Photo-Chemical Machining(PCM)
Objectives:
- To understand the basic principle of photographic and chemical etching processes PCM technology.
- To be able to arrange the PCM work in the proper process sequence.
- To be capable of perform ing the essential manufacturing processes in PCM.
1. Introduction
Photo-Chemical Machining (PCM ) is one of the major manufacturing processes for precision sheet metal components. It employs chemical etching through a photo-resist stencil as the method of mat erial removal over selected areas.
2. The PCM Processes
PCM manufacturing applies the photographic and chemical etching processes to manufacture precision sheet metal products. The process sequence can beoutlined in the following steps:-
-
Manufacture of Photo-tool
Various methods exist for the production of photo -tools. The most commonly-used method is to produce a photo -tool of the co rre ct and precise size by using a laser photo plotter to selectively expose a photographic film according to computer-aided design data.
-
Selection and Preparation of Metallic Materials for PCM
Material characteristics
The ease with which the material can be etched depends on its chemical composition, as etching is essentially a fast, controlled chemical corrosion reaction. Table 6.1 lists etchability ratings of some metals and allo ys used in PCM and reflects the fact the pure metals have a wide range of reductio n-oxid ation ("redox") potentials and corrosion resistance.
-
Sheet Metal Pre-treatment
Before lamina ted with photo -resist, the metal is thoroughly Electro lyti c alkaline cleaned to remove all dirt, rust, greases and oils so t hat good adhesion to the photo -resist to obtained. Elect rolytic alkaline dean is more rapid and reliable way fo r cleaning the surface than soak alkaline clean.
Sheet Metal Pre-treatment
Before lamina ted with photo -resist, the metal is thoroughly Electro lyti c alkaline cleaned to remove all dirt, rust, greases and oils so t hat good adhesion to the photo -resist to obtained. Elect rolytic alkaline dean is more rapid and reliable way fo r cleaning the surface than soak alkaline clean.
The processing cycles appear to be:
Fig. 1 Process flow of lamination photo-resist.
-
Lamination
Photo-resists are UV light-sensitive polymers layer that may be applied to adhere on the sheet metal surface by hot roller laminati on.
Dry Film Laminating Machining
Dry film photo -resist forms the middle layer of a three- layer sand wich structure sup p lied in rolls of various widths from 75 -600 mm and in standard thickness. The protective polyethylene layer is peeled off from the photo -resist imm ediat ely before sticking the phot o -resist on to the hot substrate. The coating is performed aut omat ically by using a laminator containing a pressur i zed and heated roll er transport system.Both sides of a sheet of metal can be coated simultaneously by using two separat e rolls of phot o-resist.
Fig. 2 Lamination of phot o-resist
-
Exposure and Development
The lam inated metal substrate , covered with photo artwork, is exposed under an UV source. The correct exposure for a photo -resist coat ing depends on its thickness. Over -exposure of a photo -resist leads to difficulties in marinating correct line widths and und er -exposure leads to problems in being able to develop the image.
During development, the unexposed negative working photo -resist will be washed away , and the exposed photo -resist would remain on the substrate to form the protective layer for etching. -
Etching and Stripping
The substrate protected with photo-resist mask is etched wit h appropriate chemical etchants. The photo -resist layer is then stripped away to yield the final precision shee t metal product.
Influence of etch timeInfluence of etch time
As etching proceeds with time, the boundary of the edge cont inually changes as shown.
Fig. 3 Development of etched profiles. ( a) Breakthrough point;(b) biconvex; (c) 'straigh t' ; (d) bio concave
See Figure 4 for the overall process fl ow of PCM.
3. Process Capabilities and Dimensional Limitation of PCM
PCM is capable of manufacturing a wide variety of precision sheet metal parts. Based on a survey of European contract PCM com pan ies and PCMI Standard Sp ecificat io n D -300, limitations have been quantified and may be used as general guidelines to evaluate the possibility of production by PCM :-
- Normally, the minimum diameter of holes (Dmin) is 1.5 to 2 times the metal thi ckness (T). For very thin metal of thickness less t ha n 0.025 mm, Dmin co uld be 2 to 4 times T since a least unde rcut will occur with the smallest depth of etch.
