The plastic back frame of office chairs is mainly manufactured through injection molding. The core process includes mold design, material preparation, injection molding, cooling and shaping, and post-processing. Among these, mold precision and material ratio are the key factors that determine product quality.
Mold Design and Machining
The back frame structure is complex, often containing curved surfaces, reinforcing ribs, and snap-fit features, requiring high-precision CNC-machined steel molds.
The mold integrates optimized cooling water channels to ensure uniform cooling of large plastic parts and prevent warping. The parting surface design must be rounded and smooth to reduce parting line marks and improve appearance quality.
Material Selection and Pretreatment
The mainstream materials are glass fiber reinforced PP or PC/ABS alloy to improve bending and impact resistance. PA6 with 20% glass fiber (PA6+20%GF) is also commonly used in high-end back frames to enhance structural strength. Plastic granules need to be dried to avoid bubbles or silver streaks during injection molding.
Injection Molding
Molten plastic is injected into a closed mold under high pressure, filling the entire cavity.
Gas-assisted molding technology reduces material usage and internal stress, making it suitable for thick-walled structures.
The gate location and parting line design directly affect the product's appearance and structural strength.
Cooling and Demolding
Cooling water or oil circulates within the mold to control the cooling rate and prevent uneven cooling that could lead to deformation.
After the back frame is fully cured, it is ejected via an ejector system; the draft angle design is crucial.
The holes in the back frame not only reduce weight but also reduce adhesion and facilitate demolding.
Post-processing and Quality Inspection
Remove burrs and trim parting lines. Some products undergo surface texturing or high-gloss treatment to enhance texture.
Conduct load-bearing, bending, and durability tests to ensure compliance with ergonomic and safety standards.