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Topic: Shrinkage of Polypropylene  (Read 4142 times)

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Offline Karkzu

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Shrinkage of Polypropylene
« on: July 23, 2017, 08:21:08 AM »
Hi, all.
 I worked polymer plant about three years ago and we manufactured from the plastic chair, dustbin but also used to polypropylene as raw material. In short,  I have a question that I wonder answer. " our product was shrinkaged while it was cooled out of injection machine. as the solution, I increase cool time of injection machine but in that case daily production quite declined. I wonder if there are different solutions, for instance: like a kind of additive
I'll glad if you could help me.. 

Offline Arkcon

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Re: Shrinkage of Polypropylene
« Reply #1 on: July 23, 2017, 08:54:16 AM »
Hrm ... an interesting problem.  We'll need some of our experts to weigh in.  However, a few general points for you to work with:

Hi, all.
 I worked polymer plant about three years ago and we manufactured from the plastic chair, dustbin but also used to polypropylene as raw material. In short,  I have a question that I wonder answer. " our product was shrinkaged while it was cooled out of injection machine.

Its a little hard to follow you here, but let's work together to get the best information to continue.  Just about everything shrinks when it solidifies (except for water and Wood's metal, if we must be absolutely correct.)  I guess you mean excessive shrinkage?  But how much did you expect?  How much is acceptable?  1mm?  0.1 mm?  0.0000001 mm?  0.000000000?  How will you easily remove it from the mold, if it doesn't shrink at least a little bit?

Quote
as the solution, I increase cool time of injection machine but in that case daily production quite declined.


Interesting.  How much did the longer cooling time help?  Did it change 0.01% shrinkage into 0.005% shrinkage?  Is that adequate?  How much cooling time increase?  Consider, if you go from 10 min cooling time, to 30 min cooling time, for slight improvement, what do you get at 20 min cooling time?  Moderate benefit?   The same benefit?  Negligible benefit?

You see, you're not being very quantitative.  We don't know how much of a problem you're having, and we don't know how much of a benefit you're getting.  You might be mistaken at every step and the results are random.  We just don't know.

Quote
I wonder if there are different solutions, for instance: like a kind of additive
I'll glad if you could help me.. 

Now this is interesting, if the input isn't simply polypropylene, you can get a variety of different effects.  But which one, and where to start?
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Corribus

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Re: Shrinkage of Polypropylene
« Reply #2 on: July 23, 2017, 11:50:28 AM »
You can try changing the injection parameters (pressure, hold time) or mold parameters during injection. PP isn't as sensitive to these parameters as polyethylene but it is still a semicrystalline polymer, and controlling crystallization kinetics plays a big role in the shrinkage. The mold shape also can be important, but probably you don't want to change that.

We do a bunch of polypropylene injection molding and it my experience this is more an art than a science. We were getting a lot of bubble formation in the neck region of our ASTM dogbones. We basically just played varied the injection parameters and made notes on how this impacted the molded parts - approaching a satisfactory condition iteratively.

EDIT: Maybe you can find this document helpful: https://www.lyondellbasell.com/globalassets/documents/polymers-technical-literature/tech-topic-mold-shrinkage.pdf?id=13948
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Karkzu

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Re: Shrinkage of Polypropylene
« Reply #3 on: July 25, 2017, 05:06:09 AM »
Its a little hard to follow you here, but let's work together to get the best information to continue.  Just about everything shrinks when it solidifies (except for water and Wood's metal, if we must be absolutely correct.)  I guess you mean excessive shrinkage?  But how much did you expect?  How much is acceptable?  1mm?  0.1 mm?  0.0000001 mm?  0.000000000?  How will you easily remove it from the mold, if it doesn't shrink at least a little bit?

you are right, I must to give more detail. As you said, I mean excessive shrinkage so about to 1.5cm - 2 cm therfore we face to montage problem. We dont want to more than 1 cm of shrinkage pp



Interesting.  How much did the longer cooling time help?  Did it change 0.01% shrinkage into 0.005% shrinkage?  Is that adequate?  How much cooling time increase?  Consider, if you go from 10 min cooling time, to 30 min cooling time, for slight improvement, what do you get at 20 min cooling time?  Moderate benefit?   The same benefit?  Negligible benefit?

We inrease cooling time aproximate  5sec, or 10sec  however this stuation disadvantage in terms of manufacturing.



Now this is interesting, if the input isn't simply polypropylene, you can get a variety of different effects.  But which one, and where to start?

Yes you re right, I already ask this question? I expect to help on this one
« Last Edit: July 25, 2017, 06:28:38 AM by Karkzu »

Offline Karkzu

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Re: Shrinkage of Polypropylene
« Reply #4 on: July 25, 2017, 05:23:46 AM »
You can try changing the injection parameters (pressure, hold time) or mold parameters during injection. PP isn't as sensitive to these parameters as polyethylene but it is still a semicrystalline polymer, and controlling crystallization kinetics plays a big role in the shrinkage. The mold shape also can be important, but probably you don't want to change that.

We do a bunch of polypropylene injection molding and it my experience this is more an art than a science. We were getting a lot of bubble formation in the neck region of our ASTM dogbones. We basically just played varied the injection parameters and made notes on how this impacted the molded parts - approaching a satisfactory condition iteratively.

EDIT: Maybe you can find this document helpful: https://www.lyondellbasell.com/globalassets/documents/polymers-technical-literature/tech-topic-mold-shrinkage.pdf?id=13948

thanks for your interst, and this link. As you said I dant want to change mold and you re right  crystalization kinetics is very important but how can I  controll crystalization kinetic I dont know. Could you give me more information this subject...

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