Mold Texture and Surface Finish in CNC Machining(plastics prototyping Enid)

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Mold texture and surface finish are critical considerations when machining molds with CNC equipment. The texture and finish of the mold cavity directly impacts the look, feel, and performance of molded parts. By selecting the right tools and following best practices, machinists can achieve the desired aesthetic and functional mold finish.
Surface Finish Requirements for Molds
Moldmakers must meet specified surface finish requirements for molded parts. The surface finish defines the texture of final molded components. Finer finishes reproduce smooth surfaces, while coarser finishes impart visible tool marks and roughness.
Typical surface finish requirements for molds include:
- Glossy plastic parts like lenses and transparent components call for polished finishes of 0.5 microns or smoother.
- Cosmetic surfaces like automotive trim and consumer product housings often require finishes of 0.5 to 1.2 microns.
- Molds for technical components like gears and bearings may have roughness averages from 1.6 to 3.2 microns.
- Textured finishes are sometimes specified for grip surfaces and decorative effects. The resulting molded parts will reproduce the mold texture.
Computer numeric control (CNC) machines equipped with the appropriate cutters can produce any of these mold surface finishes by selecting suitable machining parameters and strategies.
Cutters for Mold Texturing in CNC
Carbide end mills and ball nose cutters are commonly used to establish the base surface finish in a mold cavity or core. They rough out the form and leave a consistent tool mark pattern based on the tool size, stepover, and other settings.
Specialized cutters are then employed to improve critical mold surfaces:
- Finishing end mills with fine teeth machine to smoother finishes. They remove cusping and uniform the surface.
- Ball mills and radius cutters polish angled corners and complex contours with their spherical ends.
- Diamond cutters grind very fine finishes when required. The diamond abrasive produces polished surfaces.
In some cases, texture is applied intentionally with engraving cutters. Checkered textures, stippling, and various artistic patterns can be machined into the mold by specialty end mills. This will transfer the texture to every production part.
Programming Techniques for Mold Finishing
To produce the required mold surface finish with CNC equipment, machinists should follow best practices:
- Apply sufficient stepovers during roughing to establish texture. A stepover of around 50% of tool diameter provides uniform cusp heights.
- Use new sharp cutters for finishing passes. Worn tools leave poor surface finishes.
- Take light finishing depths of cut. Cuts under 0.005" prevent vibration and leave a consistent finish.
- Finish with small stepovers. Smooth finishing requires stepovers under 0.020" for fine surface texture.
- Employ high spindle speeds. Faster cutting speeds yield finer surface finishes.
- Use climb milling. In climb milling, the cutter rotation opposes the feed direction to give better finishes.
- Program slow feed rates for the finishing path. This allows more time to achieve the desired texture.
- Apply radius cutters and ball end mills tangentially. The smooth spherical tool profile contacts evenly this way.
With practice and proven CNC programming methods, machinists can develop techniques to produce all the mold finishes needed for plastic injection molds or die cast tooling. Every job will require experimentation to optimize the tool selection, operating parameters, and tool paths to achieve the finish specified on the component prints.
Automated Texture Measurement
Moldmakers use surface profilometers to quantitatively measure surface finish. These instruments use a stylus or laser to trace the peaks and valleys of the surface. The software then calculates key statistics like Ra roughness average, Rz maximum height, and Rq root mean square roughness.
Other handy measurement tools include:
- Optical comparators to visually examine and compare mold texture to reference standards.
- Mold replicas made by pressing plastic against the mold surface. The replica shows the surface finish.
- Microscopes to zoom in on the tool marks and grains left by the machining process.
By benchmarking the surface with quantitative data, moldmakers can validate they are achieving the specified finish levels in the mold.
Mold Polishing Methods
If surface measurements reveal the texture is insufficient, additional manual or automated polishing can be performed. Machinists have several options for smoothing mold surfaces:
- Stone polishing by hand works well for repairing small localized spots. The abrasive stones grind away peaks and tool marks.
- Small radius files and emery cloth can also improve finishes on difficult to reach surfaces.
- CNC polishing programs use abrasive, dense brushes to automatically refine large areas.
- Electro-polish through a chemical bath removes a thin layer of material for ultrasmooth finishes.
A polishing workflow involves starting with coarse abrasives and working progressively to finer polishes. This removes deeper scratches before addressing finer marks and peaks. The mold texture steadily improves through the polishing process.
Mold Texturing Best Practices
Following a structured approach ensures machinists achieve the finish requirements for molded parts:
- Confirm the surface finish specification based on part function and appearance needs.
- Select suitable tooling and operating parameters for the required finish. Lower stepovers, faster speeds, and climb milling produce better results.
- Measure and validate surface texture at multiple intervals with quantitative data.
- Adjust programming or tool paths if finish levels are deficient.
- Consider secondary manual or automated polishing if the specified finish cannot be reached through machining alone.
With experience and proven finishing methods, CNC machinists can consistently reproduce mold textures that impart the desired look and feel to plastic parts. Dialing in the optimal machine parameters and cutters for the finish is key. CNC Milling CNC Machining