Hello friends,

I intend to make a medium-sized piece (1.35mx 0.30m) with polyester resin, but I saw that it corrodes the potential risk of shrinkage during resin curing.

- The resin has the gel time of: 30 minutes and the manufacturing process would be by Hand Layup + vacuum bag or Resin nfusion.

The doubts are as follows:

"What do I have to do to stop this from slowing down?"
Because I can not have dimensional changes due to the product accessories.

Desmoulding after 4 hours of lamination cause me problems?

We have a small production line and we need to be quick in our process.

Thanks,
Dezer

Yeh you're going to struggle. The only way to really minimise it is to slow the process down, so only do a couple of layers at a time, drop the catalyst level a bit (depending on temperature), leave it in the mould for as long as possible, etc. You can also use additives like ATH, aluminium powder, calc carb, etc. But these will obviously effect other properties of the resin too.

A 4 hour demould time is just crying out for loads of warping with polyester resin.

i disagree with hanaldo in the point of doing oh in multiple steps, only a few layers each time. in my understanding of physics, this is the best way to get a warped part. polyester resin shrinks a lot during the curing, and most of the shrinkage happens during a stage, where it is already solid. so if you laminate a few layers and let them cure, the layup will shrink a bit, but consistently over the cross section. than you add more layers and let them cure. they will shrink, too, but the will be bonded to the layers wich are already cured. so the inner side of your part shrinks, while the outer side doesn't (any more). this leads th warping.
My recommendations are:
-do everything in one pour, and as quick as possible
- keep the gelcoat as thin as possible (same reason as explained above)
- keep the fibre content as high as possible (fibres resist the shrinkage). therefore infusion might be the better way
- keep the temperature low during the cure (room temperature) and try to avoid, that some parts of the mold get hotter than others (sunlight though the windows, deep sections, in wich the polyester pools  and heats up)
-use filled non-shrinkage resin, like the unimold tooling resin
-use epoxy resin (most likely no option cost wise)
last but not least I would like to say, that shrinkage is not avoidable, at least to a certain degree. so keep your ambitions realistic. even companies like koenigsegg spent hours on a jig, fitting their body parts, to get a perfect look. panel work will never be 100 percent plug and play, unless you really know you tolerances and include them into you calculations.

You are talking about a much longer time frame than I am. I am talking about keeping shrinkage at its minimum, which means controlling the exotherm. As you said, there is always going to be a degree of shrinkage in the solid state, but this occurs over a period of weeks after the initial cure. I am talking about letting the resin cure to a tack-free stage so that the exotherm stays controllable and doesn't cause higher shrinkage than would otherwise be experienced. Once the first two or three layers have gone hard and the heat has subsided, you can do the next few layers. This is generally a case of two or three hours and will have no affect on the degree of shrinkage in the solid state over the next few weeks.

If the components in question are only going to be two or three mm's thick, then there's obviously no need to do this in stages. It's just if the parts need to be five or more mm's thick that doing it all in one hit would certainly result in big exotherms and higher shrinkage.

no, I am talking about the first hour of curing. polyester reaches this "jelly stage" in wich it is no longer liquid, and cannot flow infinitely under shear stresses (the definition of a liquid). from this point on, the resin still shrinks massively, but cannot flow anymore, to compensate the shrinkage.
and I not talking about my experiences, this is state of the art university knowledge. you can find this information in "konstruieren mit faser-kunstoff-verbunden" written by Helmut schürmann, professor at TU Darmstadt. one of the best (mostly theoretical) books about composites in my opinion.

I forgot a good countermeasure against warping and shrinking:
use woven cloth instead of chopped strand mat. and use a symmetrical layup. for example layer a, then layer b, then b and finally a again.
with woven cloths you can achieve higher fibre contents, and the symmetrical layup cancels the warping forces out against each other.

+x

 

oekmont - 2/19/2018 7:30:03 AM

no, I am talking about the first hour of curing. polyester reaches this "jelly stage" in wich it is no longer liquid, and cannot flow infinitely under shear stresses (the definition of a liquid). from this point on, the resin still shrinks massively, but cannot flow anymore, to compensate the shrinkage.
and I not talking about my experiences, this is state of the art university knowledge. you can find this information in "konstruieren mit faser-kunstoff-verbunden" written by Helmut schürmann, professor at TU Darmstadt. one of the best (mostly theoretical) books about composites in my opinion.

I forgot a good countermeasure against warping and shrinking:
use woven cloth instead of chopped strand mat. and use a symmetrical layup. for example layer a, then layer b, then b and finally a again.
with woven cloths you can achieve higher fibre contents, and the symmetrical layup cancels the warping forces out against each other.

Hi my friend 
Please help me
I apply gel coat as first layer . After 2 hrs I apply second layer " mixed polyester resin  around 35% and 5 % pigment and silica powder 30 % and carbonate calcium 30 % as filler around 60 %
I demold after 1 hrs but back side of mold comes out very good but top side is not come out easily .
Please help me what is my problem ?
Also I add cobalt 0.05 % and peroxide 1.8 %
How can i remove my part easily  ?
My part thickness is 1.2 mm

Tanks so much in advance

Also, where possible, consider a filled "low shrinkage" polyester for the mould and the part. if you can reduce it with a better resin to start with then that will help and the other methods will further help.  

However low shrinkage is unlikely to mean no or negligible shrinkage.

If you really struggle, in your conditions, measure shrinkage then compensate in the pattern and mould design to account for it.  Not ideal and another added layer of complexity but can work when conditions are repeatable and known.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant