The measurement of stress might be essentially the most essential measurement in extrusion. The stress inside the extrusion die determines the output of the extruder and stress variation causes output variation. A pressure-time growth plot permits quantitative willpower of the stableness of the extrusion course of. Pressure strategies administration permits further regular extrusion.
In addition to, stress measurement is essential to safety in extrusion. Extreme pressures can occur inside the extrusion course of which will end in extreme accidents. For that motive, working an extruder with out stress measurement is awfully unsafe. Extruders should be equipped with automated shutoff. This attribute will shut off an extruder routinely when the stress reaches an important price.
For these causes, you’ll need to ensure that your stress transducer is working accurately and in a position to measuring course of variation precisely. The extrusion course of is inclined to short-term stress variation. Fast-term proper right here refers to emphasize variations that occur inside one second. If actuality, these short-term stress variations occur in every extrusion course of. To grab these variations, the stress transducer has to have a fast dynamic response.
This textual content describes a simple check out approach for stress transducers that makes use of a soften transfer indexer (MFI) modified to permit testing of stress transducers.
The MFI is a usually used check out tools to search out out the transfer habits of a molten plastic. It is primarily a ram extruder the place plastic is extruded by inserting a weight on excessive of a plunger (see Decide 1).
FIG 1 – Soften transfer indexer. Provide (all pictures): C. Rauwendaal
The MFI has a heated barrel. Plastic pellets are positioned contained within the barrel and heated until the plastic is molten and on the required check out temperature. At the moment, a weight is positioned on excessive of the plunger. This weight pushes the molten plastic out of the soften index die as a single strand with a diameter of about 1-2 mm.
The MFI price is the amount of plastic extruded in 10 minutes. The amount is expressed in grams. A soften transfer index of three signifies that 3 grams of plastic is extruded in 10 minutes. A extreme MFI price (as an illustration, 10 g/10 min.) signifies a low-viscosity plastic and a low price (as an illustration, 0.5 g/10 min.) signifies a high-viscosity plastic. The time interval “fractional soften” plastic is used for plastics with an MFI price decrease than 1.0 — these are high-viscosity plastics.
Soften Circulation Indexer Modified to Check out Pressure Transducers
On this analysis, an MFI was modified to permit testing of stress transducers. The MFI die was eradicated and altered with an adaptor with a port for a stress transducer. The MFI was full of a high-viscosity silicon oil with a viscosity of about 500 Pa.s. This viscosity is close to that of a typical extrusion-grade plastic. The consistency of this oil is very similar to that of silly putty.
The stress transducer is positioned inside the adaptor and a weight is positioned on excessive of the plunger. The burden determines the stress exerted on the stress transducer. Decide 2 reveals {a photograph} of the actual MFI discharge end. The MFI was purchased on Amazon for decrease than $2,000. The mannequin determine of the MFI was not clear. The instructions suggested that the MFI was made in China.
FIG 2 – Discharge end of a modified MFI.
Only a few factors had been encountered inside the testing of stress transducers. It was found that giant weights (5 kg and heavier) precipitated leakage on account of the clearance between the piston and the cylinder was too large. This was fixed by making a custom-made piston with an enormous outside diameter to forestall leakage. It was moreover found that the temperature sensor was not functioning inside the MFI. In addition to, the temperature sensor did not match accurately inside the sensor correctly inside the cylinder. Consequently, the MFI could not be heated. This was fixed by placing in a model new RTD temperature sensor and drilling the outlet of the sensor correctly to a diameter that ensured an excellent match of the temperature sensor.
The stress measurements had been collected by an Incióna information acquisition system. Decide 3 reveals {a photograph} of the Inciona information assortment system. Incióna is a specialised engineering and manufacturing suppliers company based in California.
The black subject on the bottom is the exact information assortment system. The sector on excessive is the power present. The measurement information is shipped to the cloud. The data is collected and displayed using Grafana. That’s a multiplatform, open-source analytics and interactive visualization internet utility. The data was downloaded as a csv file for further detailed analysis in Excel.
FIG 3 – Inciona information assortment system.
The stress on the transducer is about by the general plunger weight and the piston diameter. With the general plunger mass (Mp in kg) and the piston diameter (Dp in meter), the stress (P in Pascal) is about from the subsequent expression:
P=12.5 Mp/DP2
5 weights had been obtainable for the stress transducer testing, 0.875 kg, 0.960 kg, 1.200 kg, 1.640 kg, and 5.000 kg. By together with a second weight on excessive of the first weight, the load elevated from 0.875 to 1.835, to a few.035, to 4.675, to a most of 9.675 kg.
The stress transducer that was examined was a mixture stress/temperature sensor made by Terwin, model 2076JNN10MSP268 with quite a lot of 0-20 bar (0-3,000 psi).
It’s a “fill-free” transducer, which means there is not a fill liquid; on account of this reality, no concern about Mercury contained within the transducer. On this transducer, there is a mechanical connection between the first and second diaphragm with a cylindrical rod. This transducer was chosen because of the fast response time reported by the supplier. The response time specified by the supplier was decrease than 2 milliseconds.
Check out Course of 1
The stress transducer was examined by inserting an increasing number of larger weights on the plunger. Each stress was maintained for 1 minute. When the utmost weight (9,675 grams) was reached, the stress was maintained for 5 minutes. Then the load was eradicated one after the opposite, as soon as extra sustaining each stress for 1 minute. Thus, the pressures had been ascending inside the first 300 seconds, saved mounted for the next 300 seconds and descending for the ultimate 300 seconds.
