Ergonomic Office Chair Armrests Accessories Suppliers

How to avoid defects (such as shrinkage marks and bubbles) in the injection molding process of ergonomic office chair armrest accessories?

1. Material pretreatment and selection: Control the causes of defects from the source

Material selection and pretreatment for injection molding are the basis for avoiding shrinkage marks and bubbles. Ergonomic office chair armrest accessories usually use engineering plastics such as polypropylene (PP), nylon (PA) or ABS. The crystallinity, melt index and moisture content of such materials directly affect the molding quality.
Material moisture content control: Moisture in the raw materials is one of the main reasons for bubbles. Taking Anji Xielong Furniture Co., Ltd. as an example, its professional team will pre-treat the raw materials through a dehumidifier dryer before production to control the moisture content below 0.02% (such as PA66 needs to be dried at 120℃ for 4-6 hours) to ensure that there is no risk of gasification of the raw materials during injection molding. The advanced drying equipment introduced by the company has an intelligent humidity monitoring function, which can provide real-time feedback on the drying status and eliminate the bubble problem caused by moisture from the source.
Material fluidity optimization: If the structure of the handrail accessories is complex (such as hollow, multi-curved design), it is necessary to select materials with a moderate melt index (MI). The R&D team will adjust the material formula according to the product design. For example, while adding 30% talcum powder to PP to enhance rigidity, the melt fluidity is optimized through rheological testing to avoid insufficient local pressure caused by poor material flow, thereby reducing shrinkage marks.

2. Precise control of process parameters: coordinated optimization of temperature, pressure and time

Precise control of injection molding process parameters is the core of avoiding defects, and dynamic adjustment is required according to the structural characteristics of handrail accessories (such as uneven wall thickness and rib position design).

Refined management of temperature system
Barrel temperature: Insufficient melt temperature will lead to insufficient mold filling, while too high temperature will easily cause material degradation and produce gas. Taking ABS as an example, the barrel temperature is usually set at 200-240℃, but the barrel is temperature-controlled in sections (such as 180℃ in the feeding section, 220℃ in the compression section, and 230℃ in the metering section) through infrared temperature sensors to ensure uniform plasticization of the melt and reduce bubbles caused by temperature fluctuations.
Mold temperature: The mold temperature affects the cooling rate of the material, which in turn causes shrinkage marks. Ergonomic handrails often have wall thickness differences (such as 5mm wall thickness in the support column and 2mm in the panel). The mold temperature controller is used to control the temperature of the mold in different sections. The mold temperature in the thick-walled area is maintained at 60-80℃, and the thin-walled area is controlled at 40-50℃, so that the cooling rate of different parts is consistent and the shrinkage stress difference is reduced.

Optimization of pressure and holding pressure process
Injection pressure: The complex structure of the handrail accessories (such as the slots and threaded holes of the adjustable handrails) requires sufficient injection pressure to ensure complete filling. The servo injection molding machine can accurately control the injection pressure at 80-120MPa. For the areas prone to shrinkage such as ribs, segmented pressure control (such as 100MPa in the mold filling stage and 80MPa in the pressure holding stage) is used to avoid local depression caused by insufficient pressure.
Press holding time and pressure decay: The pressure holding stage is the key to compensating for material shrinkage. The process team found through mold flow analysis software (such as Moldflow) that the thick-walled area of ​​the handrail needs to be held for 15-20 seconds, and the pressure decays at a rate of 5%/second from the initial value of the pressure holding, which can effectively fill the shrinkage gap and reduce shrinkage marks.

Scientific setting of cooling time
Too short cooling time will cause internal stress concentration in the material and produce post-shrinkage shrinkage marks. The cooling time is calculated according to the wall thickness of the handrail accessories (such as when the average wall thickness is 3mm, the cooling time is set to 25-30 seconds), and the mold water channel optimization (such as conformal cooling water channel design) is used to ensure uniform cooling. Its advanced production equipment can monitor the cooling rate of each area of ​​the mold in real time to avoid defects caused by uneven cooling.

3. Mold design and manufacturing: Avoiding defect risks from a structural level

Mold precision directly affects the quality of injection molding. For the ergonomic design of handrail accessories (such as curved handrails and adjustable joint structures), technical measures to prevent shrinkage marks and bubbles need to be incorporated into the mold design.

