Mold Texture and Surface Finishes for CNC Machined Parts(hardness units Olivia)

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Computer numerical control (CNC) machining offers manufacturers the capability to produce parts with incredible accuracy and repeatability. One aspect of CNC machined parts that is often overlooked is the mold texture and surface finishes that can be achieved. The texture and finish will not only influence the aesthetics and feel of the part, but can also impact performance characteristics such as friction, wear resistance, light reflectivity, and more.
When designing parts for CNC machining, engineers should consider the end-use requirements and specify the appropriate mold texture or surface finish. This article will provide an overview of common textures and finishes for CNC machined molds and parts as well as guidance on selecting the ideal surface characteristics.
Visual Appearance
Mold texture can have a significant impact on the visual appearance of CNC machined plastic and metal parts. The way light interacts with the microscopic peaks and valleys of the surface influences the perceived color, glossiness, reflectivity, and masking of underlying part features.
Fine polished finishes tend to produce shiny, reflective surface qualities with crisp reproduction of underlying features. Matte and brushed finishes scatter light to produce a muted, low reflectivity surface. Textured surfaces like hairline, bead blast, and EDM finishes also scatter light but add an underlying visual texture.
Specifying the appropriate surface finish requires striking the right balance between aesthetics, light interaction, and masking. Finer finishes reproduce surface features better but can create unwanted glare. Matte finishes reduce glare but may hide critical dimensions. Textured finishes scatter light and add visual interest but can obstruct critical details.
Tactile Texture
The tactile texture of a surface refers to how it feels to the touch. Mold texture provides critical tactile feedback when users interact with a part. Fine polished finishes feel ultra-smooth to the touch while sandblasted and bead blasted textures have noticeable roughness. EDM wire burning produces a subtle topological texture.
Specifying tactile texture requires understanding user touch interactions and identifying texture characteristics that provide useful tactile feedback. For example, smooth finishes may be preferred for plastic enclosures while rougher textures provide better grip for mechanical components. Getting the right tactile texture enhances the user experience.
Coefficient of Friction
The coefficient of friction refers to how much resistance there is to motion when one surface slides over another. Mold texture significantly influences the coefficient of friction between mating surfaces.
Rougher surface finishes tend to have higher coefficients of friction. Satin, bead blast, and EDM wire burn finishes increase friction and reduce sliding between parts. High friction is desirable for mechanical components like gears that need to mesh and transmit torque. Low friction is preferred for parts that need to slide smoothly like bearings or sliding doors.
By specifying the right surface finish, engineers can tune the coefficient of friction for optimal function. Polished plastic parts or lubricant impregnated metals create low friction while rougher textures increase friction for power transmission applications.
Wear Resistance
The hardness and surface texture of mold materials also influences the wear resistance. As parts interact, micron-scale peaks on the harder surface will wear down the softer surface. The higher the peak density, the more rapid this abrasive wearing process.
Finer surface finishes allow more peaks per unit area that can accelerate wearing of mating surfaces. Glazed ceramic and polished steel finishes can quickly wear softer surfaces like aluminum. Matte and brushed finishes have lower peak density and generate less abrasive wear.
For optimal component life, engineers may specify a smoother surface finish on very hard molds and a textured finish if the mold has similar hardness to the mating parts. The right balance reduces wear rates.
Light Reflectivity
Surface texture also impacts how much light is reflected from the surface. Finely polished surfaces act like mirrors and bounce the majority of light - producing high reflectivity. Textured surfaces scatter light in different directions - reducing reflectivity.
Highly reflective surfaces may be desired for cosmetic-related parts and to produce brilliant reflections. Matte finishes are preferred to avoid glare. Optimized light scattering reduces glare but maintains sufficient reflectivity for dimensional appraisal.
Mold builders must understand the interplay between surface finish and light reflectivity to produce the right optical characteristics. Test parts with different finishes provide empirical data to inform mold surface specifications.
Fluid Flow Optimization
For molds that contain fluid flow channels, the surface finish influences flow behavior. Rough surfaces increase turbulence while smooth surfaces support laminar flow. Higher turbulence promotes mixing while laminar flow minimizes pressure drops.
To optimize flow efficiency, finer finishes are specified for straight flow channel sections. Texture may be added at channel joins to induce mixing. Optimizing channel surface finish minimizes pumping requirements while still achieving mixing when required.
Common Surface Finishes
Precision CNC machining enables a wide range of mold surface finishes to produce customized visual appearance, tactile feel, friction, wear resistance, reflectivity, and fluid flow characteristics. Here are some of the most common surface finishes for CNC machined molds:
- Polished - Produced by abrasive fine polishing. Yields highly reflective, smooth surface finish. Minimal surface texture.
- Brushed - Abrasive brushed treatment creates unidirectional satin surface finish. Reduces reflectivity while maintaining smoothness.
- Hairline - Alternating direction fine abrasion creates matt finish with non-directional hairline texture. Reduces glare reflections.
- Bead Blast - Blasted with fine glass or ceramic beads produces matte peened texture. Increased surface roughness reduces reflection.
- Sandblast - Blasting with fine abrasive media exposes coarse particle structure with muted luster. High roughness dulls reflectivity.
- EDM Wire Burn - Electrical discharge texturing produces parallel ridges from wire passes. Maintains precision form while adding micro-texture.
- Laser Etching - Laser micro-machining ablates precise texture patterns. Usually applied on polished base finish. Provides custom tactile feedback.
- Media Tumbling - Tumbled with abrasive media produces mass finishing with microscope peak rounding. Reduces abrasiveness while maintaining texture.
- Anodizing - Electrolytic oxidation creates porous metal oxide layer allowing dye infusion. Provides color tinting combined with mild etching.
- Powder Coating - Spray application of colored polymer powder with mild texture effect. Primarily influences color not base texture.
Specifying the optimal surface finish for a CNC machined mold is critical during design. Today's CNC capabilities allow for tailored texture and finish options to achieve the right visual appearance, feel, friction, wear resistance, reflectivity, and fluid flow. Consult with your machinist to explore the optimal choices for your specific mold application.
Mold texture has a profound impact on both the aesthetics and functional performance of CNC machined parts and molds. Engineers must consider the tactile feel, light interaction, friction, wear, and fluid flow characteristics required and then specify the optimal surface finish. CNC machining enables a wide range of polishing, abrasive blasting, etching, coating, and other texturing methods. A thoughtful mold texture and surface finish specification allows CNC machining to deliver optimized function and aesthetics. CNC Milling CNC Machining