A question about Density of an object...please help

[GinG]

Goood Morning All,

Just wondering if anyone can help, I work in the stretch film industry and to work out a weight of the film we do a formula length x width x thickness x 0.92(density).

Now we aree doing another prduct where the film is acctually pre-stretched by a machine, so basically taking the film down from a 17micron to say an 8micron.

Would stretching change the properties of the film and therefore change the density of the product? i.e change the potential number we need to use for the weight.

Only reason I ask is I am nt convinced the same formula works, as the reel weights are coming out no where near when using it.

Thanks

Graham

 

[Aod]

stretching the film will lower the thickness but increase the length or width proportionally. the density should not change.

 

[Vixen]

stretching the film will lower the thickness but increase the length or width proportionally. the density should not change.

Density is mass per unit volume though. If you change the thickness of a material then same mass will take up a larger volume, therefore the mass for a unit volume will decrease.

If you have an accurate way of measuring the mass of several identical pieces of film then work your equation backwards - ρ = m/V.

EDIT: If you take a piece of the film you know the density of and a piece of the new film with identical masses then the equation you need is ρ2 = (ρ1V1)/V2 where 1 denotes the known density and volume and 2 denotes the new density and volume.

 

[GinG]

Thanks Vixen, as we do not have an accurate way of measuring the density I think it will be impossible to do it.

From what you are saying, the 0.92 we would use for the unstretched film could potentialy not work for the stretched film?

 

[HaMMuS]

Thanks Vixen, as we do not have an accurate way of measuring the density I think it will be impossible to do it.

You can work out the density of each film by doing:

Mass of film roll / (Length x Width x Thickness)

But as mentioned before, you need a way of accurately measuring mass.

 

[Vixen]

Thanks Vixen, as we do not have an accurate way of measuring the density I think it will be impossible to do it.

From what you are saying, the 0.92 we would use for the unstretched film could potentialy not work for the stretched film?

That's right. If you can't measure the mass accurately then you'll need to contact the company the new film is coming from to find out density.

 

[Dave M]

You won't permanently change the density unless you change the molecular structure of the material. Most likely there's a problem with your equipment or assumptions (is it exactly 8 microns and do all films have a density of 0.92?).

 

[Amleto]

Density is mass per unit volume though. If you change the thickness of a material then same mass will take up a larger volume, therefore the mass for a unit volume will decrease.

If you have an accurate way of measuring the mass of several identical pieces of film then work your equation backwards - ρ = m/V.

EDIT: If you take a piece of the film you know the density of and a piece of the new film with identical masses then the equation you need is ρ2 = (ρ1V1)/V2 where 1 denotes the known density and volume and 2 denotes the new density and volume.

Sorry, I think you have got mixed up there. If you change the thickness, the only property you can reasonably change, in a macro sense, is the volume. The density will be the same. Otherwise by the pure act of stretching you would have changed the mass. Unless the stretching is causing a reaction of some kind in the film, the mass wont be changing.

Therefore, assuming the mass is constant from the above reasoning, i concur that

If you change the thickness of a material then same mass will take up a larger volume, therefore the mass for a unit volume will decrease.

is totally wrong. The same mass will take up the same volume.

 

[Bubo]

Unless the stretching is causing a reaction of some kind in the film, the mass wont be changing.

Since we are talking about a film, some kind of plastic type material, then it is entirely possible for its microstructure to change dramatically after being stressed. Think of that plastic stuff which used to hold a 6 pack of coke cans together. If you stretch that stuff it radically changes its structure. Sure the stuff will have the same mass before and after you stretch it, but is it guaranteed to have the same overall volume?

I suggest that if the final "reel" for the before and after products has the same diameter and the same reel width (not film thickness), and the two reels have different masses then difference simply must be that the density has changed.

 

[Wryel]

You won't permanently change the density unless you change the molecular structure of the material. Most likely there's a problem with your equipment or assumptions (is it exactly 8 microns and do all films have a density of 0.92?).

This. I'm assuming the stretching is irreversible? I.e. you get it pre-stretched and not held that way by a frame or similar?

 

[p4radox]

Depends on whether the film material is isotropic and whether the "pre-stretching" is fully plastic deformation. This might be of interest.

http://en.wikipedia.org/wiki/Poisson's_ratio

At the OP: I can't answer your question, but you might well be correct that your formula no longer works.

 

[Bubo]

Only reason I ask is I am nt convinced the same formula works, as the reel weights are coming out no where near when using it.

By the sounds of this you have the capability to measure the mass of a reel. Then do this, measure the weight of an empty reel. Measure the diameter of the inner hub in this reel(ID). Now coil up a reelsworth of the old material to a certain outer diameter (OD) and weigh that. Now coil up a reels worth of the new material to the same diameter and weigh that. Now check the width (W) of the two films, ignore the film thickness, it is irrelevant. Subract the mass if the emtpy reel from the two masses, this will give you the mass of each coiled film reel alone.

Work out the volume of each reel of film, i.e. (area of a disc of OD minus area of hub of ID)*W

You can then confirm the density of the old stuff is 0.92, and also work out the density of the new stuff.

 

[callmeBadger]

Density is mass per unit volume though. If you change the thickness of a material then same mass will take up a larger volume, therefore the mass for a unit volume will decrease.

Sorry, having trouble agreeing with this statement. When changing the thickness of the material, the volume and mass of the material will stay constant, but the surface area will increase.

Easy way to imagine for the OcUK'ers, a pint of beer in a pint glass, and a pint of beer in a bath. Both have same mass and volume just different surface area's.

edit: to OP, with the formula Length x width x thickness x density, what are the units used? cm, mm, metres? The density is a number relating to a solid cube of this material in the given unit of messure, your formula calculates the volume of the product, the last multiplier applies a mass conversion to the volume.

In your scenerio, your moving from 17 micron to 8 micron, if it is the same material just being stretched a bit, then all that is changing is the thickness (thus volume) of the film. density (being a value for 1 solid cube of the material, would remain constant just the arrangement of it would change.

This is off course assuming that the film is elastic.

 

[Bubo]

work out a weight of the film we do a formula length x width x thickness x 0.92(density).

Only reason I ask is I am nt convinced the same formula works, as the reel weights are coming out no where near when using it.

Another possible scoolboy error, since the new material is thinner you are going to be able to get a longer length of it coiled up on the reel if you are reeling it up to the same OD as before. If you are assuming the length is the same before and after then that will give a large error in your calc by itself, regardless if the density has changed. Have you measure the lengths of a reelsworth before and after?

 

[Amleto]

Since we are talking about a film, some kind of plastic type material, then it is entirely possible for its microstructure to change dramatically after being stressed. Think of that plastic stuff which used to hold a 6 pack of coke cans together. If you stretch that stuff it radically changes its structure. Sure the stuff will have the same mass before and after you stretch it, but is it guaranteed to have the same overall volume?

I suggest that if the final "reel" for the before and after products has the same diameter and the same reel width (not film thickness), and the two reels have different masses then difference simply must be that the density has changed.

I must admit, I didn't really think too deeply into the structure of the film. It's possible that if it's anything polymer-like, that the strands will un-coil and this could manifest itself in a change of volume. I don't know jack about chemistry though so...

 

[p4radox]

Sorry, having trouble agreeing with this statement. When changing the thickness of the material, the volume and mass of the material will stay constant, but the surface area will increase.

The mass will stay constant, but the volume might not. Crazy things can happen on a molecular level, especially with polymers.

 

[HaMMuS]

Surely the simplest option would be to phone up the company selling you the stretched film and asking them what the density is?
I assume the density is a reasonably important property in the film industry and so the company would have a value for it?