Specifi Gravities of Polymers

Rishi

My name is Vic Mehta. I am looking for some basic info on Blow Molding?

you can find bunch of it on the web but I am looking for a real good one.

Can you help

Vic Mehta

Thanks

Quality Engineer

Vmehta11@gmail.com

Vic,

What type of information you are looking for? A great source of information can be found on the material suppliers web site. Most suppliers have a processing guide and/or trouble shooting guide. Another good source of info can be found on blow mold equipment suppliers' web site. Thirdly, some blow mold tooling suppliers also offer information on tooling related issues and solutions. If you could be more specific on what info you are looking for, I may be able to assit.

Rishi

Thanks for your reply and willingness to help. I am looking for basic information on blow Molding. I dont know too much in depth. Like what do you do if you certain defects. How can you fix it.

We use ABS materials for making outside automobile parts. But somebody mentioned about using ASA material. Then the Melt rate comes into play. Then the cost.

Why it is common to usa ABS for most parts. I found out ABS will degrade if there is not good paint on the part, when they are exposed to UV (Sun Light)

Rishi what is your take on this?

Thanks much

Vic Mehta

Quality Engineer

Michigan, USA

Solid state (as it relates to a specific temperature as measured against water at the same temperature.)

Interesting discussion of this topic at the site below - regarding the fallability of using solids values of specific gravity as it relates to sizing shot capability in injection molding -

http://www.ides.com/articles/design/2006/sepe_01.asp

enjoy

Jim

The density of a molten polymer is lower than its frozen state. The reason being that when you increase the temperature in a polymer the chains start to separate until they have enough room to "freely flow" (freely is a bit of an overstatement). The increase in volume associated for a constant mass will get your density down.

On the other hand, your density in the frozen state does depend on your the thermal history of your sample which will influence the crystallinity and end-volume.

For a glassy polymer (which is what I think you'd be into if you make masterbatches) a ballpark approximation would be to use the thermal expansion coefficient of the resin to calculate your volume change with temperature and then use this to modify your frozen density. For the crystalline ones (and a more accurate method for your gallsy ones as well) you can try and measure your CTE above your melting point and use that to approximate your density.

Hope it helps.

Nice post; I also hadn't given much thought to discussing other non-apparent changes in final 'density' based on heat historys - burning off some portions of additives etc, further changes in lengths to the polymer chains in recurring histories etc. -

Other than the sizing of machinery and shrinkage calcs, another apparent 'problem' in the difference between frozen and various 'interpretations' of 'molten' has to do in extrusion with the behavior of the density in terms of melt flow through an orifice of a given shape - especially if you are trying to calculate flow speeds and the interfacial relationship between the material and land lengths based on the S.G. of a material at a particular stock temperature. We run into this all the time - you can tweak a die at a stock temperature and resultant density on one lot of material only to have to force the material makeup at a later time on another lot at a different temp and density -

As a wise poster once noted, who would want to be in such a business?

Jim

That's essentially true, and a good direction for the questioner.

What I do for an approximate SpG of the molten polymer is to fill the wt/gal cup with molten polymer (at whatever temperature you wish to measure). Using a hot knife blade, carefully scrape all the xs polymer off of the cup so that the molten polymer is essentially a flat surface, parallel with the lip edge of the cup. Then when the polymer solidfies, and cools, add water to the indented polymer, scrape off xs as before, and weigh how much water was added. This will give you a rough volume number for the differential between molten and solid polymer. Then you have the volumes of molten and solid polymer, and the weights.

The reason for this query was that we have a melt spinning line for making filament yarns of various polymers and since we know the capacity of the spinning metering pump (CC/revolution) and the maximum RPM to which it can be safely run. I was trying to calculate the maximum through put of the line which would mean multiplying the CC/Rev*max RPM*Specific gravity of that polymer in molten state. Thus the curiosity.

But honestly I am very happy and pleasantly surprised with the rapid response culture we have for all queries, besides the wealth of information that we are more than ready to share.

Thanks and Best Regards - Rishi Uppal

By "molten" state, I wonder if you are referring to the specific gravities of polymers in a liquid state, which implies concentrations of polymer to solvent. Do you truly mean "molten", which means "melted"? I work with many polyamides, many with N-methyl-pyrrolidone solvent...so the specific gravity changes with concentration.