Melt Pressure Technical Information

  • Mounting hole concentricity
  • Transmitter calibration curve

MPI Melt Pressure’s engineers have over 20 years of experience in solving technical problems in some of the most difficult environments for plastic melt pressure sensors and instrumentation. We have experienced just about every type of problem that can be encountered, and use this vast knowledge base to assist all our current customers.
 
This page contains answers to many of the general application FAQs our customers ask. Topics covered include:
  1. New Installation Guidelines
  2. General Operational guidelines
  3. Transducer Calibration
  4. Transmitter Calibration
  5. Pressure Convertor
  6. TIP Dimensions
  7. Manual Tuning of Melt Pressure Controllers
  8. Melt Pressure Technology Comparison

1. New Melt Pressure Transducer Installation Guidelines

A. Transmitter Tip Options

Option Code Description Abrasion Corrosion Application
S Inconel 718 diaphragm (MPI standard) Excellent Good Most
I Inconel 718 diaphragm + threads Excellent Good Temperatures above 750°F
H Hastelloy® C-276 tip + threads Fair Excellent PVC/Teflon® processing
T TiN (titanium nitride) coating Excellent Good High abrasion
C CrN (chromium nitride)  Excellent Good High abrasion
D TiAIN (titanium aluminum nitride) Excellent Excellent Best abrasion & corrosion resistance
  •  Most applications: Select Inconel diaphragm.
  • Corrosive applications (materials that produce corrosive acids): Select thread and diaphragm made of Hastelloy.
  • Abrasive materials (such as glass-filled nylon): Select D-TiAIN tip coatings

B. Hole Location

The best position in which to mount your transducer is in front of the screw where the polymer is in a good molten state and shear stress is minimized. For reclaimed extrusion lines, care must be taken that the transducer is situated far enough from hopper to ensure all pellets are melted, as partially melted pellets can damage the tip.
 
Transducer housings should be installed in a location where temperatures will not exceed 150°F.
 
Care must be taken if the transducer is mounted on the top of extruder. Heat rises and will heat the housing. If you can touch the housing without burning your hand, the location is probably acceptable. If necessary, a flex armor-style device may be recommended to move the housing away from the heat source.

C. Drilling Mounting Hole.

For best results, follow the outlines provided in the drill kit manual. Other points to be especially aware of include: 
 
a. Mounted Hole Concentricity
 
Transducer tips must be mounted in the center of the hole (within 0.002"). If the transducer tip is not centered, it may become deformed during installation or removal. This can cause performance errors or even cause the tip to fail entirely.
 
 
b. Recessing the Tip
 
Transducer tips should be installed within 0.08" to 0.1" of the interior of the extruder barrel. Any further than this can create a cavity in which polymers can collect, harden, and ultimately interfere with the pressure reading.
 
Some materials (such as nylons and polycarbonates) will actually shrink and pull the diaphragm towards to interior of the extruder. This sometimes causes the diaphragm to be ripped off. These materials should be recessed to approximately 0.05".

D. Transducer Installation

  • Use a high temp anti-seize compound to make removing the transducer easier
  • Mounting torque: 100-200inch/lbs. (max. 500inch/lbs.)
  • Polymer must be in a molten state when transducer is installed

E. Electrical Hookup

  • Care must be taken to avoid electrical noise interference from motors, heaters etc.
  • Grounding: Be sure to ground the cable on the transducer, but NOT on the instrument side
  • Twisted pairs will reduce problems from induced currents

F. Perform Zero & Span Calibration 

Follow instrument guidelines.
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2. General Operational Guidelines

A. Avoid Cold Starts

If the extruder is not heated up properly, the polymer can be in a solid state by the transducer. If the extruder is started with polymer in a solid state, the diaphragm can be ripped off.

B. Transducer Removal 

The transducer should only be removed or replaced while the machine is at operating temperature and the polymer is in a liquid state.
 
Removing the transducer from a cold extruder may cause the polymer to adhere to the diaphragm; it can then pull off or damage the diaphragm. 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 re-zeroed by the instrument’s zero potentiometer—or damage the tip extensively.
 
Always clean and check tip recession before reinserting transducer.

C. Tip Cleaning

If the transducer is removed from the barrel while it is still warm, the tip can be carefully cleaned with a clean soft cloth.

D. Hole Cleaning

Mounting holes should always be cleaned prior to installation. Any burrs or hardened polymer material may cause the diaphragm to be damaged during installation.
 
As the inner extruder barrel wears, it can reduce the size of the transducer hole. We offer cleaning kits for 1/2”-20UNF and M18x1.5 transducer holes. Using a cleaning kit will help ensure each hole is maintained at the proper size.
 

