Italix - Chemical Machining, Etching & Metal Finishing
Photo Chemical Machining

Metal from Coils

After the rolling process, the edges of the coil are slit to remove the wavy edge caused by this process. Stainless Steels have to undergo a further tension leveling (or stretch band leveling as it is often called) process. This removes any curvature across the width of the strip and along its length. However because the process is a de-coil and then re-coil operation, the characteristic of coil set is not alleviated. Coil set, which results in the material having a degree of curvature along the length of the cut sheet, is minimized by ensuring the re-coil operation is performed on coil centers that have a minimum diameter of 300mm. BS 5770 Part 4 refers to allowable curvature for Stainless Steels and although the standard is acceptable for commercial grade materials, the flatness required for photoetch must be superior to the standard. Although flatness tolerances are proportional to the thickness of the metal, a general rule of thumb is a maximum flatness deviation of less than 2% of the length of the cut sheet is a minimum requirement. Coil set is more pronounced in softer, thicker materials and special care has to be taken when these materials are sourced e.g. 1mm thick mild steel (low carbon steel) in the annealed state (105 v.p.n. max) would typically have a curvature of 30mm over a 500mm sheet length. That is if both ends were placed on a flat surface with the concave face downwards, the highest point of the curvature above the flat surface would typically be 30mm. This would make printing of the sheet (refer to image creating) with the required accuracy very difficult, if not impossible. ITALIX COMPANY, Inc D Cut Blanks are materials that have been manufactured and stored as precision strip metal on coils. Metal suppliers offer a flattening and cutting to length service. His is especially useful for metals that are prone to high degrees of coil set. The process is one of back rolling to eliminate the curvature caused by coil set and then cutting the strip to lengths, typically 500mm long. The cut sheets are then palletized as flat sheets ready for processing. As the material does not have to be recoiled the flatness is maintained. This type of supply is available for all types and gauges of metal, but is subject to minimum processing quantities and therefore not suitable for very small quantities. It is extremely important that at the rolling mill, and during the flattening operation, that the surface of the metal is kept free from machinery lubrication especially silicone oils or grease. As this can contaminate the surface of the material and ingress into the voids between the individual grains, causing processing problems at the cleaning stage (refer to metal preparation).


Mechanical Properties

Mechanical properties of the metal are unimportant to the photo etch process. Sheets need to be flat and free from surface contamination (refer to specific requirement for the process). Temper of the metal does not affect the etching process but may affect any post etch operations. Also the etching process itself does not affect in any way the temper of the material. Fully annealed material stays fully annealed and fully hardened material is easily processed without altering the hardness. The etching process acts on the material at a grain structure level (refer to component manufacture), therefore metals with an even grain structure, yield better results than those without. Grain direction of the stock material is not critical. Orientation of grain direction to component can be adjusted at the printing stage (refer to production of phototools).