Mold Texture and Surface Finish - Achieving Desired Results with CNC Machining(metal finishing chart Bob)
- source:DAHLER CNC Machining
Surface Finish Requirements in Mold Making
The surface finish on a mold cavity serves several important functions:
- Visual Appearance - The texture left on a molded part comes directly from the mold surface. The desired visual appearance and perception of quality depend on the appropriate finish.
- Part Release - An optimized texture helps the part separate cleanly from the mold after cooling. This prevents sticking that would damage the mold and part.
- Durability - Parts need to withstand wear during use. The right finish improves wear resistance.
- Dimensional Accuracy - A smoother surface improves dimensional accuracy and reduces variance between molded parts.
- Ejection without Damage - Any ejector pin marks on the part come from the mold texture. The finish must allow clean ejection without marring.
With so much riding on the mold finish, mold makers must be meticulous in achieving the desired surface texture and profile.
Advantages of CNC machining for Mold Texture
CNC machining offers key advantages for creating optimized mold surfaces:
- Precision Control - Unlike manual techniques, CNC allows for exact, repeatable movement and toolpath execution. This enhances control over the resultant finish.
- Complex Geometries - CNC can handle complex contours and surfaces that would be impossible or impractical to finish manually.
- Accuracy and Consistency - CNC machines complex 3D mold surfaces to tight tolerances based on CAD models. This accuracy is repeatable for each cavity and each batch.
- Multi-Step Finishing - CNC allows running sequential operations like roughing, semifinishing, and finishing with different tools.
- Automated Process - Once the CNC toolpaths are programmed, surface finishing can run unsupervised. This frees up skilled workers.
Key CNC Techniques for Achieving Mold Finishes
To leverage CNC's advantages for mold texture, machinists employ several specialized techniques:
- Stepdown Approaches - Stepping down incrementally with smaller cuts and finer tools creates a smoother mold finish.
- High Spindle Speeds - Using high rpms with low feed rates keeps each cutting pass light and reduces roughness.
- Tight Tolerances - Tighter tolerances in programming further limits coarseness in the final surface texture.
- Burnishing - This rubbing/polishing technique smooths out peaks using a hard tool compressed against the surface.
- Diamond Grinding/Lapping - Fixed abrasive diamond wheels impart a polished, dimensional finish.
- EDM Texturing - Electrical discharge texturing removes material without contact while avoiding stresses.
- Laser Ablating - Lasers can accurately ablate ultra-fine details into a mold surface.
The combination of CNC equipment, dexterous tooling, and smart programming allows mold makers to achieve challenging surface finishes that would be difficult or impossible with manual methods.
Simulating and Testing Results
The finishing process involves some trial and error combined with experience and expertise. To save time and materials, mold makers can now leverage CAM and CNC programming software to visualize and simulate results before machining the real mold. Powerful software helps predict and evaluate the surface finish based on assigned tooling, speeds and feeds, stepovers, stepdown passes, and other parameters. The ability to simulate, inspect, and optimize the toolpaths virtually improves efficiency, accuracy, and cost effectiveness.
Once the actual CNC machining is complete, the mold surface must be inspected to ensure it meets specifications. If any refinements are required, the mold maker can easily modify the toolpaths and run another finishing cycle. The ability to inspect and iterate quickly to achieve the right finish is key. Mold makers rely on advanced inspection equipment like profilometers and surface roughness testers. These provide quantitative data to prove the surface texture falls within specified parameters for roughness, flatness, peak-to-valley deviation, and other metrics.
In summary, computer-controlled precision paired with simulation, testing, and refinement now gives mold makers unprecedented command over the mold finishing process. CNC machining enables not just any finish, but the exact finish required to optimize mold performance and molded part quality while minimizing manual polishing and related costs. The ability to control mold surface texture with precision and repeatability helps injection molders win and retain business by meeting challenging customer specifications for part appearance, feel, longevity, and dimensional accuracy. CNC Milling CNC Machining