Process Sequencing for Mold CNC Machining

1. Introduction

Mold CNC machining is a core part of mold manufacturing. The rationality of its process sequencing directly affects machining efficiency, machining accuracy, and mold quality. Mold parts typically feature complex curved surfaces, deep cavities, narrow slots, and high-hardness materials, involving numerous CNC machining operations, diverse cutting tools, and frequent clamping. Scientific and reasonable process sequencing can reduce clamping times, shorten machining cycles, ensure machining accuracy, and is key to enhancing the competitiveness of mold manufacturing enterprises. This article systematically explains the principles, methods, and practical techniques of process sequencing for mold CNC machining.

2. Basic Principles of Process Sequencing

2.1 Datum First Principle

• Machine datum surfaces first, then other surfaces: Datum surfaces are the locating basis for subsequent machining and must be machined first
• Unified datum principle: Try to use the same datum for all operations to reduce datum conversion errors
• Datum coincidence principle: Design datum, process datum, and assembly datum should coincide as much as possible

2.2 Rough Before Fine Principle

• Rough machining first, then fine machining: Gradually remove allowance and gradually improve accuracy
• Leave allowance in rough machining: Generally leave 0.3–1.0 mm allowance, depending on part size and material
• Semi-finishing as transition: Arrange semi-finishing between rough and fine machining to ensure uniform finishing allowance

2.3 Surface Before Hole Principle

• Machine planes first, then holes: Planes serve as locating datum for holes; machining planes first helps ensure hole position accuracy
• Large holes before small holes: Large hole machining involves greater cutting forces; machining them first reduces impact on subsequent small holes
• Deep holes before shallow holes: Deep hole machining is more difficult and should be prioritized

2.4 Main Before Secondary Principle

• Machine main surfaces first, then secondary surfaces: Main surfaces have high precision requirements and should be prioritized
• Machine critical dimensions first, then general dimensions: Critical dimensions determine mold fit precision
• Machine cavity and core first, then auxiliary structures: Cavity and core are the mold's core

2.5 Minimize Clamping Times Principle

• Complete multi-face machining in one clamping: Use 5-axis linkage or rotary tables to reduce clamping errors
• Operation concentration principle: Complete as many operations as possible on one machine tool
• Unified datum clamping: Clamp multiple operations with the same datum

3. Typical Process Sequencing for Mold CNC Machining

3.1 Process Sequencing for Cavity and Core Machining

Typical Process Flow:

Detailed Operation Description:

3.2 Process Sequencing for Mold Base Plate Machining

Typical Process Flow:

Detailed Operation Description:

3.3 Process Sequencing for Slide and Lifter Machining

Typical Process Flow:

Precautions:

• Slide-to-guide rail fit surfaces require lapping, contact ratio ≥80%
• Lifter angle requires precise control, error ≤0.01 mm
• Stress relief required after heat treatment to prevent deformation

3.4 Process Sequencing for Hot Runner Plate Machining

Typical Process Flow:

Key Points:

• Nozzle hole position precision ±0.01 mm
• Runner holes require polishing to Ra0.4 μm
• Water pressure test required after assembly, 0.5 MPa held for 30 minutes

4. Practical Techniques for Process Sequencing

4.1 Rough Machining Sequencing Techniques

(1) Large Tools Fast, Small Tools Fine

• Use large-diameter tools for rough machining to improve removal efficiency
• Cutting parameters: speed 1500–3000 r/min, feed 1000–3000 mm/min
• Use contour machining strategy to ensure uniform allowance

(2) Zone Machining, Reduce Air Cutting

• Divide cavity into multiple zones and machine sequentially
• Use shortest path strategy to reduce tool retractions
• Use helical ramping or slant ramping to reduce tool wear

(3) Symmetric Machining, Balance Stress

• Use symmetric machining for symmetric parts
• Reduce cutting stress concentration and prevent deformation
• Arrange stress relief annealing after rough machining

4.2 Finishing Sequencing Techniques

(1) Steep Before Flat, Ensure Quality

• Use contour machining for steep areas first
• Use parallel machining or projection machining for flat areas afterward
• Avoid tool over-cutting in steep areas

(2) Corner Cleaning First, Avoid Tool Breakage

• Clean corners before finishing to remove residual allowance
• Use small-diameter tools for corner cleaning to avoid tool breakage during finishing
• Corner cleaning depth should be progressively increased to avoid excessive cutting forces

(3) Climb Milling Priority, Improve Surface Quality

• Use climb milling for finishing
• Improve surface quality and reduce tool wear
• Avoid conventional milling which causes rough surfaces

4.3 EDM Sequencing Techniques

(1) Rough Before Fine, Multi-Electrode Machining

• Use high current, large electrodes for rough machining
• Use low current, small electrodes for finishing
• Generally use rough, medium, and fine three electrodes

(2) Electrode Scaling, Compensate Discharge Gap

• Rough machining electrode scaling 0.2–0.5 mm
• Finishing electrode scaling 0.05–0.1 mm
• Precisely calculate electrode dimensions based on discharge gap

(3) Chip Removal Priority, Prevent Carbon Deposition

• Use tool lift chip removal method
• Set timed tool lifting to prevent carbon deposition
• Use flushing or suction to assist chip removal

4.4 Wire EDM Sequencing Techniques

(1) Threading Hole First, Then Cutting

• Machine threading holes before cutting
• Threading hole positions must be precise, error ≤0.01 mm
• Threading hole diameter generally φ1–φ3 mm

(2) Multiple Cuts, Gradually Improve Precision

• First cut: rough cut, leave allowance 0.05–0.1 mm
• Second cut: semi-finish cut, leave allowance 0.01–0.02 mm
• Third cut: finish cut, achieve final precision

(3) Prevent Deformation, Reasonable Cutting Path

• Use multi-segment cutting to reduce deformation
• Cutting path should be symmetric to balance stress
• Stress relief treatment required after cutting

5. Optimization Methods for Process Sequencing

5.1 CAM Software-Based Process Optimization

• Simulation machining: Verify process sequencing rationality through CAM software simulation
• Collision detection: Check interference between tool, holder, and workpiece
• Cutting force analysis: Optimize cutting parameters to reduce tool wear

5.2 Group Technology-Based Process Sequencing

• Similar parts grouping: Group similar parts together for unified processing
• Standardized operations: Establish standard operation library to reduce process design time
• Modular fixtures: Use modular fixtures to reduce clamping time

5.3 Lean Production-Based Process Optimization

• Eliminate waste: Reduce waiting, transportation, and rework waste
• Balanced production: Reasonably arrange operation takt times to avoid bottlenecks
• Continuous improvement: Regularly analyze process sequencing and continuously optimize

6. Common Problems and Solutions