Mold Texture and Its Importance in CNC Machining(304 stainless steel vs 316 Donald)
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- source:DAHLER CNC Machining
Surface Finish Requirements for Molds
The surface finish requirements for an injection mold depend greatly on the application. Molds for cosmetic plastic parts often demand very smooth finishes below 10 microinches to provide an attractive glossy appearance free of visible machining marks. On the other hand, textures up to 500 microinches are acceptable for non-visible surfaces or parts where appearance is less critical.
Generally, tighter finish requirements call for more time-intensive and expensive finishing processes after primary CNC machining. So it is ideal to get the best finish directly from CNC machining before doing additional hand polishing or other secondary operations. Paying close attention to parameters around tool selection, speeds/feeds, tool paths, and machine rigidity during programming can go a long way to optimize mold texture.
Cutting Tools for Optimized Mold Finishes
Choosing the right cutting tool is essential for achieving fine surface finishes in machining. Carbide end mills designed for mold making feature specific geometries and coatings to produce smooth shear cuts rather than tearing the material. They maintain sharp cutting edges even under high machining loads and temperatures.
For the finest finishes below 10 microinches, ball nose end mills around 1/16” diameter are ideal. The spherical tip profile allows the tool to contact the surface tangentially without scrubbing or damaging the texture. 2 or 3 flute ball mills yield even finer finishes by eliminating flute marks and providing smoother material removal.
Larger roughing end mills remove material more efficiently but leave coarser textures. When possible, solely using small ball nose end mills from start to finish can eliminate the need for secondary semi-finishing and finishing passes. Combining roughing, semi-finishing, and finishing operations with the appropriately sized mills ensures both fast metal removal rates and fine finishes.
Optimal Speeds and Feeds
The cutting parameters also play a critical role in mold surface finish. As a general rule of thumb, lower speeds and feeds produce smoother finishes by allowing more time for shearing rather than tearing the material. Typical finishing parameters for end mills under 1/4" diameter would be 20,000-30,000 RPM spindle speed and 8-12 IPM feed rate.
However higher speeds can actually improve finishes with ball nose end mills by reducing deflection. The tool has less time to deflect away from the programmed path before rotating to a new cutting edge. Finding the right balance between speed and feed comes down to understanding tool deflection tendencies, runout, and stability for a given machine setup.
Tool Path Strategies
How the CNC tool paths are programmed greatly affects the directionality and consistency of surface finishes. Generally, contouring toolpaths that follow along a part surface produce consistent quality and directionality. Raster paths and point-to-point movements tend to leave behind more distinct peaks and valleys that may require additional polishing.
Climb milling is preferred over conventional milling, engaging the tool from maximum to minimum chip thickness to prevent rubbing that degrades finish. When possible, avoiding retracts and unnecessary tool lifts will minimize witness marks and texture variation.
For cavity surfaces adjacent to the parting line, stepovers should be programmed so the tool starts and ends its passes at the parting line rather than mid-surface. This eliminates witness marks and ensures finish consistency across the entire cavity.
Machine Rigidity
The stiffness and vibration tendencies of the CNC machine itself also affect mold surface finish. Deflection under cutting forces will cause the end mill to rub and dwell rather than shear cleanly. Machines with box way construction provide maximum rigidity for finishing operations.
Wear on machine components like ballscrews can introduce backlash and chatter. Older machines or those in poor condition are prone to greater deflection issues. Proper maintenance and calibration is key, along with use of anti-vibration Leveling feet and supports. Heavier fixturing also damps vibration for improved finish.
Post-Machining Finishing Processes
Even with optimized machining, most mold surfaces require some secondary finishing work to meet surface finish requirements. Common techniques include:
- Hand Polishing: Using abrasive pads and polishing compounds for progressive smoothing and brightening. Great for localize improvements but time intensive over large areas.
- Media Blasting: Fires small abrasive particles at high velocity to erode peaks. Leaves a uniform matte finish ideal for stone or soft touch textures. Easy to implement but can distort sharp features.
- Electropolishing: Uses an electrolytic bath to remove microscopic peaks through controlled electrochemical dissolution. Produces excellent consistency and precision with mirror-like finishes.
- Laser Polishing: Utilizes a laser beam to selectively vaporize and melt peaks on the surface. Able to polish micro surface areas without affecting part geometry.
- Coatings: Applying PVD, CVD, or plating coatings can provide surface protection and lubricity while masking underlying roughness.
A combined approach starting with optimized CNC machining followed by a suitable finishing process usually yields the best, most cost-effective mold finishes. Paying attention to texture from the start of programming reduces unnecessary time spent refinishing later. With the right tools, parameters, and techniques, injection molders can consistently achieve fine finishes that support functional performance and aesthetic needs. CNC Milling CNC Machining