Hello everyone,
Is there a way to work with PET plastic and keep it transparent ? The crystallization behaviour makes it difficult for a beginner like me.
How is crystal clear PET created at the industrial level ?
Does the cooling time have any effect in reducing the crystallization rate and thus making it less opaque/transparent enough ?
What about other plastic types that can be found in their transparent form (bags, food containers, packaging) ? What other rules apply to them ?
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@armbouhali I did some searching and found it could be your cooling down times? have a look at this https://www.ptonline.com/columns/cooling-tips-for-crystalline-polymers
Thank you for the very clear (pun unintended) explanation.
Hey armbouhali,
I will try my best to answer your question. It may get too technical. I dont want to discourage anyone from recycling PET rather provide information that would help people focus their energy better.
In the picture your have shared has two preforms, one with a crystallized neck and one non crystallized neck. These are two different materials. The DSC graph you also shared is for the crystallized neck type. For the non crystallized type you can shift the DSC graph ~20C to the left.
Is there a way to work with PET plastic and keep it transparent ? PET is a crystalline polymer, and has a crystalinity of approxmately of ~60%. That menas it loves to crystallize.
How is crystal clear PET created at the industrial level ? PET has many grades. For bottles the grade is called PET-G. That G stands for Glycol. By adding Glycol to the polymerisation process, the Crystallisation temperature can be brought down from 140C to 120C. On industrial scale, PET is molded into preform at a very fast cooling rates (~20C/sec). From 280C to 50C in matter of seconds with the help of 8C water temperature.
Here is a videos how preforms are made
https://www.youtube.com/watch?v=a-lz0Ta-hBc
Preforms are blown into bottles,
https://www.youtube.com/watch?v=eyiu18DsItk
I am glad to see the DSC graph. I will try to explain that. I have taken the same graph and marked a few points. I hope this explanation helps.
A : The normal useage of PET bottles.
B : Glass transition temperature. Below this temperature the Polymer acts / behaves like glass i.e Brittle.
C to E : Crystallisation temperature range. To keep it simple, if the polymer is keep in this range, there will be crystals formed. The maximum crystallization rate is achieved at the peak i.e Point **D. **So to answer the clear PET question, keep the polymer out of that range. Above E crystals would melt. Once below point C, its impossible for PET to form crystals. Blowing of the Preform is donw below point C.
E : max crystallization rate.
F : 99% of the polymer is molten
G : Processing temperature of PET
Another thing about PET is that its an engineering Polymer. So its processing is really demanding. Also recycling and schredding PET needs special equipment. Any one want to use the presious plastic shredder, i wouls recommend sharpning and hardning the blades. The shredder life would increase significantly.
I hope that helped.
Hey guys, we’ve worked with PET a bit and in our experience rapid cooling is what helps to keep it from crystallizing, so it stays flexible and clear. So far we’ve only baked it in the oven, without compression (like a cake) and when it becomes liquid we pull it out carefully and dunk it straight into a bath of ice water, and swish it back and forth until it completely cools and the PET releases from the pan.
This is interesting because PET is all over the place in bottles but how to keep it transparent? Knowing this could be of big importance if one want to make things like transparent objects or sheet (plate) material. I wonder there is a good way to keep it transparent? Someone?
It’d be great if somebody who knows what they are talking about (not me!) could make a recipe book, with all the heating and cooling info!
@plastikfantastik Thank you for the article 
It seems very counter intuitive for me. Where we generally cool molded plastics quickly to give it a stable shape that is easy to work/polish/finish, but as this guy says it’s the opposite that should be done !
An even cooling is what is needed to reduce crystallization. and since plastics are poor heat conductors, they need much more time to cool evenly. To determine the minimum required cooling rate and time, we may need first to provide a circulating heat oven, then measure the maximum thickness that can be found in the mold to estimate the cooling time needed by that specific plastic.
Has anyone ever tried to slow-cool molded plastics ?
I don’t mean by slow-cooling to let the plastic cool in ambient air, the difference in temperature is already large enough to trigger the crystallization. I mean cooling using an oven by slowly decreasing temperatures until the state where the polymer cools completely under a specific temperature (to find) is reached.
Here’s an interesting topic for you guys who have the compression machine already built. this time I guess the mold should also be heated and cooled along with the melted plastic (maybe the way the compression v1.0 worked in the past ?
By the time I have the necessary tools set up, I hope somebody else starts working on this issue soon
If clarity is an issue, sometimes the problem comes from the cooling process. Uneven cooling can lead to haziness, so checking mold temperatures might help. Also, if you’re using recycled material, contaminants or variations in the resin mix could affect transparency.
A fresh pet preform with high-quality virgin resin should typically give you crystal-clear results, so material choice is definitely something to consider.