- The minimum metal land width (Wmin) between ape rtures is relat ed with the metal thickness (T). Fo r T < 0.125 mm, the Wmin is equal to 1 to 1.25T; for T > 0.125 mm, Wmin is equal to 1.25 to 2T.
- Both minimum inside corner radius (R min) and minimum outside corner radius (Rmin) are dependent on metal thickness too . Theoretically, Rmin = T and Rmin = 0.75T.
- Although the maximum metal thickness varies with the metal being etched and the profi le (edge) requirements. A general rule of thumb for metal thickness lim it is 2 mm.
- The dimensional tolerances, an important specification,are 0.1T.
- The centre-to-centre and pitch- to -pitch dimension of PCM has a tolerance of 0.05% of the sp ecified dim ension, e.g. for a 10 mm pitch, a tolerance of 0.005 mm can be easily achieved by PCM.
4. Comparison of PCM to Metal Stamping
PCM is often compared with metal stamping because both methods produce precision sheet metal parts.
- In nature, PCM has a relatively high part cost and a very low artwork and tooling cost. In contrast, precision metal stamping has an extremely low unit production cost but a very high tooling cost, especially for complicated parts. Thus PCM has an economical edge for complex parts and small to medium quantity. For less complex parts and very large quantit y, precision metal stamping would have the cost advantage.
- PCM lead time is short, it takes about 2 -3 days to produ ce sample quantities , while in meta I stamping, it normally takes several weeks to make the tooling. If there is any design change required, PCM can modify the photo tool easily at low cost within a short period of time.This gives PCM a definite cost and lead t ime advantages over metal st amp ing in making protot ypes.
- The etched components from PCM are burr free. Metal stamping requires furt her abras ive treatments to eliminate the burrs and to finish the components.
- PCM is a process which does not affect the physical or chem ical properties inherent in the sheet metal. This makes PCM the prominent process in magnetic material fabricati on since it would not affect the magnetic permeability. Metal stamping will reduce the magnetic permeability sign ificant ly and cause stresses for w hich a time consuming and costly annea ling process is needed to restore the permeabilit y.
- Metal stamping cannot process -pre hardened sheet metal. Metal hardness has never been a concern in PCM manufacturing, which can process sheet metal of any hardness.
- Some other aspects of PCM and metal stamping can be compared in the followings:-
In conclusion, PCM is most suit able for thin gauge , high accuracy and small to medium quantity job orders. Metal stamping is more appropriate for the production of very large quantity. Conseq uent ly, PCM serves as the best method to produce prototypes in sheet metal industry . It has definite advantages over metal stamping in photo tooling cost, lead time and ease of incorpo rat e design changes.
5. Typical Applications of PCM
A wide range of functional products can be made by PCM in electro nic, computer, metal working and precision engineering industries. Some typical applications worthy of mentioning are as follows:
- Colour TV receiver tube aperture mask, commonly known as shadow mask, normally comprises of some 300,000 perfectly etched slots. PCM is the only feasible method for the shadow mask manufacturing.
- Integrated circuit lead fame, an electronics component with a complex geometry, is broadly fabricated by PCM because of the rapid change in IC designs and also the geometry becomes more and more complex.
- Stainless steel spring and magnetic head for floppy diskette drive are commonly fabricat ed by PCM.
- Metal stencils for SMT can only be made by PCM as accurate dimensions are strictly required.
- Metal mesh screen , used in Juiceextractor , 1s another PCM major application.
- Metal toy models, a sm all batch production of great number of different designs, are typ ical products made by PCM.
- Prototypes for precision sheet metal engineering indust ry, e.g. watch dials.
- Decorative and graphic pro ducts are commonly made by PCM due to the low volume and normally complex pattern design.