Surface textures on molding tools are significant for reasons other than aesthetics. Here are a few examples of topics to think about: Different textures will influence how moldable a part is. Some increase heat transfer between the tool wall and the resin, necessitating the use of alternative cooling solutions. Other texturing will cause the part to cling inside the mold until the draft angle is increased. Because heavily textured surfaces require more dwell time to assure full packing, manufacturing may be slowed. Furthermore, highly polished surfaces are only suitable for specific types of plastic. To achieve the greatest outcomes, a full design for manufacturing consultation should be performed early in the development stage.
Let’s take a closer look at one specific surface texturing application: how are textures applied to plastic injection mold tools. The issue can be complicated, therefore we’ll limit ourselves to the five most prevalent ways. Laser Etching, Sanding & Polishing, Chemical Photoetching, Media Blasting, and EDM Spark Erosion are a few examples. as we go over these procedures and talk about how they affect completed part quality, production time, cost, and other factors.
#1 Etching With A Laser
With laser etching, practically any texture can now be mapped onto any curved surface. The success of this method is dependent on the integration of two technologies. The first is sophisticated 3D computer modeling. This enables a designer to virtually align a texture map with the outlines of essentially any moldable shape. The second technology is a five-axis computer-controlled robotic arm. This allows the laser to accurately track over the surface’s structure and reach undercuts and other ‘hidden’ places. This technique is most commonly used in the automobile industry, where larger forms, such as an automotive dashboard, require small patterns that are continuous over a long duration. It is more expensive and takes more time to generate the required 3D CAD models, but for some applications, this is the only option.
#2 Polishing & Sanding
There are always some tool marks left behind after rough machining. If not handled, these marks will be transmitted to the resin part surface during molding, which is undesirable. To eliminate these marks and other minor surface flaws, professional specialists sand and polish utilizing a range of rotary tools, diamond burrs, sandpapers, files, and other abrasives. Textures span from mirrored surfaces to coarsely textured surfaces, and everything in between. It takes care to avoid breaking the mold or changing its proportions too much. One drawback is that it is difficult or impossible to access deep holes, pockets, or other intricate features by hand, necessitating the use of other procedures.
#3 Photochemical Etching
Many plastic parts feature intricate in-molded patterns and textures. Some are designed to look like stone, leather, or wood grain, while others are more abstract geometrical designs. But how do these patterns get onto a molding tool? Photoetching is used to do this. First, a photoresist, or light-sensitive chemical, is applied to the tool wall. The required pattern is then optically projected onto the tool surface, and any areas that come into contact with UV radiation are miraculously healed. The leftover photoresist is washed away, leaving a film mask behind. After that, the mold is immersed in acid, which etches away the vulnerable portions, resulting in the appropriate texture. To account for the type of metal, the complexity of the design, and the depth of surface penetration induced by the acid, the procedure must be carefully managed. Etching is a quick and inexpensive way to create delicate textures for practically any appearance. However, there are certain limits. Etching will not operate on undercuts or other regions where the optical line of sight is obstructed. Overbroad, curved surface areas, designs can become geometrically deformed.
#4 Blasting Of Media
Spraying various types of dry or wet abrasive media against the tool wall is accomplished using high-pressure air. Silica (sand), aluminum oxide, glass or plastic beads, and even walnut shells are examples of these media. This is an excellent method for cleaning the tool while also imparting a uniform matte or satin finish. The finished appearance is determined by the type of medium employed, as well as the air pressure, volume, and spray pattern. Blasting is a quick and low-cost method that uses little raw material. Another benefit is that it may be controlled by competent operators, allowing a matte-finished area to butt up against a textured design with no overlap or interference.
#5 Spark Erosion In EDM
Electro discharge machining, also known as spark erosion, involves the employment of a graphite or copper electrode in an electrolytic solution of water or oil. The electrode produces a spark, which impacts the tool wall. This region reaches melting point but is then cooled by the liquid bath, resulting in a thermal shock that fragments the metal into tiny particles that are then flushed away. All of this occurs in a fraction of a second. Spark erosion can create very tight tolerances on both hard and soft metals. It’s great for deep thin slots, sharp concave corners, debossed lettering, and other features that would be difficult or impossible to machine otherwise. In addition, a very fine and smooth finish can be attained, eliminating the need for time-consuming and costly hand polishing. Excessive melt, termed as slag,’ can sometimes be found in the eroded area. Large EDM electrodes were originally used to provide a textured surface to mold tools, and this finish was known as a “sparked finish,” but this technology has been mostly supplanted by chemical etching; all that remains is the name.
