Solutions for Transducer Field Returns

The Transducer Must Be Mounted Correctly

Damage to a pressure transducer is frequently the result of their installation into an improperly machined hole.  In driving a transducer into an insufficiently large or eccentric hole, the transducer diaphragm could be crushed and subsequently, the instrument will not function.

Tool kits offered for machining the mounting hole will assist in ensuring the holes are appropriately sized.  A mounting torque of 100 to 200 inch pounds (for ½-20 UNF) is vital to create an adequate seal. However, too much mounting torque will result in seizing.

A high temperature anti-seize compound should be used on the transducer threads ahead of installation to preclude seizing related issues. Transducers installed at a mounting torque beyond 500 inch pounds will be problematic to remove, even when anti-seize compound is applied.

Make Sure that the Mounting Hole Thread Size is Correct

The abrasion caused by screwing a transducer into a mounting hole with an improper thread size will cause damage to the instrument’s threads. This damage may inhibit a high quality, tight seal, leading to material leakage, and the instrument will not function effectively or safely.

To avoid thread galling, the appropriate dimensions for the mounting hole must be used. The threads are usually the industry standard of ½ - 20 UNF 2B.  A mounting well gage plug should be employed to confirm that the mounting hole is appropriately machined and cleaned.

The Mounting Holes Must Be Uncontaminated

It is vital that the transducer mounting holes are kept free from contamination and any accumulation of plastic. Prior to cleansing of an extruder, all of the transducers must be detached from the barrel to avoid instances of damage. Following their removal, there is a risk of plastic flowing into the mounting holes and hardening.

If removal of this hardened plastic residue is not carried out, extensive tip damage will occur on re-insertion of the transducers. The contaminant plastic can be removed with a cleaning tool kit. It is worth noting that overly frequent cleaning may create holes which are too deep and thus cause damage to the transducer tip.  If this is noticed, spacers should be employed to raise the transducer.

Choose an Appropriate Location

Transducers can be positioned at a number of different points: in the barrel, before a screen changer, before and after a melt pump, or in the die. When a transducer is situated too far upstream in the barrel, un-melted plastic pellets could scrape against the transducer tip, causing damage.

If a transducer is situated too far back in the mounting hole, an immobile pool of melted plastic will accumulate between the transducer tip and the screw flights.  Eventually, this plastic will degrade to carbon, which will inhibit the transmission of a true pressure signal. Conversely, if the transducer reaches too far into the barrel, the screw flights could shave off the unit’s sensor tip.

Take Care When Cleaning the Transducer

Transducers should be detached prior to cleaning an extruder barrel using either a wire brush or specialized cleaning compounds.  These can both cause great damage to the transducer diaphragm. Removal of the transducer should be carried out while the barrel is at a higher temperature and the tip should be wiped clean with a non-abrasive cloth. At this point, the transducer hole should also be cleaned using a cleaning drill/guide sleeve.

Avoid Cold Starts

The transducer and extruder can both suffer damage if the extruder is not brought up to operating temperature prior to operating the machinery.  An adequate “soak time” must be allowed for the plastic to convert from its solid to molten state.

Furthermore, it is important to note that where a transducer is detached from a cold extruder, material may stick to the transducer tip, resulting in the diaphragm tearing from the unit.  Ensure that the barrel is of a sufficiently warm temperature so that any plastic present will be softened prior to removal of the transducer.

Avoid Causing Overpressure in the Transducer

Although transducers are intended to tolerate 1.5 times overpressure, it is important to circumvent the risk of applying too much pressure by ensuring that you are employing the appropriate model designed for your range of extrusion operation pressures.

A sensible criterion is to make use of transducers that are designed to tolerate two times the rated pressure in your process. This way, the extruder would have to be functioning at an immensely high and non-safe pressure level to cause to transducer to fail.

Diaphragm Damaged by a Sharp Edge

Causes:

  • Damage through sharp edge contact (screwdriver or knife)
  • Dropped
  • Contact with degraded polymer in hole

Solutions:

  • Employ protective cap (transport, storage)
  • Use gage plug to inspect hole for burrs or hardened plastic
  • Employ Dynisco cleaning tool to cleanse hole

Diaphragm Ground Off

Causes:

  • Contact with abrasive material during process
  • Use of wire brush or wheel to cleanse

Solutions:

  • Dymax coating for abrasive applications
  • Use cloth to wipe plastic from top before cooling
  • Always avoid wire brush or cleaning wheel

Diaphragm Torn

Causes:

  • Allowing material to reach a cold or hardened state before removal
  • Shrinkage of adhesive materials
  • Sensor positioned over flite (fatigue)

Solutions:

  • Take out before cooling to prevent adhesion
  • Double-strength Diaphragm (T80) or TiN coated diaphragm
  • Reposition sensor

Diaphragm is Missing

Causes:

  • Removal in cold material condition
  • Shrinkage of adhesive constituents
  • Diaphragm fused to the material

Solutions:

  • Take out before cooling to prevent adhesion
  • Double-strength Diaphragm (T80) or TiN coated diaphragm

Diaphragm is Wavy; the Edge is Crushed on the Sensor Tip

Causes:

  • High shear tension
  • Lateral crushing during installation (cross thread)
  • Non-concentric mounting hole

Solutions:

  • Inspect the install positioning
  • Double strength Diaphragm (option T80)
  • Use gage plug to inspect hole for burrs or hardened plastic
  • Employ Dynisco cleaning tool to cleanse hole

Seal Surface is Damaged

Causes:

  • Inferior mounting hole or cross-threading

Solutions:

  • Use gage plug to inspect hole for burrs or hardened plastic
  • Confirm veracity of threads with gage plug
  • Employ Dynisco cleaning tool to cleanse hole

Shaft Bent

Causes:

  • External mechanical impact on stem

Solutions:

  • Stem is overly long – contract stem to align with depth
  • Over-torqued

Shaft Torn Off

Causes:

  • Mechanical impact
  • Thread has been fused in the hole (galled)

Solutions:

  • Apply Anti-seize to thread ahead of installation
  • Follow torque stipulations
  • Reduce galling with Hastalloy threads

Flexible Connection (Capillary) Broken

Causes:

  • Bend radius too tight
  • Damage on installation
  • Capillary cut or stretched to capacity

Solutions:

  • Use with great care as a result of exposed capillary
  • Elongate flex, or use 435XL version

Thread Damaged

Causes:

  • Inadequate mounting hole
  • Forced insertion/removal

Solutions:

  • Use gage plug to inspect hole for burrs or hardened plastic
  • Employ Dynisco cleaning tool to cleanse hole

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.

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