I am doing some plates recently for testing with different number of layers and resins just to try and understand what makes the difference in the strength of the product etc… doing different layup scenarios and the conclusion i have made so far is this.
With about the same overal amount of carbon fiber and same orientation of fibers etc and despite that the resins that i use for typical layup are the same or of slightly superior characteristics of the resin used in the prepreg that i use , the prepreg samples that i have created outperform by far - havent test them yet to tell you exact numbers- the ones made with the classic method.
Is this something to be expected?
or i am doing something wrong?
Not an easy question. What do you call the classic method? Hand layup (with vacbag?)
Void content and Vf are the two main things wich are important for laminate quality, besides the properties of the matrix and reinforcement fabric itself. With prepregs you have to use a higher pressue than wet bagging, so the consolidation is better(and probably void content too with the appropiate cure schedule. The same thing can be achieved with resin infusion, maybe even a lower void content in some specific applications.
When using autoclave’s, the quality will be even better.
Yes ,the method i am comparing it is hand layup with vaccum bag…
And because i am talking simple plate samples which is a an easy shape and so on it is dificult to believe that i do something wrong… the quantity and the void in the resin should be regulated with the vaccum baging … or not?
The pressure that i use in both of the samples is -1 bar…
And the thing with the resin infusion is that the resins that are of high viscocity and they are good for this process are usually of pour characteristics at list the ones i have come across, this is why i havent tried it yet… but it is in my plans…
And something else… the prepreg makes difference noise when i tick it with my finger nail for example … is this normal?
It makes more metalic noise and the normal hand layup makes more plastic noise
The noise difference is the Vf ratio. The more metallic it sounds, the less resin is in there (pretty cool 
). Vacuum bagging will not remove all air from your laminate.
And how then do I remove the air from the laminate ?
With careful layup ?
With vacuumed resin before I make a the layup ? (with vacuum chamber)
With doing Infusion instead of layup??
Most of the differences you see are caused by the resin system itself. As a rough rule of thumb when the viscosity of the resin is higher, it has the potential to have better properties.
Because most modern prepregs are made from resins which are solids at room temperature they will have better properties. That is why you hear of hot melt prepreg, where the resin is melted to apply it to the fabric. The old way was to dissolve the resin in a solvent that needed to be evaporated off.
Also why infusion resins are more expensive and often have lower properties which go with their lower viscosity.
Increased temperature at cure will also improve properties. This works with wet layup as well.
the consistency of process for pre preg makes a generally superior sample.
with the pre preg there is no guess work for the tech when laying up. When you laid up the wet layup panel, you had to mix the resin, weight the material, should weigh the resin to match to the fabric, degass the resin, carefully wet out the fabric, ensure no dry spots, no bubbles, smooth out, not distort the weave, and no debulking.
with the prepreg you don’t have to do anything but layup, ensure no air is entrapped, debulk on regular cycles, and then bag. The time is spent on making a good part, not ensuring there is enough or not too much resin, etc.
I also agree higly with AVT, if you use two different resins, then you can’t compare the processes very well. Different cure temps, resins, fabric, etc makes a direct comparison difficult.
I also think that infusion would generally make a better pieces, but the setup time and mistakes are inevitable until the process is repeated and fixed.
If your doing wet layup, there are a large number of variables. I have picked a few that you may not have considered.
- The quality of the finished part is greatly influenced by the perforated film and breather you choose.
By changing your vac stack, it’s possible to get a better ( or worse!) Vf and lower void content.
If you are having difficulties across all of your samples I think that it may be better to not base your results on a single vac stack setup. There are many different perf variations, and from experience, it is worth checking the vacuum process and tuning it to the resin & layup.
For optimal properties from any layup, you must control temp and humidity. It is often overlooked, but has a big detrimental effect if you exceed recommended parameters. Ideally, you want the humidity as low as possible.
How are you measuring your Vacuum for curing? - Digitally, on the part, not on the pump I hope.
You can change the characteristics by how soon the vac is applied to the laminate.
Again, temp whilst curing plays it’s part too.
Have you post cured the resin before testing? You can gain a few more % by upping the post cure temps sometimes.
It is possible to make wet layup samples that perform to prepreg numbers, maybe not as practical as using prepreg.
What tests are you performing to verify the characteristics of the laminate?
Pre preg is far better and always makes a high tensile sound in comparison to wet laid carbon.
I always tell my customers that pre preg is literally twice as good.
That might have been so 10 years ago. Actual infusion resins are very good and sometimes better than prepreg resins.
Why? Very easy. Most prepregs are used in aerospace industry, and they have certifeied resin systems they can not change easily. Infusion resind are developed very good in the last year because they are used in the wind energy sector and must have very good properties.
But in the end this is all a question about the price, a cheap infusion resin will never be as good as an expensive prepreg resin and the other way.
But a wet layup will never be as good as a prepreg part.
The grade of carbon fibre used can also greatly improve mechanical properties once you have the processing perfected.
Surely it depends on the resin…
What characteristics are you referring to?
To add to this; Generally speaking the carbon fibres used in of prepreg are of higher quality than dry fibres. With companies becoming more vertically integrated the fibre producer is now also the weaver and prepreger.
A prepreg will provide a more constant product than wetlam, in most cases. Prepreg resin is mixed and applied to fibre by computer controlled equipment, and undergoes a lot of QA process to ensure it is in spec. When mixing resin by hand there will be variations in the ratio, and when applying it there will be variations in the volume fraction.
Another area of prepreg superiority is when manufacturing parts, prepreg is pretty much idiot proof. I have tested that theory many times, some of my old staff were idiots of the highest order. As long as the plys were put down in the right order/place, parts would be constantly the same weight and strength. This meant I could hire unskilled staff and pay them half what I would have needed to for a high skilled laminator. Seems harsh but that is the nature of production.
I have to agree with moke here. prepreg defiantly has the big advantage of process control even with idiots that you just don’t get with wetlam. so long as no shortcuts are taken with the cure cycle that is. it still doesn’t stop people leaving plastic film in the middle of the laminate though!!
Absolutely.
Materially speaking it is very easy to get prepreg numbers from wet lam for testing. Much harder to do it on the shop floor repeatedly.
The material is only half the story. In this game, it’s vitally important to have solid processes.
As for less than optimally skilled people and prepreg- Backer, contamination, bridging/voids & wrinkles in the laminate plus increased wastage are the usual stumbling blocks as well as struggling with reading and interpreting drawings to real basics like using a tape measure!
I found that by using a few skilled guys in the right places the risks could be minimized. I had one on cutting and nesting and creating a laminate pack and another on bagging. Getting prepreg with backing on one side only halve that risk and keeping the backing paper for a count at the end makes sure that all plys are used and backing removed.
Always good to have skill where you need it! Sometimes you are left “in it” though!
I like to watch a team grow and progress. It’s fun to see how certain individuals can pick it up fast, absorb information and love to learn.