Blank curing problem.

[MikeinSC]

So I'm having some issues this week with blanks curing properly. And of course, it's at a time when I am trying to fill a big order. The failure rate is too high. I'm hoping to find a reason why this is occurring since I am not able to figure it out.

Fresh Silmar41. Fresh MEKP. New, and in some cases hand washed, PVC molds. Temp holding around 70, humidity 50-60%. The process and materials used are the same as I have done in the past. I've only done a few thousand blanks at this point and have had good success until now with my process. I use 6 drops MEKP per 2 ounces by weight Silmar41, about 1.75 ounces by volume. That has always worked. I posted to a Facebook group yesterday about this and received a common response of too much MEKP and too much heat so I dropped down to 5 drops. The results are the same except the blanks aren't hardened yet.
The surface crackles are merely on the surface only and turn away easily. The rest of the blank is unaffected but it creates a product that couldn't be sold. And i wouldn't sell it.

 

[Brooks803]

It could be a couple different things Mike. Or a combination.

How fresh is the MEKP? When you shake the bottle do bubbles form at the top? It's a good sign if it does. I wouldn't reduce the amount used at all with temps only around 70. Once it gets into the 80's and higher you might think about it.

Are you using any new pigments or increasing the amounts? Over saturation or something that isn't compatible can cause that issue.

I'm willing to bet the cause is not enough heat. When they wrinkle like that I've found it to be due to a drop in temp during hardening.

Do a couple of tests with straight clear blanks. Do 1 with everything as you did the ones in the pics. Then do another with more MEKP. See if you get a difference.

 

[MikeinSC]

I made a clear cast Thursday night using 6 drops, no issues with that. Most single color cast come out OK. MEKP was sent with the last order 6 weeks ago. I opened a new bottle last week when the previous one ran out and cracked open the other new bottle last night, no difference noted there. Colorants are the same, no change there.

Do you think it's uneven heating in the blank?

 

[jttheclockman]

Mike, if i read this right you are using 6 drops of hardener per 2 ounces of resin or 3 drops per ounce. I use 4 to 6 drops per ounce any time I cast. Have not done mixed colors so I can not address that. Where did you get the resin from.???

Mixing of colors will always slow down hardening. There has to be something that has been changed if you are doing the same type casting as before. Hope you figure it out.

I would change the molds and use something else. Maybe do square molds and cut into blanks. To me looking at the first photo the resin is reacting to the mold product or cleaning agent used. That is why I like to use clear plastic tubes and they get turned away when the blank gets turned down. You can see what is happening and no reusing to contaminate the next pour.

 

[Brian G]

New, and in some cases hand washed, PVC molds.

I'm still at fewer than 150 total color casts with Silmar and PVC, but I notice I get that effect when I use new PVC molds and maybe a little too much PAM as a release. Once I run a second batch, the problem goes away.

Maybe not much help, but the quoted part caught my attention.

 

[MikeinSC]

Brian,
I'm not using any mold release. I hand washed some trying to sort out this issue. My thought was that whatever mold release is used, if any, when the pipes are made could be causing a reaction. So I figured I'd wash some in an attempt to rule out that variable of either mold release or some other contamination.

 

[MikeinSC]

JT,
It's all fresh PVC. I get my resin from US Composites so I have no doubt that it's not an age issue. The process I'm using has worked consistently with no issue until this week when i get a large order for a retailer. If I had an enemy, I'd swear this is the moment they were waiting for.

 

[jttheclockman]

JT,
It's all fresh PVC. I get my resin from US Composites so I have no doubt that it's not an age issue. The process I'm using has worked consistently with no issue until this week when i get a large order for a retailer. If I had an enemy, I'd swear this is the moment they were waiting for.

You have to try eliminating the problem and I would start with the molds. I agree you do not have to worry about the source of the resin. Never had a problem with them. Good luck and keep us informed as to what you find.

 

[PSNCO]

I get that. Mostly when I'm using 5/8" tan PVC. Turn it down and the blank is fine. The issue is if the ruts run too deep. I have yet to figure out why this happens, but it happen most of the time in the tan thinner walled PVC pipe.


Oh and the 5/8" blanks will turn out like that while the 1/2" and 3/4" blanks from the same pour turn out fine

 

[MikeinSC]

JT,
I've been trying for the past week to figure it out. I've got 150-175 test blanks as a result.
What would you change about the PVC?

