Today's KNOWLEDGE Share: Integral Hinge Material Characteristics

Today's KNOWLEDGE Share:

Integral Hinge Material Characteristics:
I have worked on a number of projects, both material selection and failure analysis, involving an integral hinge. An integral hinge, also known as a living hinge, is a thin flexible web that connects two relatively rigid wall sections. The living hinge is formed when partially oriented plastic is drawn cold and flexed for the first time. Because of its molecular structure and flexural fatigue properties, polypropylene is widely used in integral hinge applications. Well oriented polypropylene webs are considered to have virtually unlimited fold endurance, assuming appropriate part design, resin, and molding conditions are utilized.

When reviewing the commercially available polypropylene resins, it is helpful to consider which compositional characteristics maximize the material’s utility for a living hinge application.

Typically, fillers and reinforcements compounded into polypropylene will negatively impact the performance of the integral hinge. Fillers and reinforcements reduce the elongation at yield of the material, which accounts for this reduction in performance.

A superior molded integral hinge relies on the ability to freeze polymer orientation during molding prior to cold drawing. A number of polymer variables affect the amount of frozen orientation.

Molecular Weight: High molecular weight is desirable in a living hinge application. However, lower melt flow rate resins can be more difficult to mold properly.

Molecular Weight Distribution: A polypropylene resin with a broad molecular weight distribution results in superior integral hinge performance. Longer polymer chains cannot relax as easily as short chains.

Nucleation: Nucleation enhances hinge quality by accelerating the process of freezing the polypropylene orientation. However, it is important to note that if there is melt flow hesitation, nucleation will reduce the performance of the integral hinge. It is very important that the material, and molding parameters be balanced to produce an optimally functional integral hinge.

Polymer Type: Given their higher tensile strength, polypropylene homopolymers generally produce better integral hinges. Homopolymers are characterized by relatively high stiffness, even at elevated temperatures. However, they exhibit brittle behavior at temperatures below 5 °C (41 °F). Polypropylene random copolymers exhibit excellent transparency, and offer reduced stress whitening within the hinge. They generally exhibit cold temperature performance that is better than homopolymers. Polypropylene heterophasic copolymers (block copolymers or impact copolymers) are characterized by good impact properties down to temperatures below freezing.

Source:The Madison group
Visit MY BLOG https://lnkd.in/fcSeK9e

#plastics #polymerscience #polypropylene #hinge #design #impact

Today's KNOWLEDGE Share: Why are Charcoal and Salt Added in the Earth Pit?

Today's KNOWLEDGE Share:

Why are Charcoal and Salt Added in the Earth Pit?

The combination of salt and charcoal is the perfect mixture which makes the ionic bonding for moisture in the earth pit. When the moisture increases in the soil, it increases the conductivity of the earth or ground conductor to the grounding rod or earth plat buried in the earth pit. That’s why the mixture of charcoal and salt is the best combination to put in the earth pit to maintain the low resistance.

An alternate layer of salt and charcoal is used to increase the effective area of the earth which leads to decrease the earth’s resistance.

As discussed above, the mixture of salt and charcoal as an alternate layer in the earth pit absorbs the moisture from the soil and surroundings. Additionally, the salt makes a perfect bonding with water, soil and charcoal. Therefore, the combination of charcoal and salt decreases the resistance and increases the conductivity of the earth pit. This way, the fault current can easily flow from the metallic body of the machine through the grounding conductor (earth continuity conductor) to the earthing lead and earth electrode (earth plat) buried in the earth pit.

The mixture of coal and powdered charcoal can maintain the moisture around the soil for a long time period. Hence, it reduces the resistance of earth pit and soil. This way, in case of fault, a less resistive path is available for fault current to flow to the ground. Thus, it provides proper protection to electrical machines as well as against the electric shock to a human body in contact with the metallic body of electrical appliances.

As the resistances may vary according to the different types of soil, thus it is important to check and test the conductivity of the soil before making an earth pit for grounding rods and earth electrodes (generally GI pipe or plate).

