Today's KNOWLEDGE Share: How Strong is Your Material?

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, this is not captured on plastic material datasheets. For a particular grade of polycarbonate, the tensile strength as determined by standard tensile testing was determined to be 64 MPa, a good match with the datasheet value. However, this is not how the component manufactured from the polycarbonate will be stressed – it is expected to last longer in field use than the 90 seconds it took for the tensile test, and it will be under continuous static loading or dynamic loading depending on the specific application. Based upon realistic conditions, creep and fatigue tests were performed. The testing determined that the actual material strength was 20 MPa for 5 years under continuous loading and 31 MPa at 750,000 cycles, the expected service life. Obviously these are far less than the 64 MPa identified as the tensile strength.

 

Source:The Madison Group

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#plastics #plasticsengineering #polycarbonate #failure #engineering #design #failureanalysis #fatigue #stess #creep #tensile #loading

Today's KNOWLEDGE Share:THIN RUNNER

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, you increase stress and thus promote slip and consequently lower the melt residence time and the associated risk of thermal degradation.

Years ago, we took advantage of Flow Analysis (with a Slip model included in the constitutive laws) and the above idea to design a hot runner system for PVC that proved to work for weeks without any melt degradation.

This may not apply to many other materials and will not be very relevant in cold runners where residence time will never exceed cycle-time, but I thought it was good food for thought anyway. It could apply to the nozzle of a molding machine by the way.

Source:VITO LEO

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#plastics #polymers #injectionmolding #thermal #extrusion #shear #hotrunner #meltflow #meltdegradation #PVCmolding # #design

Today's KNOWLEDGE Share: Comparing the properties of composite materials!!!

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 and Robert O Ritchie.

Credit:#managingcomposites #thenativelab

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#composites #cfrp #gfrp #nfrp #impact #strength #mechanical #properties #fibers #resin #silk

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