Hello, this is Gordon Styles. If you’re new to our channel, I’m the founder and CEO of Star Rapid. I’ve been involved in rapid prototyping and new product introduction for over 35 years, and I’m delighted to welcome you back to another exciting installment of Serious Engineering for Serious Engineers.
In a previous episode we talked about surface texture as it relates to general manufacturing and product development. If you haven’t seen it (and you should), please check out the link here or at the end of the video.
In this episode we’re going to take a deeper dive into one specific application for surface texturing – how are textures applied to plastic injection mould tools?
The subject can be complex, but we’ll stick to the five most common methods. These are: sanding and polishing, EDM spark erosion, media blasting, chemical photoetching, and laser etching.
Join me as we review these processes and discuss their implications for finished part quality, production time, cost, and more.
- Sanding and Polishing
After rough machining there are always some tool marks left behind. If untreated, these marks would then get transferred to the resin part surface during moulding, and of course that’s not desirable.
So, sanding and polishing is done by skilled technicians using a variety of rotary tools, diamond burrs, sandpapers, files and other abrasives to remove these marks and other minor surface imperfections. The textures range from mirrored surfaces to coarsely grained, or anything in between.
It requires skill to avoid damaging the mould or altering its dimensions excessively. One limitation is that it’s difficult or impossible to access deep holes, pockets or other complex features by hand, so other methods are needed.
- EDM Spark Erosion
Electron discharge machining, also known as spark erosion, uses a graphite or copper electrode placed in an electrolytic bath of water or oil. The electrode discharges a spark that strikes the tool wall. This area reaches melting temperature but is then cooled by the liquid bath, resulting in a thermal shock that causes the metal to fragment into tiny particles which are then flushed away. All of this happens in a small fraction of a second.
Spark erosion works on hard and soft metals alike and can produce very tight tolerances. It’s ideal for making deep thin slots, sharp concave corners, debossed lettering and other features that would otherwise be hard or impossible to machine conventionally. Also, a very fine and smooth finish can be achieved, which eliminates time-consuming and expensive hand polishing.
Sometimes the eroded area has excessive melt, known as ‘slag’. This should not be confused with ‘slag’. Look it up.
Large EDM electrodes were once used to apply a textured surface on mould tools, and that finish was known as a “sparked finish”, but this technique has largely been replaced with chemical etching; all that remains is the name.
- Media Blasting
High pressure air is used to spray various types of dry or wet abrasive media against the tool wall. Examples of these media would be silica (sand), aluminium oxide, glass or plastic beads, and even walnut shells. This is a great way to clean the tool whilst also imparting a uniform matte or satin finish. The type of media used, as well as the air pressure, volume and spray pattern determine the finished look.
Blasting is fast and inexpensive and consumes little raw material. Another advantage is that it can be controlled by skilled operators, so a matte-finished area can butt up against a textured pattern with no overlap or interference between them; this, by the way, should not be confused with butting up against.
- Chemical Photoetching
Many plastic parts are designed with sophisticated in-molded patterns and textures. Some imitate the look of stone, leather or wood grain, while others produce more abstract geometrical designs. But how are these patterns applied to a mould tool? This is done by photoetching.
First, the tool wall is coated with a photoresist, or light-sensitive chemical. The desired pattern is then optically projected onto the tool surface and any areas struck by UV light are miraculously cured.
The remaining photoresist gets washed away, leaving behind a film mask.
The mould is then bathed in acid, which etches away the unprotected areas, thus creating the desired texture. The process must be carefully controlled to account for the type of metal, complexity of the design, and the depth of surface penetration caused by the acid.
Etching is fast, cost-effective and can produce very fine textures to produce almost any look. But there are some limitations. Etching doesn’t work on undercuts or other areas where the optical line-of-sight is blocked. And patterns can get geometrically distorted over large, curved surface areas.
- Laser Etching
It’s now possible to map almost any texture onto any curved surface with laser etching. The success of this process depends on combining two technologies.
The first is advanced 3D computer modeling. This lets a designer take a map of the texture and virtually align it with the contours of almost any moldable shape.
The second technology is a computer-controlled robotic arm with 5 axes of movement. This lets the laser accurately track over the topology of the surface and it can also reach undercuts and other ‘hidden’ areas.
This solution is most often found in the automotive field, where bigger forms – like an automotive dashboard – must have fine patterns that are consistent over a long length.
It’s more expensive and it takes time to create the necessary 3D cad models, but for some applications this is the only way to go.
What are the implications for you?
Surface textures on mould tools are important for reasons that go beyond aesthetics. To give just a few examples of things to consider: Different textures will affect how moldable a part may be. Some increase heat transfer between the tool wall and the resin, so different cooling strategies may be needed. Other textures will cause the part to stick inside the mold unless there is a greater draft angle. Heavily textured surfaces will need longer dwell time to ensure complete packing, so this may slow down production. And highly polished surfaces are only appropriate for certain types of plastic. So, to get the best results it’s important to perform a comprehensive design for manufacturing consultation early in the development stage.
That’s all the time we have for today. In our next episode we’ll cover surface coatings for mould tools so stay tuned for that.
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