Milli a new composite mill kit

[badhabit]

With a fear of ending in yet another "mother vs. daugther"-discussion on this subject: Why the obsession with the dry-packing when you have such poor results? With such an easy form, why not just wet mix and then vacuum-bag it? I've never had these issues and results turn out close to perfect 99% of the time...

[peteeng]

Hi Thomas - I only have had poor results on this particular long thin test piece. All my other casts have been very good. The dry pack approach is to sidestep the issue of entraining air in the mix. My preferred process is infusion this gives excellent results. I think trying to wet mix stack a 6mm thick mould then get a bag on it is more trouble then a dry pack approach. The current mould suffered a few design issues. 1) the side strips stood a bit proud and stopped the top mould from coming down fully 2) not enough screws in the top mould. To remedy that a dry pack, vacuum bag approach and vacuum fill will sort it and use plastic mould edges vs the timber..... Peter

and to clarify the casting issue a little further. Its easy to cast any object and get an apparent 99% good cast (wet or dry approach). The aim of this cast is to make a representative object for a stiffness test, which is a bit more challenging then production of a generic part. I would say most people don't publish their trials and tribulations only their successes but here they are...

[Zorbit]

When measuring deflection on such a thin test piece are you getting a less than optimal result because of the extra resin on the surfaces ? An accurate test would require you to cut the piece to size, to allow aggregate to be evenly distributed right through, but this would weaken the material too.

[peteeng]

Hi Zorbit - On this piece (no1) the top surface is very poor (the bottom surface is excellent) and the geometry is poor. Next one will be better. Cutting alox is a bit difficult. Its effectively a grinding wheel. I will get some finer alox and make a two size mix down the track. I have done this with alox/aluminium see prior. The surface is a bit less dense but for now its as good as can be expected. I may build a thicker No2. But my aim was to show a sample side by side with the aluminium extrusion... I think the material is very well distributed and my target of 70GPa is well below theoretical so averages should be OK for now.... Making large samples is easier but doesn't achieve what I wanted....Peter

[peteeng]

Evening all - I have been thinking about this cast material and the next mould. For the mould I shall get some 25x6 aluminium flat bar for the mould surround. This will be better then plastic and will be better for the CSA grout coupon to follow. I have played with the piece I have and I feel even the next piece maybe "soft". In another thread a person has had EG samples made and lab tested and found the modulus to be down in bending. All the main figures published are from compression tests and these test high.... We shall see soon I imagine... This skepticism made me create a decision table for some materials and steel wins out. I played with some of the numbers and St still won so maybe that's the best direction if I can't get 70GPa out of CSA or ALOX... The main characteristics for the table I selected are stiffness first, then cost then dampness then density. These are then given weights and normalised as averages, then added to create the biggest number wins scenario... Steel is cheap and stiff is the driver here and scores 2.11 then UHPC 2.10 then alox/EP then Al.... To make steel damp I can laminate it, but I think I'd go with laminated aluminium so it can be machined on a router....

I also came across Uni of Pennsylvania doing interesting structural optimisation work... I downloaded their software and will have a play...

https://psl.design.upenn.edu/

Keep making Peter

[peteeng]

Morning all - I've been tidying up a bunch of small jobs that are growing moss. So my lists and headspace are now more manageable. Came across this blooper video. So many dial gauges broken!!



Peter