Precision CNC Machining Processes for Mold Components
This technical overview details advanced CNC machining techniques for manufacturing high-precision mold components, focusing on turning, milling, and engraving processes that are essential for producing quality injection molding large parts and other precision components.
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From design to finished product, achieving micron-level precision in every component.
CNC Turning of Circular Core Inserts
Figure 1-24 shows a circular plastic mold core insert, whose surface consists of cylindrical, conical, and arc surfaces. The material used is P20 with a quenched and tempered hardness of 28~30HRC. Due to the small size of the core and its relatively low heat treatment hardness, for ease of processing and heat treatment, the material can undergo quenching and tempering heat treatment after blanking before machining. The process begins with rough turning, and finally finishes with precision turning on a CNC lathe.
To facilitate machining and clamping, the length should be extended by 10mm during blanking, with this additional length left at the right end to serve as a center hole drilling and clamping section, which will be removed after finishing. This approach ensures better stability during the machining process, which is particularly important when producing injection molding large parts where dimensional accuracy is critical.
The mechanical processing procedure for this core is as follows: blanking → quenching and tempering heat treatment → rough turning and center hole drilling → CNC turning finishing → polishing of working surfaces → fitter finishing of end faces → inspection.
To ensure part accuracy, the left end is clamped using a self-centering chuck, while the right end is supported by a live center. The core turning process is completed in a single setup on the CNC lathe, divided into semi-finishing and finishing steps. This setup minimizes errors that can occur from multiple clampings, a crucial factor in maintaining precision for components used in injection molding large parts.
The CNC turning process begins with semi-finishing operations that establish the basic shape while leaving appropriate allowances for the final finishing pass. Cutting parameters are carefully selected based on the P20 material properties, typically using carbide inserts with specific geometries optimized for steel turning applications.
During the finishing phase, cutting speeds and feeds are adjusted to achieve the required surface finish specifications. Coolant is applied continuously throughout both phases to control heat buildup, reduce tool wear, and improve surface quality—all essential considerations when producing components that will be used in critical applications such as injection molding large parts.
Figure 1-24: Circular Plastic Mold Core Insert
The core insert features cylindrical, conical, and arc surfaces requiring precise CNC turning operations.
Key Specifications
- Material: P20 tool steel
- Hardness: 28~30HRC after heat treatment
- Surface finish: Various Ra values including 0.8, 1.6, and 63
- Additional length: 10mm for clamping purposes
CNC Turning Process Steps
1. Material Preparation and Heat Treatment
The P20 steel blank is cut to size with the additional 10mm length, then undergoes quenching and tempering to achieve the 28~30HRC hardness range. This heat treatment ensures optimal material properties for subsequent machining operations, which is especially important for components used in injection molding large parts where material stability is crucial.
2. Rough Turning Operations
Initial turning operations establish the basic shape, remove excess material, and create the extended clamping section. Center holes are drilled at both ends to facilitate subsequent machining operations with proper alignment.
3. Setup on CNC Lathe
The workpiece is mounted in a self-centering chuck at the left end with a live center supporting the right end, ensuring maximum stability during machining. This setup minimizes deflection during cutting, which is essential for maintaining dimensional accuracy in components that will be used in precision applications like injection molding large parts.
4. Semi-Finishing CNC Turning
The CNC lathe performs semi-finishing passes on all cylindrical, conical, and arc surfaces, leaving a small machining allowance (typically 0.1-0.3mm) for the final finishing operation. Cutting parameters are optimized for material removal rate while maintaining acceptable surface quality.
5. Finishing CNC Turning
Final turning operations achieve the precise dimensions and surface finishes specified. Special attention is paid to maintaining the required tolerances and surface roughness values, which directly impact the performance and longevity of the mold when producing injection molding large parts.
6. Post-Machining Operations
After CNC turning, the clamping section is removed, working surfaces are polished, and the end faces are finished by a fitter. The completed component undergoes rigorous inspection to ensure all specifications are met before being integrated into the final mold assembly, a critical step in ensuring the quality of injection molding large parts.
CNC Milling of Mold Cavities
Figure 1-25: Soap Box Injection Mold Cavity Insert
The cavity insert features complex 3D surfaces, text elements, and various functional features requiring precise CNC milling.
Cavity Features and Challenges
- Complex 3D curved surfaces throughout the molding area
- Recessed text elements on one cavity surface
- Arc-faced grooves at the top
- Semicircular runner system
- Locating platforms and cooling channels
Figure 1-25 shows a soap box injection mold cavity insert. As a molding component, it requires high dimensional accuracy and surface finish. One part of the cavity surface containing text features a leather-like texture, while the other half has a sandblasted finish. The material used is P20 with a hardness of 30~32HRC, which provides an excellent balance of machinability and wear resistance for producing injection molding large parts.
As can be seen in Figure 1-25, the molding portion of the cavity consists entirely of 3D curved surfaces, with one cavity surface featuring recessed text. The top has arc-faced grooves, the runner system is semicircular, and there are locating platforms around the perimeter with cooling channels. The back of the insert contains only mounting screw holes and cooling channel holes, which are relatively simple to machine.
The focus is on machining the front face, particularly the curved surfaces of the molding portion. Therefore, the primary machining process for this part is CNC milling, which offers the precision and flexibility needed to create complex geometries required for high-quality injection molding large parts.
Due to the moderate hardness requirements of the part, the blank undergoes quenching and tempering heat treatment after下料. It is then milled into a hexagonal shape on a conventional milling machine, with all six faces ground to size on a surface grinder. Since the threaded holes and cooling channels have relatively low precision requirements, they can be produced by fitters using layout methods.
