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Physical properties differences of machined vs molded

Physical properties differences of machined vs molded

Physical properties differences of machined vs molded

We currently have valve seats machined from extruded Nylon 6/6 rod. We are investigating the possibility of having these molded. Our concern is the change of physical properties that may occur. Does anyone have information on this subject.


RE: Physical properties differences of machined vs molded

Changes do occur for several reasons, these being:-
  1) Rod is extruded, and a high molecular weight (high  viscosity) material must be used in the extrusion process so as to be able to hold the profile in the moulten state, whereas lower molecular weight (low viscosity) material is normaly used in injection moulding. High molecular weight gives improved toughness
  2) Rod is extruded in a thick section, when compared to typical injection mouldings, this leads to a very slow cooling and therefore solidifying (or freezing) rate. This causes a high level of crystalisation, with the formation of large crystals. Injection moulding grades generally have a neucleating agent to increase crystalisation in the much shorter freezing times typical of injection moulding, but this grows a very large number of very small crystals.
  3) The different cooling rates, and therefore crystalisation  and shrinkage from the skin to the core of rod, leads to internal stresses at a level not seen in typical injection mouldings. This is exasivated by differential moisture contents accross the rod as it absorbs moisture at its surface.
  4) When machineing the rod, you remove the surface layer, which is more amorphous, and has some molecular alignment in the direction fo flow.
  5) When machineing the rod, you create notches which act as stress risers, increasing the chance of breakage.
  6) It is rare that a machined prototype is the exact same design as the final moulded part, as the machinest generally takes a lot more time to perfect his work when making a permanent mould vs a temporary, experimental prototype. The moulded part design will normally pay attention to eliminating unnecessary material, so as to save material costs and improve cycle time, whereas, in a prototype from barstock, the machineist simply removes any material that is in the way, regards functionality of the part.

I know it is complicated, with a number of contradictions.

More details about the design and conditions of use would help in giveing a straight answer.

e-mail me direct with drawings, photo's etc if you like


RE: Physical properties differences of machined vs molded

The part is a simple washer type seat.
It measures .680 OD x .375 ID x .085 THK
in service it sees about 3000 psi on the face and about a 385 lb. load from the ball along the sealing surface (about .405 diameter).

Do you feel the compressive strentgh will change enough to alter the resultant coined surface created by contact of the ball to the seat during pressurization?

Thanks for the help

RE: Physical properties differences of machined vs molded

I expect the compressive strength will probably increase slightly, but tensile will most probably decrease slightly, and elongation at break will most certainly decrease noticably if not dramaticaly.

Make sure the nylon is conditioned to the environment in which it will be used. Nylon is hydroscopic, and does take a long time to reach equilibrium.

If nylon is conditioned in 100% RH, it can loose up to 70% of it's flex mod compared to dry as moulded nylon.  

I expect that the compressive strength will also be dramatically effected.

I can try to find some data, but it has been buried for quite a few years.


RE: Physical properties differences of machined vs molded

do you suspect any changes in coefficients of friction.
We have concerns with the actuation forces resultant of the valve ball loaded on the nylon seat when pressurized.
It sounds like your telling me these seats in a molded condition may be relatively harder or more brittle.
Do you think if they are too brittle and do not seal on the ball, the nylon could be conditioned in some way?

RE: Physical properties differences of machined vs molded

They will be relatively harder and more brittle, but not to a huge degree. Also, the tougher extruded material will have lots of notches from the machining, which will counteract the inherent extra toughness.

Moisture conditioning to the environment is essential if you want any indication of real world results from your tests.

Unfilled nylon 6.6 changes up to 300% in some properties when fully conditioned vs dry as moulded.

Also, from memory, the anealing temperature for nylon 6.6 dry is about 180 deg C, but imersed in water, the anealing temp is below 70 deg C, so if accelerating moisture conditioning by heating in water, be carefull to consider the effects of anealing as well.

Moisture is rapidly attracted to the surface of nylon, as it is quite hydroscopic, but the diffusion rate through the nylon into the core of the moulding is very slow, and at room temperature, takes many monthe to approach equilibrium.

Equilibrium at 50% RH is about 3% moisture content, and this causes a linear swelling of about 1%

All this is from memory, so the figures might not be dead accurate, but they will be in the ballpark.

I do have a lot of time, RH, Temperature, section thickness related graphs, but it is quite a bit of work to dig them out.


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