Just like it says. I cannot understand why injecting the plastic would make it that much stronger:
Injected tensile strength = 20 KSI
https://www.protolabs.com/media/1011443/hylon-66-n1033hl.pdf
Machined tensile strength = 12 KSI
https://www.protolabs.com/media/1014751/nylon66_30gf_natural.pdf
Both filled with roughly the same proportion. Both glass fiber reinforced (as opposed to filled)
Any thoughts?
I thought this thread was going to be about my dad's old .22... The nylon 66.
I have a couple thoughts on this, but I haven't gone back to confirm or do any calculations.
1) The machined material is extruded so polymer chains and glass fibers are aligned differently than injection molded. Think fiberglass cloth vs fiberglass mat.
2) Probably a process result, but the machined material is approx 3% lower density, so less material per volume.
3) Glass content is about 10% different. (33% vs 30%).
4) Molded parts will have a "skin" over the surface and the glass will not be at the surface. Machines components expose the glass during machining and can alter friction and strength.
On edit: it might be useful to take a look at unfilled versions to see how much the glass adds, to estimate the effect on adding glass and what replacing 3% of the plastic with glass fiber might do.
The unfilled, injected molded stuff is back at 12 KSI, where the machined GF stuff is
In reply to No Time :
I thought about your #1 from forging/casting, but I don't think it's possible to reduce that much strength, plus, even when injected, there is no real way to align the fibers.
On your #3, it's again possible, but it's 10% reduction in GF for a near 50% reduction in strength.
In reply to tuna55 :
I wouldn't be surprised if that 10% reduction is in fact responsible for most of that 50% reduction in strength.
When you make matrix calculations for composites, you really see the HUGE impact that a little extra matrix (epoxy, poly or any other plastic) has on the strength and rigidity of a composite part. And real life testing backs this up.
Since this is a composite part in effect, I think the same applies here.
I'll quote myself from another thread.
"The second problems comes from the construction of the parts themselves. A lot of pre-made composite parts under-perform constantly in tests because of an excess of adhesives. Basically, it means, that the builders use too much epoxy between the fabric. And the strength goes down really quickly when you have too much epoxy. So a lot of parts are closer to a carbon reinforced plastic parts than an actual CF composite construction and will be outperformed by a basic 6061-T6 part. IMHO, if you want a CF structure that will perform, you need some pre-impregnated fabric laid down by pros and then cured in an autoclave. I have never seen good results without that. "
Could also be due to different filler orientation and/or filler fiber lengths used.
Only about a 2-3 % gain for a 10% gain in GF from this other material by their datasheets. I would not directly compare them, but within their own material datasheets you can confirm this at a variety of places.
The outer surface of any object works the hardest, no? Any fibers which crossed that boundary before machining are truncated. So we're more or less guaranteed that the shortest fibers in the matrix will be at the surface. Which I guess on the face would be more about rigidity than strength. OTOH, depending on the length of the fibers, a variable amount of the total cross section will be affected by this fiber truncation (i.e. the longer the fibers were initially, the further through the part they'll reach on average, and thus the larger a proportion of the cross section will have some of its fibers cut at the outline of the part). I know nothing about how well bonded the material is to the glass and whether snipping a fiber in half actually has any meaningful effect on strength.
Or there's just something they're not telling you about the matrix that causes the difference. The Wikipedia page on 6/6 Nylon suggests that there is a lot of process to production, and I imagine the variables affect outcomes.
That's my undereducated contribution; no idea whether it's near the mark.
As mentioned above, there is an influence from the molding process aligning the fibers as well as the outer skin on the part. Both of these can be leveraged to add strength to a molded part that wouldn't be there in a machined version.
Wouldn't the fibers be severed once machined, thereby reducing the length and therefore strength?
Given my limited plastics knowledge, one coop short of an AAS, I suspect the fiber orientation is much better when injected under how ever many tons of pressure needed to fill the mold, which is a product calculation of the part surface area.
You'll never get the same results of a wet layup due to the lack of pressure on the part as in the injected part.
Hey this was my area of specialization in my mechanical engineering degree program.
Fiber reinforcement in plastics are usually critically influenced by the interface of the fibers and matrix of plastic. By machining the surface there is some unknown variance introduced due to the cutting process. We can't know the extent of disturbing any of the fiber/matrix connection within the remaining material. If I recall correctly the reduced strength is an engineering standard to take into account the variability of strength post machining. Still I do recall confirming the variance in tensile tests and shear testing.
There are some fancy reinforcement media that doesn't have this drop off but I don't recall the specific materials.
Also as someone pointed out earlier there is a lot of load bearing performed by an object's surface. Causing millions of micro tears between fibers and matrix at the machining interface is essentially creating millions of stress concentration failure points.
The only solution we studied was the application of a coating or some how resealing the machined surface.
At the end of the day, the only way I was taught to know a composite will work is through FEA and real part testing.
Lots of good reasons from a mechanical and the rheological perspective. Filled polymers are inherently heterogenous metals which are always homogeneous. Typically injected molded filled polymers will have 4-6 flow layers that are easily viewed under a microscope.
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