February 28, 2022
Melt Pump Tooling Control by Peter Turk
Melt Pump Tooling Control by Peter Turk
Possibly the most challenging component care for is the melt pump and its adapters. For medical device extrusion tubing the main goal of a melt pump is to both dampen out the screw pulses as well as support the processes’ ability to achieve the tight tolerances called for in medical tubing.
Melt pumps are very tricky to assemble as often the gears will not spin freely once assembled and need to be disassembled, re-polished and then reassembled. When a pump gets too easy to assemble and spins too freely this should be a warning that the pump is worn. The wear will result in pump leakage and ‘blow by’ which makes the pump ineffectual at achieving its purpose. Pump performance can be monitored by observing the screw pulses showing up at the extruder head and/or inlet melt pressure transducers. The same pulse should be dampened and hopefully eliminated as observed on the outlet/downstream transducer.
Melt pumps are very tricky to assemble as often the gears will not spin freely once assembled and need to be disassembled, re-polished and then reassembled. When a pump gets too easy to assemble and spins too freely this should be a warning that the pump is worn. The wear will result in pump leakage and ‘blow by’ which makes the pump ineffectual at achieving its purpose. Pump performance can be monitored by observing the screw pulses showing up at the extruder head and/or inlet melt pressure transducers. The same pulse should be dampened and hopefully eliminated as observed on the outlet/downstream transducer.
Care during cleaning will help minimize wear and damage and ensure easy assembly. Most of the parts are going to need to be polished with fine Scotchbrite pads and cleaned thoroughly (after polishing) with clean rags and IPA. Soaking parts with IPA during assembly and continually rotating the gears can also help in easier assembly.
Monitoring the tolerances of the melt pump components is important. Critical tolerances can be supplied by the melt pump supplier so that they can be measured. Basically, the stack ‘height’ of the gears (on their shafts if separate) and bearings/seals should match up to the main body width with tolerance. Bores in bushings and the OD of the gear shafts can also wear and potentially gall if not cleaned well. Most of these components are manufactured to tight tolerances with minimal allowance for wear.
Pump components must also be monitored for visual damage. Inspection for galling as well as gear teeth chips and breaks are some of the defects that must be checked for. Below are examples of damaged components, mostly chips, likely due to poor handling during cleaning due to parts not being carefully placed into cleaning baskets. Though it is not likely, there is also potential for foreign material to get in the pump and causing damage, but this typically means the screen pack was compromised too.
Monitoring the tolerances of the melt pump components is important. Critical tolerances can be supplied by the melt pump supplier so that they can be measured. Basically, the stack ‘height’ of the gears (on their shafts if separate) and bearings/seals should match up to the main body width with tolerance. Bores in bushings and the OD of the gear shafts can also wear and potentially gall if not cleaned well. Most of these components are manufactured to tight tolerances with minimal allowance for wear.
Pump components must also be monitored for visual damage. Inspection for galling as well as gear teeth chips and breaks are some of the defects that must be checked for. Below are examples of damaged components, mostly chips, likely due to poor handling during cleaning due to parts not being carefully placed into cleaning baskets. Though it is not likely, there is also potential for foreign material to get in the pump and causing damage, but this typically means the screen pack was compromised too.
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Mixing and matching of melt pump components is unacceptable as this will result in poor performance and potential damage as components of varying dimensions will affect the pumps performance. An example would be mismatched drive and driven gears leading to excessive blow by on the narrower gear. As already noted, these parts are manufactured to tight tolerances and pump wear will most likely vary from pump to pump, as well as with age.
Care for the adapters (aka inlet and outlet flanges) is also important. Care for these is often neglected as their ‘function’ is minimal. However, these adapters usually support the inlet and outlet pressure transducers for the melt pump operation. I have seen $80-90,000 annual expense in pressure transducer damage (replacement) due to worn out bores in the flanges. Once these flanges soften up after multiple cleanings, the (fine) threads of the transducers get damaged, subsequently damaging the transducer threads, and often the tip and diaphragm (distortion) due to misalignment.
Dynisco supplies a cleaning and measuring kit for the purpose of maintaining the transducer bores. A transducer blank with bluing ink applied (though a Sharpie works just as well) should be inserted into all transducer bores periodically to ensure the bore and threads are not damaged. Special care must be taken to ensure that the transducer tip area there is no uneven wear of the bluing ink. Uneven wear indicates the transducer is not threading in well centered. This will lead to transducer tip deformation. The threads must also be inspected to ensure no damage. There are other cleaning tools within these kits, and caution should be taken to ensure that only a trained technician use these tools.
Care for the adapters (aka inlet and outlet flanges) is also important. Care for these is often neglected as their ‘function’ is minimal. However, these adapters usually support the inlet and outlet pressure transducers for the melt pump operation. I have seen $80-90,000 annual expense in pressure transducer damage (replacement) due to worn out bores in the flanges. Once these flanges soften up after multiple cleanings, the (fine) threads of the transducers get damaged, subsequently damaging the transducer threads, and often the tip and diaphragm (distortion) due to misalignment.
Dynisco supplies a cleaning and measuring kit for the purpose of maintaining the transducer bores. A transducer blank with bluing ink applied (though a Sharpie works just as well) should be inserted into all transducer bores periodically to ensure the bore and threads are not damaged. Special care must be taken to ensure that the transducer tip area there is no uneven wear of the bluing ink. Uneven wear indicates the transducer is not threading in well centered. This will lead to transducer tip deformation. The threads must also be inspected to ensure no damage. There are other cleaning tools within these kits, and caution should be taken to ensure that only a trained technician use these tools.
Tip area to inspect after bluing to ensure no uneven wear when seated
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Transducer cleaning kit. From left to right, a bore and seat cleaner, a thread cleaner, and a blank.
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Additionally, with the melt pump adapters and any connecting components, sealing surfaces need to be kept clean and sharp so that there are no dead spots that can lead to foreign material defects in the tubing.
© Copyright Extrusion Consulting, Inc., 2022. All rights reserved.
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© Copyright Extrusion Consulting Inc. 2023 All rights reserved.