An important ingredient of "The ISI Advantage" is the vast storehouse of knowledge and on-site experience accumulated by ISI's people during more than 25 years of visiting extrusion plants which, by any measure, are among the most hostile environments for equipment and instrumentation, including our own melt pressure sensors. Observing firs-hand how these conditions can effect operating efficiency has given ISI personnel the experience to advise plant operators on ways to make their process environments safer and more efficient by avoiding or eliminating conditions that create downtime and erode productivity and profitability. The purpose of this discussion is to familiarize you with these conditions so that you can be a more effective problem-solver. ISI personnel are experts in solving problems which commonly plaque extrusion plant operations in these five areas:

1. Noise Interference

2. Mounted Holes

3. Transducer Installation

4. Diaphragm Considerations

5. Transducer Calibration

6. Transmitter Calibration

NOISE INTERFERENCE

Noise interference is often misunderstood or misdiagnosed because it seems so mysterious and complex. In reality, the problem is quite easy to solve by following well-known and simple procedures.

Noise problems caused by electrical interference are usually traceable to poor grounding, inadequate and/or insufficient electromagnetic protection in the signal processor and/or the readout instrument.

ELECTROMAGNETIC PROTECTION:

Every single instrument and transducer (transmitter) that carries the ISI name has received the C.E. mark of approval. This means that ISI products are immune to electromagnetic energy and noise caused by motors, switches, heaters, compressors, walkie-talkies and unbalanced factory loads. DC motors are particularly troublesome, but independent outside testing has shown that ISI products are exceptionally well protected against energy spikes induced by rapid switching of the DC motor control current. (Pulse width modulation.)

GROUNDING:

Because all melt pressure sensors are grounded at the tip, they are in fact grounded when they come into direct contact with the extruder. For this reason, it is imperative that the shield of the cable which connects the transducer to the instrument be attached to the sensor via the connector. This can be accomplished simply by attaching the shield under on of the screw terminals located on the transducer. Do not connect the shield to the readout instrument.

It is important that the shield be grounded at one end. Otherwise, a ground loop may be created through the shield which would defeat the purpose of a single-ended grounded shield called a "Faraday Cage," (for its developer Michael Faraday.) This common technique is used to extract noise, collect it via the shield and bring it harmlessly to ground.

DUAL GROUNDING:

There are some instances where grounding at both ends of the cable seems to be the only solution. In these cases, care must be taken to ensure that a ground loop does not exist. In order for a ground loop to exist, a potential difference must exist at the transducer and the instrument. (See Figure 1)

If V1 = V2 there is no current flow. If V1 is greater or less than V2, current Ig will flow through the shield.

UNBALANCED Y CONNECTION:

Many extrusion plants use a power service known as a Y connection to power up the plant and its equipment

L1, L2 and L3 represent the loads that are spread throughout a plant. If they are unbalance, as often happens, a current (In) will flow through the neutral. This current is, in fact, a sine wave but its frequency is three times the line frequency, or 180 cycles. Depending upon the amplitude, this neutral current can raise havoc with instruments, since the ground is varying, and not rock-stable as it should be. This current can be read with a conventional current meter.

TWISTED PAIRS:

It is always good practice to use twisted pairs for all excitation and/or signal leads. A magnetic field can induce current flow in the signal/excitation leads if a good shield (Faraday Cage) is not present. Consequently, twisting the wire leads provides a second line of defense by causing these induced currents to be phase-shifted by 180°, essentially canceling out the induced currents.

2. MOUNTED HOLES    back to top

CONCENTRICITY:

It is extremely important that the threads and mounting wells be concentric to within 0.002". This means that on a diameter line, 0.002" per side must be maintained.

If the edge of the transducer comes into contact with the side of the hole, a significant amount or torque could be developed when inserting or removing the transducer, with the result that the tip will be crushed and hence could cause damage to the diaphragm. It could even cause the weld at the diaphragm to fail.

CLEANLINESS:

Mounting holes should always be cleaned prior to installation. They must be free of burrs and any degraded material from prior extrusions.

3. TRANSDUCER INTALLATION    back to top

The installation of a transducer is always a consideration and certain rules must be followed:

SET BACK:

ISI recommends that the transducer be set back about 0.02" by using washers or shims. This will prevent damage to the diaphragm caused by high shear stress along the face of the transducer, which can actually push the diaphragm forward. This damage will show up as a wave along the front of the diaphragm. Care must be taken to ensure that this set back is not greater than 0.02". Otherwise, degraded polymer can collect inside the depression and interfere with the pressure measurement.

