight of the article (or articles for a multicavity mould) plus runners that could be injection moulded. The latter is set at 85% of the shot weight for articles with low requirement, e.g. figurines; 75% of shot weight for articles with high requirement, e.g. crystal parts. The discrepancy is due the much higher injection pressure when there is a mould. High requirement moulding uses high injection pressure. Example 3: Figurines made of UPVC (S.G. 1.38) with a combined weight of figurine plus runners of 4 oz. are to be moulded. What size of machine is sufficient? Shot weight in terms of PS = 4 * 1.05/1.38 = 3.04 oz. Using the 85% guide line, the machine shot weight needed = 3.04/0.85 =3.58 oz. 2.1.5 Selecting a machine which is not too big An injection moulding machine of a specified shot weight can be used to mould article(s) including the runners weighing from 35% to 85% of the shot weight. The lower limit comes from bending on the platens, barrel resident time of the resin and electric power consumption per kg of processed material. A small article using a small mould puts undue bending on the mould platens, causing them to deflect (which affects product quality), and to break in the extreme. If a big machine is used to mould small articles, the melt in the barrel could degrade due to unduly long residence time. Barrel residence time could be estimated as follows. Barrel residence time =
(weight of melt in barrel * cycle time) / (actual shot weight) Weight of melt in the barrel is estimated to be the weight in two times the injection volume. Moulding small parts with a big machine is inefficient in energy usage per kg of material processed, also known as specific power consumption. Example 4: The same figurine in example 3 is to be moulded in a big machine. What is the biggest machine that could be used? Using the 35% rule, the biggest machine that could be used has a shot weight = 3.04/0.35 = 8.7 oz. Example 5: What is the residence time of UPVC (S.G. 1.38) in a machine with screw diameter of 55 mm, injection stroke of 250 mm, shot weight (PS) of 567 g, and a cycle time of 10 s moulding shots weighing 260 g? Volume of melt in the barrel is estimated to be two times the injection volume = 2 * 3.1416 * 5.5 * 5.5 * 25 / 4 = 1188 cm3 Barrel residence time = 1188 * 1.38 * 10 / 260 = 63 s Having multicavities per mould to increase the articles' weight and to increase the mould size are solutions to using bigger machines. Alternatively, lowering the barrel temperature would help avoid degradation due to long residence time. 2.2 Clamping force Clamping force is an important attribute of the clamping unit of a PIMM. It is the maximum force the machine is capable of to keep the mould closed against the cavity pressure during injection. Insufficient clamping force gives rise to flash at the mould joint. Most PIMMs today use their clamping force (in tonnes) in their model name, e.g. ME125III. It is advisable to use a sufficient clamping force below the maximum. See section 3.11.8. The sufficient clamping force is proportional to the projected area of the cavity. Projected cavity area is the cavity area projected onto the plane at the mould parting surface. In this article, tonne is used to denote metric tonne (which is 1000 kg) to distinguish it from (short) ton (which is 2000 lb.) used in USA. Machine specifications use ton in both cases. One can tell them apart by the use of Imperial or metric system for the rest of the specifications. See section 5. The clamping force needed could be estimated in several ways. The conservative method is to multiply the projected cavity area by a constant which is different for each material. For example, for GPPS, the constant is 1.0 to 2.0 tonnes/in2 for thick wall articles, 3.0 to 4.0 tonnes/in2 for thin wall articles. 