Thickness and post curing of high temperature mould


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Dentex
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Hi guys,

I am almost finished with my pattern and I have ordered materials for high temperature mould that will be used for pre preg. I cannot find some answers so I hope you can help me.

First thing is, since I still don't have oven and won't finish it in next 1 or 1 and a half month, can I make mould and post cure it when I finish oven, or I need to post cure it as soon as possible?

My plan was to brush approx. 1mm of gelcoat, brush some resin afterwards and add chopped carbon on sharp angles and reinforce everything with 12 layers of 200g black stuff twill. In total, mould should have thickness around 4-5mm for each part of mould. Is that enough? I am not comfortable using thicker fabrics due to complexity of pattern.

Also, is it maybe possible to "level" some curves with mix of high temp epoxy and milled carbon fiber and pour it as next layer after gelcoat and before fabric? That would greatly reduce amount of work needed for laying fabric reinforcements.
Edited 4 Months Ago by Dentex
Hanaldo
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Dentex - 10/3/2020 8:43:16 PM
Hi guys,

I am almost finished with my pattern and I have ordered materials for high temperature mould that will be used for pre preg. I cannot find some answers so I hope you can help me.

First thing is, since I still don't have oven and won't finish it in next 1 or 1 and a half month, can I make mould and post cure it when I finish oven, or I need to post cure it as soon as possible?

My plan was to brush approx. 1mm of gelcoat, brush some resin afterwards and add chopped carbon on sharp angles and reinforce everything with 12 layers of 200g black stuff twill. In total, mould should have thickness around 4-5mm for each part of mould. Is that enough? I am not comfortable using thicker fabrics due to complexity of pattern.

Also, is it maybe possible to "level" some curves with mix of high temp epoxy and milled carbon fiber and pour it as next layer after gelcoat and before fabric? That would greatly reduce amount of work needed for laying fabric reinforcements.



1. No problem to delay the post-cure.

2. More than enough for a small carbon/epoxy mould.

3. Yes its an option. I try not to for pre-preg tools, because you end up with a mould with quite varied thicknesses in cross section, which means the mould heats up and cools down unevenly and can cause issues with surface finish and distorting. In reality, it likely wont make a massive difference.
Dentex
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Thanks for reply Hanaldo. 

I was afraid that delayed post cure will maybe result in "poorer" performance of already cured resin but if it can be delayed that's great news for me.

I don't know what is considered small but mould would be 35cm ( 14 inches) wide and 130cm long (52 inches). I was considering pouring epoxy with milled carbon because I am afraid that laying up so many layers will have bubbles between that will burst through gelcoat.

Since I plan to make carbon product for sale I can tolerate small imperfections because part will be clear coated but a lot of bubbles would be an issue for sure. Can I run with torch across every layer before adding next one to pop as much bubbles?
Hanaldo
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Dentex - 10/3/2020 10:29:05 PM
Thanks for reply Hanaldo. 

I was afraid that delayed post cure will maybe result in "poorer" performance of already cured resin but if it can be delayed that's great news for me.

I don't know what is considered small but mould would be 35cm ( 14 inches) wide and 130cm long (52 inches). I was considering pouring epoxy with milled carbon because I am afraid that laying up so many layers will have bubbles between that will burst through gelcoat.

Since I plan to make carbon product for sale I can tolerate small imperfections because part will be clear coated but a lot of bubbles would be an issue for sure. Can I run with torch across every layer before adding next one to pop as much bubbles?



You can sort of consider a mould uncured at any temperature it hasn't previously been exposed to. So it doesn't really matter how long a mould sits at room temperature, the cross-linking can't progress until it is exposed to heat to excite the molecules. You could lay up a mould and put it into use at room temperature for 5 years, and then one day decide you want to use it for pre-preg and give it a post-cure and that would be fine. The one consideration is that it is always best to post-cure as high as your pattern materials will allow while the mould is still supported by the pattern, and in a lot of cases with high temperature resins you can't demould until it has been post-cured at a certain minimum temperature. However if you can resist the temptation to demould before your oven is finished, then this isn't a concern for you.

I would consider anything under 1sqm to be pretty small. At 130cm long, you will want to be thinking about the torsional rigidity of your mould, but 4-5mm thick carbon is going to be pretty rigid, and hopefully your part has some geometry there that will help.