Analysis of Check out Data of Check out Course of 1
The measured stress versus time graph is confirmed in Decide 4. Pressure is plotted alongside the vertical axis in psi, time is plotted alongside the horizontal axis in seconds. The pressures improve in 5 steps over a interval of 300 seconds. Each stress is maintained for 1 minute. The utmost stress is reached after 300 seconds. This stress is maintained for 300 seconds after which weights are taken off with as soon as extra each stress maintained for 1 minute.
FIG 4 – Pressure vs. time in check out course of 1.
Decide 4 reveals that when the 875-gram weight is added, the stress will improve slowly over the 1-minute interval and does not attain a gradual price inside 1 minute. The identical pattern occurs when the second 960-gram weight is added. When the third 1,200-gram weight is added, the stress will get close to reaching a gradual price after 1 minute. When the fourth 1,640-gram weight is added, the stress almost reaches a gradual price after 1 minute.
When the fifth 5,000-gram weight is added, the stress reaches a gradual price in about 30 seconds. This stress is maintained for 300 seconds. This information reveals {{that a}} common stress price is reached in decrease than 1 minute solely when the 5,000-gram weight is added. For all completely different weights, it takes longer than 1 minute for the stress to realize common price.
These outcomes level out that for pressures below 100 psi the response time for the transducer is longer than 1 minute. The response time for pressures rising to 200 psi the response time is lowered to about half a minute. Apparently, the response time specified by the supplier was decrease than 0.002 seconds. Due to this the measured response time is about 4 to 6 orders of magnitude higher than the specified values — a disturbing discrepancy!
The pressure-time curves for the descending stress roughly adjust to the curves for ascending stress with regard to the time it takes to realize common values. In descending stress, the common values deviate from these obtained with ascending stress. For instance, with a weight of 4,675 grams the common stress in ascending mode is about 100 psi and 130 in descending mode. That signifies considerable hysteresis inside the stress measurement.
Check out Course of 2
Throughout the second check out course of, the stress transducer was examined by starting with the most important weight (5,000 grams) after which inserting an increasing number of smaller weights on the plunger. Each stress was maintained for 1 minute. When the utmost weight (9,675 grams) was reached, the stress was maintained for 5 minutes. Then the weights had been eradicated one after the opposite, as soon as extra sustaining each stress for 1 minute. The two check out procedures are confirmed in Decide 5 as a bar chart.
FIG 5 – Check out procedures 1 and a few displaying weight vs. check out amount.
Decide 6 reveals stress versus time check out course of 2, revealing a definite pressure-time pattern for course of 2 compared with course of 1. With an increase in weight from 0 to 5,000 grams, the stress ranges off in about 20 seconds. With an increase in weight from 8,800 to 9,675 grams, the stress ranges off in about 5-10 seconds. With course of 1, the response events ranged from 30 seconds to better than 1 minute. With course of 2, the response events ranged from 5-20 seconds.
FIG 6 – Pressure vs. time in check out course of 2.
Clearly, the measured response events rely strongly on the sequence of weights added or away from the plunger. It should be well-known, nonetheless, that even with the lower response events measured with course of 2, the measured values are nonetheless orders of magnitude elevated than the specified values. Decide 7 reveals stress vs. time for the first 120 seconds of course of 2.
FIG 7 – Pressure vs. time for the first 120 seconds.
When the 5,000-gram weight is added it takes about 2 seconds in order so as to add the load as confirmed by the peak between 23 and 25 seconds. The stress will improve shortly between 25 and 26 seconds after which ranges off over the next 10 seconds. If we define the response time as a result of the time for the sensor finding out to realize 99.3% of the general step change, the response time is about 10 seconds.
The response time specified by the supplier of the stress transducer is 0.002 seconds. That’s three to 4 orders of magnitude earlier than the measured response time using the soften transfer indexer.
Future Work
The MFI tester is restricted inside the pressures which may be generated inside the cylinder. The utmost stress is about 200-250 psi (1.4-1.7 MPa). Higher pressures may be created using the lever principle.
Decide 8 reveals how the drive on the plunger may be magnified by inserting a weight a distance L1 from a pivot degree with the plunger a distance L2 from the pivot degree. This affiliation permits the drive to be magnified by a component L1/L2. A sensible price of L1/L2 is 5.
FIG 8. Energy magnifier.
The gradual response time of the stress transducer examined makes it robust to analysis short-term stress variations inside the extrusion course of. On this context, short-term variations are fluctuations that occur inside one second. These short-term variations occur in every extrusion course of. It is essential to have a measurement system that allows detection of these short-term variations. For that motive, you’ll need to obtain good information on the response time of stress transducers used inside the extrusion course of. Pressure transducers with response events higher than 1 second are normally not applicable for determining short-term stress variations inside the extrusion course of.
The soften transfer indexer provides a helpful approach to verify stress transducers. MFI testers are extensively obtainable and so they’re low-cost. The MFI tester permits assortment of serious stress information in a relatively transient time. It provides affordable measures of the response time of a stress transducer. This information is essential inside the interpretation of stress information measured in an extrusion course of.
ABOUT THE AUTHOR: Dr. Chris Rauwendaal is a well-known author, lecturer, researcher, entrepreneur and advertising guide inside the self-discipline of extrusion. He holds fairly just a few patents and has written better than 200 articles and 7 books related to extrusion, mixing, injection molding and statistical course of administration. A fellow of the Society of Plastics Engineers (SPE), he is the developer of the CRD, VIP and ASM mixing utilized sciences that benefit from sturdy elongational transfer to boost mixing in extrusion and molding. Rauwendaal moreover developed the HHT (high-heat-transfer) extruder screw designed to boost cooling in foam tandem and completely different extrusion operations. In 1990, he primarily based and stays to be president of Rauwendaal Extrusion Engineering. Rauwendaal was inducted into the Plastics Hall of Fame in 2024. Contact: 530-269-1082; chris@rauwendaal.com; rauwendaal.com.