Mold Texture in CNC Machining: Achieving the Right Finish for Your Parts(6061 aluminum vs 7075 Ivan)
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This article provides an overview of mold textures in CNC machining and how to choose the right finish for your molds and molded parts.
What is Mold Texture?
Mold texture refers to the roughness or smoothness of the mold cavity surface. It is quantified by surface finish measurements like Ra (roughness average) or Rz (peak to valley height). A mirror-like finish has an Ra value below 0.1 microns, while textured finishes range from 0.5 to 25+ microns in roughness.
The texture directly impacts how plastic flows into the mold and replicates the surface. Smooth finishes allow plastic to flow and pack out easily, replicating every detail. More texture provides more friction against the plastic, inhibiting flow.
Benefits of Smooth Mold Finishes
A smooth surface produced by fine machining, grinding or polishing provides many benefits for molded parts:
- Improved surface quality - Parts achieve a glossy, reflective surface without visible flow lines or surface defects. Smooth molds replicate fine details better.
- Easier plastic flow - The low friction of a polished surface allows plastic to flow and pack out easier in the mold. This prevents defects from improper filling.
- Increased durability - Glossy, high-quality surface finishes better resist scratches, wear and chemical damage during service life.
- Lower ejection force - The decreased friction between the plastic and mold reduces sticking, allowing parts to be ejected more easily without distorting or damaging details.
Applications like plastic lenses, reflectors, transparent parts, and products requiring an aesthetic finish benefit from the highest surface polish on CNC machined molds. Injection molds require a surface finish of at least Ra 0.4 microns. Typical polishing is done with diamond abrasives to achieve sub-micron finishes.
Benefits of Textured Mold Finishes
While polished finishes are ideal for many applications, textured surfaces provide some unique advantages:
- Hiding surface defects - The topography of textured finishes helps hide any minor flow lines, sinks, or other irregularities in molded parts.
- Lower costs - Texturing eliminates secondary polishing operations. A machined EDM surface may be suitable for textured parts.
- Controlled plastic flow - Texture can slow and control the flow front for even filling of detailed areas. This reduces jetting and improves venting.
- Bonding surfaces - Parts like footwear, grips, and handles benefit from molded-in textures for improved bonding to other materials like rubber.
- Aesthetic textured effects - Many products require an intentionally rough finish like orange peel, stippling, graining or etching. This is created by matching the mold and part texture.
Common texturing methods include chemical etching, media blasting, EDM texturing, laser texturing, and directly machining patterns like diamond knurling. Mold surfaces are also roughened with tools like engraved rollers and electrical discharge texturing.
How Mold Texture Affects Injection Molded Parts
Mold texture directly influences the surface finish, appearance, and quality of injection molded parts. Some key considerations when selecting the right mold finish include:
- Gloss/appearance - From ultra-gloss to matte, the mold finish determines the visual appearance and light reflection.
- Surface defects - A smoother mold surface reduces or eliminates visible flow lines, jetting, sink marks and other irregularities.
- Replication - Fine details and textures on the mold will be more accurately replicated onto parts with a polished mirror-like finish.
- Ejection - Sticking and distortion is reduced with a smoother surface. But texture can assist with controlled ejection in deep cavities.
- Dimensional accuracy - Highly polished surfaces allow for more precise replication of part dimensions. Texturing increases uncertainty.
- Post-mold processing - Textured finishes may eliminate the need for painting or other secondary finishing steps after molding.
Optimizing Mold Surface Finish
Determining the ideal mold texture is a matter of balancing quality, appearance, cost and performance needs for each application. Here are some best practices:
- Critical appearance areas like visible surfaces are polished, while texturing is only applied to concealed areas if beneficial.
- Optical parts demand the highest surface polish on the mold.
- Intricate parts are polished to accurately reproduce fine details. Simple geometries can tolerate more texture.
- Take into account plastic color - lighter colors show texture more than dark colors.
- Running multiple cavities - ensure uniform texturing/polishing across cavities for consistency.
- Prototype molds may utilize a cheaper EDM finish suitable for short runs.
- Align any texturing direction to the flow front and part ejection.
With the right CNC machined mold texture for your application, you can achieve the surface finish, appearance, and performance required for your injection molded components. Collaborating with an experienced injection mold maker helps optimize the mold finish. CNC Milling CNC Machining