Nylon and polycarbonate are particularly troublesome if the set back is greater than 0.02. As the material cools down, it tends to stick to the diaphragm and, as it shrinks, it actually pulls the diaphragm into a bowed-out position. Damage will occur when the transducer is removed or when the next extrusion cycle begins. In this situation, the diaphragm will most likely be pulled off.

If possible in nylon and polycarbonate extrusion, it is better to have the diaphragm as close to flush as possible, providing the transducer is not directly over a screw flight which puts it into the path of high shear stress. When installing or removing the transducer, always keep the extruder at temperature.

PLACEMENT:

Positioning of the transducer should always be a consideration. The best position is forward of the screw where the polymer is in its most molten state. This also happens to be the point at which shear stress is minimal.

When transducers are used in reclaimed extrusion lines, they should be downstream of the feed hopper. Un-melted pellets that are close to the hopper can inflict severe diaphragm damage by producing stress cracks caused by point loading of the pellet.

TRANSDUCER STAIN GAUGE HOUSING:

The transducer strain gauge housing should always be away from the hot zone. In general, if you can place your hand on the stain gauge housing and leave it there, it is not too hot. Short-stem rigid transducers should not be mounted on top of the extruder. Rising heat from the extruder (the chimney effect) will invariably increase the housing temperature past its acceptable limits (170°F to 200°F).

4. DIAPHRAGM CONSIDERATIONS     back to top

high shear stress: One additional solution to high stress on the diaphragm is to make it thicker. However, if it becomes too thick, it gets stiffer and loses all flexibility and becomes less pressure-sensitive and more temperature-sensitive.; (An 0.008" thick diaphragm is recommended, but zero shift, which is normally 15 PSIG/100°F, will now double to 30 PSIG/100°F.)

CORROSIVE MATERIAL:

Many extrusion products require that corrosive materials be introduced into process. Needless to say, these acids raise havoc with stainless steel to a point where the diaphragm fails. In these corrosive situations it is best to use Hastelloy, a nickel-based alloy. It is always good practice to manufacture both the threads and the diaphragm out of Hastelloy.

ABRASIVE MATERIAL:

Abrasive material such as 30% glass-filled nylon is so abrasive that it wears down the transducer so that it take on the shape of the internal curvature of the barrel. The best protection is ISI's Modification 261, which was designed specifically for ceramic extrusion, ceramic being a more abrasive material than glass-filled nylon.

OTHER TYPES:

The selection of a coating is determined by the abrasiveness or corrosiveness of the material (Consult the Diaphragm Material & Coating Selection Chart which can be found on the following page.)

5. TRANSDUCER CALIBRATION    back to top

If possible, a calibration check of the transducer/transmitter should be performed on a regular basis. ISO 9000 standards dictate frequent calibration checks.

TRANSDUCER CALIBRATION:

When a transducer is shipped from ISI, the user receives a certificate that records three parameters as shown:

PARAMETER
Zero Balance
Full cale Sensitivity
R-Cal @ 80%

TYPICAL
0.3 mv/v
3.33 mv/v
2.66 mv/v

A full 10-point calibration is not normally needed by customers but it can be provided, if required, at an additional cost. These three points may be used as a reference to determine if the transducer is still within specification.

A visual examination of a transducer diaphragm should be done before proceeding with the calibration check to determine if the diaphragm is flat and free from any damage. A quick check of the three parameters may now be done. The data should be recorded and compared against the data of the transducer as received. The following table illustrates the data presentation:

The deviation in this illustration is zero, indicating that the transducer is fine. A deviation of ±0.25% is acceptable. Please note that the data are given a millivolts/volts and not in millivolts. This done in order to eliminate the possibility of the use of different power supply voltage, which will give different output volttages. Outputs in millivolts/volts in independent of the supply voltage. The user may want to go further and check the linearity of the transducer if a complete calibration system such as the IS CST2000 is available. Generally speaking, the shape of the curve is not important as long as the non-linearity is within the original tolerance of the transducer as shipped. Figure 4 depicts the normal definition of a calibration curve.

