Injection nozzle blockage and solutions
Injection nozzle blockage is a common problem in injection molding production, manifesting as improper melt ejection or unstable ejection volume, leading to product shortages, dimensional deviations, and even mold damage. The cause of blockage is closely related to raw material characteristics, nozzle structure, process parameters, and equipment maintenance. Rapidly diagnosing and resolving blockage issues can reduce downtime and lower production costs. An effective solution requires a combination of preventative measures and troubleshooting procedures to fundamentally minimize the likelihood of blockage.
Impurities and foreign matter in the raw materials are the main cause of nozzle clogging, including unmelted particles, metal debris, and dust. These impurities accumulate in the narrow nozzle channel (3-8mm diameter), gradually causing blockage. Preventive measures include: screening the raw materials before storage (40-80 mesh ) to remove large impurities; installing a magnetic separator above the hopper to absorb metal debris; and regularly cleaning the barrel and feed port to prevent material accumulation and carbonization. For example, a PE product production operation experienced frequent nozzle clogging. Inspection revealed woven bag fragments mixed in with the raw materials. After installing an 80-mesh screen, the number of clogging events decreased from five per day to once per month. When impurities clog the nozzle, disassemble the nozzle and clean it with a copper wire brush or a special needle. Never use hard tools (such as screwdrivers) to avoid scratching the nozzle bore.
Decomposition products of high-viscosity or heat-sensitive plastics (such as PVC, POM, and PC) can easily cause nozzle clogging. PVC decomposes easily at temperatures above 200°C, producing hydrogen chloride gas. This reacts with metals to form chlorides, which adhere to the nozzle’s inner wall and form hard scale. POM decomposes into formaldehyde polymers, which can also cause clogging. Adjusting process parameters can prevent decomposition. For PVC, the nozzle temperature should be controlled between 160-190°C, and for POM, between 180-210°C. Also, shorten the melt’s residence time in the barrel (≤5 minutes). If decomposition products are detected in the nozzle (such as black spots or scorch marks), immediately lower the nozzle temperature and perform 3-5 consecutive emptyings to expel the products. If this is ineffective, disassemble the nozzle and clean it with alcohol or acetone to remove the scale. For example, a clogged nozzle in a PVC pipe fitting mold revealed dark brown scale on the inner wall after disassembly. After soaking in acetone for 2 hours, scrub it clean and resume normal operation.
An improper nozzle design can easily lead to material stagnation and blockage. For example, if the nozzle’s inner bore has a right-angle transition, an excessively large reduction, or dead corners, the melt’s flow rate slows down, and prolonged stagnation can lead to cooling and solidification. A reasonable nozzle structure should feature a streamlined design with an inner bore taper of 5°-10° and a transition radius R ≥ 1mm, avoiding right-angle or abrupt reductions. For high-viscosity plastics, the nozzle aperture should be increased (for example, from 4mm to 6mm) to reduce the flow gradient and minimize stagnation. For example, a PC lens mold’s original nozzle had a 3mm aperture and frequently clogged. After switching to a 5mm aperture and increasing the transition radius, the clogging problem was completely resolved. Furthermore, the clearance between the nozzle and the mold’s main channel bushing should be ≤ 0.1mm to prevent melt leakage and accumulation in the gap, causing secondary blockage.
Improper process parameter settings can also cause nozzle blockage. For example, low nozzle temperature can cause premature melt cooling, while excessive backpressure can cause excessive melt shearing, resulting in charring. The nozzle temperature should be 5-10°C higher than the end-of-barrel temperature to ensure good melt fluidity within the nozzle. For example, if the end-of-barrel temperature for PP is 200°C, the nozzle temperature should be set to 205-210°C. Backpressure is generally controlled between 5 and 15 bar. Excessive backpressure increases the melt temperature (3-5°C higher for every 10 bar increase in backpressure), potentially leading to decomposition of heat-sensitive plastics. If blockage is suspected to be caused by improper parameters, increase the nozzle temperature by 10-15°C, maintain this for 10 minutes, and then perform continuous injection to observe if the nozzle is unobstructed. If this fails, reduce the backpressure to below 5 bar to reduce shear heat and try again. For example, a nozzle blockage in an ABS product due to excessive backpressure (25 bar) was resolved by reducing the backpressure to 10 bar and increasing the nozzle temperature by 10°C.
Nozzle wear and corrosion caused by long-term use can exacerbate clogging. Wear causes the inner bore surface to roughen (Ra ≥ 1.6μm), making it prone to melt adhesion and accumulation. Corrosion (such as that caused by PVC decomposition products) can cause pitting in the inner bore, creating stagnation areas. Preventative measures include regularly inspecting nozzle wear (every 50,000 molds) and regrinding if the inner bore roughness exceeds Ra 1.0μm. For corrosive materials, choose corrosion-resistant nozzles (such as those with chrome plating or Hastelloy). When nozzle wear is severe and cannot be repaired, it should be replaced promptly. During replacement, ensure the concentricity of the new nozzle with the barrel (deviation ≤ 0.05mm) to avoid material stagnation caused by eccentricity. For example, a POM mold nozzle wore out after 100,000 molds, with an inner bore roughness of Ra 2.0μm. After replacing the nozzle and grinding it to Ra 0.4μm, clogging frequency was significantly reduced.
