Barrel and screw cleaning
Barrel and screw cleaning is a critical step before material changes or shutdowns in injection molding production. Its purpose is to remove residual melt, degradation products, and impurities, preventing performance degradation or cosmetic defects caused by the mixing of different materials. Incomplete cleaning can lead to a range of problems. For example, in the production of PC lenses, residual ABS material in the barrel resulted in black spots on the lenses, resulting in a 12% defect rate. Similarly, residual POM material in the screw caused a 15% decrease in strength during assembly of a PA66 gear. The appropriate cleaning method should be selected based on the material’s characteristics. Thermally stable materials (such as PP and PE) can be cleaned directly, while heat-sensitive materials (such as PVC and POM) require specialized cleaning agents to prevent decomposition at high temperatures and the production of corrosive substances. The effectiveness of cleaning can be verified by observing the color and clarity of the discharged material until it matches that of the new material. Using this method, one production line increased the first-time qualified product rate after a material change from 60% to 95%.
Manual cleaning is suitable for small-batch material changes or severe contamination, requiring screw disassembly and thorough cleaning. While time-consuming, this method provides thorough cleaning and is suitable for precision injection molding. The specific steps include: After disassembling the screw and barrel, use a copper brush (avoid scratching the surface) to remove any surface residue. Carbon deposits in corners can be gently polished with 80-120 grit sandpaper. In one case, this method removed a 0.2mm thick layer of degradation on the barrel’s inner wall. After cleaning, wipe with a cotton cloth dipped in alcohol to remove any remaining impurities. Finally, perform a pre-assembly dimensional inspection to ensure the screw-barrel clearance meets the standard (typically 0.1-0.3mm). Manual cleaning reduced the screw’s surface roughness from Ra1.6μm to Ra0.8μm on an optical mold, preventing melt retention caused by surface roughness and eliminating silver streaks on the molded part. Manual cleaning cycles are long (approximately 2-4 hours), making it suitable for scheduled production downtime.
Using cleaning compounds for in-line cleaning is an efficient method for batch production, particularly suitable for color changes or minor contamination of the same type of material, and can be completed without stopping the machine. Cleaning compounds are categorized into chemical and physical types. Chemical compounds contain detergents that dissolve biodegradable materials and are suitable for removing carbon deposits. Using chemical cleaning compounds on a PVC production line significantly reduced the black biodegradable materials in the barrel. Physical cleaning compounds, through their high viscosity, carry impurities and are suitable for materials such as PP and PE. Using physical cleaning compounds on a PP lunch box production line, color change time was reduced from 30 minutes to 15 minutes. During cleaning, the barrel temperature should be controlled 10-20°C above the normal processing temperature to improve material fluidity. The screw should also be rotated at a low speed (30-50 rpm) to ensure adequate contact between the cleaning compound and the inner wall. Test data shows that using cleaning compounds can reduce residual amounts to below 0.1%, significantly lower than the 0.5% achieved with manual cleaning, and also reduces screw wear.
The cleaning process for material changes should be designed according to the “gradual transition” principle to avoid cleaning difficulties caused by material compatibility issues, especially when switching from high-viscosity to low-viscosity materials. When switching from PC (high viscosity) to ABS (medium viscosity), a small amount of PE (transition material) should be added first to allow its fluidity to dislodge PC residue, followed by the addition of ABS cleaning compound. Using this process, an electronic housing production line reduced cleaning time by 20% without any mixing defects. When switching from glass fiber-containing materials to standard materials, the cleaning time should be extended and the screw speed increased. For example, when switching from PA66+GF30 to PP, adding 5kg of cleaning compound and increasing the screw speed to 80rpm effectively removed residual glass fiber and prevented surface scratches on the plastic parts. Post-cleaning verification is crucial. The color, gloss, and mechanical properties of the plastic parts can be checked during 3-5 consecutive production runs. An automotive parts production line verified cleaning compliance through tensile strength testing (deviation < 5%).
Cleaning special materials requires targeted measures to prevent cleaning obstacles or equipment damage caused by material properties. PVC easily decomposes at high temperatures, producing HCl gas. Therefore, during cleaning, the barrel temperature should be lowered (160-180°C) and an alkaline cleaning compound should be used to neutralize the acid. This method was used by a PVC pipe production line to prevent corrosion of the barrel inner wall and extend equipment life. When cleaning POM materials, avoid using fluorine-containing cleaning compounds to prevent chemical reactions and bubbles. A POM gear production line eliminated bubble defects in its plastic parts after switching to polyethylene cleaning compounds. For biodegradable materials (such as PLA), nitrogen protection should be applied after cleaning to prevent residual material from absorbing moisture and degrading. This measure reduced the scrap rate on a PLA packaging production line from 15% to 3% the next time the machine was started. After cleaning, the barrel temperature should be lowered to room temperature or a small amount of fresh material should be introduced to prevent oxidation of the inner wall caused by the high temperature of the empty barrel.
