Melted cheese-like flow -- What is it called and its cause?

[thepopasmurf]

There are some plastics I am using that when melted, pull apart in a stringy way. My best comparison is like melted cheese in a sandwich.

I want to know more about this, but other than looking up 'polymer rheology', I'm not sure what I should call this type of flow. I want to know what causes it and if the molecular weight of the polymer is important.

 

[Baluncore]

Thermoforming. Vacuum forming. https://en.wikipedia.org/wiki/Thermoforming
The string patterns form when unsupported thinner areas heat and soften more rapidly, then merge with the nearby string of cooler material.

 

[Asymptotic]

Is this the effect?
Partially degraded polymers (i.e. - an mixture of monomers and longer chains) often have this property.
High tack polymers tend to be stringier. Exxon Escorez and other tackfier additives can be blended with other polymers to enhance this effect.

 

[thepopasmurf]

@Asymptotic, this effect seems close to what I am looking for. The wikipedia entry doesn't give much to build on, I guess 'stretching' and 'fingering' are the best things to look at.

 

[Nidum]

In general it is non-Newtonian fluid flow . Most commonly encountered as visco-elastic flow though there are several other interesting variants .

 

[Chestermiller]

The degree of viscoelasticity increases with increasing molecular weight and poly-dispersivity.

 

[MTU_Husky]

Hey PopaSmurf,

A little more information is needed here such as the type of polymer you are using and is it glass filled, does it have lubricants or other additives?

The term for that type of flow is "laminar flow" and all polymers flow this way. essentially you have long polymer chains flowing side by side like two sheets of paper. There is no such thing as turbulent flow in plastics.

Molecular weight is very important in plastics as it dictates the quality of stiffness in the plastic as well as other properties. It will also give you an idea of how hard your polymer is to "push" (what pressures will I need to inject).

The string cheese effect you are describing sounds like de-lamination. This can be caused by degrading the material (temperatures are too high in the barrel or in the dryer), too much moisture being in your material or too high of a shear rate (injecting too fast).

I hope this helps!

Husky

 

[Chestermiller]

Hey PopaSmurf,

A little more information is needed here such as the type of polymer you are using and is it glass filled, does it have lubricants or other additives?

The term for that type of flow is "laminar flow" and all polymers flow this way. essentially you have long polymer chains flowing side by side like two sheets of paper. There is no such thing as turbulent flow in plastics.

This is not correct. It is easily possible to achieve turbulent flow for polymers with low degree of polymerization.

 

[MTU_Husky]

This is not correct. It is easily possible to achieve turbulent flow for polymers with low degree of polymerization.

I definitely misread the original post as I thought they were referring to injection molding plastic but after re-reading it they just stated this happens when melting the plastic. Do we know what method/process they are using to melt?

It will not let me update my original post so I will correct it here. There is no turbulent flow in injection molding thermoplastics and the reason I say that is because I have calculated the Reynolds number for several different polymers using a fill velocity of 400 in/s and the highest Reynold value I can calculate is around 160. The crazy part is I am using relatively low viscosity (about 5 - 10 poise taken from a viscosity vs. shear rate curve at 80,000 1/s), a 0.06" gate diameter and an extremely fast injection speeds (the fastest I have ever worked with is 35 in/s and I am using 400 in/s in my calculations), which all should raise my Reynolds number vs normal operating conditions.

I would be interested to see a polymer process where they would see turbulent flow. What industries and processes use these polymers and what does the turbulent flow achieve for them? If you have a generic or trade name I would be interested in comparing to my industry.

 

[Chestermiller]

I definitely misread the original post as I thought they were referring to injection molding plastic but after re-reading it they just stated this happens when melting the plastic. Do we know what method/process they are using to melt?

It will not let me update my original post so I will correct it here. There is no turbulent flow in injection molding thermoplastics and the reason I say that is because I have calculated the Reynolds number for several different polymers using a fill velocity of 400 in/s and the highest Reynold value I can calculate is around 160. The crazy part is I am using relatively low viscosity (about 5 - 10 poise taken from a viscosity vs. shear rate curve at 80,000 1/s), a 0.06" gate diameter and an extremely fast injection speeds (the fastest I have ever worked with is 35 in/s and I am using 400 in/s in my calculations), which all should raise my Reynolds number vs normal operating conditions.

I would be interested to see a polymer process where they would see turbulent flow. What industries and processes use these polymers and what does the turbulent flow achieve for them? If you have a generic or trade name I would be interested in comparing to my industry.

I wasn't talking about any particular process. I was just saying that it is possible for polymers to exhibit turbulent flow.