Test Your Rheology IQ

>> Saturday, 20 September 2008

Win T-shirt, I lost (only 40%, I will try some day), have fun!

Test Your Rheology IQ


Mold balancing part 2/2

Mold balancing is design and mold manufacturing problem, machine, set up factors have lower impact. We can not skip factors result from viscosity changing, which are filling stage set up and polymer rheology. Factors which have impact on polymer flow parameters (temperature, pressure, viscosity). It is important to secure symmetry distribution in velocity, shear rate, temperature, viscosity, for mold balancing. Title and body refer to mold, injection mold, but in the same way can be analyze any tool in polymer processing (e.g. extrusion die). Interesting problem is flow balancing in extrusion blow molding and parison centering, measured by wall thickness (e.g. botlle), the problem will be described soon.

How mold balance impact parts?

Each cavity is filled and packed in different stage for imbalanced mold. One cavity can be full filled during injection (over pack) and other partly filled during holding, when we fill shrinkage losses. Effect we receive parts with different stresses distribution, shrinkage, and dimensions.

How find reasons in mold imbalance ?

Imbalance for molding cycle to cycle can be machine and viscosity difference. Factors influence imbalance for the same cycle, can be divided for:

geometrical (geometrical differences between cavities, nozzle gate dimensions and shape),
thermal (design and heating and cooling system function),
mechanical (damage, blocked cavities)

How check mold balance?

Procedure you will find in part 1/2.

When we do the mold balance test?

The best time is to do it when you validate your mold during commissioning. I establish that all your molds are balanced otherwise we can not talk about process stability and capability.

How to deal with imbalanced mold?

Identify the reason. In most cases setting up: mold, hot runner or nozzle temperature is sufficient. If not follow the rule always chose mold adjustment not molding parameters. Mold adjustment you will always do once, parameters you have to always monitor, any deviation and you will receive bad part.


Bimodal high density polyethylene, HDPE

>> Wednesday, 17 September 2008

We will try to imagine how bimodal molecular weight distribution rise. We will focus on two main properties: processing and mechanical. We start from normal (unimodal) molecular weight distribution (please, start to analyze the graph). Macromolecules with short chains, are easiest to process, polymer has lower viscosity, long chains are responsible for mechanical properties the grow. Considering, what left (middle), can be replaced. The idea was simple, eliminating average properties, we receive polymer with key properties.

Return to the most important, processing and final product properties, considering molecular weight distribution. Short fractions or single monomers are gaseous substance, waxes, their impact is questionable (easy degradation, responsible for bad odor) and appear for unimodal distribution, changing the distribution we eliminate them. Long fractions generally affect mechanical properties, e.g. stiffness, it is possible to reduce wall thickness (shorter cooling time, than cycle time, higher output) with the same results in drop and top load tests, etc. increase ESCR. Bimodal, HDPE is specially recommended for aggressive substances, e.g. in household segment. It has low die swell, Barus effect close to 1, designing the forming instrumentation we know the key parameter, on the other hand changing from unimodal to bimodal grade will probably cause investment in new instrumentation (from large cross-section to smaller, not possible!).

Summary, main properties for bimodal HDPE:
• higher stiffness (higher density),
•higher chemical resistance, ESCR (Environmental Stress Cracking Resistance),
• higher impact,
• low die swell (Barus effect),
• low degradation,
• no bad odor.

Bimodal HDPE has the same product application as unimodal grade, e.g. films, bottles, tubes, caps, etc. and it is a great example in developing on polymer segment.


Polymer viscoelasticity part 2/2

>> Tuesday, 16 September 2008

Once more, technical issue, from Massachusetts Institute of Technology web page click right on Rheological Behavior of Fluids than save target as on your drive, and run using Realplayer

Conclusion and complement for part 1/2, polymer viscoelasticity effects: Weissenberg and Barus.

Weissenberg effect during shear flow between two coaxial cylinders appear climbing up along the rotating rod for no-newtonian fluids, that is melted polymers. The same effect can be observe during stain an paint mixing and can not be observe for newtonian fluids, e.g. water.Weissenberg effect is a result in generating during shear flow extra stain, called normal stresses. Barus effect appears during extrusion and it is observed by swelling extruded material leaving nozzle. For melted polymers the effect is big and describe as relation between flow and nozzle diameter, it is beetwen 1,2-2,5. Depends on flow capacity, tool geometry, but the most on relation channel length/diameter and polymer properties, less observed for bimodal polyethylene about it in the next act. The same as Weissenberg effect, Barus effect it is due to a non-zero first normal stress difference.


