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.