The Key to Success: Proper Extruder Maintenance – Barrels & Screws

September 7, 2023

Proper extrusion line maintenance is often the unsung hero in the plastics extrusion industry. It’s one of those things that can easily be overlooked until a problem arises. In the realm of extrusion, where precision and consistency are paramount, neglecting maintenance can lead to costly disruptions in your production process. This article will delve into the critical aspects of extrusion maintenance, focusing on barrels, screws, wear indicators, visual inspections, and measurements.

Let’s begin by discussing barrels and screws, the heart and soul of any extrusion system. The clearance between the screw and barrel is crucial to ensure optimal performance. Generally, the recommended clearance for a screw is 0.001 to 0.0015 inches per side per inch in diameter. Smaller machines, like those with ½-inch or ¾-inch screws, may require slightly more clearance due to size constraints and tolerance stack-ups.

However, excessive clearance in larger extruders can result in a drop-off in output. When this happens, operators may need to increase the screw’s RPM to maintain the desired line rates, leading to rising melt temperatures.

This drop-off in output may not be as noticeable in smaller machines, especially if they are not designed for high RPM. Some smaller extruders run at lower speeds, and the loss in output may go unnoticed. Nonetheless, three critical indicators suggest wear and require immediate attention:

Gels and Filter Failures: Gels become problematic when running natural materials, especially in applications like catheter shafts and balloons. This issue can indicate excessive clearance at the front end of the machine. Premature filter failures and slightly elevated melt temperatures may also occur.

New Process Instabilities: The appearance of new process instabilities could signal excessive wear near the feed throat opening. This is an area where substantial torque is applied to the screw. Some manufacturers, like Graham Engineering’s AK Brand, offer replaceable feed throat inserts to address this specific wear point.

Increasing RPM Rates: If your process records show increasing RPM rates while maintaining the same output levels, it’s a clear sign of wear and inefficiency.

Visual Inspection and Maintenance

Knowing your extruder is a critical factor in its maintenance. Different machines have different maintenance requirements. For instance, machines using inserts in the feed throats usually oversize the insert, while others undercut the screw. Performing a visual inspection of your screws is a proactive step in identifying potential issues.

Here are some key visual inspection points:

Screw Flights: Screw flights should maintain their squared edges. Placing a screw on a precision granite block is a quick way to visually inspect flight rounding and straightness.

Chrome-Plated Screws: Chrome-plated screws should not exhibit flaking, peeling, or worn chrome. Any such signs may indicate the need for replacement.

17-4 Screws: These screws typically darken in appearance over time but should do so uniformly. Inconsistent darkening may signify hardening issues.

Inconel Screws: Under high loads, Inconel screws can twist. Accurate length measurements can help detect this issue.

Flight Cracking: Check for any cracking on the screw flights, especially in critical areas like the hub, screw feed section, and feed throat. Scoring in these locations may indicate wear on the rear bushing at the upstream side of the feed throat, necessitating replacement.

Axial Deflection: Excessive axial deflection of the screw is a concern, especially in small screws with tangential feed sections or grooves. Operators should avoid forcing a small extruder with failed solids conveyance, as this can lead to screw deflection and potential damage.

Scoring and Barrel Compatibility

Scoring on the flights further up the screw can indicate that the screw is no longer straight enough to fit correctly into the barrel. To test this, insert it backward if a clean screw does not slide freely into a clean barrel. If it slides in backward without issues, the problem likely lies in the back end of the screw, warranting further inspection.

Cleaning Small Screws

Cleaning small screws is a critical aspect of maintenance. It’s vital to ensure that those responsible for cleaning these screws push them out a few flights at a time and brush them off near the barrel opening, particularly for screws with small root diameters (typically ½ to 1 inch).

Gauze on Barrel Brushes

If you use gauze on barrel brushes, clear out the quill for the gearbox regularly. Neglecting this part of maintenance can lead to unexpected issues down the line.

Measurements Matter

Lastly, measurements are a vital part of extruder maintenance. They provide precise data on the condition of your equipment. Regularly measuring and documenting key parameters, such as clearance, wear, and straightness, can help you identify issues early and prevent costly breakdowns.

Conclusion

In conclusion, proper extruder maintenance is the key to consistent and efficient production. By paying attention to the barrel and screw clearances, monitoring wear indicators, conducting visual inspections, and keeping up with measurements, you can ensure that your extrusion process runs smoothly and delivers high-quality products. Don’t wait for problems to arise; invest in maintenance to secure your extrusion success.