PCM Process

Photochemical machining (PCM) is an important process in the production of precise parts, mainly sheets and foils. Materials suitable for etching include: aluminum, brass, copper, magnesium, molybdenum, nickel steels, chromium, lead, nickel, silver, phosphor bronze, stainless steel and zinc.

Coating the metal sheet with photoresist sensitizes it. When this sensitized metal is put into the double-sided phototools and is exposed to UV light on both sides, an image is formed in the photoresist. This is then developed in a liquid formulation to form an adherent, durable image on both sides of the metal.

Various methods exist for the production of phototools. The most commonly used method is to produce a phototool of the correct size by using a laser photo-plotter to selectively expose a photographic film according to computer-aided design data.

Photoresists are UV light-sensitive polymers. The etching is performed by an acid cocktail that attacks the surface of the raw material on both sides. Before coating the metal with photoresist, the metal is thoroughly cleaned to remove all dirt, rust, greases and oils so that good adhesion to the photo-resist is obtained. The final element of the PCM process is to strip off the resist, ensure that the metal is clean and that the dimensional specifications requested have been satisfied.

Advantages of photochemical machining

Low-cost tooling

Quick delivery

Retention of mechanical properties

Step-and-repeat of design

Tooling changes at a minimum coast

Burr-free parts

Technical Data

Dimensions
Because of the nature of the etching process and the undercutting at the edges of the resist patterns on the surface, all dimensions, tolerances and configurations are functions of the thickness of the stock being etched, the material, and to a lesser extent, the process variables. In the section on tolerances and materials that follows, tolerances are given for a variety of metals, thicknesses, overall pattern sizes, etc. These are generally applicable to equipment, processes, metals and configurations currently being used. They do not, however, express the ultimate capabilities of photochemical machining.

Metal Thickness
Metal	0.05 (0.002")	0.2 (0.008")	0.5 (0.02")	0.8 (0.03")	1 (0.04")	1.5 (0.06")	Tolerance ±
Alluminium	0.02	0.03	0.06	0.09	0.15	contact
Copper	0.01	0.02	0.04	0.06	0.1	chemograf
Nickel	0.01	0.02	0.04	0.06	0.15
Steel	0.01	0.02	0.04	0.06	0.15
Stainless Steel	0.015	0.03	0.08	0.1

Relationship of beval to matel thickness: An etched equally from two sides, a bevel is produced. As a general rule, this bevel is approximately 10% to 20% of material thickness (T).

1. Double-side etching A=10% to 15%T	2. Single-side etching A=20% to 40%T

Inner and outer corner radius (R)
Minimum inner radius = 75%T
Minimum outer radius = 50%T

Relationship of hole size
to matel thickness:
As a general rule, it is normally stated that the diameter of a hole cannot be less then the matel thickness. This relationship however, does very as the matel thickness changes.

Practical hole sizes attained
from sample thicknesses

thicknesses (t)	Practical hole sizes (D)
0.0005"	conect
0.0010"	Chemograf
0.0020"	0.0030"
0.0050"	0.0060"
0.0070"	0.0080"
0.0100"	0.0120"
0.0200"	0.0260"

Center-To-Center Dimensions
Generally speaking, for small pieces, it is possible to etch Photo Chemically Machined parts which will tend to duplicate the center-to-center dimensions which exist on production artwork.
Because of limitations in rhe area of artwork. preparation, the following table IV gives practical center-to-center dimensions tolerances for finished parts.
Most of the tolerance is used up on material shifts in the etching process.

The PCM Process

1. Preparation on Raw material	2. Lamination photoresist	3. Masc attachment	4. Ultra-violet light exposure

5. Developing after removing the phototool	6. Quality control	7. Etching	8. The product