April 26, 2022

Proper extruder maintenance and extrusion line maintenance is one of the keys to success, yet it is often overlooked until there is a problem. In this post, we’re going to take a look at some common areas one can examine to ensure that their extruder is running to the best of its abilities. If you have any questions regarding your extruder and its maintenance, don’t hesitate to contact us by reaching out to our Technical Support Team.

Barrels and Screws

In general, barrel clearance for a screw is 0.001” to 0.0015” per side per inch in diameter (smaller ½”, ¾” machines, it is usually a little more due to size constraints and tolerance stack-ups). In larger extruders, excessive clearance will show as a drop-off in output where screw RPM needs to be brought up to maintain the proper line rates and melt temperatures begin to climb.

This drop-off may not be as noticeable in the smaller machines as they may not have processes designed for high RPM. Many are run “in the mud” (a term for the old DC drive machines) and may not see a screw RPM above 20, 30, or 40, so the loss in output may not be as noticeable. But there are some indicators that a problem may require attention much sooner than one would think.

Three critical indicators that there may be wear:

  1. Gels start to become a problem. This usually happens when running natural materials for catheter shafts and balloons. Gel issues can indicate excessive clearance at the front end of the machine. If using a candle filter, filter failure may happen prematurely depending on the surface area of the filter. You might also see slightly higher melt temperatures in the barrel.
  2. New process instabilities. This could indicate excessive wear just upstream of the feed throat opening, where a good deal of the torque is applied to the screw. This is one of the reasons why Graham Engineering’s American Kuhne Brand originally came out with a replaceable feed throat insert, as only the insert is needed instead of an entire casting.
  3. Process records show increasing RPM rates at the same output levels.

Knowing the equipment is a crucial factor in extruder maintenance. Those machines that use inserts in the feed throats will usually oversize the insert and not undercut the screw in the feed section (usually smaller machines), while others undercut the screw.

Perform a visual inspection of your screws. Screw flights should maintain their squared edges. Placing a (smaller) screw on a precision granite block is a quick way to inspect flight rounding and straightness visually. Chrome-plated screws should not have flaking/peeling/worn chrome. 17-4 screws that usually darken in appearance should do so uniformly (possible hardening issues if it doesn’t). Inconel screws can twist under high loads (accurate length measurements can pick this up), and in all cases, flights should not have any cracking. Pay attention to the hub, screw feed section, and feed throat – scoring in these locations can indicate that the rear busing at the upstream side of the feed throat is worn and should be replaced. There could also be too much axial deflection of the screw. Small screws with tangential feed sections or grooves can also do this, primarily if used beyond the particular need. Another primary source of this is operators trying to force-feed a small extruder that has failed solids conveyance, as it’s easy to add too much axial load and deflect the screw. We recommend not doing this – experience is an expensive teacher.

Scoring of the flights further up in the screw can indicate that it is no longer straight enough to fit into the barrel correctly. If a clean screw does not freely slide into a clean barrel, take the screw out and put it in backward. If it slides in freely backward, the problem is usually in the back end of the screw (time to get out the dykem marking ink and find the problem if nothing can be seen visually). If it doesn’t, it may need to be straightened or replaced.

Sometimes cleaning small screws incorrectly can cause problems. Ensure those charged with cleaning small screws push them out a few flights at a time and brush them off close to the barrel opening (especially ½” -1” that will have small root diameters).

If using gauze on barrel brushes, ensure that the quill for the gearbox is cleared out.

Measurements

There are a few factors that determine how often you should measure the screw and barrel:

  1. Type of barrel (Xaloy 102, 306, 309, 800 series or equivalents)
  2. Screw materials (4140, 4140 with Colmonoy hard facing, 17-4, Inconel, CPM, etc.)
  3. Materials extruded (natural resins, barium/bismuth/tungsten filled)
  4. Machine utilization (run hours) and pounds processed
  5. Temperature profiles used (torque/motor load)
  6. Cleaning methods

In general, measuring barrels and screws is relatively easy. The equipment needed for it is not a huge capital expense (and less costly than trying to identify and resolve “phantom” problems or waiting for failure). All you need to get started is a flight micrometer and a bore gauge. We would also recommend a borescope for visual inspections of the inside of the barrel, feed throat, and rear bushing. Especially when not using a modular designed machine to reduce its time for inspection. Also, be wary of small devices with tangential feed sections. They place a great deal of axial load on the screw that will cause deflection in the feed section that can prematurely score/wear the downstream exit of the feed opening and the upstream bushing used to center the back end of the screw in the gearbox quill.