Gate position and size optimization
The gate position should avoid pressure attenuation caused by excessive melt flow, and the exhaust path should be considered. When designing the handrail mold, the mold team uses a latent gate or a fan gate, and sets the gate in the thick wall area (such as the handrail support seat) to ensure balanced melt filling. For example, the gate diameter of a certain adjustable handrail mold is set to 1.5mm and the length is 2mm, which can effectively control the melt flow rate and avoid turbulent air intake caused by a small gate.

Fine design of exhaust system
Bubbles are mostly caused by the inability to discharge gas in the mold. Exhaust grooves (depth 0.02-0.03mm, width 5-10mm) are opened on the mold parting surface, core, etc., and breathable steel (porosity 15-20%) is set at dead corners that are difficult to exhaust (such as the bottom of the rib position) to ensure that the gas is discharged in time during mold filling. In addition, the company uses mold flow analysis to predict the gas gathering area and optimize the exhaust structure in a targeted manner to increase the mold exhaust efficiency by more than 30%.

Mold surface treatment and temperature uniformity
The roughness of the mold surface affects the melt flow resistance. The mold cavity is mirror polished (Ra≤0.2μm) to reduce turbulence during melt flow and reduce the risk of gas entrapment. At the same time, through the "series + parallel" hybrid design of the mold water channel, the mold temperature fluctuation is ensured to be ≤±2℃ to avoid bubbles caused by local overheating or shrinkage marks caused by cold materials.

4. Dynamic monitoring and quality inspection of the production process: prevention of defects in the whole process

The stability of injection molding depends on real-time monitoring and quality feedback of the production process, and defects are controlled through the dual mechanism of "online monitoring + offline inspection".

Online process parameter monitoring
The company's intelligent injection molding machine is equipped with a PLC control system, which collects real-time data on parameters such as barrel temperature, injection pressure, and holding pressure (sampling frequency 100Hz), and automatically alarms and adjusts when the parameter fluctuation exceeds ±5%. For example, when it is detected that the holding pressure fluctuation of a batch of handrail accessories exceeds the set value, the system will automatically increase the holding pressure compensation amount to avoid shrinkage marks caused by parameter drift.

Offline defect detection technology
Visual inspection and non-destructive testing: Quality inspectors conduct 100% visual inspection of handrail accessories, focusing on areas prone to shrinkage such as ribs and corners, and use ultrasonic flaw detectors to detect internal bubbles (bubbles with a diameter of ≥0.5mm can be identified). The quality inspection team of Anji Xielong Furniture Co., Ltd. has been professionally trained and strictly follows the ISO 9001 quality standard to ensure that the defect detection rate reaches more than 99%.
Destructive testing and data analysis: Regularly conduct destructive testing (such as tensile testing and impact testing) on ​​products to analyze whether there are stress concentrations caused by bubbles or shrinkage marks in the internal structure of the material. The test data is analyzed by the SPC (statistical process control) method. If the shrinkage rate of a batch exceeds 0.5%, the process parameters are immediately traced and optimized.

5. Process optimization and innovation: continuous improvement based on feedback

The avoidance of injection molding defects is a continuous optimization process, relying on professional R&D teams and advanced technologies to continuously iterate process solutions.

Mold trial and process verification
Before the new product goes into production, the company will use 3D printing to make a mold prototype, conduct a small batch of mold trials (50-100 pieces), use a high-speed camera to record the mold filling process, analyze whether the melt flow generates vortices that cause bubbles, and optimize the gate position and process parameters through the mold trial data, reducing the defect rate during formal production by more than 60%.

Application of new technologies
Introduce an in-mold pressure sensor (accuracy ±0.1MPa) to monitor the pressure distribution during the mold filling stage in real time, combine the AI ​​algorithm to predict the risk area of ​​shrinkage marks, and automatically adjust the pressure holding strategy. For example, when the sensor detects that the pressure in a certain area of ​​the handrail is insufficient, the system will automatically increase the pressure holding time of the area by 1-2 seconds to compensate for the shrinkage of the material. In addition, explore the use of micro-foam injection molding technology to reduce the material density by injecting nitrogen, while reducing the shrinkage rate, and in principle reduce the generation of shrinkage marks.