6. TRANSMITTER CALIBRATION    back to top

The transmitter calibration is not as straightforward as calibrating a transducer. This is due to the fact that an amplifier is built into the transmitter. The purpose of the amplifier is to give the user the ability to re-zero the transmitter at temperature. This means that zero at room temperature will be negative after it is removed from the extruder, making it difficult to check the original zero balance, which was , prior to installation, noted and recorded at room temperature. The user should do a visual diaphragm inspection and then perform a 5- or 10- point calibration check.

SENSING DIAPHRAGM

The ISI fluid transducer is a sealed system. The sensing diaphragm is one of the most important components of the transducer.. The standard diaphragm is a one-piece construction, machined out of 15-5PH stainless steel and is coated with Armoloy to increase the surface hardness. It has a nominal thickness of 0.0045 inches. ISI offers other types of diaphragms suited for use in harsh or abusive application - see diaphragm material chart.

If the sensing diaphragm becomes damaged; ie: dented, gouged or deformed in any way, the transducer will no longer provide accurate reading and should be returned for repair and/or replacement.

MOUNTING HOLE

A bad transducer mounting hole is one of the key causes of damaged to the transducer. Extruder barrel wear should be monitored. As the extruder barrel wears, the transducer tip will get closer and closer to the barrel bore until flush with the sidewall. Once the tip is flush, it will then wear at the same rate as the extruder barrel. To alleviate this problem, ISI recommends that the transducer be recessed 0.015 inches from the barrel bore. A copper shim can be manufactured (to seal on the 45 degree seating surface) that will act as a spacer backing off the transducer tip from the inside barrel bore.

If the transducer tip protrudes into the extruder barrel, it is subject to unnecessary wear and is vulnerable to damage by a passing screw flight.

Another common mounting hole problem, that is a result of barrel wear, is roll over. Over time the opening onto the barrel is reduced from the recommended dimension of .312"/.314". If a transducer is installed in this under-size hole, the tip becomes crimped or forced into a cone shape, deforming the sensing diaphragm and damaging the transducer.

CLEANING & GAGING THE MOUNTED HOLE

The mounting hole should be cleaned and gaged prior to the installation of a new transducer or replacement of an existing transducer. A three piece cleaning tool kit is available from ISI in order to clean the mounting hole and also includes a gage plug in order to check the integrity of the hole. Part number is CT-3.

MOUNTING THREADS

The transducer mounting threads should be 1/2-2ounf-2b and should be cleaned and inspected prior to the installation or reinstallation of a transducer. The use of a high temperature anti-seize compound will help in preventing galling and will allow easy removal of the transducer. Copper graphite anti-seize compounds should be used such as C5-a by Felpro.

MOUNTING TORQUE

Although the transducer is capable of being installed at a mounting torque of up to 500 inch/pounds without damage or zero shift, an adequate seal is made with 100-200 inch/pounds. This can be achieved by installing finger tight and tightening a quarter-turn with a wrench. Excessive mounting torque will cause seizing and make the transducer difficult to remove.

OPERATING TEMPERATURE

The transducer tip is capable of operating at temperatures up to 750°F. (400°C). The transducer electronics are capable of operating at temperatures of up to 160°F (70°C). It is important that the electronic housing not be placed on top of a heater or heat source.

COLD STARTS

One of the most common causes of diaphragm damage is cold starts. A combined temperature and pressure transducer (Models 0102, 0162, 0112, 0122) can be used to insure that the extruder barrel is up to temperature prior to the rotation of the extruder screw.

REMOVAL/REPLACEMENT

The transducer should only be removed or replaced while the machine is at operating temperature and the polymer is liquid. Removal of the transducer from a cold extruder will cause diaphragm damage due to polymer adhesion. A transducer should never be installed into a mounting hole where there is solid polymer. If the diaphragm is forced against a solid, the transducer can easily be overloaded and left with a very high zero offset - too high to be rezeroed by the zero potentiometer on the instrument.

* Used in applications which are extremely abrasive - such as: ceramics or glass filled nylon

The standard ISI transducer diaphragm is machined out of a single piece of type 15-5PH Stainless Steel, heat treated and then Armoloy coated. This material gives ISI transducers the traverse strength and toughness needed for most standard applications.

There are, however, certain extrusion processes that require different types of diaphragm materials and/or coatings. ISI is abe to supply customers with diaphragms and coatings specifically suited to their needs and applications.