polymerguru

Today's KNOWLEDGE Share: Shear Rate Sorry to disappoint some of you, but there is NO such a thing as Maximum admissible Shear Rate for a material in Injection Molding.   30+ years back, Colin Austin (Moldflow founder) wanted to put a recommendation in the code documentation and, having no clear answers from suppliers ,he decided to list some values that were loosely based on a wild guess exercise. Actually an estimated typical shear rate (at some standard T) for a critical Shear Stress equal to an arbitrary fraction (abt 10%) of a Stress at break in the solid state. How wild is that ?? Shear rate does not destroy a polymer. If chains break, that is due to Shear Stress, not shear rate. If they degrade thermally, it is the result of accumulated shear-heating along the flow. In both cases, shear rates alone CANNOT resolve the risk of damaging the material. At low temperature (high viscosity hence higher stress) a lower shear rate can be more dangerous than a higher rate at a higher te

Today's KNOWLEDGE Share: How Strong is Your Material? Do you know the true strength of your material? With plastics, that is a trick question.   Because of their molecular structure, thermoplastic materials have different properties compared to other materials, like metals. ·    The polymer molecules consist of very long chains – high molecular weight. ·    The individual polymer chains are entangled in each other. ·    The polymer chains are mobile and can slide past each other because they do not share chemical bonds with the other chains around them.   Because of this, plastics exhibit viscoelastic behavior, displaying aspects of both elastic and viscous performance. Attributed to their viscoelastic character, the properties of plastics, including tensile strength, will vary depending on the conditions of the stress loading. This means the “strength” of the material will vary over temperature, time under load, duration of dynamic loading, strain rate, and more.   Unfortunately,

Today's KNOWLEDGE Share: THIN RUNNER I am saying that sometimes, you will avoid thermal degradation in runners by going thinner ! How is that possible ? Materials like RPVC have a strong tendency to degrade in extrusion and even more in Injection Molding. Some lubricants are typically present in the compounds to promote the slip of the melt on the reciprocating screw surfaces. Such slip will also be present in runners, in particular in hot runners (considered as a dangerous option in PVC molding by most). Slip is very well described by a power law relating Slip Velocity to the applied Shear Stress. So the onset of slip is triggered by a sufficiently high stress, very much like motion is triggered in a static friction coefficient context. This means that in a larger runner, where Shear Stress is much lower, the melt may well NOT slip, resulting in a very long (infinite) residence time of the top melt layers in contact with the hot runner chamber. By making your hot runners thinner,

Today's KNOWLEDGE Share: Comparing the properties of composite materials!!! A very useful method of doing this is by plotting them as ''Ashby charts'', which represent each material on the chart as ellipses or ''bubbles'', whose width and height are determined by the range of the value of the properties.  This Ashby plot shows the comparison of the impact strength at room temperature of polymer composites reinforced by glass fibers, carbon fibers, plant fibers and silk fibers. The corresponding composites are respectively abbreviated as GFRP (Glass Fiber Reinforced Plastics), CFRP (Carbon Fiber Reinforced Plastics), PFRP (Plant Fiber Reinforced Plastics) and SFRP (Silk Fiber Reinforced Plastics).  We find it amazing how this type of plot can make our life much easier! Source: Article "Enhancing the Mechanical Toughness of Epoxy-Resin Composites Using Natural Silk Reinforcements", written by Kang Yang, SuJun Wu, Juan Guan, Zhengzhong Shao an

Today's KNOWLEDGE Share: Viscoelasticity I am always looking for new ways to understand the viscoelastic properties of polymeric materials. I found an interesting explanation in the brochure, “Dynamic Mechanical Analysis Basics”, from PerkinElmer, Inc. It was focused on understanding the concepts of viscoelasticity as related to storage modulus, E’; loss modulus, E”; and the tan of the phase angle known as damping, the ratio of the loss modulus to the storage modulus, Damping = tan δ = E”/E’. These can be difficult concepts to comprehend. One place to start is Young’s modulus. Young's modulus, E, is the modulus of elasticity, a mechanical property that describes the stiffness of a solid material when the force is applied lengthwise. It quantifies the relationship between stress (σ) and axial strain (ε) in the linear elastic region of a material, E= σ/ε. This is a basic concept, but polymeric materials are viscoelastic and non-linear, and their behavior requires further explanat

Today's KNOWLEDGE Share: What are the differences between thermoplastic and thermosetting plastics?  The easy answer is that a thermoset plastic cannot be remolded after being cured, while a thermoplastic one can be reheated and remolded. But why?  As with many questions related to materials engineering, to answer that we have to zoom in a little bit and understand a bit more about chemistry at a molecular level.  In a thermoplastic, strong bonds link monomers into polymer chains, however, these long monomers are joined to one another by weak bonds! These bonds can easily break apart when the plastic is heated and quickly reform again as it cools.  Thermosetting plastics, on the other hand, have monomers that are cross-linked, thus, have extremely strong bonds!  Polymer matrix composites can have either thermoplastic or thermoset matrices.  Source: #managingcomposites Visit MY BLOG http://polymerguru.blogspot.com #composites #thermoplastic #thermoset