Today's KNOWLEDGE Share: Viscoelasticity

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 explanation.

The PerkinElmer explanation used an elastic superball. If we drop a super ball, the ball doesn’t bounce back exactly to our hand because of losses to internal motion and friction. The amount of bounce can be related to the storage modulus, E’, a measure of how elastic the material acts under these conditions of temperature, load, and frequency. The lost height can be related to the loss modulus, E”.

We conducted this superball drop experiment on two different balls. The superballs were dropped from a height of 44 in. and the rebound was measured. The results, shown in the table below, revealed a difference between the two superballs. Ball A produced more rebound, with a higher bounce back, compared with Ball B. This indicated that Ball A had a greater elastic response with less loss and less damping compared with Ball B. The bounce test illustrated a clear performance difference between the two balls.

The variation in the properties of the two superballs was explained by compositional differences between the two materials. The balls were analyzed using Fourier transform infrared spectroscopy (FTIR). Both balls were produced from polybutadiene-based resins. However, the Ball B material also includes a significant level of calcium carbonate. This filler accounted for the lower elastic response / greater loss in the Ball B performance. The next logical step would be to quantify the filler loading through thermogravimetric analysis.

There is a more formal way to measure the storage modulus, loss modulus, and damping or tan δ. Dynamic mechanical analysis (DMA) can effectively assess these important characteristics of polymeric materials.

Source:The Madison Group

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#plastics #rubber #elastomers #ftir #mechanical #viscoelasticity #modulus

Today's KNOWLEDGE Share: The differences between thermoplastic and thermosetting plastics

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

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#composites #thermoplastic #thermoset

Today's KNOWLEDGE Share:Injection Molding Machine:

Today's KNOWLEDGE Share:

Injection Molding Machine:

Historically, early injection molding machines would essentially be pressure controlled.Many good parts have been made under such process control. So, it is not all bad !

However, note that when molding an end-gated fairly long part, a constant pressure fill translates into an ever decreasing melt front velocity, as the pressure drop builds up.

This in turns corresponds to a decreasing average temperature of the melt front along the flow.

Such a decreasing Temperature will create an increasingly strong degree of molecular orientation when moving away from the gate. The part, especially when using semi-crystalline grades, will have a strong gradient of mechanical properties along the flow which could be as serious as showing good ductility near the gate and severe brittleness far from the gate.

Source:Vito Leo

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#injectionmolding #mechanicalengineering #moldingsimulation

#plastics #plasticsengineering #brittleness #ductility #meltflow

Today's KNOWLEDGE Share:Bio-based acrylonitrile (ACN)for carbon fiber

Today's KNOWLEDGE Share:

Bio-based acrylonitrile (ACN)for carbon fiber manufacture:

"As part of its research with bio-based ACN, Southern Research conducted a life cycle assessment (LCA), comparing biomass-to-ACN manufacture to petroleum-to-ACN manufacture. Results said bio-based ACN manufacture offers a carbon footprint of -1.57 pounds equivalent CO2 per pound of finished product, compared to 3.5 pounds equivalent CO2 per pound of finished product for petroleum-based ACN manufacture. In short, the bio-based feedstock allows for a process that conserves carbon emissions." 

"Regarding cost, Southern Research’s process is sensitive to the purity of the sugars feedstock, and the higher the feedstock quality, the more expensive it is. Southern Research says,it was getting ready to commission a small-scale production plant and looking for carbon fiber manufacturers willing to assess the quality of its ACN." 

Source:#managingcomposites

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#carbonfiber #acrylonitrile #co2emissions #carbonfootprint #biobased #biomass

Today's KNOWLEDGE Share:Plastic fork

Today's KNOWLEDGE Share:

Plastic fork

I was travelling a few weeks ago and between flights grabbed some dinner at the Atlanta airport. I got my food and along with it came a plastic fork – nothing unusual there. As I looked around from my table, I noticed that there were signs indicating that the utensils were made from compostable material and that they were environmentally friendly. Cool, I thought. As I was eating, I noticed molded-in writing on my fork. On the top surface the fork was labelled as “Compostable Commercially*”. I thought it was funny that there was an asterisk. What could that be about I wondered, until I noticed even smaller molded-in printing on the reverse side. The reverse side markings indicated “*Facilities may not exist in your area.” Ahh – the asterisk is explained.

My curiosity was peaked - what is this *compostable* fork made from? I took the fork and when I returned to the office, I analyzed it. The first test I ran was Fourier transform infrared spectroscopy (FTIR). This identified the fork material as polylactic acid (PLA). This made sense because I found that PLA is compostable under controlled conditions.

Next, the material was analyzed using differential scanning calorimetry (DSC). This thermal analysis technique identified a glass transition temperature (Tg) of 65 °C and a melting point (Tm) of 177 °C. Both of these were consistent with a PLA resin.

Finally, thermogravimetric analysis was utilized, and this indicated that the base polymer underwent thermal degradation at 349 °C. Further, the composition was characterized with a nominal filler content of 15%.

On the non-technical side, I noticed that there were no special receptacles at the airport to collect these compostable utensils, and that people were throwing them into the garbage. From the little I know; I do not think that these PLA forks will compost in a landfill.

Source:The Madison Group

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#plastics #polymerengineering #polymerscience #materialsscience #forks #pla #compostable #ftir #dsc #tga #analysis #biobased #molded #food #printing #utensils

Today's KNOWLEDGE Share: Unbalanced Force

Today's KNOWLEDGE Share:

Unbalanced Force:

Based on consulting requests, I realize that a lot of people forget that huge forces are developed during the molding process, as a result of pressure levels exceeding often 1000 bar/100MPa.

That amounts to 1 metric Ton of equivalent force applied to each square cm of tool surface.That is why clamp tonnage numbers are what they are of course.

But, no matter how good your steel or tool design is, metal will bend significantly when subjected to huge unbalanced forces.

And, even more surprisingly, for balanced forces, the cavity will expand by "compressing" the steel by quite a few microns !

You can run a quick FEA to check that, by applying 1000-2000 bar on a piece of steel.

Of course tubular shaped parts will readily see significant core shift problems as soon as flow is slightly unbalanced, since a differential of a few Tons-force can quickly appear if flow is not perfectly balanced. The problem here is, of course, that the more the core deflects, the more the unbalance grows. So it is a bad case of positive feedback leading to catastrophic results (unexpected weldlines in the thinned side towards which the core has been bent/pushed).

Don't underestimate the importance of these effects in molding.

While coupling Flow Analysis with stress analysis on the steel structure can supposedly model this, it is very challenging to describe the complex tool assembly. And such coupled approaches can be very challenging numerically. So, while core-shifting predictions are now quite standard, full tool deflections are usually neglected in simulations. And the clear tendency of steel compressibility to lead to overpack is never accounted for.

Source:Vito Leo

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#plastics #polymerscience #plasticsengineering #tooling #steel  #polymerprocessing #injectionmoulding