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| Author | Topic: Tensile Strength |
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shayne31 Senior Member Posts: 18 |
posted October 19, 2006 11:01 AM
  
Good Morning, I've recently become immersed in a raging debate as to what has the largest influence on Tensile Strength of ae extruded product (in this case PA6/6). It has been my understanding that material porosity, resulting from inadequate devolitization and moisture content, can have an inverse effect. I also beleive that filler integrity will influence this characteristic i.e. fiberglass strand length integrity. I was hoping someone within this forum could shed some light on the subject of Tensile Strength. Better yey, is anyone aware any empirical truths regarding mechanical profiles and their influence on Tensile Strength? Cheers ------------------ IP: Logged |
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Tom C Moderator Posts: 782 |
posted October 20, 2006 11:36 AM
Shayne, Your question could take a book length answer, but I'll try to keep it short. Lots of things affect strength of polymers. The basic strength of the polymer backbone, molecular weight, orientation and molecular entanglement will be the most influencial on the tensile strength of a particular polymer. Impurities, low molecular weight components, and voids will reduce the ability to evenly transfer stress throughout the part, thus reducing the ultimate tensile strength potential. Fillers with an aspect ratio, such as fiberglass, can increase the tensile strength of a polymer if the polymer can tranfer stress to the filler, and distribute that stress over a wider area. It is kind of like a tug of war, many hands (the polymer) hold onto the rope (the fiberglass) which tranfers the stress into the rope. No one pair of hands feels the total force on the rope, but we know the total force of all the hands can produce so much force that the rope can break. Since fiberglass has very strong tensile properties it will not break very easily and high tensile strength product will result. Problems come into play when the rope is short (broken fiberglass) because not many hands can hold the rope. Under high stress the hands may lose their grip before the rope breaks, lowering the total force or tensile strength. Also the participants on the rope may have weak arms with the unfortunate analogy being their arms are ripped off. Participants on slippery ground may lose their footing and slide away (low levels of entangelment). In the worse case senerios, no participatns show up or the rope seems lubricated. This results in very poor material properties. This is a lack a chemical couplings with the fiberglass. What rope to select, how to keep the rope long, how to get many strong players on the rope, are all tricks of the trade to produce high tensile property polymers. Please contact me personally if your company requires some help in this area. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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shayne31 Senior Member Posts: 18 |
posted October 27, 2006 11:15 AM
Thank you for your reply, Tom. We've recently discovered that not all of the polymer is being melted prior to glass introduction at the side feeder. As I understand, this will have a tremendous adverse effect on the tensile strength of any product. To use your previous analogy, would that result in several shorter, weaker ropes being pulled independently? ------------------ IP: Logged |
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Tom C Moderator Posts: 782 |
posted October 27, 2006 06:36 PM
Just makes a lot of short ropes. Short fiberglass is pulled from the polymer maxtrix more easily becasue fewer polymer molecules are holding onto the fiberglass. Those molecules are stressed to the breaking point well before the fiberglass will break. So the tensile strength reflects the tensile strength of the polymer rather than the fiberglass. When the fiberglass is long there is the potential for the polymer molecules to hold onto the fiberglass until the fiberglass's yield point, thus increassing the tensile strength. I have designed many twin screws of the fully intermeshing co-rotating style (W&P style) to help with this specific problem. Not only does the polymer need to be fully melted, but also heated to a temperatue where the polymer does not freeze up when the fiberglass is added. The screw should also employ distributive rather than dispersive mixing of the fiberglass in order obtain fiberglass wetting, but not breakage. I use and sell Polytech's TXS twin screw simulation program which calculates all of the above parameters for particular screw designs. Results are excellent. Recently 40% Nylon 6 glass filled was produced on a 47MM twin screw at 900RPM 1000 lbs/hr using a design developed on the software. A 70MM W&P SC making glass filled with similar issues as yours was improved from 800/hr and poor properties to 1800/hr and excellent properties with help of the software.
------------------ Tom Cunningham [This message has been edited by Tom C (edited October 27, 2006).] IP: Logged |
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shayne31 Senior Member Posts: 18 |
posted October 31, 2006 03:26 PM
Thanks again Tom. I've heard of the TXS twin screw simulation program and am quite interested in experimenting with it. Could you provide me with a quote for the software? ------------------ IP: Logged |
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Tom C Moderator Posts: 782 |
posted October 31, 2006 03:45 PM
Please email me so I have your address. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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