1/2" thick Fortron plate

[carlbud]

We have built a cavity to mold a 6" X 6" X 1/2 thick plate of Fortron with 40% glass (similar to ryton R4. We are having trouble with some slight porosity in the center of the parts. We then made the cavity smaller in steps to the point that the cavity is now 3" X 3". The runners are 5/16 dia.and the gate is a full length fan gate along one edge and is .175 thick where it enters the cavity. We have added overflow tabs around the part to get rid of some of the cold material at the end of the part. We have pretty much eliminated the trapped gas voids that we had in the beginning but we still have slight porosity in the center of the .500 thickness. We have tried a variety of injection speeds and pressures along with hold pressures and times. We are running it in a 230 ton press. Our customer wants to use these plates to machine small connectors and the porosity shows up when they cut them. In sampling we skim cut the plates .030 at a pass and the first .170 of depth looks great. When we get past that the material show areas of a slightly different color and some porosity.
In some discussions with the material supplier he mentioned shrinkage voids but I have not been able to get in touch with him again to discuss further. I am thinking that this is what we are struggling with since the porosity seems to be only in the center of the plate. It makes sense that the material will cool faster on the outer surfaces and thus leave nowhere for the hotter material to shrink as it cools without creating voids. We have never molded anything this thick so we have not experienced this before. Also the fact that the customer cuts it up and needs the material to be solid all the way through is a unique situation.
Does anyone have any general advice on how we might improve this situation through molding parameters or tool modification?

Thanks,
Carl

 

[btrueblood]

From this reference,

http://www.cpchem.com/bl/rytonpps/en-us/Documents/RytonDesignGuide.pdf

under the "wall thickness" header comes this quote:

"In very thick sections, the surface stops shrinking as it solidifies while the interior bulk continues cooling and shrinking. The
difference in shrinkage can pull the material apart leaving sink marks and shrink voids which reduce overall strength. Maximum
allowable wall thickness depends on the Ryton® PPS compound selected. Most Ryton® PPS parts should not have wall thickness
greater than 0.375 in. (9.52 mm)."

You might try a mineral reinforced material, as the same reference shows these materials to have lower shrinkage.

 

[MikeHalloran]

Mold it 6" x 6" x 1-1/8", and split the plaques with a saw.


Mike Halloran
Pembroke Pines, FL, USA

 

[dgallup]

I think anything that thick would have giant voids.

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[MikeHalloran]

Yes, it would have huge voids. The trick is putting them where you can cut them out as waste.

Delrin can be ordered in fairly big slabs. Because it's a condensation polymer, all the slabs have a central porous plane. So it you need a part that's nonporous, and 1/2" thick, you buy a slab that's ~1-1/4" thick, and you slice off two good 1/2" faces with a bandsaw, leaving the central porous plane as scrap.

This stuff will have voids from shrinkage, not condensation, but the same tactic may work. The trick is to adjust the molding process to keep the majority of the voids in the central plane of the slab.


Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

Many years ago (20) we inherited tooling which made, essentially, "slabs" of PPS. They are up to 1/2" thick and vary in L x W from about 6" long to 2.5" wide. They are vanes for chemical pumps.

When we got the tooling, the customer said he was having major breakage problems - not had one since. I assumed it was due to incorrect processing (i.e. tool too cold). These parts are not machined.

The parts have a gate at towards the end edge of the long side of approx 8mm x 4mm. Runners 8mm round. Tool temp is around 150C (I know the material suppliers say 135C min, but I like to use towards top end values), injection speed slowish, long hold time. And: always dry the material to ensure any surface moisture is rid of. +150C if your dryers will go that high (ours do not, but they are high vacuum so we use the max of 120C)

Also as above posts - likely to be unavoidable shrinkage voids

Stupid question, but why don't you just mould the connectors? Must be magnitudes "in position cost" cheaper.

Cheers

H



www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[carlbud]

Thanks guys for the responses. You confirm my thoughts that these voids are probably unavoidable. They are very small so what we are getting is actually quite good. My customer machines a lot of small quantities of parts so a mold is typically not justifiable if they can get material and machine them. They can purchase ryton plates and for years that is what they have been using but there is a mil spec on the material callout for their customer and ryton is no longer certified to the mil spec but Fortron is.
We are using a high mold temp already (350F) and we are drying the material well but only at 250F.

Thanks for the help.
Carl