Mold Texture in CNC Machining - Achieving the Desired Surface Finish(hot runner vs cold runner Bard)
- source:NEWRGY CNC Machining
What is Mold Texture?
Mold texture refers to the microscopic irregularities left on the surface of a CNC machined mold after manufacturing. It is quantified by surface roughness parameters such as Ra, Rz and Rq. A perfectly smooth surface is impossible to achieve in practice. There will always be some level of irregularity depending on the machining process and parameters used.
The texture left on a mold surface affects how accurately it transfers details onto the molded product. It also impacts release properties, wear resistance, friction, light reflection and bonding of coatings. Therefore, selecting the right mold texture for the application is essential.
How Mold Texture is Achieved in CNC Machining
The mold texture is primarily determined by the chosen machining process and parameters. Here are the key factors:
- Cutting Tool: The geometry, material, coatings and wear state of the cutting tool influences surface finish. A sharp, precision tool generally produces a smoother surface.
- Feeds and Speeds: Higher feeds and speeds typically result in rougher surfaces, while lower parameters create finer finishes. Slower machining may be required for glossy surfaces.
- Cut Depth: Final finishing passes with shallow cut depths yield finer textures. Roughing operations involve deeper cuts and rougher surfaces.
- Tool Path: Tool paths optimized to avoid sharp changes in direction produce better surface finish. Contour tool paths generally provide superior results over zig-zag paths.
- Coolant: Effective use of cutting fluids is essential for removing chips, reducing heat and obtaining fine finishes.
- Vibrations: Minimizing vibrations in the machine, fixture and workpiece is key to preventing waviness and other defects.
- Post Processing: Subsequent polishing and finishing processes further enhance the mold texture. Media blasting, honing, lapping and more can be utilized.
Selecting the Right Mold Texture
Determining the appropriate mold texture is important to ensure optimal product quality and process efficiency. Here are some key considerations:
- Product Appearance: Glossy plastic parts require mirror-like mold finishes. Matte appearances need rougher textures to diffuse light reflections.
- Part Accuracy: Micro details and sharp corners demand very smooth mold surfaces. More tolerance is allowed for basic geometrical shapes.
- Release Properties: Textures with properly angled peaks allow easy ejection of molded parts with minimal draft angles.
- Wear Resistance: Hard coatings deposited on finer mold textures last longer against abrasive melt flow and thermal cycling.
- Friction: Parts that slide against the mold require lower roughness to minimize friction, wear and ejection forces. Static parts can have rougher finishes.
- Contamination Risks: Rough porous textures trap and transfer contaminants onto critical molded surfaces like optics and connectors. Smooth glass-like finishes are preferred.
- Cost: Achieving ultra-fine surface finishes is more time consuming and expensive. A balance between cost and required quality must be achieved.
Achieving Micron-Level Surface Finishes
Today's advanced CNC machining centers in combination with precision tooling and techniques make it possible to achieve micron level mold finishes comparable to polished stone. Here are some tips:
- Use micro-grain carbide endmills and ballnose cutters for finishing operations. Diamond-coated tools also provide superb finishes.
- Reduce axial and radial depth of cuts to 0.01 - 0.02mm during final passes.
- Apply specialized vibration dampening toolholders and shrink fit chucks.
- Machine on platforms with active vibration control.
- Keep feed rates low and use spline toolpaths for finishing.
- Use specialist surface measurement devices to quantify results.
- Protect delicate surface textures post-machining to avoid handling damage.
With the right CNC machining strategy, tooling and quality control, molds with tailored textures for the perfect look and functionality can be manufactured repeatably. The mold texture has a direct impact on product performance and economics, making it a key consideration in design and machining. CNC Milling CNC Machining