Foreign metal objects entering the barrel, severe carbonization of raw material, heavy wear on the barrel or screw requiring overhaul... All these situations call for disassembling and reassembling the screw or barrel. This is a hands‑on task that truly tests a technician’s practical skills.
Never underestimate the disassembly and reassembly of the screw and barrel. People with different skill levels produce very different results.
It is inadvisable to rely on brute force, using just a large sledgehammer and a few strong workers as if doing rough labor, hoping that “brute force works miracles.”
Because disassembling and reassembling the screw and barrel requires skill, focusing on temperature control, mechanical fit, and precision alignment.
Rough disassembly and reassembly can cause irreversible damage, such as scoring the screw, seizing the flange, compromising equipment accuracy, or even worse, causing the screw to break or the nozzle to leak plastic during the next startup.
Today, we skip the empty theories and focus on practical, hands‑on knowledge to show you how to scientifically disassemble and reassemble the screw and barrel. Let’s get straight to the practical advice.
Section 1. Preparation in Advance: Know Exactly What You Are Doing
Under no circumstances should you just pick up a tool and start working immediately.
Take care of a few details first; it is never too late to start after that.
Step 1, which is also the most critical step: Purging the barrel
First, purge the residual raw material from the barrel. Generally, a high‑viscosity purging compound is used. For details, see How to Clean the Screw and Barrel of an Injection Molding Machine.
For materials like PC or materials with nylon, they must be cleaned out. Otherwise, as soon as the barrel temperature drops even slightly, the screw and the barrel will stick together tightly, making it impossible to pull the screw out.
Also, for materials like POM that easily generate irritating gases, if you dismantle without purging first, not only will you be choked by the fumes, but the entire workshop will be filled with an irritating smell, and everyone in the workshop will complain about you.
Prepare your tools: High‑temperature‑resistant anti‑seize grease, a knocking copper bar (never strike any part of the equipment directly with an iron hammer), special wrenches, a heavy‑duty torque wrench, and high‑temperature‑resistant gloves.
Safety warning: Place a warning sign reading “Maintenance in Progress – Keep Clear” in a prominent position on the machine.
Section 2. The Physics Principle During Disassembly: Thermal Expansion and Contraction
The hot disassembly method is key to the disassembly process:
You cannot wait until the barrel has completely cooled down; otherwise, the screw may truly be “done for.”
1. Removing the screw
a. First, raise the temperature of each barrel zone to 10–20 °C above the melting temperature of the raw material, and wait 10 minutes for the material inside the barrel to soften completely.
b. Switch off the heaters, put on heat‑resistant gloves, and remove the nozzle heater band.
c. Remove the flange: loosen the flange bolts in a cross‑diagonal sequence and in stages so the flange does not deform from uneven force on one side.
d. Remove the screw tail cover.
e. Take off the flange.
f. Extract the screw.
The screw cannot be pulled out entirely in one go. For a small screw, generally pull it out about two‑thirds, place cardboard underneath for support, and have two people work together, one at the front and one at the back, to lift the screw out.
Then, strike while the iron is hot: remove the sticky plastic residue from the screw and clean off any carbonized black spots with a copper brush.
If the plastic becomes hard and difficult to clean once it cools, apply slight local heating with a torch, but do not direct the flame at a single spot on the screw for a prolonged period to avoid causing localized annealing of the screw.
On some machines, the injection unit can rotate by a certain angle, so you do not need to remove the mold before removing the screw.
2. Removing the screw tip
Remove the screw tip. Pay attention: the screw tip has a left‑hand thread (reverse thread).
Use a copper scraper and copper brush prepared in advance to quickly clean the residual plastic from the threaded area. Once it cools, it becomes difficult to clean.
3. Removing heater bands: Turn off the power to the heater bands. Remove the heater bands and thermocouple wires one by one and label them clearly. Match each heater band to its corresponding terminal connection – do not mix them up.
4. Removing the barrel: Sling the barrel with lifting straps. First, remove the large flange nuts (or clamps) that fix the barrel. Then press the injection forward key to push the barrel out, after which you can lift the barrel away.
5. Reassembly is key: achieving precision reset
Some may say: “Just put everything back the way you took it apart.” However, reassembly requires careful attention to tolerances and fits.
a. Precision confirmation and lubrication
Precision confirmation requires using an inside micrometer and an outside micrometer.
Measure the inner diameter of the barrel and the outer diameter of the screw. The radial clearance (single‑side fit clearance) usually varies with machine size and is generally required to be within 0.05 mm–0.15 mm; it must not exceed this range.
The threaded holes of the flange and the nozzle threads are all exposed to high temperatures for long periods and can seize up over time. Apply a layer of high‑temperature grease containing tiny spherical particles onto the threads. This will make disassembly easier next time.
b. Concentricity of the screw and barrel
Alignment calibration is critical: After installing the barrel and screw, first measure the concentricity between the screw tail and the drive shaft. The concentricity deviation must be within 0.03 mm to be acceptable.
Otherwise, the screw will wobble inside the barrel during operation, and within just a few months the screw will score the barrel and the screw itself will wear.
Use a torque wrench to tighten the bolts to the specified torque value appropriate for the machine tonnage. Tighten in a diagonal pattern in multiple passes, gradually increasing the torque to ensure the bolts are evenly loaded and securely fastened.
c. The screw three‑piece set
The screw three‑piece set (screw tip, check ring, spacer). Clean their mating surfaces thoroughly so that no residue remains. Otherwise, tiny gaps will exist at the contact surfaces. Under high injection pressure, the check ring will not be able to lock completely and form a perfect seal; it will fail to contain the melt, and plastic will “squeeze” through the gaps.
6. How to commission and verify at startup
Cold machine verification: Do not add any material and do not turn on the heating. Slowly rotate the hydraulic motor at low speed. There must be no “rustling” metallic rubbing noise and no binding or seizing.
Hot machine verification: Reinstall all heater bands, raise the barrel temperature to the processing temperature, and then soak for 30 minutes so that both the barrel and the screw fully expand with the heat.
Start the motor and rotate the screw at 30–50 rpm. Confirm that the current or pressure gauge needle remains steady and does not fluctuate. Add raw material and perform an air shot; the injected melt must be smooth and free of any foreign matter.
A machine never lies. If you treat the screw and barrel gently, they will repay you with stable dimensions and a good qualification rate. If you treat them roughly and simply – without applying grease, without measuring concentricity – then one day, at a critical moment in production when orders are due, they may repay you with a loud “snap” and a costly break.