You've somewhat touched on the critical aspect of making wet-laid carbon/epoxy moulds - those air voids can be quite difficult to get rid of, and they will ruin your day like nothing else. It is only that first layer behind the gelcoat that is a major concern, and from there any voids shouldn't affect the surface of the mould unless they are very large. Unfortunately using a torch or a heat gun isn't likely to get rid of voids on the other side of the reinforcement, which is what you are concerned about. A combination of good layup technique, a finned consolidation roller and a bristle roller are the best methods to get rid of them.  
Dentex
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Unfortunately my pattern is routed from MDF and I am not really comfortable going above room temperature because I am afraid of warping it. 

When talking about torsional rigidity, I am considering placing steel ribs across length or something similar if possible but model is quite complex with couple long grooves so as you said, it's very likely that it will be stiff enough on it's own. Also, I don't think I will be able to use roller because grooves are 10mm wide so I guess I would need 5mm diameter roller at least so brush is my only option..


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Dentex - 10/3/2020 11:30:13 PM
Unfortunately my pattern is routed from MDF and I am not really comfortable going above room temperature because I am afraid of warping it. 

When talking about torsional rigidity, I am considering placing steel ribs across length or something similar if possible but model is quite complex with couple long grooves so as you said, it's very likely that it will be stiff enough on it's own. Also, I don't think I will be able to use roller because grooves are 10mm wide so I guess I would need 5mm diameter roller at least so brush is my only option..



I use MDF for patterns all the time, it can go to 50-60 quite happily and not cause any issues.

Steel ribs on a pre-preg tool are another iffy situation. The difference in CTE means they can end up warping your mould or printing through onto the surface. Personally, I prefer to just make moulds monolithic and rigid enough to be free-standing. I still feel that at the size you are doing, 4-5mm carbon is going to be very rigid, especially on a complex part. It generally isn't something you need to worry too much about unless your mould is oddly flat.

Not being able to use a roller could be an issue, I have not managed to make one single successful wet-laid epoxy mould without using a roller. You can get very small diameter rollers, I think I have a 3mm one here. If you can't roll it, I would suggest casting those grooves solid may be a better option than just trying to do it with a brush. Woven reinforcements just trap voids like nothing else, and with carbon you can't see them so you need to be very methodical about getting them out.
Dentex
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I understand. I guess I will try and "bake" it up to 50°C and hope for the best.

CNC routing where I live is quite expensive for a student budget so I am trying to do everything from first try. I will try to find and order small rollers, if not, I will go with casting resin into those grooves. I wouldn't cast everything, just those grooves that are approx. 75cm long, 1cm wide and 0.5cm deep on lowest part of groove. 

Do you think that there would be really much difference in heating rates so to speak, in carbon fabric + epoxy and mixture of epoxy + milled (and maybe fine chopped) carbon fiber? 
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In those thicknesses, not so much. I doubt you would have any issues in making those grooves solid. Could even add some aluminium powder to the casting mix, which would help with conducting heat. 

I didn't realise the grooves were that small, in practise with wet-lat you will likely fill those pretty much solid with resin regardless. 
Dentex
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I can post a picture of pattern so you might have some other toughts if that's ok.

Not to open another thread, is it ok to use high temp epoxy to finish plug and sand + polish it before mould making? 

Will it stick to mould during post curing (not going above 50°C and release agent applied)? 
Edited 4 Months Ago by Dentex
Hanaldo
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Dentex - 10/4/2020 7:46:45 PM
I can post a picture of pattern so you might have some other toughts if that's ok.

Not to open another thread, is it ok to use high temp epoxy to finish plug and sand + polish it before mould making? 

Will it stick to mould during post curing (not going above 50°C and release agent applied)? 

It is ok in the sense that it will work, but it is not a good idea - epoxy is very difficult to sand and polish, you won't get a great surface from it. Much better off using a conventional 2k coating system, like a 2k primer and then 2k topcoat. If you don't have spray equipment, perhaps look at Preval sprayers. If you really need to brush, then something like EC's Pattern Primer and High Gloss coatings are the only thing I would use.

Feel free to post a photo, would likely help.

GO

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