1.0 tonne/in2 = 0.155 tonne/cm2 = 15.4 MN/m2. Table 3 lists the constants for commonly used resins. Example 6: A GPPS cup of diameter 79 mm is to be moulded. The cup is 0.6 mm at its thinnest section. Find a conservative clamping force which would be sufficient. The projected area of the cup (and runner) is 3.1416 * 7.92 / 4 = 49 cm2. This cup belongs to the thin wall domain. The conservative clamping force is 0.62 * 49 = 30.4 tonnes A more accurate method takes into account the flow path length and wall thickness. Flow path is the length travelled by the resin from the sprue gate to the furthest point in the mould cavity. See Figure 1. If the wall thickness of a part varies, take its minimum wall thickness. Example 7: The same GPPS cup has a flow path length of 104 mm. Find a more accurate clamping force needed. Flow path to thickness ratio = 104 / 0.6 = 173. From Figure 2, at 0.6 mm wall thickness, the cavity pressure is 550 bar. From the conversion tables in section 5, 1 bar = 1.02 kg/cm2. The clamping force = 550 * 1.02 * 49 = 27,500 kg = 27.5 tonnes. The above calculation has not accounted for viscosity. It turns out to be still correct as the viscosity factor for GPPS is 1.0. The viscosity factor for common resins is listed in Table 4. Example 8: The same cup as in the above example is to be made out of ABS. Find the clamping force needed. Using the viscosity factor of 1.5, the clamping force needed = 1.5 * 27.5 tonnes = 41.3 tonnes. The most accurate estimate of clamping force is done by computer simulation after the mould is designed. An example of such a package is C-MOLD. A simplified version called Project Engineer is available for downloading for a 30-day trial period from AC Technology's web site at http://www.amold.com. Resin | tonnes/in2 | tonnes/cm2 | MN/m2 | PS (GPPS) | 1.0 - 2.0 | 0.155 - 0.31 | 15.4 - 30.9 | PS (GPPS) (thin walls) | 3.0 - 4.0 | 0.465 - 0.62 | 46.3 - 61.8 | HIPS | 1.0 - 2.0 | 0.155 - 0.31 | 15.4 - 30.9 | HIPS (thin walls) | 2.5 - 3.5 | 0.388 - 0.543 | 38.6 - 54.0 | ABS | 2.5 - 4.0 | 0.388 - 0.62 | 38.6 - 61.8 | AS (SAN) | 2.5 - 3.0 | 0.388 - 0.465 | 38.6 - 46.3 | AS (SAN) (long flows) | 3.0 - 4.0 | 0.465 - 0.62 | 46.3 - 61.8 | LDPE | 1.0 - 2.0 | 0.155 - 0.31 | 15.4 - 30.9 | HDPE | 1.5 - 2.5 | 0.233 - 0.388 | 23.2 - 38.6 | HDPE (long flows) | 2.5 - 3.5 | 0.388 - 0.543 | 38.6 - 54.0 | PP (Homo/Copolymer) | 1.5 - 2.5 | 0.233 - 0.388 | 23.3 - 38.6 | PP (H/Co) (long flows) | 2.5 - 3.5 | 0.388 - 0.543 | 38.6 - 54.0 | PPVC | 1.5 - 2.5 | 0.233 - 0.388 | 23.3 - 38.6 | UPVC | 2.0 - 3.0 | 0.31 - 0.465 | 30.9 - 46.3 | PA6, PA66 | 4.0 - 5.0 | 0.62 - 0.775 | 61.8 - 77.2 | PMMA | 2.0 - 4.0 | 0.31 - 0.62 | 30.9 - 61.8 | PC | 3.0 - 5.0 | 0.465 - 0.775 | 46.3 - 77.2 | POM (Homo/Copolymer) | 3.0 - 5.0 | 0.465 - 0.775 | 46.3 - 77.2 | PET (Amorphous) | 2.0 - 2.5 | 0.31 - 0.388 | 30.9 - 38.6 | PET (Crystalline) | 4.0 - 6.0 | 0.62 - 0.93 | 61.8 - 92.6 | PBT | 3.0 - 4.0 | 0.465 - 0.62 | 46.3 - 61.8 | CA | 1.0 - 2.0 | 0.155 - 0.31 | 15.4 - 30.9 | PPO-M (unreinforced) | 2.0 - 3.0 | 0.31 - 0.465 | 30.9 - 46.3 | PPO-M (reinforced) | 4.0 - 5.0 | 0.62 - 0.775 | 61.8 - 77.2 | PPS | 2.0 - 3.0 | 0.31 - 0.465 | 30.9 - 46.3 |
Table 3. Simple clamping force estimation
Figure 1. Flow path length is measured from tip of sprue to an extremity of the article
Figure 2. Cavity pressure as a function of wall thickness and flow path length
Thermoplastics | Viscosity factor | GPPS (PS) | 1 | PP | 1 - 1.2 | PE | 1 - 1.3 | Nylons (PA6 or PA66), POM | 1.2 - 1.4 | Cellulosics | 1.3 - 1.5 | ABS, ASA, SAN | 1.3 - 1.5 | PMMA | 1.5 - 1.7 | PC, PES, PSU | 1.7 - 2.0 | PVC | 2 |
Table 4. Viscosity factor Section 2.3 to 2.15 describe other attributes of the injection unit. |
