Shrink factor for really thick piece?


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    Default Shrink factor for really thick piece?

    I'm going to mold a gear with a diameter of ca 88mm (little over 3 inches) and a thickness of 8mm (little less than 1/3").
    I don't have any experience with molding parts that thick.
    The material is POM. How much will it shrink approximately, or in other words how many percent should I scale up the cavity of the mold to get a dimensionally accurate part?

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  2. #2

    Default Re: Shrink factor for really thick piece?

    .020"/" is what is typically used for acetal. What type of press will this be going into? I had seen on another forum that you were using some what of a benchtop machine, is that still the case? I ask be cause I wonder about the ability of a benchtop machine being able to fully fill and pack out the part without defects as it is a pretty thick part you have there. Is the part allow for you to core it out and then add ribbing to strengthen it back up? Otherwise with such a thick part you should make a dummy cavity and measure the result for shrinkage



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    Default Re: Shrink factor for really thick piece?

    Luckily I have a better machine in the meantime. I'm using a Dr. Boy 50. Unfortunately the part needs to be solid because it will be shrunk fit to a shaft. I previously got the gears machined from a solid rod of POM.

    Just making sure that I understand correctly, I would need to enlarge the disk diameter (3") to about 3.060"? Seams like a lot.



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    Default Re: Shrink factor for really thick piece?

    Quote Originally Posted by cordvision View Post
    Luckily I have a better machine in the meantime. I'm using a Dr. Boy 50. Unfortunately the part needs to be solid because it will be shrunk fit to a shaft. I previously got the gears machined from a solid rod of POM.

    Just making sure that I understand correctly, I would need to enlarge the disk diameter (3") to about 3.060"? Seams like a lot.
    Correct, may seem like a lot but it will shrink that much and maybe even more considering the wall thickness. BTW, you wont just enlarge the OD of the gear, you applied shrinkage in all axes. I guess I'd have to see the gear and where it's getting shrunk onto, but still don't see why the part couldn't be cored out like shown here, the inside diameter is still beefy.

    Attached Thumbnails Attached Thumbnails Shrink factor for really thick piece?-gear-ribbing-jpg  


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    Default Re: Shrink factor for really thick piece?

    The problem is that I will have to mill the center hole after injection molding because they need to fit many different shaft diameters.
    I'll just test injection mold a disk with the same diameter and see what happens. Thanks for all the help



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    Default Re: Shrink factor for really thick piece?

    Almost done with the test mold insert. Just need to cut some vents. Not even going to make any ejection system. Fingers crossed that it works.
    Shrink factor for really thick piece?-img_5559-jpg



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    Default Re: Shrink factor for really thick piece?

    Just a quick safety note..don't let that acetal mix with any PVC. I don't know what you run in your shop, but they don't like each other.

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    Default Re: Shrink factor for really thick piece?

    Quote Originally Posted by jchal3 View Post
    Just a quick safety note..don't let that acetal mix with any PVC. I don't know what you run in your shop, but they don't like each other.

    Sent from my Z958 using Tapatalk
    I'm very glad you mentioned that. I'm not using PVC but TPE's. I did some re-search and came across this: "Never mix POM with TPEs or TPVs such as Santoprene or PVC". What is actually the problem with mixing them?



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    Default Re: Shrink factor for really thick piece?

    I believe it puts off mustard gas or nerve gas I can remember for sure. I just remember it was a big no no

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    Default Re: Shrink factor for really thick piece?

    The "C" in PVC is chloride. I know if you burn it in a laser, it's dangerous...



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    Default Re: Shrink factor for really thick piece?

    Test injected a few pieces, the surface finish is terrible:
    http://www.cnczone.com/forums/moldma...ml#post1923742



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    Default Re: Shrink factor for really thick piece?

    Quote Originally Posted by cordvision View Post
    I'm very glad you mentioned that. I'm not using PVC but TPE's. I did some re-search and came across this: "Never mix POM with TPEs or TPVs such as Santoprene or PVC". What is actually the problem with mixing them?
    The only problem would be that they would not mix, because they have different heat requirement's, most plastics can mix if they are correctly formulated, that's what is called engineered plastic, you can buy them already mixed

    The only problem with PVC is if you burn it, apart from that it's the most used plastic in the world, you would never be using PVC anyway because you need a different Barrel and Screw for your machine to run it

    Mactec54


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    Default Re: Shrink factor for really thick piece?

    Quote Originally Posted by jchal3 View Post
    I believe it puts off mustard gas or nerve gas I can remember for sure. I just remember it was a big no no

    Sent from my Z958 using Tapatalk
    I suggest you do some research, before posting, such incorrect information

    Mactec54


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    Default Re: Shrink factor for really thick piece?

    Mactec54, you are right next time I'll keep my mouth shut and hope nothing happens. Wouldn't wanna tell someone to be careful or put an extra emphasis on safety and be wrong.

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    Default Re: Shrink factor for really thick piece?

    Did a little research, and I was wrong. It's not a gas release, it's risk of explosion. See the link below..again sorry to raise a red flag safety is for losers
    Processing PVC and Acetal - Plastics Training & Technology Blog

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    Default Re: Shrink factor for really thick piece?

    Quote Originally Posted by jchal3 View Post
    Did a little research, and I was wrong. It's not a gas release, it's risk of explosion. See the link below..again sorry to raise a red flag safety is for losers
    Processing PVC and Acetal - Plastics Training & Technology Blog

    Sent from my Z958 using Tapatalk
    Again miss information there has to be extreme conditions for any thing like this to happen, and they do release Gases this is someone just saying what could happen, there is no safety issue, you can't make any form of gas from these two materials, without extreme conditions, which can not happen with normal use, and how these materials are used, I have used both of these materials on a regular basis for years, there are may plastics that are worse than these two, but again you need extreme conditions before they become a safety hazard

    Mactec54


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    Default Re: Shrink factor for really thick piece?

    If you're only making a few of these and don't care about the look use a 'chill' on some locators.



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Shrink factor for really thick piece?

Shrink factor for really thick piece?