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Hi Sebastian,
We've encountered this problem a few times. I think you'll be able to understand how it happens. It's practically impossible to get the carbon fibres down onto the surface following this fine detail on the mould surface and as a result the fabric creates numerous 'bridges' spanning the detail of the embossed letters. Once the fabric is 'down' onto the surrounding flat areas it then basically clamps the fabric and locks it in place, leaving these bridges (shortcuts) over the embossed detail in place. When you then vacuum bag the prepreg - hoping and assuming - that the vacuum will press the carbon into the letter detail, what you actually find is that the pressure of the bag on the flat areas just continues to lock the fabric (and the bridges) in place. Basically, the pressure acting on the small areas where the fabric has bridges, that you might hope would drive the fabric into those areas, is no where near sufficient to overcome the pressure on the much larger area of surrounding fabric which is locking it in place and preventing it from 'supplying' any more fabric surface area into the embossed detail of the letters. Phew! I hope that makes sense.
So, what can you do about it? There are a few things you could try:
1) Localised positive pressure during lamination
What you really need to try to do is get the carbon down following the detail of the letters before it touches and sticks down too well to the surrounding areas. To do this, as best as you can, start laminating from the logo outwards. As soon as you've thoroughly tacked down the logo but before you've done much lacking down of the surrounding area the ideal situation would be to get pressure on the logo area of the laminate which the pressure on the surrounding areas. You could do this with a clamping arrangement using a plate, a compressable medium (like silicone rubber) and some G-clamps. This could be tightened down locally on the logo and would force the carbon to conform to the detail before it's locked-off by the surrounding areas.
2) Cut lines
You could CNC cut or manually cut the logo out itself and place it down into the mould first. This might be too difficult or fiddly but if it is possible then it would certainly overcome the problem of the pin holes and wouldn't be noticeable on the finished part because the cuts are on corners.
3) Debulking the first ply
You mentioned that you were debulking after the backing ply. I think in this situation it would be better to debulk after the surface ply however you will still be fighting the underlying problem which is that under vacuum the surrounding material is so well clamped down that it is almost impossible for it to 'slip' and feed more fabric surface area into the bridges.
4) Localised positive pressure during cure
This would work along the same principle as (1) in that you would apply a greater pressure to the detail area than you have on the surrounding area in the hope that this will allow the main area to 'slip' and feed fabric surface into the details. You would set this up by vacuum bagging the normal way and then setting up the additional pressure using a compressible medium (silicone/rubber sheet) with a plate and clamps which would put additional pressure onto the laminate (and vacuum bag) in this area. One danger to this though would be a 'witness mark' showing on the finished part which shows where the extent of the additional pressure was applied.
I hope this gives you some things to try.
Matt Statham
Easy Composites / Carbon Mods - Technical Sales
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