How to remove the screw from an injection molding machine

To remove the screw from an injection molding machine, heat the barrel to processing temperature, purge it empty, take off the nozzle and the barrel head, unscrew the screw tip while it is hot, release the screw from its drive, and then push or pull the screw out of the front of the barrel while supporting its weight. The exact method depends on the machine, so follow the machine manual; the steps below are the general sequence and the precautions that apply to most horizontal machines.

The screw is item 4 in the drawing. For what each part does, see the barrel and screw guide.

Before you start

• Read the machine manual for the screw removal method, the tools it calls for, and the torques for refitting.
• Plan the material. Purge out any heat-sensitive material, such as PVC or POM, with a stable purging material before you start, because it degrades while the barrel stays hot.
• Tools. A screw head wrench that matches the tip, a wrench for the nozzle flats, a torque wrench, brass brushes and scrapers, cotton rags and a cleaning rod, high-temperature anti-seize compound, and supports or V-blocks for the screw.
• Lifting. A sling and a crane or hoist rated for the screw's weight; long screws need support at more than one point.
• Protection. Heat-resistant gloves, a face shield and long sleeves.
• Safety. Parts of the job need the machine powered, with heaters on and the injection unit moving in manual mode. Follow your site's lockout procedure for every step done inside the machine.

Removing the screw step by step

1. Heat and soak. Bring the barrel to processing temperature and let it soak so all the plastic is molten.
2. Purge empty. Close the hopper slide gate and run the screw until no more material comes out. Keep the heaters on.
3. Retract the injection unit fully from the mold. On machines whose injection unit swivels, swing it out to give room at the front.
4. Remove the nozzle. Take off its heater band and thermocouple and unscrew it hot. See the nozzle guide. Never stand in front of the nozzle while the barrel is heating.
5. Remove the barrel head. Support it, loosen the bolts a little at a time in a cross pattern, and lift it off. See the barrel head guide.
6. Expose the screw tip. In manual mode, at low speed and pressure, inject forward until the tip stands out of the barrel.
7. Remove the screw tip. Unscrew it hot with the screw head wrench; most tips have a left-hand thread. Slide off the check ring and the seat ring. See the screw tip guide. On some machines it is easier to remove the tip after the screw is out, but do it before the tip cools.
8. Release the drive end. At the rear, remove the parts that hold the screw shank in the drive coupling, such as a split collar, retaining ring or keys. The design varies by machine.
9. Push the screw out. Use the injection stroke, with a pusher or extension piece if the manual calls for one, to move the screw forward out of the barrel.
10. Pull it clear. Take the weight with the sling as the screw comes out, so it does not drop onto the edge of the bore or bend. Lay it on supports spaced along its length.

Cleaning and inspecting while the screw is out

Clean the screw while it is still warm, with brass brushes and scrapers, and clean the bore with cotton rags on a rod until they come out clean. Steel tools scratch the flights and the bore.

While everything is apart, inspect:

• The screw flights, root and drive end, and the screw's straightness.
• The barrel bore, for scoring and uneven wear.
• The tip, check ring and seat ring, for wear on the sealing faces.
• The faces of the barrel head and the barrel, the nozzle and its tip.

Measure the screw flights and the bore to decide whether the screw, the barrel, or only the tip set needs replacing. Our guide to measuring barrel and screw wear explains how.

Refitting the screw

1. Clean the bore, the screw, the drive coupling and all sealing faces. No plastic or debris may stay on them.
2. Lubricate the drive end only if the manual says to.
3. Support the screw on the sling and slide it into the barrel, at the barrel temperature the manual specifies, keeping it level so the flights do not catch on the edge of the bore.
4. Engage the drive end in its spline or key, and refit the coupling parts.
5. Fit the seat ring, check ring and screw tip as a matched set, with anti-seize on the thread only, and tighten the tip to the torque in the manual.
6. Refit the barrel head with clean faces, tightening the bolts in sequence to the specified torque.
7. Refit the nozzle, heater bands and thermocouples.
8. Heat the barrel to temperature and let it soak before turning the screw. Start the screw slowly, check the barrel head and nozzle for leaks, and purge before production.

Common mistakes

• Turning or injecting before the barrel has soaked, which can break the tip or twist the screw.
• Forcing a tip with a left-hand thread the wrong way.
• Scraping the screw or the bore with steel tools.
• Lifting a long screw at one point, which can bend it.
• Leaving plastic on the sealing faces, so the barrel head or the tip leaks.
• Reusing a worn or mismatched tip, ring and seat set.

What to send for replacement parts

If the inspection shows wear, send the measurements and photos with your request. The screw guide lists what to send for a new screw, and the screw tip page what to send for a tip set. We supply injection screws and the screw head wrenches listed above. Request a quote with the details.