Metal etching is a subtractive manufacturing process used to create intricate designs, textures, or functional features on metal surfaces. When applied to mold-making, etching enables the production of high-precision molds for applications such as injection molding, embossing, or microfluidic devices. This guide provides a detailed overview of the materials, steps, and considerations involved in etching metal molds.
Materials and Tools Required
Metal Substrate:
Common choices: Stainless steel (grade 304/316), aluminum, copper, or tool steel.
Thickness: Typically 0.1–5 mm, depending on mold depth requirements.
Resist Material:
Photoresist: Light-sensitive polymer films (e.g., dry film photoresist) or liquid photoresists (e.g., AZ 4620).
Alternative Resists: Wax, vinyl, or polymer coatings for non-photochemical methods.
Etching Chemicals:
Ferric Chloride (FeCl₃): For copper, brass, and nickel alloys.
Nitric Acid (HNO₃): For stainless steel and titanium (diluted to 10–30% concentration).
Hydrochloric Acid (HCl): Often mixed with HNO₃ for aluminum.
Equipment:
UV exposure unit.
Developing tank and etching tank (or spray etcher for industrial setups).
Hotplate or oven for resist curing.
Precision cutting tools (for post-processing).
Safety Gear:
Gloves (nitrile/neoprene), goggles, lab coat, and fume extraction system.
Step-by-Step Process
1. Design Preparation
Use CAD software (e.g., AutoCAD, SolidWorks) to create the mold design.
Convert the design into a high-contrast photomask (film or glass) with the pattern to be etched.
Critical Consideration: The photomask must be a mirror image of the final design to account for resist application on the metal surface.
2. Metal Preparation
Cleaning: Degrease the metal using acetone or alkaline cleaners. Remove oxides with a mild acid (e.g., 5% HCl).
Surface Roughening: For better resist adhesion, lightly sandblast or etch the surface (optional).
3. Resist Application
Dry Film Photoresist:
Laminate the photoresist film onto the metal using a heated roller (70–90°C).
Ensure no air bubbles remain.
Liquid Photoresist:
Spin-coat or spray the resist onto the metal.
Cure in an oven at 60–80°C for 15–30 minutes.
4. Exposure
Align the photomask over the resist-coated metal.
Expose to UV light (365–405 nm wavelength) for 30–120 seconds.
Result: UV light hardens the resist in exposed areas; unexposed areas remain soluble.
5. Developing
Immerse the metal in a developer solution (e.g., 1% sodium carbonate for dry film resist).
Gently agitate to dissolve unexposed resist, revealing the bare metal in the pattern areas.
Rinse with deionized water and dry.
6. Etching
Immersion Etching:
Submerge the metal in the etchant (e.g., FeCl₃ at 40–50°C for stainless steel).
Etch rate: ~0.001–0.003 inches/minute. Agitate the solution to ensure uniformity.
Spray Etching:
Faster and more precise; uses pressurized nozzles to spray etchant.
Monitor Progress: Remove periodically to check depth using a profilometer.
7. Resist Removal and Cleaning
Strip residual resist using acetone or specialized strippers (e.g., NaOH solution).
Neutralize acid traces with a baking soda rinse.
Polish the etched surface if required (e.g., for injection molds).
8. Post-Processing
Surface Treatment: Apply hardening treatments (e.g., nitriding) for wear resistance.
Quality Control: Inspect for undercuts, depth consistency, and edge definition.
Applications of Etched Molds
Injection Molds: Textured surfaces for plastic parts.
Embossing Dies: Patterns for leather, paper, or metal foils.
Microfluidic Chips: Precision channels for lab-on-a-chip devices.
Decorative Molds: Artistic designs for jewelry or automotive trim.
Challenges and Solutions
Undercutting: Occurs when etchant attacks beneath the resist.
Solution: Use fresher etchant, reduce etching time, or employ a resist with better adhesion.
Uneven Etching: Caused by temperature fluctuations or poor agitation.
Solution: Use spray etching or automated agitation systems.
Resist Failure: Peeling due to improper cleaning or curing.
Solution: Optimize surface preparation and curing parameters.
Safety Considerations
Ventilation: Always etch in a fume hood or well-ventilated area.
Chemical Handling: Neutralize spent etchant (e.g., FeCl₃ with NaOH) before disposal.
PPE: Wear acid-resistant gloves and eye protection.
Conclusion
Metal etching offers unparalleled precision for creating complex mold geometries that are difficult to achieve with mechanical methods. By optimizing resist application, etching parameters, and post-processing, manufacturers can produce durable, high-quality molds for diverse industries. While the process requires careful handling of hazardous chemicals, its advantages in cost-effectiveness (especially for small batches) and design flexibility make it indispensable in modern manufacturing.
