Injection molding loading and feeding management
Injection molding loading and feeding management is crucial for ensuring production continuity, reducing raw material waste, and improving product quality. It encompasses the entire process, including raw material storage, transportation, metering, and drying. A scientific management system can control raw material loss to less than 0.5%, preventing product defects (such as black specks, silver streaks, and dimensional fluctuations) caused by raw material contamination, moisture absorption, or metering errors. This process requires the development of standardized operating procedures that combine raw material characteristics (such as hygroscopicity and flowability), production plans, and equipment performance to ensure stable and controllable loading and feeding.
Raw material storage and pretreatment are fundamental to material handling management, and targeted measures must be implemented based on the raw material type. Bag-packed granules should be stored in a dry and ventilated warehouse (humidity ≤ 60%, temperature 15-30°C), stacked no higher than 10 layers to prevent the bottom layer from agglomerating due to pressure. Powdered materials in barrels (such as PVC powder) should be sealed, and the barrel lids should be fitted with silicone seals to prevent moisture absorption and the ingress of impurities. Hygroscopic raw materials (such as PA and PC) should be pre-dried in dedicated drying hoppers, with drying parameters strictly adhered to standards: PA66 should be dried at 120-140°C for 4-6 hours; PC at 120°C for 8-12 hours. The moisture content after drying should be ≤ 0.02%. For example, after exposure to 75% humidity for 2 hours, the moisture content of a PA66 raw material increased from 0.01% to 0.08%, resulting in silver streaks in the molded product. After re-drying for 4 hours, the moisture content dropped to 0.015%, eliminating the defect. Raw material pretreatment also includes screening and removing impurities, filtering large particles through 80-120 mesh screens, and using magnetic separation devices (magnetic force ≥8000 Gauss) to absorb metal debris to ensure the cleanliness of the raw materials.
The selection and maintenance of feeding equipment directly impacts conveying efficiency and material quality. Commonly used equipment includes vacuum, spring, and screw loaders. Vacuum loaders (vacuum degree -0.04 to -0.06 MPa) are suitable for most granular materials. They are dust-free and highly automated, with a conveying capacity of 50 to 500 kg/h, making them suitable for small and medium-sized injection molding machines. Spring loaders offer high conveying speeds (up to 1000 kg/h) but are prone to dust generation, making them suitable for powdered or recycled materials. Screw loaders are suitable for high-viscosity materials or when metering is required. Feeding pipes should be made of smooth stainless steel (roughness Ra ≤ 0.8 μm). The diameter should be selected based on the conveying volume (50 to 100 mm), with a bend radius ≥ 3 times the pipe diameter to minimize material retention. For example, a vacuum feeder in one workshop had trouble conveying PA6 raw materials due to excessive pipe elbows (three 90° elbows). Replacing the pipes with larger-radius elbows (R = 150mm) improved conveying efficiency by 30%, with no residual material. Feeding equipment requires daily filter cleaning to prevent clogging, weekly seal inspections to prevent leaks, and monthly calibration of feed rate (deviation ≤ 2%).
Precise control of feed metering is crucial for ensuring consistent product quality. Excessive metering deviations can lead to product weight fluctuations (over 1%), further impacting dimensions and performance. For single-screw injection molding machines, the feed section temperature must be 50-100°C below the raw material melting point (e.g., 50-70°C for PP) to prevent premature melting and the formation of “bridging.” Feed rate is controlled through a combination of screw speed and back pressure. Excessively fast screw speeds (>150 rpm) can lead to unstable metering, while slow speeds can affect production efficiency. For example, in the production of a certain PP product, increasing the screw speed from 100 rpm to 160 rpm resulted in an increase in the standard deviation of product weight from 0.5g to 1.2g. By reducing the speed to 120 rpm and increasing the back pressure by 5 bar, the standard deviation decreased to 0.3g. For multi-component raw materials (such as masterbatch mixing), a loss-in-weight scale (with an accuracy of ±0.1%) is used to automatically feed the materials proportionally to ensure uniform mixing. During the feeding process, the material level in the hopper needs to be monitored in real time, and automatic material replenishment can be achieved through a material level sensor (infrared or capacitive) to avoid material shortage and shutdown caused by an empty hopper.
The proportion and quality of recycled materials must be strictly controlled during the feeding process. Recycled materials (scraps and rejects) must be crushed, cleaned, and dried to a uniform particle size ( 3-5mm). The proportion of recycled materials mixed with virgin material should be determined based on product requirements: ≤20% for exterior parts, ≤30% for structural parts, and ≤50% for internal components. The melt flow rate (MI) of recycled materials must be tested before addition. The deviation from the MI of virgin material must be ≤10%, otherwise the molding process stability will be affected. For example, the MI of recycled materials in an ABS product is 18g/10min (compared to 20g/10min for virgin material). At a mixing ratio of 30%, the product surface exhibits no noticeable defects. However, increasing the ratio to 40% results in decreased melt flowability, leading to underfilling and the need to increase the barrel temperature by 5-10°C to compensate. A separate conveying channel should be provided for recycled materials to prevent cross-contamination with virgin material. Each change in the recycled material ratio requires a trial run of 3-5 molds. Only after the product performance is verified and qualified can mass production begin.
Monitoring and recording the loading and feeding process is crucial for quality traceability. A comprehensive management ledger should be established. Records should include: raw material batch number, loading time, loading quantity, drying parameters, recycled material ratio, equipment operating status, etc., with a retention period of 1 year or longer. An online monitoring system should be installed to collect real-time data on parameters such as hopper level, drying temperature, and conveying pressure. Automatic alarms should be triggered when anomalies occur (such as a material level falling below the set value or a temperature deviation exceeding 5°C), with a response time of 10 seconds or less. For example, one workshop uses an online moisture meter to monitor the moisture content of raw materials after drying. When the measured value exceeds 0.03%, the system automatically extends the drying time and notifies the operator, reducing the defective product rate due to excessive moisture from 2% to 0.3%. Regular calibration of the feeding system, including weekly calibration of the metering device using an electronic scale (with an accuracy of 0.1g), and monthly testing of the entire raw material conveying process, ensure the stability and traceability of the loading and feeding process.