Molecular weight in polymer processing

>> Monday, 15 September 2008

A polymer is a set of molecules with very high molecular weight called macromolecules. Macromolecules are composed of basic units called monomers. Monomers are groups of the same linked atoms. Molecular weight is a sum of atoms weight for molecule.
Molecular weight impacts everything, viscosity, strength, thermal resistance, light transmition, but not density. Macromolecules are different, different in length, start from monomers and end up with “huge” macromolecules. This is the statistical (e.g. normal distribution) process of production, every lot (even bag!) from the same plant, from the same method, can be different in molecular weight distribution and average molecular weight.

Different fractions have different impact on processing and application properties. Short fractions, or single monomers are gaseous substances, waxes and the advantages are problematic (easy degradation, there are responsible for bad odor) With molecular weight grow, viscosity will grow, mechanical properties will grow, etc. chains will be longer.
Average molecular weight and the range have to be consider during choosing the grade for processing methods, e.g. extrusion blow molding one of the most important is melt strength which protect against sagging. It will be possible for grades with grater viscosity, high molecular weight, etc. for low shear rate.
Technique still benefits natural polymers, e.g. caoutchouc from elastomers, from polymers cellulose and proteins and it derivative.

PS. Picture shows: molecular chain of polyethylene and comes from: University of Florida, USA


Mold balancing part 1/2

>> Sunday, 14 September 2008

Does your mold balanced? There will be the part of just preparing problem for mold balancing. Fundaments with mechanism in part 2/2 which formally is the first part.

How to check mold balance?
The eases way is to weigh parts from mold. I suggest go to the shop floor take the full shot and weigh on the most precise available weight. Date you can compile in the form presents below. The best you can do is to take more than one shot and count the average from the same cavity. You have two ways, try both, count deviation from maximum weight (first way) and from average weight (second way) for all cavities. Formulas are in the table, where indexes; m – maximum weight from all cavities, n – weight for the variable cavity, a – average weight for all cavities.

Few conclusions from the above example. Cavities 7 and 10 are extreme first is the most second the least filled. Deviation from average gives the tendency +/- from average. Cavities 9 and 10 are the worst balanced. I would assume 5% as boundary between cavities and allow mold to be acceptance during commissioning.
The same you can do for not full filled mold, assume, 50% filled, there will be more visual rating.
When you analyze your case include the cavity, hot runner and cooling system location.


High impact polystyrene, HIPS

>> Saturday, 13 September 2008

Short and in the theme about mass thermoplastic polymer as high impact polystyrene, HIPS is. As I know there is not always known but all the power is in the name. Let’s try to decode. Impact is the resistance for cracking. This properties HIPS owes polybutadiene rubber. And that’s all. When you compare light transmission, HIPS is translucent and general purpose polystyrene, GPPS transparent.


Polymer viscoelasticity part 1/2

There will be screenshots from Rheological Behavior of Fluids movie applied, it is available from Massachusetts Institute of Technology web page, Realplayer is an requirement.
The movie is s great introduction for no-newtonian fluids (as polymers are) properties. There are many experiments which will be key for polymer processing understanding where this properties emerge. The movie history reaches latest 60th (fundaments for fluid mechanics and rheology are the same) it is not short more than 22 minutes but it is worth seeing more than once. As you listen carefully you will find answers for problems from: newtonian fluids, no-newtonian fluids (Bingham, pseudoplastic, dilatant fluids), viscoelasticity effects, Weissenberg effect, Barus effect, time dependent effects
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, resist shear flow and stain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. Viscosity we know already. Elasticity is material return ability to their original state once the stress is removed.

Polymers have both solid and fluid properties and are time dependent (because of its molecular structure). Viscoelasticity gauge in polymer processing is relation between characteristic material time and characteristic process time defined as Deborah number.

Back to the movie, the first example shows how time scale is important, ball (viscoelastic material, why, find below) for very short time, process time behaves as elastic solid (Deborah number tends to infinity).

For long time, process time ball behaves as viscous fluid ( Deborah number tends to zero).

In part 2/2 there will be about viscoelasticity effects, that is Weissenberg effect and Barus effect.

Once more, technical issue, from Massachusetts Institute of Technology web page click right on Rheological Behavior of Fluids than save target as on your drive, and run using Realplayer





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