INTRODUCTION OF CHEMICAL MILLING
Chemical milling is of particular importance in the airplane and in the aerospace industry, where it is used to reduce the thicknesses of the dishes that surround walls of rocket and airplanes, striving to improve rigidity to the weight ratio.
Chemical milling is also used in metal industries to lose weight, webs and ribs of parts produced by forging, from the formation of metal sheets or by casting.
THINNING OF PARTS BY CHEMICAL MILLING
Furthermore, the process has many applications relating to the improvement of the characteristics of the surface, as the following:
Removing the Ti oxide (α-case) from Ti forgings and superplastic formed parts.
Eliminating the decarburized layer from low-alloy steel forgings.
Elimination of the recast layer from parts machined by EDM.
Removal of burrs from conventionally machined parts of complex shapes.
In Chemical milling, a special coating called Maskant protects the areas of which the metal must not be removed.
The process is used to produce pockets and contours.
CHEMICAL MILLING STEPS
Chemical milling is composed of the following steps:
CHEMICAL MILLING SETUP
Preparation of cleaning the surface of the workpiece, mechanical or chemistry, to provide good adhesion of masking material.
Mask with a strippable mask that adheres to the surface and resists chemical abrasion during etching.
Scribing of the mask using special models to expose the areas to be etched.
The selected mask type depends on the size of the work, the number of pieces and the desired resolution of the details.
Silk screens for the surface cuts of the nearby dimensional tolerances.
After engraving, the work is rinsed and the mask is stripped manually, mechanically or chemically.
The work is washed by deionized water and then dried with nitrogen.
During the Chemical milling, the depth of the etching is controlled at the time of the immersion.
Etchants used are very corrosive, and therefore must be managed with adequate safety precautions.
Vapors and gas produced by the chemical reaction must be controlled for environmental protection.
A Agitator is used for fluid agitation.
Temperatures typical of the reagent vary from 37°C to 85°C, which must be checked within ± 5 °C to achieve uniform processing.
Faster etching rates occur at higher etchant temperatures and concentrations.
When the mask is used, the processing action proceeds both inwardly from the opening of the mask and, sideways, below the mask, thus creating the ETCH (EF) factor, which is represented as,
(EF = du / Te).
This ratio should be considered when writing the mask using the models.
A typical EF of 1: 1 takes place in a cutting depth of 1.27 mm.
Deeper cuts can reduce this ratio to 1: 3.
TOOLS FOR CHEMICAL MILLING
The tools for the Chemical milling are relatively cheap and simple to modify.
Four types of tools are required:
Maskants, etchants, templates and writing accessories.
Synthetic or rubber base materials are often used as maskants.
They should have the following properties:
Sufficiently resistant to support handling.
Inerts to the chemical reagent used.
Capable of supporting the heat generated by etching.
Adhered well to the work surface.
Capable of removing easily after etching.
Multiple markant, are frequently used to increase the etchant resistance and to avoid the formation of pinholes on the machined surfaces.
You can also get deeper cuts that require longer exposure time.
Brush, Roller and Electrocurrent, as well as adhesive tapes, can be used to apply masks.
Spraying the mask on the workpiece through a silk screen in which the desired design is imposed combines the maskant application with the writing operation, since peeling is not required.
Therefore, product quality has improved, due to the ability to generate more fine details.
Etchants are highly concentrated acid or alkaline solutions maintained within a controlled composition and chemical temperature range.
These are able to react with the workpiece material to produce a metallic salt that dissolves in the solution.
Working glass or germanium, HF or HF + HNO3 acid solutions are used as etchant.
An appropriate etchant must provide the following requirements:
Good surface finish of the work.
Uniformity Metal removal.
Control of selective and intergranular attack (IgA).
Low cost capacity and availability.
Ability to regenerate, or easily neutralize and dispose its waste products.
These include tanks, hooks, brackets, racks and fixtures.
Weight reduction is possible in complex contours that are difficult to be machined.
Various parts can be worked simultaneously.
Simultaneous removal of material from all surfaces, improves productivity and reduces wrapping.
No burr formation.
There are no induced stresses, thus minimizing distortion and allowing the processing of delicate parts.
Very low capital cost of the instrument, and also an minor tooling cost.
Rapid implementation of design changes.
A less qualified operator is required.
Low scrap rate.
Only shallow cuts are practical. Deep tight cuts are difficult to produce.
The management and disposal of etchants can be problematic.
Masking, scribing and stripping are repetitive, time consumer and tedious.
Surface imperfections, if any, are reproduced.
For best results, metallurgical homogeneous surfaces are required.
Porous castings yield uneven etched surfaces.