Keep in mind that the minimum and ideal resistance of the earthing and grounding system should be at least 1 Ω. If the resistance is more than 1-5+ ohms, you may increase the size of earthing lead and earth continuity conductor. Make sure to put water from time to time in the GI pipe connected to the earth pit which makes sure proper moisture around the earth plat. Hence, the earthing & grounding system for protection purposes works smoothly.

Source:Technical Engineering Portal (Linkedin)

Visit MY BLOG http://polymerguru.blogspot.com

#charcoal #salt #earth #conductivity #conductor #electrode

Today's KNOWLEDGE Share: History of fiberglass

Today's KNOWLEDGE Share:

History of fiberglass

The use of fiberglass dates back to 1836 when Ignace Dubus-Bonnel received the world’s first patent on a method of making them. At the time, fiberglass was hard to make thin enough to be completely flexible, and no reliable method of mass production existed. 

These problems would only be solved in 1932 by Dale Kleist, a graduate student who was working part-time at Owens-Illinois as a researcher. The company wanted to make glass blocks for architectural use, and its researchers were looking for a way to seal the two halves of a block together so that moisture couldn’t get inside. 

He decided to try a metal-spraying gun with molten glass instead of bronze and discovered that it created a shower of ultrafine, thread-like glass fibers. Owens-Illinois immediately recognized that this was an excellent way to make glass wool for insulation and that it might be adaptable for other applications. 

Four years and the researchers were turning out individual strands long and flexible enough to be woven into cloth. The cloth was remarkably strong, and it could be cut and folded just like ordinary fabrics. 

Source:

The Fiberglass Story, written by Michael Lamm/

#managingcomposites

Visit MY BLOG http://polymerguru.blogspot.com

#fiberglass #composites

Today's KNOWLEDGE Share: Valve Liner Cracking

Today's KNOWLEDGE Share:

Valve Liner Cracking

I recently wrapped up a failure analysis on a valve liner that had cracked while in service. The valve liner had been machined from perfluoro alkoxy (PFA). In this case, almost no background information was available on the valve application. My client was several steps removed from the installation. This can significantly hamper the failure analysis, limiting the ability to interpret test results.

The visual examination revealed a significant level of rust and adherent gritty debris on the valve liner. A circumferential crack was present within a design corner of the plastic liner. The cracking exhibited a continuous irregular crack pattern indicative of multiple radial cracks, not a circumferential crack. No signs of macro ductility were apparent, and the observed features were characteristic of brittle fracture.

The fracture surface displayed features indicating multiple individual cracks initiating at the liner design corner. The cracking extended radially into the part wall, and the coalescence of the individual cracks formed the circumferential fracture orientation.

The scanning electron microscopic (SEM) examination confirmed the presence of multiple crack origins, separated by ridgelike features corresponding to crack unions. A significant level of micro ductility was apparent within the crack origins, as indicated by stretched flaps. Striation bands of stretched flaps corresponding to progressive crack propagation were evident extending out from the crack origin area. The features were characteristic of dynamic fatigue. These features extended out from the origin through the mid-crack fracture surface. On the opposite wall of the liner from the crack origins, a significant level of stretching and deformation was apparent. This represented substantial micro ductility, and corresponded to mechanical overload within the final fracture zone.

Overall, the fractographic evaluation indicated that the valve liner failed through dynamic fatigue associated with alternating cycles of cracking and arrest. Based upon the observed striations, it is thought that this was low cycle fatigue, less than 10,000 cycles. However, such assessments can be erroneous, as crack initiation can account for up to 80% of the cycles.

Analytical testing including Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) produced results characteristic of a PFA resin, with no indication of material irregularities.

Background information detailing the service conditions would have been helpful to identify the source of the stress responsible for the failure, as well as contributing environmental factors.

Source:The Madison Group

Visit MY BLOG http://polymerguru.blogspot.com

#plastics #failureanalysis #cracking #pfa #valves #fractography #sem #dsc

Today's KNOWLEDGE Share: Nanoparticle in carbon fiber composites

Today's KNOWLEDGE Share:

Nanoparticle in carbon fiber composites

Check out this amazing microscopy! 

