A melt pressure sensor is screwed back into the extruder after maintenance. The output signal is generally present, but it responds more slowly to process changes than before. In another case, a removed sensor shows clear signs of rubbing or abrasion on its tip. The question quickly arises as to whether the sensor itself is defective. However, the actual cause may already lie in the axial position of the sensor diaphragm within the mounting hole.
For reliable melt pressure measurement, the sensor tip must be positioned so that the process pressure acts directly on the separating diaphragm without an unnecessary dead space. If the diaphragm sits too far back, a small pocket forms in front of the sensor where polymer melt can stagnate and thermally degrade over time. If the sensor tip protrudes too far into the melt channel or extruder barrel, the diaphragm is exposed unnecessarily to flow and abrasive components. In the worst case, even the extruder screw can mechanically damage the sensor tip.
The correct installation depth is not determined by the thread alone. With a typical Dynisco process connection using 1/2-20 UNF, the geometrically correct 45° sealing seat is particularly important. Its position determines how far the screwed-in sensor tip extends toward the process. Deposits, an incorrectly machined hole or repeated aggressive reworking can alter this geometry.
If unusual measurement behavior or repeated diaphragm damage occurs, the sensor itself should therefore not be the only component checked. The equally important question is: where is the sensor diaphragm actually positioned relative to the internal contour of the extruder or die?
Why is diaphragm position so important?
A melt pressure sensor measures the pressure of hot polymer melt through a metallic separating diaphragm at the sensor tip. To ensure that the measured pressure is as representative of the process as possible, this diaphragm must have direct hydraulic contact with the melt. At the same time, it should remain mechanically protected within the intended contour of the measuring point.
An arbitrary position within the mounting hole is therefore not acceptable. If the tip sits several millimeters behind the actual internal contour, a narrow channel or dead space is created between the main flow and the diaphragm. The melt in this area is exchanged less effectively than in the main flow. If the sensor protrudes too far, however, the sensitive diaphragm is located directly in a mechanically and hydraulically more heavily loaded area.
For typical applications, Dynisco specifies a position with only a slight recess relative to the internal process contour. As an approximate guide, around 0.010", or approximately 0.25 mm, recessed is stated. However, this value should not be used as a universal installation dimension for every sensor version. The installation drawing of the specific sensor and the corresponding mounting hole are always decisive.
How should a melt pressure sensor generally be positioned?
When correctly installed, the separating diaphragm is positioned very close to the internal contour of the extruder barrel, die or melt channel without protruding unnecessarily into the main flow. This allows the process pressure to reach the diaphragm directly while protecting it from direct mechanical contact.
| Diaphragm position | Effect | Typical assessment |
|---|---|---|
| Too far recessed | Dead space in front of the diaphragm, stagnant melt possible | Measurement signal may become slower or distorted |
| Correct position | Direct pressure contact with protected diaphragm | Intended operating condition |
| Protruding too far into the process | Increased mechanical and abrasive stress | Possible diaphragm or sensor damage |
It is important that the correct position is not adjusted by simply tightening the sensor more or less based on visual judgment. The defined mechanical position is established by the intended sealing surface. With a correctly machined connection, proper tightening automatically places the sensor in the intended axial position.
What happens if the sensor sits too far back?
If the sensor tip sits too far back in the mounting hole, an additional cavity is created in front of the diaphragm. The melt within this area is not exchanged at the same rate as the melt in the main flow. This can create a stagnant polymer zone.
This situation is particularly unfavorable at high process temperatures. If plastic remains in the dead space for a prolonged period, it may thermally degrade, discolor, crosslink or carbonize. Deposits in front of the sensor diaphragm can then increasingly impair pressure transmission. The measurement signal may respond more slowly to process changes or behave in a way that initially appears to indicate a sensor fault.
During a material change, old polymer may also remain in this area. While the extruder itself may already be processing a new material, residues from the previous product may still remain directly in front of the diaphragm. This is another reason why dead spaces should be avoided at melt pressure measuring points.
What happens if the sensor protrudes too far into the process?
The opposite situation is even more critical mechanically. If the sensor tip protrudes beyond the intended internal contour, the diaphragm is exposed more directly to the melt flow. With filled or abrasive plastics, this can significantly increase diaphragm wear.