 

[Brooks803]

I made a clear cast Thursday night using 6 drops, no issues with that. Most single color cast come out OK. MEKP was sent with the last order 6 weeks ago. I opened a new bottle last week when the previous one ran out and cracked open the other new bottle last night, no difference noted there. Colorants are the same, no change there.

Do you think it's uneven heating in the blank?

Uneven heat is probably causing the wrinkling. But I think the overall temp needs to be higher.

Have you tried post curing the flexibility blanks to see if they'll actually harden?

 

[MikeinSC]

I've checked on them throughout the day and they are curing.

 

[Brooks803]

If they're curing then I still say a higher constant heat source.

 

[MikeinSC]

I think I need an incubator

 

[MikeinSC]

The test pieces I made earlier today with higher mekp came out ok. These two however....

Same pour, same everything, one is smooth with no problems coming out of the mold while the other has something going on.

 

[jttheclockman]

JT,
I've been trying for the past week to figure it out. I've got 150-175 test blanks as a result.
What would you change about the PVC?

Get rid of it. Make some blanks in a square container. The cheapest thing would be to make a box and line it with wax paper. People have made molds out of tin foil. Why do the blanks have to be round??? Go to hobby lobby and find some plastic molds. Go to walmart and get some plastic trays or microwave containers. That is what I would do but as I said I cast in clear plastic tubes. This way it eliminates the pvc and go from there. I would as I said increase the drops. Too low for me.

 

[Sylvanite]

If they're curing then I still say a higher constant heat source.

I agree. I suspect that the wrinkling occurs because the resin is cooling (and therefore shrinking) before it has hardened. PR curing is a heat-accelerated, exothermic reaction. If you don't have a high enough initial temperature (70-80 degrees F), or a large enough thermal mass, or enough hardener (1%-2% MEKP by volume plus or minus), or properly promoted resin (which is the manufacturer's responsibility - Silmar 41 should be pre-promoted with Cobalt naphthanate), then the casting may not generate enough heat to sustain the reaction to completion.

Try adding more MEKP, or putting the pour someplace warm while it cures (I often set mine out in the sun). The cast should be warm to the touch until fully hard.

Good luck,
Eric

 

[Curly]

Whatever steps you take to improve the process do one at a time. If you do two or three things like more MEKP, new moulds, and warmer temps you never really know what the fix was.

 

[MikeinSC]

It seems that more MEKP was the answer and Jonathon was correct.

I don't know what has changed but enough of something changed enough to give me a days long headache.

 

[Brooks803]

It could be a chain reaction type of event. I tried pvc a few times. One thing I noticed is that it doesn't retain as much heat as my silicone molds. Since my molds hold 6 blanks at a time it gets hotter faster and longer. Which helps the curing. Lower temps and spacing between the pvc in conjuction with the lesser MEKP earlier is most likely the cause.

 

[edicehouse]

It could be a chain reaction type of event. I tried pvc a few times. One thing I noticed is that it doesn't retain as much heat as my silicone molds. Since my molds hold 6 blanks at a time it gets hotter faster and longer. Which helps the curing. Lower temps and spacing between the pvc in conjuction with the lesser MEKP earlier is most likely the cause.

Not sure what the weather has been in SC, but here in VA it will be mid 60's during the day and drop to 35 at night.

 

[Brooks803]

It could be a chain reaction type of event. I tried pvc a few times. One thing I noticed is that it doesn't retain as much heat as my silicone molds. Since my molds hold 6 blanks at a time it gets hotter faster and longer. Which helps the curing. Lower temps and spacing between the pvc in conjuction with the lesser MEKP earlier is most likely the cause.

Not sure what the weather has been in SC, but here in VA it will be mid 60's during the day and drop to 35 at night.

Lately it's been hitting low 70's during the day and low 50's at night.

 

[tomtedesco]

I had the same thing happen on some PR I was casting. Several came out of the mold rubbery, not tacky just bendable. I placed them is a toaster oven for about 40 min at 125 degrees and they all hardened up.

 

[mike4066]

I had my first blanks come out in a similar wrinkled state. I think it was a combination of being too cold overnight (I was pouring before I went to bed) and not enough mekp. I started putting them in a warming box (bucket with a light to keep the temp up) and the problem went away. I did have an issue of it getting to hot and causing bubbles so I wired my heat source (light bulb) to a water heater thermometer that automatically turns the light off @ 80-85 degrees.