How to Clean the Screw and Barrel of an Injection Molding Machine

In everyday injection molding production, whether the screw and barrel are clean directly determines the optical appearance of the molded parts, their overall mechanical strength, and practical issues like how quickly color or material changeovers can be carried out.

In many cases, the root cause of scrap rates that just won't come down lies in the residues inside the barrel—carbonized black specks, discolored fibers, and contaminants left behind by materials subjected to prolonged high-temperature degradation.

Truly professional purging is never as simple as just running material through the barrel. It involves a great deal—principles of rheology, thermodynamics, and mechanics all come into play; it is a comprehensive treatment process.

Chapter 1: The Physicochemical Nature of Screw Contamination

To improve purging efficiency, you first need to understand exactly how these contaminants adhere to the surface.

1.1 Formation of the Carbonized Layer
Material tends to accumulate in the dead spots of the screw, such as the root of the screw flight, the crevices inside the check ring (non-return valve), and similar areas. After prolonged heating, it gradually oxidizes and cracks, eventually forming an extremely hard carbonized layer.

1.2 Color Deposition and Polar Adsorption
Many pigments, such as carbon black and organic red, are inherently highly polar and easily adhere tenaciously to the tiny pits on the metal surface. In addition, polar materials like PA and EVOH also have a strong affinity for metal. As a result, when switching materials, we often encounter persistent color trailing that is difficult to eliminate—a particularly troublesome problem.

Chapter 2: Classification and Technical Mechanisms of Mainstream Purging Methods

The purging methods currently available in the industry can be broadly categorized into four types. Each has its own suitable application scenarios, and the physical principles behind them are different.

2.1 Physical Displacement Method
The principle is fairly straightforward: it relies on the viscosity difference between the purging compound and the residual material in the barrel to achieve displacement.

You can remember this principle of high-viscosity extrusion: choose a material with a lower Melt Index (MI) and higher overall viscosity than the production material, such as high-molecular-weight PE or a specially formulated purging compound.

The core point is that as the high-viscosity melt is extruded, it generates greater shear stress against the barrel wall, effectively scraping off residues adhering to the inner wall, layer by layer.

In actual operation, it is recommended to use a pulse purging method—sharply adjusting the screw speed up and down—using the resulting pressure fluctuations to dislodge contaminants from the dead spots.

2.2 Chemical Decomposition Method
This method primarily relies on the active ingredients in the purging compound to exert chemical effects at high temperatures.

The swelling agents and surfactants contained in the purging compound gradually penetrate into the various crevices and dead spots of the screw. As the temperature rises, they expand and break the molecular chains of the firmly bonded carbonized deposits, softening the contaminants so they can mix into the melt and be expelled together.

This approach works exceptionally well for stubborn carbonized black specks and residues from heat-sensitive materials. One thing to note, however, is that chemical purging cannot be rushed. It is best to allow a few minutes of soaking time for the chemical reaction to fully take its course; otherwise, the effectiveness will be greatly diminished.

2.3 Physical Abrasive Method
The principle involves mixing fine, hard particles—such as glass fiber, calcium carbonate, and soft ceramic particles—into a carrier resin.

When the screw rotates, these particles act like flowing sandpaper, gradually grinding away the stubborn stains tightly bonded to the metal surface.

But the risk here must be carefully managed: the hardness of the selected particles must absolutely not exceed the hardness of the screw's surface nitrided layer. The general standard is to stay below HV1000. Otherwise, instead of cleaning, you will grind down the precision of the screw itself, which does more harm than good.

2.4 Extreme Disassembly Method
This one is very straightforward: stop the machine, pull out the entire screw, and clean it manually.

This method is generally used only when the contaminant buildup is extremely severe, causing blockages, or during scheduled deep-maintenance overhauls.

Another crucial point to remember: never use a steel wire brush to scrape the surface during cleaning. The correct practice is to use a copper brush or a copper scraper. If conditions allow, placing the screw directly into an ultrasonic cleaning bath causes the least damage to the equipment.

Chapter 3: Specialized Purging Solutions for Different Material Scenarios

Purging approaches for different materials are completely different; one single method cannot be applied all the way through.

3.1 The “Cooling and Sealing” Principle for Heat-Sensitive Materials (PVC, POM)
PVC, if left at high temperatures for too long, will decompose and release hydrogen chloride gas, causing severe corrosion to the screw.

Therefore, when purging this type of material, you must absolutely avoid arbitrarily raising the temperature. Use a dedicated PVC purging compound to rapidly flush out the residual material at the normal processing temperature. Afterward, fill the barrel with a stable material like PE or PP at a low speed to prevent any remaining material from continuing to decompose and damage the equipment after shutdown.

3.2 Color Changeover: Switching from Dark to Light Colors
This is the most common and challenging scenario on the shop floor.

A stepwise method can generally be used: first, do a base purge using a natural (uncolored) material of the same color family, and then follow up with a dedicated chemical purging compound for deep cleaning.
A temperature gradient technique can also be applied: appropriately raise the temperature in the middle and rear zones of the barrel by 20 to 30 degrees Celsius to reduce the material's adhesion resistance, and then extrude with high back pressure. This will significantly speed up the color change.

3.3 Transitioning from High-Temperature Materials (PEEK, PPS) to Standard Materials
This is a pitfall many people step into. Ordinary purging compounds, upon encountering such high temperatures, will instantly burn and form lumps right inside, making the barrel dirtier the more you try to clean it.

The correct approach is to reduce the temperature gradually in steps. First, use a high-temperature purging compound rated for temperatures above 400°C. Then, gradually step down the barrel temperature, and during this cooling process, switch in stages to medium-temperature, and finally low-temperature, standard materials as transitions.

Chapter 4: Five Key Engineering Parameters to Enhance Purging Efficiency

When setting up a purging process, these five parameters must be watched closely. All the differences are hidden in the details.

4.1 Back Pressure
Remember one core principle: back pressure must be increased during purging.

When back pressure is raised, the melt packs more fully inside the screw, the compression effect is better, and entrapped air can be expelled smoothly. The purging compound can then conform more tightly to the dead spots at the root of the screw flight, resulting in a cleaner purge. It is generally recommended to set the purging back pressure to about 1.5 to 2 times that of normal production.

4.2 Screw Speed
The screw speed should not remain fixed; alternating between high and low speeds yields the best results.

High speed rotation generates sufficient shear heat to strip away contaminants faster; low speed allows longer residence time for chemical reactions to gradually take effect. Switching back and forth between high and low speeds creates turbulence inside the barrel, helping to clean out many dead spots that are normally unreachable.

4.3 Nozzle Contact Pressure
When conditions permit and safety is ensured, try to keep the nozzle in tight contact during purging. Use the high back pressure to build up and hold pressure inside the barrel, harnessing the accumulated potential energy of the pressurized melt to expel even deep-seated residues.