Smooth, highly-polished surfaces are not always desired on plastics parts. For example, automotive interior plastics with textured surfaces proliferate, to both reduce shine and enhance esthetics. Almost any texture pattern can be applied to a mold by a variety of methods.
As the outside surface is typically the cavity of the mold, and a common method of machining the cavity today is electrical discharge machining, a growing number of molds use edm to provide surface texture. While this method could result in high stresses in the cavity, it works best when a single electrode can be used as a finisher for the completed cavity.
The more common method, one which has unlimited texturing options, is acid etching. This form of mold finishing, also referred to as graining or texturing, allows for subtle patterns, from a light stipple effect to heavy wood and leather grain effects. Patterns can be faithfully reproduced to duplicate a master in single- or multiple-cavity molds.
Texturing by acid etching, stated simply, is accomplished by protecting the portion of the mold that should not be corroded away, while exposing the surface that should be corroded with an acid. If an area of the mold needs to be depressed (e.g., for standing plastic on the part), the area around the feature would be masked while the textured area is exposed to acid.
The resist material varies between texturing suppliers, but wax such as jeweler’s wax, laquers, and rubberized paints are frequently used. However, any medium that is easy to apply and remove, and withstands attacks from aggressive acids, could be used. Large areas of the mold, and especially parts of the mold surrounding the textured area, are protected by tape.
After the mold has been prepared, the mold is immersed in acid. Typically, ferric, hydrochloric, nitric, or sulfuric acid is used.
The process time is dictated by the type of acid, mold material, and texture depth and pattern, but typically does not take long. The greatest amount of time is spent preparing and masking the mold for acid immersion, not the process itself. If the mold is too large for the tub, acid may be poured into the cavity itself or even sprayed on.
Most tool steels can be textured. Additionally, non-ferrous mold materials, including aluminum, copper-beryllium, and NiSiCr copper alloys generally will texture well. However, it is necessary to properly identify the type of material to the texturing source so that the best combination of acids and resist material can be determined.
The basic steps in this acid-etching process call for the specification of a texture pattern and desired depth and a mold-surface finish prepared with 320-grit paper (600-grit if using a fine pattern).
The texturing supplier will then: degrease the mold surface and vapor-hone or bead-blast it to ensure cleanliness; tape or mask off all non-plastic-forming areas; apply the resist or wax pattern protecting the mold areas that will not be exposed to the acid; select the acid that is matched to the mold material and texture pattern; immerse the component in acid and time the exposure; remove it from the bath and neutralize the reaction with water; and measure the depth of the texture and re-immerse for greater depth, if needed.
When the desired effect is achieved, the component is rinsed with water to stop the chemical reaction. The part is air-dried, the rub (masking) is removed from the surface, and the component is thoroughly dried. The surface is then scrubbed, the ash removed, and degreased. The surface is glass-bead-blasted or vapor-honed, depending upon the final surface gloss desired. Finally, the mold is sprayed with an acid-neutralizer and rust preventative to stop post-treatment oxidation and rusting.
The etching process normally can be completed in a week, depending more upon the amount of masking for the pattern than any other factor. Some patterns require the masking and exposure process to be repeated for the desired effect.