MPI Melt Pressure Technical Information

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Melt Pressure Transducer
- Technical Information

MPI MELT PRESSURE engineers bring over 20 years of experience visiting and solving technical problems in the plastic extrusion industry in some of the most difficult environments for plastic melt pressure sensors and instrumentation. During that time we have experienced just about all the types of problems that can be encountered, and use this vast knowledge base to assist all our current customers.

SOME of the general application questions our customers ask are provided on this page.

1. New Installation Guidlines

2. General Operational guidlines

3. Transducer Calibration

4. Transmitter Calibration

5. Pressure Convertor

6. TIP Dimensions

7. Melt Pressure Controller - Manual Tuning

8. Melt Pressure Technology - Comparison

1. NEW MELT PRESSURE TRANSDUCER INSTALLATION GUIDELINES.

A. DIAPHRAGM SELECTION

Diaphragm Material
Application Performance

Diamond Particulate on Inconel
All Applications
Ultimate Coating for High Abrasion

Good Corrosion Resistance

Triple Tip - Inconel 718

3X Thicker Diaphragm

Minimum 3000psi

Excellent Abrasion Resistance

Good Corrosion Resistance

Inconel 718

MPI Standard Diaphragm

3X harder than stainless steel

All applications

Excellent Abrasion Resistance

Good Corrosion Resistance

HASTALLOY C-276

PVC or TEFLON processing

or any process generating :

HCL, HF, HBn, HI

Max Temp - 600oF

Fair Abrasion Resistance

Excellent Corrosion Resistance

Titanium Nitride
High Abrasion
Good Abrasion Resistance

Fair Corrosion Resistance

Armoloy
Low Abrasion Applications
Fair Abrasion Resistance

Fair Corrosion Resistance

Electroless Nickel
Low Abrasion Application
Fair Abrasion Resistance

Fair Corrosion Resistance

MOST APPLICATIONS:

Select Inconel Diaphragm.

CORROSIVE MATERIALS (That produce corrosive acids)

Select thread and diaphragm made of Hastelloy.

ABRASIVE MATERIAL (Such as glass filled nylon):

Select Diamond Particulate tip coatings.

B. HOLE LOCATION

The best position to mount the 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. Partially melted pellets can damage the tip.

Transducer Housing should be installed such that it doesn't get beyond 150F.

Care must be taken if transducer is mounted on the top of extruder. Heat rises and will heat the housing. If you can touch the housing, it is probably fine. If necessary, flex armour style may be recommended to get housing away from heat.

D. DRILLING MOUNTING HOLE.

Follow outlines in the DRILL KIT MANUAL

Some points to take care.

a. MOUNTED HOLES CONCENTRICITY:

  The transducer tip must be mounted in the center of the hole (within 0.002").

  If the tip is not in the center, the transducer tip can be deformed during installation or removal.
  This can increase the error, or even cause the tip to fail prematurely.

b. RECESSING OF THE TIP

The transducer tip should be installed within 0.08" to 0.1" of the interior of the extuder barrel. Any further, any this can create a cavity for polymers to collect, harden and then 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 approx 0.05".

E. TRANSDUCER INSTALLATION

Use a high temp anti seize compound to make it easier to remove transducer.

Mounting torque - 100-200inch/lbs (max 500inch/lbs)

Polymer must be in a molten state when transducer is installed.

F. ELECTRICAL HOOKUP

Care must be taken to avoid electrical noise interference from motors, heaters etc..

Grounding - It is important to ground the cable on the transducer, but NOT on the instrument side

Twisted Pairs - Reduces problems from induced currents.

G. PERFORM ZERO AND SPAN CALIBRATION

Follow instrument guidelines.

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 then started, 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.

Removal of the transducer from a cold extruder may cause the polymer to adhere to the diaphragm , and 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 rezeroed by the zero potentiometer on the instrument or even damage the tip altogether.

ALWAYS CLEAN and CHECK TIP RECESSION before reinserting transducer.

C. TIP CLEANING

If the transducer is removed from the barrel and 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.

Cleaning kits are available from MPI for 1/2-20UNF and M18x1.5 transducer holes and this will help ensure hole is maintained at the right 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 be 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.

3. TRANSDUCER CALIBRATION

If possible, a calibration check of the transducer/transmitter should be performed on a regular basis - yearly or every 6 months. The unit should be performed by MPI or a 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 the transducer/transmitter should be performed on a regular basis - yearly or every 6 months. The unit should be performed by MPI or a qualified calibration facility.

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

5. PRESSURE CONVERTOR TABLE

psi

bar

Kg/cm2

kpa

Mpa

1

0.068948

0.070307

6.8948

0.0068948

14.504

1

1.0197

100

0.1

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 = 0.068948bar = 0.070307Kg/cm2 = 6.8948kpa = 0.0068948Mpa

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

7. MELT PRESSURE CONTROLLER - Manual Tuning

Tuning the melt pressure controller on an extruder must be performed with much care, 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 with 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%

- check by entering setpoint changes - observe

4. If control is too slow to respond - reduce PB to 40%

- check by entering setpoint changes - observe

5. If control is too slow to respond - reduce Integral to 9sec

- 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 having problems - CONTACT THE FACTORY.

8. MELT PRESSURE TECHNOLOGY - COMPARISON

PARAMETER

MERCURY FILL

PUSH ROD

OIL FILL

Diaphragm Thickness

0.0045"

0.005"-0.010"

0.0045"

Fatigue Resistance

Excellent

Good

Excellent

Max Temperature ( deg F)

800

1000

660

Fill Material

Mercury

Push Rod

Oil

Fill Classification

Hazardous

Non-Hazardous

Non-Hazardous

OK in Food Application

No

Yes

Yes

OK in Medical Application

No

Yes

Yes

Temperature Drift (psi / 100F)

15

10

30

Calibrations

Instrument & Sensor Calibrations
Range of Manufacturers
Omron
RKC
Eurotherm
Athena

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