Installation in the extruder barrel is particularly critical. If the tip protrudes far enough into the area swept by the screw flights, the rotating extruder screw can directly contact the sensor tip. Such a collision can damage the diaphragm or shear off the entire sensor tip. If a sensor is repeatedly mechanically damaged, it should therefore not simply be replaced without first checking the mounting hole.
Even a slight excess protrusion can reduce service life without direct contact with the screw. The diaphragm is exposed more strongly to flow, shear and abrasive components. Correct installation depth is therefore especially important with glass-fiber-reinforced or mineral-filled plastics.
What role do the thread and 45° sealing seat play?
For many Dynisco melt pressure sensors, the 1/2-20 UNF process connection has become a standard. However, with this design, the actual process seal is not formed by the thread itself, but by a defined conical or 45° seating surface in the mounting hole.
The geometry of this seat therefore has two functions: it seals the measuring point against the hot polymer melt and at the same time defines the axial end position of the sensor. If the sealing surface is incorrectly positioned, damaged or machined too deeply, the position of the sensor tip changes accordingly.
| Area of the mounting hole | Function | Typical problem |
|---|---|---|
1/2-20 UNF-2B thread |
Mechanical mounting | Damaged or non-concentric thread places lateral stress on the sensor tip |
| 45° sealing seat | Process sealing and axial positioning | If too deep or damaged, the installation position changes |
| Sensor tip bore | Clearance for the sensor tip | Polymer residues, burrs or incorrect diameter can squeeze the tip |
| Transition to melt channel | Direct contact with the process melt | Excessive recess creates dead space; excessive protrusion increases mechanical stress |
It is therefore not sufficient merely to check whether the sensor can be screwed smoothly into the thread. Thread, bore diameter, concentricity and sealing seat together form the complete measuring point.
Why can the installation depth change?
A mounting hole that was originally manufactured correctly can change over its service life. Especially after a sensor is removed from a hot extruder, polymer can flow into the open connection and solidify there. If the new sensor is subsequently installed without completely cleaning the hole, it can contact the plastic residue before reaching its intended sealing surface. The sensor tip then sits too far back and the sensor may also become mechanically stressed.
The opposite can also occur. If the mounting hole is aggressively reworked during every maintenance operation using unsuitable drills, abrasives or other tools, metal can gradually be removed from the 45° seating surface. The sensor can then be screwed in farther than intended. Dynisco specifically points out that repeated or excessive cleaning can produce an excessively deep mounting hole.
Other possible causes include a mounting hole that was incorrectly manufactured from the beginning, a non-concentric connection, burrs, incorrect thread dimensions or the use of a sensor type whose process connection or tip geometry does not match the existing mounting point.
Checking the mounting hole with a gauge plug
For recurring problems, a suitable test plug or gauge plug provides much more useful information than checking the connection with a normal threaded bolt. The gauge plug replicates the relevant geometry of the intended sensor connection and therefore allows the thread, sealing surface and clearance of the sensor tip bore to be checked.
For inspection, Dynisco recommends, among other things, applying marking compound to the relevant surfaces of the gauge plug and inserting it into the cleaned mounting hole. In a correctly machined hole, contact should occur specifically on the intended 45° sealing surface. If contact marks appear on other surfaces, this may indicate incorrect geometry or lateral loading of the sensor tip.
For the widely used 1/2-20 UNF connection, the Dynisco gauge plug 200908 is available, for example. The Dynisco Cleaning Tool Kit 200100 also includes a gauge plug and tools for controlled cleaning of the relevant bore areas.
When can spacers be useful?
If a mounting hole has already been machined too deeply, it may not always be necessary to replace the complete extruder component immediately. For suitable Dynisco connections, shaped spacers are available that raise the sensor slightly in the axial direction. Dynisco offers, for example, a flanged copper spacer with part number 633511.
Such a spacer changes the position of the 45° seat and therefore prevents the sensor from being screwed too far toward the melt channel. However, only components specifically intended for this purpose should be used, and the resulting diaphragm position must be checked. A conventional washer beneath the hexagon is not an equivalent substitute because the process seal must still be correctly formed by the intended sealing seat.
