Mold Textures and Surface Finishes Achievable with CNC Machining(spring rubber Murray)

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In injection molding and other molding processes, the texture and surface finish of the mold cavity directly impacts the visual and tactile properties of the molded parts. With computer numerical control (CNC) machining, a wide range of mold surface textures can be produced to create parts with appealing aesthetic qualities and tactile feels.
Surface Finish Metrics
Surface finish is quantified using parameters like Ra, Rz, and RSm. Ra is the arithmetic average deviation of the surface from the mean line, describing overall roughness. Rz indicates peak-to-valley roughness, showing the distance between the highest peak and lowest valley. RSm provides the mean width of the profile elements, indicating the spacing of irregularities.
While mold polishing can reduce these values, the mold fabrication process largely determines the starting surface texture. CNC machining is capable of achieving very smooth finishes prior to hand polishing.
Uniform Directionality
In addition to roughness values, the directionality of the surface texture also impacts part appearance and feel. CNC machining can create unidirectional textures by aligning tool paths with the contour. This prevents a chaotic, inconsistent surface texture.
Tool Selection
The choice of tooling has a significant impact on surface finish in CNC machining. Smaller corner radii produce lower roughness values, while larger radii are more durable and less prone to chipping. Using new, sharp tools is critical for achieving fine finishes. Diamond-coated end mills can last longer than uncoated carbide in abrasive materials like hardened steel.
Machining Strategies
Both conventional and climb milling can produce excellent finishes in CNC machining. Conventional milling tends to rub more on the surface, while climb milling cuts more cleanly. But adjustments to chip load, feed rates, and cutting tools can optimize both strategies for fine finishes. Staggering the tool path between passes helps evenly distribute any lines or markings from the process for improved consistency.
Adding a finish pass with decreased feed rate and depth of cut reduces roughness after the primary material removal. Finally, choosing a smooth tool path pattern which moves cleanly in and out of the workpiece avoids dwell marks and other visible tool path artifacts on the mold surface.
Effects of Mold Material
The mold material also affects the starting surface condition after CNC machining. Pre-hardened steels generally allow smoother machining than fully hardened materials. The hardened steels require diamond-coated tooling and optimized machining parameters. Aluminum tools can be machined quickly with conventional tools but have lower durability than steel.
Post-Machining Processing
Additional surface conditioning processes can further improve mold texture and feel. Abrasive flow machining extrudes viscoelastic media filled with abrasive substances through the cavity to produce a stable uniform finish. Electropolishing applies an electrolytic solution to smooth and polish the steel surface to an almost mirror-like luster. Laser polishing can also reduce Ra values for precision optical surfaces.
Benefits of CNC Texture Control
Consistent surface textures from CNC machining offer many benefits for injection molding and other applications:
- Improved part appearance with evenly distributed tool marks and uniform directionality. Avoiding chaotic textures provides a clean, high-quality look.
- Tactile part properties can be tuned by adjusting roughness parameters and introducing directional patterns. This allows engineering the feel of grips, knobs, and other handled components.
- Micro textures can increase bond strength for overmolded components or improve wear resistance. Controlled surfaces improve function beyond just aesthetic appearance.
- Structured textures prevent stacked molded parts from sticking together right out of the mold. Small bumps or lines allow venting channels between stuck surfaces.
With careful tool selection, optimized machining parameters, and post-processing techniques, CNC machining can achieve molded surface finishes that would otherwise require extensive hand polishing or texturing. The process control facilitates repeatability and part quality. Adjustable surface patterns and finishes expand the design possibilities for molded components. CNC's texture capabilities allow effective engineering of both aesthetic properties and functional performance. CNC Milling CNC Machining