Dimension Stability of PA66 Ultramid Natural

[cwlim85]

Hello there, anyone knows about the dimension stability of the above mentioned material?
I am facing some oversize issue when moulded part was kept for few months...

But there is one thing that i am still confuse... when masterbatch was added into PA66 Ultramid Natural material, the moulded part does not have any dimension issue. Can anyone advice on this?

[rickbatey]

Remember that colorant is a contaminant , just one that is tolerated by the polymer. The metal powder used to color the resin affect the shrinkage of the part. You will find that certain colors create even more shrinkage than others because of the dye used. I've seen much more shrinkage with say a yellow versus a blue. Not too sure as to the exact cause, but it was all about the color.
Rick.

[M&M]

If it is over a span of time I would point to moisture the part has absorbed, it will make the part grow a few percent.

[Louis]

For sure, the part will grow some with time depending on the amount of moisture it absorbs. In your case, the colorant seems to be affecting the rate at which the moisture is absorbed.

The other thought is that the molding process itself has changed to accommodate the virgin resin versus the colored resin, perhaps over packing the part. Since you mentioned that the change was noticed after a few months, that is probably not the issue.

If it is an assembly issue, place the parts in a desiccant oven to bring them back to size, but be aware that they will swell up again with time and humidity.

[cwlim85]

Hello Louis,

Still investigating if the colorant added affect on moisture absorbtion.
Besides desicant oven, is there any other method that we can use to maintain the dimension stability (assembly process at customer site)?

[cwlim85]

Hello Rickbatey,

Do you mean that if colorant / masterbatch was added into the resin, the injected part will encounter shrinkage?

[rickbatey]

I meant just what I said: the powdered metal used to dye the product can often affect final dimensions. I worked in textiles many years ago. We had a few molds that required a specific process depending on color ran. Some even had to be watered differently.
Even if the color changes the moisture absorption rate, your final dimensions will change. Changes to the process to mix the color/resin can affect the dimensions. When/how are you mixing the resin and color? That may be the root cause. Also the carrier resin in the color.
Rick.

[Louis]

Hello Louis,

Still investigating if the colorant added affect on moisture absorbtion.
Besides desicant oven, is there any other method that we can use to maintain the dimension stability (assembly process at customer site)?

You might try packing parts in moisture proof bags. Perhaps do an in-house test where you take parts from the same run fresh off the press and pack some while leaving others open to ambient air.

That should prove out the theory.

You never mentioned if the parts that had a master batch added were "aged" the same as those that were kept a few months that grew in size.

[Will Call]

I agree with Louis but you will also want to test impact or tensil properties. If I recall Dry PA66 can be brittle if some moisture isn't introduced post molding.

[Len]

I agree with Louis but you will also want to test impact or tensil properties. If I recall Dry PA66 can be brittle if some moisture isn't introduced post molding.

Agree w/ all above. The term is DAM Dry As Molded. There are special foil lined bags for storing parts just after molding to prevent moisture interaction. All part tested DAM will be brittle compared to parts allowed to gain moisture. DAM = lower elongation, higher tensile, lower impact. non-DAM = higher elongation, lower tensile and modulus, and better impact. HDT will be better w/ DAM parts. I love writing that... DAM!

Why not try a mineral filled PA66? These parts will have much better dimensional stability compared to unfilled parts. The triack is to use a treated filled, like Amino-silane treated kaolin. This treatment will preserve impact, in fact improve it long w/ HDT properties, while still having lower shinkage.

20 or 40% Amino treated kaolin would be my recommendation, if you are willing to use an alternate, filled PA66 material.


Len

[Louis]

Assemble them dry and in-size and let the ambient air do the rest.

Has anyone ever checked the assembled parts and failed them for size?

[Will Call]

I want to thorw in a question. If you pack them dry as molded and keep them dry doesn't the material become brittle? I've seen products molded and extruded in PA66 where the supplier poured a cup of water into the box. When I asked why he told me that it added moisture back to the material to keep them from breaking.

[Louis]

I want to thorw in a question. If you pack them dry as molded and keep them dry doesn't the material become brittle? I've seen products molded and extruded in PA66 where the supplier poured a cup of water into the box. When I asked why he told me that it added moisture back to the material to keep them from breaking.

Yes and No.

Dry parts are more brittle and moisture solves that at the expense of dimensions.
If the issue is inspecting parts for dimensions prior to use and then rejecting them, and not one of functionality when assembled, ship dry should work.
If it is a question of the parts not assembling as they are too big when reaching equilibrium, dry should work, UNLESS there is stress placed on the parts during assembly.
If the parts assemble OK when dry, they will reach equilibrium in use.

But none of our posts have addressed the original posters question, stability of PA66 with and without master batch. I should think that it would not vary once molded as the key ingredient is PA66, so moisture will be absorbed and the size will change on both mixes. The issue might be that the tool runs differently with and without the master batch, causing different pack rates that results in a size as molded change.

That change may be small enough to allow the parts to pass dimensional specifications as molded, but large enough to allow them to swell out of tolerance with age.

[rickbatey]

Guys, colorant is normally powdered metals. The different particle sizes and the metal used will change the moisture absorption rate. You can even has issues due to the carrier resin in the color can cause issues.
We still don't have any actual process data to look at: so we're all throwing darts in the dark.
Rick.