chemical machining principle and working principles

Chemical machining principle, applications, advantages, disadvantages

1. Chemical Machining Oldest nontraditional machining process. Material is removed from a surface by chemical dissolution using chemical reagents or etchants like acids and alkaline solutions.

2. Process characteristics and process parameters and MRR (cutting speed) Process Characteristics Process parameters and typical Material Removal Rate or Cutting speed Chemical Machining Shallow removal (up to 13 mm ) on a large flat or curved surfaces; blanking of thin sheets; low tooling and equipment cost; suitable for low production runs 0.025-0.1mm/min

3. Steps in Chemical Machining Cleaning of workpiece - to ensure that material will be removed uniformly from the surfaces to be etched=> using alkaline solution and then with fresh water Masking - maskant , chemically resistant to the etchant, applied to portions of the work surface not to be etched Etching - the material removal step; part immersed in an etchant that chemically attacks unmasked portions; part removed and washed when desired amount of material has been removed Demasking - maskant removed from the part Demasking

4. Maskant materials : Neoprene , polyvinyl chloride , polyethylene and other polymers Silk screen masks –shallow cuts –close dimensions Sythetic or rubber base materials (polymers and polyethylene) Maskant

5. Maskants properties and method Maskants should have the following properties Be tough enough to withstand handling Adhere well to the workpiece surface Scribe easily Be inert to the chemical reagent used Be able to withstand the heat generated by etching Be removed easily Be inexpensive after etching Dip, Brush, Spray, roller and electrocoating as well as adhesive tapes can be used to apply mask.

6. Factors to be considered during the selection of Maskant /resist Chemical resistance required No. Of parts to be produced Detail or resolution required Shape and size of component Ease of removal Economics

7. Etchants Etchants are acid or alkalie solution maintained within a controlled range of chemical composition and temperature The purpose of an etchant is to dissolve a metal by turning it into a metallic salt which then goes into the solution Etchant selection is dependent on certain factors. They are Surface finish required Uniformity of metal removal Material to be etched Depth of Etch Type of resist ( maskant ) used Rate of Material Removal Potential damage to the metallurgical properties of work Cost

8. Etch factor Etch factor= Undercut/ Depth of cut Undercut depends on Depth of cut The type & Strength of the etchant, and The work material The quantification of undercut is known as Etch factor Depth of cut is controlled by immersion time

9. Etchant selection – depends on work material, desired depth and rate of etch and surface finish requirements

10. Methods of MAsking Cut and peel Photographic resist/ Maskant Screen resist/ Printing

11. Cut and peel maskant Maskants used are Neoprene, Butyl or Vinyl based materials Applied either by Dipping the component in the tank Spraying the mask on the part or flow coating Thickness of coating : 0.025-0.13mm Maskant is applied to entire surface Cut,scribing or peeled off- using template as guide Etching :13mm max, Accuracy: +/- 0.13 to +/- 0.75mm Time of etching controls the depth of material removal Rescribing Application : Large sized work piece, chemical industries, aircraft industries, missile industries Good for batch production for products with large depth >1.5mm to be etched.

12. Spray etching process

13. Etch profiles dependent on time of etch A wide range of etch profile is possible in CHM process

14. Disadvantages If adhesion of maskant is improper,=> debonding => stray etching The maskant and workpiece has to be thoroughly cleaned Cleaning of porous material is very difficult Very low MRR To achieve higher MRR=> agitate and heat the etchant

15. Cut and Peel of Maskant - Chemical Machining To achieve higher MRR  Agitate and heat the etchant How to maintain high MRR? Filtration of etchant Periodical addition of new etchant Discarding some percent of used etchant

16. Screen Resist/printing Maskant applied by silk screening method Maskant is painted through a stainless steel mesh containing stencil onto surface areas that are not to be etched The screen has the area blocked off that are to be selectively etched Press the screen against the surface of the part and roll up the maskant Remove the screen and dry the part by baking The exposed unmasked area is the one form which material is to be etched Good for high volume production Low accuracy (< ±0.2mm) Comparatively low etching depth(<1.5mm) Part Size: <1.2m x 1.2m Parts having only flat surface or simple contours Etch depth; < 1.5mm

17. Illustration of screens used to mask for chemical machining

18. Photo Resist Maskant : Masking materials contain photosensitive chemicals Maskant is applied to work surface and exposed to light through a negative image of areas to be etched These areas are then removed using photographic developing techniques Remaining areas are vulnerable to etching Applications: Can be used for materials upto 0.8mm thick Used for automatic processing of high volume components Produce complicated accurate shapes Accuracy : 0.025mm Repeatability of 0.005mm

19. Is defined as the process of chemically eroding material to produce blind details(pockets, channels or to remove material from all surfaces of a part for the purpose of weight reduction Used largely in the aircraft industry Applicable to large parts where substantial amounts of metal are removed Also used to fabricate micro-electronic devices- wet etching Cut and peel maskant method employed As depth increases, surface finish becomes worse Metallurgical damage very small MRR=0.0025-0.1mm/min Chemical Milling

20. Sequence of processing steps in chemical milling Clean raw part Apply maskant Scribe, cut and peel maskant from areas to be etched Etch Remove maskant and clean to yield finished part : 3

21. Uses chemical erosion to cut very thin sheet-metal parts, down to 0.001 in. ( 0.03mm )and/or for intricate cutting patterns Produces burr free parts Photoresist or screen resist method applied Maximum stock thickness ~0.030 in.( 0.76mm ) Hardened or brittle materials can be processed Chemical Blanking

22. . Steps in chemical blanking: (1) clean raw material,(2) apply resist( maskant ) by painting through screen, (3) etch partially completed, (4) etch (completed), and (5) remove resist and clean to yield finished part

23. A chemical machining process used for making flat panels that have lettering and/or artwork on one side Can be used to make raised or recessed lettering by reversing the portions of the panel to be etched Masking done by either photoresist or screen resist methods Filling operation to apply paint or other coating follows etching Chemical Engraving

24. Chemical machining in which the photoresist masking method is used Employed in metalworking when close tolerances and/or intricate patterns are required Used extensively in electronics industry (makes VLSI possible) Photoresist materials in current use are sensitive to UV light, but not other wavelengths Photochemical Machining (PCM)

25. Sequence of processing in photochemical machining Clean raw part, (2) apply resist( maskant ) by dipping, spraying or painting (3) Place negative on resist, (4) expose to ultraviolet light; (5) develop to remove resist from areas to be etched, (6) etch(shown partially etched), (7) etch (completed) and (8) remove resist and clean to yield finished part.

26. Products manufactured by chemical machining process

27. Chemical Milling=> Production of blind holes( Pockets, Channels etc.,) Chemical Blanking=> Production of through cavities( Trough holes, slots, etc.,)