The subsequent key operations are performed on a CNC milling machine. Because the entire front face of the insert contains no right-angle internal recesses or narrow grooves, all operations—including outer contouring, forming surfaces, runners, and locating platforms—can be completed in a single setup with both semi-finishing and finishing passes. This single-setup approach minimizes alignment errors, which is crucial for maintaining the dimensional integrity required for injection molding large parts.
After milling, the cavity undergoes surface treatment. The text-containing portion of the cavity receives a leather-like texture through a texturing process, which essentially involves chemical etching (covered in Chapter 6). The non-text portion features a sandblasted finish, which can be achieved through electrical discharge machining. Finally, fitters perform polishing on other areas as needed to ensure the cavity meets all surface finish requirements.
The CNC milling process for this cavity insert utilizes advanced CAM software to generate toolpaths that accurately follow the complex 3D surfaces. Multiple cutting tools of varying sizes and geometries are used to reach all areas of the part, with each tool carefully selected based on the specific feature being machined. This level of precision ensures that the final mold will produce consistent, high-quality parts, whether for small components or injection molding large parts.
CNC Milling Process Parameters and Considerations
Cutting Tools Selection
- Ball-nose end mills for 3D surface contouring
- Flat end mills for planar surfaces and roughing
- Tapered tools for difficult-to-reach areas
- Carbide tooling for improved wear resistance
Optimal Cutting Parameters
- Spindle speeds: 3000-6000 RPM for P20 steel
- Feed rates: 500-1500 mm/min based on tool size
- Depth of cut: 0.5-2 mm for roughing, 0.1-0.3 mm for finishing
- Coolant: Flood cooling to prevent heat buildup
Quality Control Measures
- In-process inspection after critical operations
- Coordinate Measuring Machine (CMM) verification
- Surface roughness testing at multiple locations
- Dimensional checks against CAD models
CNC Engraving of Cavity Features
As shown in Figure 1-25, the cavity insert contains reverse text patterns shown in Figure 1-26, including the Chinese characters "陕西科技大学" and the English letters "SHAANXI UNIVERSITY OF SCIENCE & TECHNOLOGY". All text patterns have a depth of 0.2mm, with the English letters having a stroke width of 1mm. These detailed features require precise CNC engraving techniques to ensure clarity and accuracy, which is especially important for branding and identification purposes in injection molding large parts.
For CNC engraving of text, the process typically involves first using a relatively coarse engraving tool for roughing, followed by a finer tool for finishing. Therefore, for this part, engraving begins with a 1mm round tool for roughing, followed by a 0.2mm ball nose tool for final finishing. This two-step approach balances efficiency with precision, ensuring that the text features are both accurately positioned and sharply defined.
The CNC engraving process starts with creating a detailed toolpath from the text vector data. This toolpath must precisely follow the contours of each character while maintaining the required depth of 0.2mm. The roughing pass with the 1mm tool removes most of the material efficiently, leaving a small allowance for the finishing pass. This approach minimizes tool wear and ensures that the finer 0.2mm tool can achieve the required detail without excessive stress.
For shallow, fine text patterns like these, chemical etching is also commonly used. Additionally, laser engraving is becoming increasingly popular in modern mold manufacturing for processing text and patterns, offering advantages in certain applications. However, CNC engraving remains preferred for this application due to its ability to achieve precise depth control and excellent surface finish, which are important considerations for injection molding large parts where text clarity can significantly impact product quality.
The choice between CNC engraving, chemical etching, and laser engraving depends on several factors including the material, text size, required depth, production volume, and surface finish requirements. For this soap box mold application, CNC engraving was selected as the optimal process due to its precision, repeatability, and compatibility with the P20 material.
During the engraving process, special attention is paid to maintaining consistent depth across all text elements. This requires careful calibration of the machine and may involve using touch probes to set accurate work offsets. The spindle speed and feed rate are optimized for the small tools used, typically operating at higher speeds (8000-15000 RPM) with reduced feed rates to ensure accuracy and surface quality.
After engraving, the text features are inspected using magnification to ensure they meet the design specifications. Any burrs or imperfections are carefully removed by hand, taking care not to alter the text geometry. This final inspection step is crucial, as the text will be replicated on every part produced by the mold, making quality control essential for maintaining brand integrity in injection molding large parts and small components alike.
Figure 1-26: Text Patterns on Cavity Insert
The insert features Chinese and English text patterns with 0.2mm depth, requiring precise CNC engraving.
Engraving Specifications
| Text Depth | 0.2mm |
| English Letter Stroke Width | 1mm |
| Roughing Tool | 1mm round tool |
| Finishing Tool | 0.2mm ball nose tool |
| Alternative Processes | Chemical etching, laser engraving |
Comparison of Text Machining Processes
| Process | Advantages | Disadvantages | Best For |
|---|---|---|---|
| CNC Engraving |
|
|
High-precision text, shallow features, injection molding large parts |
| Chemical Etching |
|
|
Shallow text, complex patterns, high-volume production |
| Laser Engraving |
|
|
Very fine text, decorative elements, prototype development |
The precision machining processes outlined—CNC turning for circular core inserts, CNC milling for complex cavity features, and CNC engraving for detailed text elements—represent the state-of-the-art in mold component manufacturing. These processes ensure that molds meet the strict tolerances and surface finish requirements necessary for producing high-quality plastic parts, from small components to injection molding large parts. By combining advanced machine tools with optimized cutting parameters and careful process control, manufacturers can achieve consistent results that meet the demanding requirements of modern injection molding applications.
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