With screws, measure all the flights and root diameters, identifying the type and direction of measurement. Log the values. Note any visual anomalies as well. Use a granite block for verifying straightness if there is no other method available. Verify barrier flight and mixer differentials as these can be critical for performance.

Control panel and wiring

Control panel wiring is often overlooked in machine maintenance programs. Circuits continually heat up and cool down, even in the PLC-controlled machines. It is good to have maintenance staff go through the panel and ensure all wire terminal strip and relay connections are tight, especially in higher load AC heat zone wiring.  Please ensure power is turned off, discharged, and locked out prior to this activity. Most manuals will have torque settings for wire connections. If not, be careful not to overtighten the connections and strip the threads or the screw heads.

Ensure that all thermocouples are inserted and touching the barrel steel casing. Verify that the TC wires are still grounded to the end of the sheath via continuity to the armored case (this is always to avoid gremlins).

Barrel Fans, barrel heaters

Barrel fans always seem to be a catch-all for pellets, cleaning artifacts, and other foreign objects. Clean them out regularly. Depending on the design of the machine, pellets can fall onto the barrel. Clean the burnt resin from the heaters and other barrel areas as needed.

Gearbox

Read the manual and establish a proper procedure for the gearbox’s oil change (and don’t forget the break-in change for new machines if required). When changing the oil, have maintenance use a magnet in the old oil or run it through a screen to inspect for any large shards of metal and foreign matter. Take the time to run some purge oil through the casing to flush all the old oil out.

Breaker Plates and Sealing surfaces

Inspect breaker plate registers of the barrel and the tooling flanges to ensure the seats are flat, and the corners are not rounded. Make sure breaker plates have flat/parallel seats, the entry holes are well detailed, and the entry opening matches up well with the opening in the barrel.

Feed throat cooling

Ensure that your feed throat cooling channels are free from debris and deposits to ensure sufficient and uniform cooling. If you’re using hard water, this becomes very important. Contact your manufacturer for the best way of cleaning the cooling channels of your casting.

Upstream and Downstream equipment

There is a wide range of upstream and downstream equipment in extrusion lines, and it would be impossible to cover everything.

If you are in the medical device/pharma industry, you are in luck. All your maintenance requirements should have been spelled out in your Equipment Installation protocols. If not, an excellent place to start this outline begins with the installation of the equipment.

Synopsis

Many organizations perform their extruder maintenance based on the calendar, even when using Computer Maintenance Management (CMM) software. Preventative maintenance costs money, and over maintaining your extruder will cost you even more. These costs are why we recommend you focus on the run-time of the extruder versus the date of its last maintenance—for example, changing the oil of a gearbox that has only been run 300 hours during a calendar year.

Whenever possible, convert the calendar-based practice to run time. It would be best to consider installing a run hour meter wired into the stop/start circuits in older machines. Many devices are available with Bluetooth or WiFi connectivity to report values into a supervisory web server or CMM software systems. Newer machines with PLC controls already have this feature and can be wired into plant networks. There are also sensors available today to monitor machine vibration patterns and alert your maintenance team when something does not seem right. These are excellent options for mission-critical equipment. Most of these will connect wirelessly to your network and CMM software.

Suppose you do not use CMM software (many smaller organizations default to manual methods, which is fine). In that case, the sooner one is installed, the easier it will be to manage as an organization grows (and the less time it will take to backfill it with a large inventory of equipment that can tie up personnel for months to enter them correctly. Many low-cost options are available today (remember that preventive extruder maintenance is a primary input for ISO 13485).

 

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