Shibo - Injection Mold Graining (For techies)

fastsach

One of my trips to Japan was for 8 weeks of training on the manufacture and maintenance of high pressure injection molds. Two of these 8 weeks was for the application and repair of shibo (grained) surfaces. The only molds we worked on were for front and rear bumpers on the Sentras but most plastic bumpers now are smooth and not grained.
If you look at the plastic in your car's interior you will see very few surfaces that are smooth. Most are textured with a very fine pebbely surface and others are a leather simulation. All intended to give the car interior a softer feel and remove as much harshness as possible. Here is basically how (18 years ago) this is achieved.
In conjuction with styling a designer picks the proper texture, say for an A pillar finisher, from a catalog of black and white pictures. This is then speced on the print for the injection mold and is the job of the mold maker to apply. The applicable female surface of the mold is machined and then hand stoned with increasingly finer grit stones and then same with emery cloths until a very fine finish is obtained. Not quite mirror but close. No scratches or other imperfections. This is then inspected by another individual that gives the go ahead to grain. But first practice shots are made to produce early production parts to prove out the mold and supply engineers with parts to make the first couple of prototypes mainly to check fits.
If you want to see something ugly you should see an early car interior with parts that are not textured. All smooth and butt ugly but fine for fitments.
The texture is selected from the catalog and a translucent slide is obtained to match it. Black and white. A special paper is hung from hangers but is smoothed against a surface. The slide is placed in a high powered projecter and the paper is illuminated for a set period of time. The black areas are left intact and the white areas are burned through to produce "holes" in the paper that reflect the given texture.
You end up with a piece of paper large enough for the applied female cavity of the mold and it finely perforated. At this point a man appears that finishes the application which is highly artistic and our guy was a deaf mute. A great sense of humor but that is another story. Can't recall his name but we will call him Yoshi.
Yoshi sprays the paper with a semi adhesive spray and starts to stick it to the mold surface. What makes this hard is that no mold has straight or flat surfaces and the radii and such must have the paper sectioned and cut to make the texture appear seamless. They tried to teach us to do it knowing we could not just to show us how hard it was.
This can take several days for a bumper and after it's done it's now time for the ink. The day of inking Yoshi was very late and badly hung over but nobody spoke (mainly through writing) to him about this. This took about three days. He then took a small roller and carefully rolled semisolid ink on to the paper and this dried overnight. The next day he carefully peeled the paper off which left an exact negative of the required texture on the mold surface.
Still with me?
At this point with a magnifying glasses, bright light and a set of small brushes and needles he fixes any imperfections. ANY imperfection will make the part scrap by inspectors and one must always remember this mold will make thousands of parts and anything wrong will be replicated many times. An incredible work of art actually.
Then a bath and drain set up is installed and acid is finely sprayed on the surface. The acid eats away everywhere there is no ink so you end up with peaks and vallys that is the texture. Clean with water and lightly oil.
The surface is finished and test shots are made. If OK and there are no fitment adjustments the mold is finished and sent to the plant for production.
Never saw Yoshi again.

The finished surface is incredibly sensitive because there is no heat treatment here. If you were to drop something as small as a dime on it, and the texture was very fine, it would leave a mark and have to be repaired.

That is what we did learn. How to repair and it was extremely difficult to do as we had to weld the damaged area, stone and polish, apply ink and resurface with punches and hammers and not disturb the surrounding area and then blend it all. I can see why Yoshi liked big saki.

This was fun to explain and hope you enjoy how this is done. Next time you sit in your car look around and feel a new appreciation for the piece of art that surrounds you.

Fastsach

CBRich

I once owned a GM vehicle. They have none of these issues as every surface was hard, flat plastic. --zing--