Mold Texture in CNC Machining: Achieving the Desired Surface Finish(cad cam software Odelette)
- source:DAHLER CNC Machining
Surface Finish Importance in Mold Making
The texture and finish of a mold cavity directly affects the surface quality of molded parts. A rough surface on the mold will transfer and appear on the parts. Smooth mold surfaces are usually desired to create glossy, attractive parts with a consistent surface texture. The mold finish also impacts release and ejection of parts from the mold. A rougher surface provides more friction which could cause parts to stick in the mold.
How Toolpaths Affect Mold Finish
The CNC toolpaths used to machine the mold greatly impact the resulting surface finish. Toolpaths control the path followed by the cutter as material is removed. Optimal toolpath strategies are required to obtain the required finish while maximizing machining efficiency.
Some factors related to toolpaths that influence finish include:
- Stepover distance between passes - The closer the stepover distance, the finer the finish. A smaller stepover requires more passes and longer machining time.
- Toolpath pattern - Common patterns like zig-zag and contours leave different finish effects. The orientation of patterns also affects finish.
- Cut direction – Climb vs conventional milling impacts surface finish. Climb milling generally leaves a finer finish.
- Feed rate – Higher feeds can leave more visible tool marks. Reduced feeds help obtain a smoother finish.
- Tool type – The geometry, material, and coating of the cutting tool impacts finish. Ball end mills generally provide a finer finish than square end mills.
These factors need to be balanced to create an optimal toolpath strategy that achieves the required finish quality without excessive machining time.
Finishing Operations to Enhance Texture
Often, roughing toolpaths are used to efficiently remove the bulk of material. Separate finishing operations are then required to improve the mold finish to the final specification. Common finishing techniques include:
- Finish passes – Additional light finishing passes with smaller stepover distances to smooth the rough finish left behind by initial toolpaths. This is the most basic finishing technique.
- Scallops – Crescent shaped toolmarks left behind by ball end mills can be smoothed by using a flat end mill at a lower stepover.
- Re-machining with climb milling – Critical areas can be re-machined using only climb milling to reduce visible tool marks.
- Polishing – Abrasive polishing with progressively finer grit can be used to smooth and shine machined surfaces. Hand polishing or CNC-automated polishing may be used depending on mold geometry.
- EDM – Electrical discharge machining (EDM) uses controlled sparking to erode away tiny amounts of material. This can eliminate tool marks and polish the mold surface.
- Vibration finishing – Low-amplitude high-frequency vibration has been used to refine mold finish after machining.
- Media blasting – Blasting the surface with media such as sand or glass beads can provide a uniform fine finish.
By incorporating suitable finishing operations into the CNC machining process, molds with the desired finish specification can be achieved.
Factors that Influence Mold Surface Finish
There are many variables in the machining process that impact the final mold texture. Being aware of these factors enables the optimization of machining parameters and strategies to obtain the target finish. The key factors include:
- Tool condition – Worn or damaged cutting tools will leave poorer finishes. Regular tool replacement helps maintain finish quality.
- Rigidity – Deflection of the tool or workpiece reduces finish. A rigid setup prevents vibration and chatter marks.
- Coolant – Effective coolant use improves finish by flushing away chips and reducing thermal effects.
- Speeds/feeds – Optimized cutting parameters help achieve required finish while maximizing productivity. This requires finding the right balance of speed and feed for the operation.
- Toolpath patterns – As covered earlier, the arrangement of toolpaths greatly influences resulting finish.
- Stepdown – Taking lighter final finishing depths of cut will improve finish. However, this increases cycle time.
- Machine condition – Precision machines in good condition with minimal runout provide better finish capabilities.
By optimizing these factors, it’s possible to improve mold surface quality to the desired specification through the CNC machining process.
Inspection Methods for Mold Texture
To confirm that finish requirements are met, molds must be inspected after machining. Some common methods for evaluating surface texture include:
- Visual inspection – Carefully looking at the surface under bright lighting can reveal overall finish quality, noticeable toolmarks, and defects. This simple technique catches obvious issues.
- Tactile – Running a fingernail or fingertip over the surface feels subtle variations in the texture.
- Profilometer – This instrument uses a stylus to trace the surface variations, generating quantitative finish measurements like Ra surface roughness average.
- Interferometer – Interferometric optical techniques enable highly sensitive non-contact measurement of surface texture. Interferometers can map 3D finish topography.
- Mold replicas – A mold replica involves creating a plastic or wax impression of the mold surface. Inspecting the replica enables checking finish quality in fine detail.
Mold makers use these quality control techniques to validate that finish requirements are achieved after CNC machining. Proper finish inspection confirms the mold will produce parts with the desired surface characteristics.
Achieving the target mold texture and finish through CNC machining requires an optimized machining strategy and process parameters. However, with the right techniques it’s possible to create molds that produce smooth, attractive, easy-to-release molded components. Paying close attention to toolpath patterns, finishing operations, machining variables, and inspection allows mold makers to consistently hit the desired finish specification. CNC Milling CNC Machining