Spacers are also not a general method for compensating for arbitrary mounting-hole errors. With major geometry errors, damaged threads, poor concentricity or an unsuitable sensor type, the measuring point must be professionally reworked or redesigned.
Which measurement errors can be caused by incorrect positioning?
Incorrect installation depth does not necessarily create a constant offset that can simply be removed by a zero adjustment. Depending on the cause, it can instead alter dynamic pressure transmission, thermal loading or even the mechanical integrity of the diaphragm.
| Observation | Possible relationship to installation depth | Next check |
|---|---|---|
| Pressure signal responds unusually slowly | Sensor too far recessed, polymer deposit in front of diaphragm | Remove sensor and inspect mounting hole for dead space or residues |
| Signal shows significant zero shift after reinstallation | Sensor is sitting on polymer residue or experiencing lateral stress | Clean hole and check with gauge plug |
| Measured values become increasingly implausible | Carbonized melt or damaged diaphragm | Inspect sensor tip and mounting hole separately |
| Diaphragm shows severe abrasion | Sensor may protrude too far into the melt flow | Check installation depth and process material |
| Sensor tip mechanically sheared off or severely deformed | Possible contact with screw or another moving component | Check mounting-hole geometry before installing a new sensor |
For this reason, an abnormal sensor should not be assessed solely on the basis of its electrical signal. In melt pressure measurement, the mechanical installation is an integral part of the measuring chain.
Practical example: pressure signal becomes sluggish after maintenance
A Dynisco melt pressure sensor operates reliably on an extruder for several months. During maintenance, the sensor is removed. After restart, the device still shows plausible pressure values, but rapid process changes appear much more damped on the display than before.
A sensor fault is initially suspected. When the sensor is removed again, however, hardened plastic is found in the lower area of the mounting hole. During the previous installation, the sensor was therefore unable to screw fully down to the intended 45° sealing seat. Its diaphragm was positioned farther behind the internal contour than intended, and an additional polymer pocket had formed between the melt channel and sensor tip.
The mounting hole is cleaned while warm using the intended cleaning tool and then checked with a gauge plug. The sensor can now once again be installed down to the correct sealing seat. After restart, the pressure measurement responds much more directly to process changes.
This example shows why an electrically functional sensor can still provide poor process measurement. Measurement quality depends not only on the transducer itself, but also on how the diaphragm is mechanically positioned relative to the melt channel.
Systematic inspection of the measuring point
- Safely shut down the system and completely relieve the process pressure.
- Remove the sensor at an appropriate process temperature in accordance with the manufacturer’s instructions: Solidified polymer must not damage the diaphragm when the sensor is withdrawn.
- Inspect the sensor tip: Check for dents, abrasion, cracks, a deformed diaphragm and adhering polymer.
- Clean the mounting hole: Use only suitable tools and avoid unnecessary removal of metal from the sealing seat.
- Check the thread, tip bore and 45° sealing seat using the correct gauge plug.
- Compare the installation dimensions with the drawing for the specific Dynisco sensor version.
- If the mounting hole is too deep, check whether a manufacturer-approved spacer can be used.
- Install the sensor using the specified tightening torque and avoid unnecessary mechanical side loads.
- After heating and complete pressure relief, check the zero point and then observe the dynamic measurement behavior.
Common mistakes
- Simply tightening the sensor farther: Installation depth must not be corrected by excessive tightening torque.
- Overlooking hardened polymer in the mounting hole: The sensor may then fail to reach its intended sealing seat.
- Repeatedly “clearing” the mounting hole with a conventional drill: This can gradually deepen the 45° sealing surface.
- Selecting the sensor solely by the thread: The process connection and complete tip geometry must match the mounting hole.
- Using arbitrary washers as spacers: Spacers must match the sealing geometry of the sensor connection.
- Simply replacing a damaged sensor: If the mounting hole is the cause, the replacement sensor will be damaged again.
- Setting the diaphragm flush by eye: The manufacturer’s drawing and correct position of the 45° sealing seat are decisive.
Dynisco sensors and accessories for the mounting hole
Dynisco offers numerous sensor designs for melt pressure measurement in extruders, dies and other plastics-processing systems. One example is the Dynisco PT462. The sensor has a rigid sensing stem with a flexible capillary and is available, among other versions, with a 1/2-20 UNF-2A process connection. The spatial separation of the strain-gauge housing from the hot sensor tip makes the series suitable for melt temperatures up to 400 °C.