This picture shows residues of a nano particle toughened epoxy matrix adhering to a fractured carbon fiber! 

Source: Leibniz-Institut Für Verbundwerkstoffe

#managingcomposites

Visit MY BLOG http://polymerguru.blogspot.com

#composites #carbonfiber #epoxy #microscopy

Today's KNOWLEDGE Share:Failure Analysis of Electrical Connectors

Today's KNOWLEDGE Share:

Failure Analysis of Electrical Connectors

The metal leads on low voltage electrical connectors cracked during outdoor exposure testing. Initially, a metallurgical failure analysis was performed, concluding that the leads failed due to pitting and stress corrosion cracking (SCC).

As part of the metallurgical analysis, the lead wires were identified as a 65% Cu/35% Zn yellow brass with an exterior silver plating and a nickel underplating. These results were verified as part of the continued evaluation I performed through SEM-EDS analysis and elemental mapping.

The plastic base material was specified as an unfilled polypropylene, formulated with tetrabromobisphenol A bis(dibromopropyl ether) a brominated flame retardant; and antimony oxide a synergistic flame retardant additive. This was confirmed through Fourier transform infrared spectroscopy (FTIR) and EDS.

Visual and microscopical examinations confirmed the cracked leads. The connectors also showed the presence of white and blue-green deposits, as corrosion debris, surrounding the failed leads.

The connectors were inspected via scanning electron microscopy (SEM) and tested using energy dispersive X-ray spectroscopy (EDS). The plastic core showed a high concentration of carbon; moderate levels of bromine, oxygen, and antimony; and a trace of chlorine. The carbon was principally present as polypropylene. Some of the carbon, the bromine, the oxygen, the antimony, and the chlorine represented the flame retardant package.

The SEM examination confirmed the presence of corrosion debris, as a mud cracked morphology, consistent with the deposition of metallic corrosion product. Elemental analysis of these surface deposits showed high concentrations of copper and zinc, and a substantial increase in the level of oxygen. The presence of bromine was also indicated within the corrosion deposits.

The SEM examination of the plastic bases adjacent to the failed leads revealed needle-like particles, with relatively high concentrations of oxygen and bromine. The antimony content within the needles was not elevated relative to the base plastic. These results and the form of the particles indicated that the needles represented brominated flame retardant that had migrated from the base plastic onto the surface - a phenomenon referred to as bloom.

The needle-like particles observed on the failed connectors were identified as the brominated flame retardant, which had bloomed to the surface during the exposure testing. The brominated flame retardant acted as a corrosive agent in conjunction with the connector lead wires. Under conditions of weathering and exposure to moisture, it is possible that the brominated flame retardant produced degradation products that would be even more corrosive to the lead wires.

Source:The Madison Group

Visit MY BLOG http://polymerguru.blogspot.com

#plastics #failureanalysis #ftir #polymers #polypropylene #sem #fractography #electrical #corrosion

Today's KNOWLEDGE Share:How thin are carbon fibers?

Today's KNOWLEDGE Share:

How thin are carbon fibers? 

The answer is pretty simple, carbon fibers have a diameter between 5 and 10 micrometers. But that is kind of hard to visualize, right? So let's compare them to something all humans have (some more than others), hair!

Luckily, we have a very nice picture that makes it very easy to draw a comparison. Crazy to realize how carbon fibers are thin, right? In case you are wondering, a human hair is about 40-120 microns in diameter. 

But if you think that human hair is a very weak material, you are wrong! It consists almost entirely of a protein called Keratin which has about half of the ultimate tensile strength of steel (200 MPa). If you are tensile testing a single strand, you will only measure about 100 grams of force. 

Going back to carbon fibers, it is crucial for them to have small diameters, because it allows greater graphite content. This way, the probability of having a concentration of defects in the 3D structure is considerably reduced. The mechanical properties of these fibers are inversely proportional to their filament diameter. 

Source:mccomposites

Visit MY BLOG http://polymerguru.blogspot.com

#composites #fibers #carbonfiber #graphite #materialsscience