polymer processing

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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 st...

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 (sho...

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 pol...

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 g...

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 a...

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.

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 whe...