Flexural Modulus, MPa

[quiteone]

A number associated with the flexibilty or stiffness of a material.The lower the number the more flexible, the higher the less flexible, my question is
Dose this number affect the shrinkage of plastic materials,
I.E the higher the number the more shrinkage you get??

Have just sampled two new plastic resins with the FM# higher than the regular resin that i use and both have shown more shrinkage, than the regular resin.
Any thoughts on this. All 3 materials share the same MFI and density etc the only differance is the FM# regular material is 1150 new materials are 1400 and 1450.

 

[CoryPad]

How are you defining shrinkage?


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Flex mod of different polymer types has no interaction with shrinkage, now MFI nor density.

However, if the polymer is the identical chemical type, and is a semi crystalline material, the degree of crystallinity effects both flex mod and shrinkage, but only to a minor degree.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[quiteone]

Shrinkage as in part size
the part size being 1.3250 +/-0.003. With the regular material i get a mean of 1.3239 but with the other materials i get a mean of 1.3204 and 1.3212 which the parts out of spec, but going back to the regular material the size returns to spec.
All three materials are PP and identical in every way except for the flex mod number.
The degree of crystallinity affects both flex mod and shrinkage, ok, is this something that can be tested to determine its effects and if so how?

 

[patprimmer]

PP comes in various grades of co-polymer, and also with different additive packages. Even different pigments can effect the nucleation of crystals.

The moulding conditions and the amount of time after moulding can both effect crystalinity as can post treatments like annealing or thermal cycling due to use or other processes.



Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[CoryPad]

quiteone,

I think you are not quite understanding structure-property relationships. Materials have certain structures that provide certain properties. The crystallinity of the material is the structure; the modulus, shrinkage, melt flow rating, etc. are the properties. Crystallinty can be measured directly using x-ray diffraction, which is almost prohibitively expensive. It can be assessed indirectly using many other techniques, like density, strength, modulus, DMA, etc.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Cory

The problems I see here are that quietone is trying to draw conclusions from very small changes in properties and is trying to hold very tight tolerances.

As you say, x-ray diffraction is well beyond the means of almost everyone, and other methods can be lost in the smokescreen of cause and effect, however a change crystalinity will effect the properties in question in the manner described.

Whether or not quietone really is being supplied with exactly the same compound every time, or whether there are other variables is in my opinion the real question at this stage.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dwightdixon]

The normal effects of filler or reinforcers is to decrease the amount of shrinkage in PP because the mineral does not shrink. PP is still classified as semi-cyrstaline. The amount of crystalinity is easily measured with a DSC run. Many labs have equipment. The molding conditions have to held very closely to achieve the degree of accuracy you are asking for. This may be beyond the capability of your machine or facility. The sample size, method of measurement, and how you take random samples, may also be effecting what results you think you are getting.

 

[quiteone]

your right the tolerances trying to be held are very tight for the type of part that I am making, the machine, process have been questioned as I have never experianced anything like this before, I,ve been able to change materials like for like on previous injection moulding jobs and have no trouble.

Thanks for the help folks I'll let you know the outcome.

Regards
Quiteone