E. Check Tip Recession Before Reinserting

As the extruder barrel wears, the transducer tip will get closer and closer to the inner barrel wall. If it is allowed to become flush with the extruder inner barrel wall, it will wear at the same rate as the inner wall. MPI recommends maintaining a minimum 0.02" recession. Copper shims can be made to push the tip away from inner wall.

F. Keep Transducers Dry

Do not allow oil or water to come in contact with transducer or cables.
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3. Transducer Calibration

If possible, a calibration check of your transducer should be performed on a regular basis: every six months, or yearly. This process should be performed by MPI or another qualified calibration facility.
 
When a transducer is shipped from MPI, the user receives a certificate that records three parameters as shown:
Parameter Value
Zero 0.1mV/V
Full Scale 3.33mV/V
RCal (80%) 2.66mV/V

Data is provided as mV/V format to account for different instrument supply voltages. This can be used to help calibrate the instrument.

4. Transmitter Calibration

If possible, a calibration check of your transmitter should be performed on a regular basis: every six months, or yearly. Calibration should be performed by MPI or another qualified calibration facility.


A 10-point calibration can be performed by MPI to produce a very accurate curve that can increase the system accuracy. Return to Top of Page

5. Pressure Converter Table

psi bar Kg/cm2 kpa Mpa
1 0.068948 0.070307 6.8948 0.0068948
14.504 1 1.0197 100 0.01
14.223 0.98066 1 98.066 0.098066
0.14504 0.01 0.010197 1 0.001
145.04 10 10.197 1000 1
1psi = 0.068948 bar = 0.070307 Kg/cm2 = 6.8948 kpa = 0.0068948Mpa Return to Top of Page

6. Tip Dimensions

Thread A B C D1 D2 D3
1/2-20UNF 0.22"-0.01"
5.56mm-0.26
0.44"
11.2mm
0.62"
15.74mm
0.31"-0.002"
7.8mm-0.025
0.41"-0.001"
10.5mm-0.025
0.42"
10.67mm
M18x1.5 0.24"-0.01"
6mm-0.26
0.58"
14.8mm
0.75"
19mm
0.394"-0.002"
10.0mm-0.05
0.63"-0.003"
16.0mm-0.03
0.63"
16.0mm
M14x1.5 0.21"-0.01"
5.4mm-0.26
0.65"
16.4mm
0.42"
10.6mm
0.31"-0.002"
7.8mm-0.025
0.46"-0.001"
11.8mm-0.025
0.47"
12mm
M10x1 0.244"-0.01"
6.20mm-0.26
0.425"
10.79mm
0.38"
9.7mm
0.2365"-0.002"
6mm-0.05
0.335"-0.001"
8.51mm-0.05
0.334"
8.48mm
 
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7. Manual Tuning of Melt Pressure Controllers

Extreme care must be taken when tuning the melt pressure controller on an extruder, or damage to the extruder can occur.
 
Standard autotune controllers cannot be utilized because they introduce a step change in the controller output, then monitor the process value (pressure). Extruder drives cannot handle a large sudden change in output without suffering some damage.
 
Standard manual tuning methods cannot be used because they, too, introduce step output changes.
Luckily, most melt pressure controller loops can be tuned by starting with one set of values and making slight changes.
 
Starting Values
  • Proportional band: 60%Span
  • Integral: 15sec
  • Derivative: 0sec
  • Input Filter: 2sec
  • Output rate limiter: 15sec for 100% change
Steps to Successful Tuning
  1. Enter starting values; loop should be very stable
  2. Enter changes to pressure setpoint; observe
  3. If control is too slow to respond, reduce PB to 50%
    1. Check by entering setpoint changes; observe
  4. If control is too slow to respond, reduce PB to 40%
    1. Check by entering setpoint changes; observe
  5. If control is too slow to respond; reduce Integral to 9sec
    1. Check by entering setpoint changes; observe
  6. Keep reducing until achieving desired response
  7. If initial settings are unstable, increase input filter to 5sec and increase PB and Integral until stable
If still experiencing difficulties problems, contact the factory.
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8. Melt Pressure Technology Comparison

PARAMETER MERCURY FILL NaK FILL OIL FILL
Diaphragm thickness 0.0045" 0.005"-0.010" 0.0045"
Fatigue resistance Excellent Excellent Excellent
Max. temperature (F) 800° 1,000° 660°
Fill classification Hazardous Non-hazardous Non-hazardous
Approved for food applications No Yes Yes
Approved for medical application No Yes Yes
Temperature drift (psi/100°F) 15 10 30
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