For reliable installation, the correct accessories are at least as important as the sensor itself. For the standard connection, available accessories include the Dynisco Cleaning Tool Kit 200100, the Gauge Plug 200908 and suitable spacers. These components allow the mounting hole to be cleaned and checked without unnecessarily altering its critical geometry.
Suitable melt pressure sensors, melt pressure transmitters and accessories can be found under Dynisco products at ICS Schneider. Information on the example used here can be found under Dynisco PT462 melt pressure sensor. Suitable spacers and gauge plugs are also available in the Dynisco sensor accessories section.
Conclusion
The position of the sensor diaphragm within a melt pressure mounting hole is a crucial part of the measuring-point geometry. If the sensor sits too far back, a dead space forms in front of the diaphragm where melt can stagnate, thermally degrade and impair pressure transmission.
If the sensor tip protrudes too far into the process, on the other hand, the mechanical load on the diaphragm increases. Abrasive polymer melts can accelerate wear. In an extruder, a sensor tip that protrudes significantly can, in extreme cases, even collide with the screw and be sheared off.
The cause often does not lie in the sensor itself. Polymer residues, an incorrectly manufactured mounting hole, a damaged 45° sealing seat or repeated aggressive cleaning can alter the actual installation position. A gauge plug and the correct cleaning tool are therefore important diagnostic aids.
For reliable melt pressure measurement, the following therefore applies: always treat the mounting hole and sensor as one system, check the diaphragm position against the specific manufacturer’s drawing, avoid both dead spaces and excessive sensor protrusion, and before installing a replacement sensor first ensure that the thread, 45° sealing seat and sensor-tip bore are geometrically correct.
FAQ: Installation depth of Dynisco melt pressure sensors
How far should a Dynisco melt pressure sensor protrude into the extruder?
The exact position depends on the sensor and mounting hole. For typical Dynisco applications, the sensor diaphragm is specified as only slightly recessed relative to the internal contour. As an approximate guide, around 0.010″ or approximately 0.25 mm is stated. However, the drawing for the specific sensor is always decisive.
What happens if the melt pressure sensor is installed too shallow or too far back?
A dead space can form in front of the sensor diaphragm where polymer melt stagnates. The material may thermally degrade or carbonize, thereby impairing pressure transmission or dynamic measurement behavior.
What happens if the sensor protrudes too far into the extruder?
The diaphragm is exposed more strongly to flow and abrasive materials. If the protrusion is sufficient, the extruder screw can also mechanically contact and damage the sensor tip.
Does the 1/2-20 UNF thread determine the installation depth?
Not by itself. With typical Dynisco connections, a 45° seating surface provides the process seal and at the same time defines the axial end position of the sensor. This surface must therefore also be geometrically correct.
Can hardened polymer in the mounting hole affect the measured value?
Yes. Polymer residues can prevent the sensor from reaching its intended seat, apply direct stress to the diaphragm or partially block the pressure path in front of the diaphragm.
How do you check a Dynisco mounting hole?
A gauge plug that matches the connection geometry can be used for this purpose. It allows the thread, sealing seat and clearance of the sensor-tip area to be checked much more reliably than with a simple visual inspection.
Can frequent cleaning damage the mounting hole?
Yes. If unsuitable or overly aggressive tools remove metal, the position of the sealing seat in particular can change. The sensor may then protrude too far into the process.
What can be done if the mounting hole is already too deep?
Shaped spacers are available for suitable Dynisco connections to raise the sensor axially. Whether this solution is suitable for the specific measuring point must be checked based on the connection geometry.
Why should a damaged sensor not simply be replaced with a new one?
If an incorrect mounting hole, polymer residue or incorrect installation depth caused the damage, the replacement sensor would be exposed to the same problem. The measuring point should therefore be checked before the new sensor is installed.
Which specific Dynisco sensor is suitable for typical extrusion applications?
One example is the Dynisco PT462 with a flexible capillary section and rigid sensing stem. It is available, among other versions, with the widely used 1/2-20 UNF process connection and is designed for melt temperatures up to 400 °C.