How to Make Precise Parts & Mold Cavities
Introduction
Precision manufacturing is essential across industries like aerospace, automotive, medical devices, and consumer products. Engineers strive to produce parts and products with tight dimensional tolerances, excellent surface finishes, and repeatable accuracy.
For plastic parts, injection molds must also meet stringent requirements to yield flawless production quality. A mold cavity needs highly polished surfaces to prevent defects. Cooling channels, ejector pins, and other features require precision engineering.
Achieving this level of accuracy and repeatability requires skilled technicians, precision tools, and processes optimized for ultra-fine tolerances. Both CNC machining and EDM (electrical discharge machining) are indispensable for precision parts and mold making.
This article will provide an overview of these technologies and best practices for utilizing CNC machining and EDM to create precise parts and injection mold cavities needed for today’s high-performance products.
CNC Machining Overview
CNC (computer numerical control) machining uses computer-controlled tools to cut and form parts from blocks of material. The process removes material through subtractive methods like milling, turning, drilling, and grinding. By following programmed instructions, CNC machines can produce complex shapes with very tight tolerances repeatedly and accurately.
CNC technology first emerged in the 1950s and has evolved enormously. Today’s CNC machines offer:
- 3 to 5-axis capabilities for intricate part geometries
- Precision down to microns (0.001 mm) or better
- Automated tool changing for minimal setup time
- Capacity for small intricate components up to large-scale parts
- Support for hard and soft metals, plastics, foam, wood, and more
Modern CNC machining centers provide very high precision thanks to advanced controls, tighter tolerances, better motors, and precise ball screw mechanisms. The software also helps compensate for inaccuracies in real-world machining.
EDM Overview
EDM (electrical discharge machining) is an exact method using controlled electrical sparks to erode material. No cutting forces are involved. EDM is especially valuable for mold making, small or fragile parts, and hard-to-machine conductive materials.
In the EDM process, the workpiece and electrode are submerged in a dielectric fluid. When brought close together, electrical voltage creates brief discharges that melt and vaporize minute amounts of material. By repeating hundreds of thousands of times a second, EDM slowly but accurately erodes the desired forms and details.
Key benefits of EDM include:
- Micron precision levels down to 0.0005 mm
- No contact or force makes it suitable for delicate parts
- Complex 3D geometries and cavities
- Works for hard, brittle conductive materials
- Minimal burrs and excellent surface finishes
Major Types of EDM
Several EDM variants exist. Here are three of the most common.
- Sink EDM: Also called cavity type or CNC Sinker EDM, this method uses an electrode to erode a desired shape in the workpiece. Complex 3D forms, tapers, and cavities are possible.
- Wire EDM: A thin continuously fed wire follows a programmed path to cut through conductive materials. Allows 2D profiles and tapered edges. Useful for intricate contours.
- Hole Drilling EDM: Also called CNC Drilling EDM, small electrodes drill extremely precise holes using EDM sparking. Achieves accuracy and intricacy difficult with standard drilling.
Best Practices for Precision CNC Machining
Engineers have developed various techniques and strategies to achieve the highest accuracy with CNC machining. Here are some best practices.
Use Precision Machine Tools
The machine tool itself must provide extreme precision and repeatability. Look for:
- High-quality components like ball screws, way surfaces, spindles, and serving motors are rated for minimal runout and backlash
- Dampening features that reduce vibration
- Thermal compensation to account for thermal expansion and contraction
- Software options for precision path contouring and compensation
Choose Materials Wisely
The workpiece material influences several factors from tool life to heat treatment response. Harder materials like tool steel generally allow better precision but can cause tool wear. Materials should match design needs.
Implement Proper Fixturing
Securely fixturing parts without distortion is crucial for precision machining. Fixtures must locate and support the part while the clamping force remains minimal. Custom fixtures ensure proper holding while avoiding stresses.
Use New Sharp Tooling
Worn tools deflect more under pressure, cause increased vibration, and produce wider cuts. Maintaining an arsenal of sharp tooling is key. Plus, sharper tools require lower cutting forces. Proper speeds and feeds also help tools cut cleanly.
Apply Strategic Operations
Order operations methodically to minimize distortion from clamping or stresses. Rough parts leave extra material for finish passes. Allow roughing to relieve internal stresses before final precision cuts.
Eliminate Thermal Effects
Flood coolant controls heat buildup. But also let parts normalize back to ambient temperature before precision finishing. Preventing thermal growth during machining ensures accuracy.
Automate and Monitor
Automating machining operations promotes consistency. In-process metrology tools (measurement probes, laser systems, etc.) provide real-time cut monitoring to immediately detect any deviations.
Perform Vibration Dampening
Excess vibration during machining reduces accuracy. Methods like vibration monitoring, tool path optimization, noise suppression pads, and isolation systems help minimize undesirable chatter and vibration.
EDM Strategies for Precision Mold Making
EDM brings immense value to mold making where micro-smooth surfaces free of imperfections are mandatory. Here are some ways EDM enables precision mold and die fabrication.
Complex Cavities
EDM efficiently produces complex concave and convex geometries regardless of depth via the shaped electrode. Undercuts and angular forms including tapering walls are possible.
Extreme Accuracy
EDM offers micron precision levels. Tighter tolerances result in minimal flash and improved molded part quality. EDM also creates highly uniform surface finishes.
Conformal Cooling Channels
EDM can erode curved cooling lines that follow cavity contours rather than straight drilled holes. This provides optimal cooling for molded part consistency and reduced cycle times.
Hard Materials
Mold steels over 60 HRC that are difficult to machine conventionally can be formed easily with EDM since no cutting forces exist. This facilitates very durable long-life mold tooling.
Split Surface Textures
Different surface finishes on core and cavity halves are possible by changing electrodes. This allows mold surfaces optimized for release while preventing ejector pin marks.
Rapid Prototyping
EDM combined with graphite electrodes made in-house via CNC machining enables fast turnaround of mold cavities for prototype validation builds.
Polishing Process
EDM removes only 0.025-0.125mm per pass. Using a series of progressively smoother electrodes (step-down process) creates a high-polish finish mold surface.
Injection Nozzle and Venting
Precisely formed nozzle channels and venting details not possible by drilling are added via shaped electrodes. This prevents trapped air defects in molded parts.
Ejector Pins, Slides, Lifters
EDM accurately creates ejector pin holes and forms lifter slides and other movable mold components needed for reliable part ejection without damage.
Conclusion
Today’s high-performance products require manufacturing systems delivering ultra-fine precision, consistency and overall quality. By applying best practices around work-holding, tooling, operations, and automation, CNC machining can produce parts within microns. EDM extends even greater accuracy through controlled erosion along with the ability to create complex cavities and surface finishes needed for injection molding.
Mastering precision machining and EDM for critical applications takes extensive experience. Finding a qualified supplier with proven expertise in micro-tolerance machining and mold work is key. With skilled resources and the right technology, incredibly precise components and molds can be created to enable the products we use daily.