Technical Guide to Gymnasium Chair Mould Processing

1. Introduction

With the vigorous development of the national fitness industry, the construction of large venues such as gymnasiums and stadiums is increasing day by day. As an important part of venue supporting facilities, the demand for gymnasium chairs continues to grow. Gymnasium chairs are usually produced by plastic injection molding, and the mould, as the core tooling for injection molding, directly determines the quality, production efficiency, and manufacturing cost of the seats. This article systematically introduces the key technical points of gymnasium chair mould processing.

2. Characteristics of Gymnasium Chair Moulds

2.1 Product Structural Features

Gymnasium chairs generally consist of a seat and a backrest, with some products also including armrests. Their structural features include:

• Complex Curved Surfaces: Both the seat and backrest use ergonomic curved designs to ensure seating comfort.
• Uneven Wall Thickness: Load-bearing areas have greater wall thickness, while edge areas have smaller thickness.
• Numerous Reinforcing Ribs: The bottom is usually provided with crisscrossing reinforcing ribs to ensure structural strength.
• Large Batch Size: The number of seats in a single venue can reach thousands to tens of thousands, requiring long mould life.

2.2 Mould Structural Features

In response to the above product characteristics, gymnasium chair moulds usually have the following structures:

• Adopting large gate injection or hot runner systems to ensure uniform filling.
• Equipping with multi-point ejection mechanisms to prevent product deformation.
• Equipping with conformal cooling channels to shorten the molding cycle.
• Mould materials mostly use pre-hardened mould steels such as P20, 718H, or S136.

3. Mould Processing Workflow

3.1 Preliminary Preparation Stage

(1) Product Analysis and DFM Review

Before mould processing, a Design for Manufacturability (DFM) analysis must be conducted on the 3D model of the seat, focusing on checking:

• Whether the draft angle is sufficient (generally ≥1°)
• Whether the wall thickness is uniform (recommended 2.5–4mm)
• Whether the fillets are reasonable (avoid sharp corners causing stress concentration)
• Whether the reinforcing rib thickness is 60%–70% of the main wall thickness

(2) Mould Structure Design

Use software such as UG, Pro/E, or SolidWorks to complete the 3D mould design, including parting surface determination, gating system design, cooling system layout, and ejection mechanism design.

3.2 Cavity and Core Processing

(1) Rough Machining

Use CNC milling machines or machining centers for large-scale roughing to remove most of the stock. Typical cutting parameters are:

• Tool: φ16–φ32 carbide end mill
• Speed: 1500–3000 r/min
• Feed: 1000–3000 mm/min
• Stock allowance: 0.5–1.0mm

(2) Semi-Finishing

Use φ8–φ12 tools for constant-height contour milling to further remove stock, leaving a finishing allowance of 0.2–0.3mm.

(3) Finishing

Finishing is the key process to ensure the quality of the seat's curved surfaces:

• High-Speed Milling (HSM): Speed 8000–20000 r/min, using φ6–φ10 ball-nose cutters, step-over 0.1–0.3mm
• Electrical Discharge Machining (EDM): Used for deep cavities, narrow slots, and corner cleaning; electrodes made of copper or graphite
• Polishing: Curved surfaces need to be polished to Ra0.4–0.8μm to ensure a smooth seat surface

3.3 Cooling System Processing

The cooling system of gymnasium chair moulds significantly affects production efficiency:

• Use deep-hole drilling to process straight cooling channels, with a diameter of φ8–φ12mm
• For complex curved areas, 3D-printed conformal cooling inserts can be used to make cooling channels conform to the product contour
• The distance from cooling channels to the cavity surface is generally controlled at 15–25mm

3.4 Ejection System Processing

• Ejector pin holes are processed by slow wire-cut EDM or EDM, with fit accuracy H7
• Ejector plate holes are processed by CNC milling
• Return spring holes and limit block mounting positions need precise machining

3.5 Mould Base and Auxiliary Parts Processing

• Mould base plates are processed by CNC milling and grinding to ensure parallelism within 0.02mm
• Guide pillar and guide bush holes are processed by jig boring machines, with fit accuracy H7/g6
• Standard parts such as sprue bushings and locating rings are processed according to drawings or purchased externally

4. Key Processing Technical Points

4.1 Curved Surface Processing Accuracy Control

The seat's curved surface directly affects seating comfort and appearance quality. During processing, attention should be paid to:

• Use 5-axis simultaneous machining centers for complex curved surfaces to reduce setups
• Use on-line inspection systems to monitor processing accuracy in real time
• After finishing, perform Coordinate Measuring Machine (CMM) verification, with tolerances controlled within ±0.05mm

4.2 Deformation Control

Gymnasium chair moulds are relatively large and prone to deformation during processing:

• Perform stress-relief annealing after rough machining
• Use symmetrical machining to balance cutting stress
• Use a small depth of cut, fast feed strategy during finishing to reduce cutting forces

4.3 Surface Quality Control

• The surface roughness of the cavity directly affects the seat's appearance and needs to be polished to a mirror finish
• Use diamond polishing paste for progressive polishing (from coarse to fine: 400# → 800# → 1500# → 3000#)
• Textured surfaces (such as faux leather grain) need to be processed by chemical etching

4.4 Heat Treatment Process

• Cavity and core materials generally require quenching hardness of HRC48–52
• Use vacuum quenching to reduce oxidation and decarburization
• Tempering 2–3 times to eliminate internal stress

5. Mould Debugging and Acceptance

5.1 Trial Moulding

• Use an injection molding machine for trial runs to verify filling, packing, and cooling effects
• Check whether the product has defects such as short shots, flash, sink marks, or deformation
• Adjust gate size, cooling time, and ejection parameters based on trial results

5.2 Acceptance Criteria

• Product dimensions meet drawing requirements (tolerance ±0.2mm)
• No visible surface defects, roughness meets standards
• Mould movements are smooth, without jamming
• 500 consecutive molding cycles without abnormalities

6. Technology Development Trends

6.1 Additive Manufacturing Technology

Metal 3D printing (SLM) technology can be used to manufacture conformal cooling inserts, increasing cooling efficiency by more than 30% and shortening molding cycles by 20%–40%.

6.2 Intelligent Machining

• CAD/CAM Integration: Seamless connection from design to machining
• Adaptive Machining: Automatically adjust feed speed based on real-time cutting forces
• Digital Twin: Optimize machining process parameters through virtual simulation

6.3 High-Precision Inspection Technology

• On-line laser scanning inspection
• Industrial CT non-destructive testing for internal defects
• AI visual inspection for surface quality

7. Conclusion

Gymnasium chair mould processing is a comprehensive technology involving multiple processes such as CNC machining, EDM, wire cutting, polishing, and heat treatment. As construction standards for sports venues continue to rise, higher requirements are placed on the precision, lifespan, and production efficiency of seat moulds. Mould manufacturing enterprises should actively introduce advanced technologies such as high-speed machining, additive manufacturing, and intelligent inspection to continuously improve processing levels and provide high-quality mould support for sports facility construction.