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Last Updated ( Sunday, 30 September 2007 )
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How to select Plastic Injection Moulding Machine 2 |
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Written by Administrator
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Sunday, 30 September 2007 |
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2.1 Shot weight Shot weight is an important attribute of the injection unit of a PIMM. Expressed in ounces or grams, this is by far the most commonly used single attribute to select a plastic injection moulding machine. It is not unreasonable to say that it has been abused. The reason is simple. A moulder has an article at hand to be moulded. Once the plastic material is selected, it has a weight. A PIMM with sufficient shot weight is then selected. 2.1.1 Definition of shot weight The shot weight is the measured (therefore actual) weight of the plastic 'injected' when the nozzle is free-standing (not held against the mould). The plastic used is usually polystyrene with a specific gravity (S.G.) of 1.05. This is specified in the specification as PS. 2.1.2 Shot weight in terms of the resin to be used If the article to be moulded is made of a resin different than PS, then the shot weight in the specification could not be used immediately, but must be calculated as follows: Shot weight in terms of a resin = c * b/1.05 where b = S.G. of the resin, c = shot weight in terms of PS (S.G. = 1.05) Table 2 lists the S.G. of some common resins. Example 1: POM has an S.G. of 1.42. It is to be moulded in a PIMM with a shot weight of 8 oz (in PS). This machine has a shot weight of
8 * 1.42 / 1.05 = 10.8 oz of POM. Example 2: PP has an S.G. of 0.86. It is to be moulded in a PIMM with a shot weight of 8 oz (in PS). This machine has a shot weight of
8 * 0.86 / 1.05 = 6.6 oz of PP. An 8 oz. (in PS) PIMM would not have provided the capacity needed by 8 oz. of PP. Resin | Abbreviation | S.G. at room temperature | General Purpose Polystyrene | GPPS (PS) | 1.04 - 1.09 | High Impact Polystyrene | HIPS | 1.14 - 1.20 | Acrylonitrile Butadiene Styrene | ABS | 1.01 - 1.08 | Acrylonitrile Styrene | AS (SAN) | 1.06 - 1.10 | Low Density Polyethylene | LDPE | 0.89 - 0.93 | High Density Polyethylene | HDPE | 0.94 - 0.98 | Polypropylene | PP | 0.85 - 0.92 | Plasticized Polyvinyl Chloride (soft) | PPVC | 1.19 - 1.35 | Unplasticized Polyvinyl Chloride (rigid) | UPVC | 1.38 - 1.41 | Polyamide-6 | PA-6 | 1.12 - 1.15 | Polyamide-66 | PA-66 | 1.13 - 1.16 | Polymethyl Methacrylate | PMMA | 1.16 - 1.20 | Polycarbonate | PC | 1.20 - 1.22 | Polyoxymethylene (Polyacetal) | POM | 1.41 - 1.43 | Polyethylene Terephthalate | PET | 1.29 - 1.41 | Polybutylene Terephthalate | PBT | 1.30 - 1.38 | Cellulose Acetate | CA | 1.25 - 1.35 | Polyphenylene Oxide, modified | PPO-M | 1.04 - 1.10 | Polyphenylene Sulfide | PPS | 1.28 - 1.32 |
Table 2. Specific gravity of resins at room temperature 2.1.3 Relation of shot weight to injection volume Shot weight is not equal to injection volume times the S.G. of PS. Shot weight is measured. Injection volume (see section 2.9) is theoretical. Injection volume times the S.G. of PS provides a higher value than shot weight due leakage pass the screw during injection. Also, the non-return valve at the tip of the screw moves backward a little before it reaches the closed position. Some manufacturers prefer to use injection volume as the starting point to state the shot weight of their machines, instead of using measured shot weight. See section 2.9 |
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Last Updated ( Sunday, 30 September 2007 )
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