Inconsistent Shrinkage Rates

Hey all,
I’m working on a project that has all of the engineers in my department stumped and we are all searching for answers.
We have a part that we are molding in Nylon and are getting varying shrinkage rates, enough that we are now looking at making a set of core/cavity inserts for our mold for the 3rd time.

The Nylon we are using is a pure/prime/virgin batch that has a listed shrinkage rate of .020” per inch. I designed our first iteration using that in mind and the parts came out way too big. I measured all the parts and found that they actually had a shrinkage of around .009”/inch.
We turned around and remade the mold again using this new shrinkage rate, however now the parts are showing only a .005”/in shrinkage.
This most recent batch used the exact same batch of material, use the same machine settings/parameters in terms of cycle time, temp, etc. We are scratching our heads and grasping at straws trying to determine the source for this variance in shrinkage rates.

This uses Nylene 4114. We dried it over a 24 hour period prior to each use.

Has anybody run into something similar or does anyone have a potential cause?

I appreciate any of your support,
Thanks

I'm not very familiar with nylon resin and shrink rate, but I've come across the need for shrink studies on some of my molded parts. Timed intervals of measurement to examine shrink, and knowing when they've stopped moving. Easy enough experiment for someone to run in the background.

Not too much experience with Nylon but it is a hygroscopic material - wont this perhaps cause some of your issues? The longer the part is out the mould the more moisture it absorbs. When we pack our nylon parts we add some water to the bag so that it anneals.

Nylon is a semi-crystalline material so you can expect anisotropic shrink. Is this a filled grade? If not, a shrink rate of only .005 in/in consistently on all dimensions suggests that the part is not crystallizing at all, which may mean your mold temp is much too low. Also watch your residence time - thermal degradation results in shorter chain lengths and less crystallinity.

how is your product wall thickness, if they are uniform, different part wall thickness has different shrinkage. The best way is to make it uniform and you can get desired part shrinkage. if it cannot be, make steel safe will be the best way to resolve it. do you run moldflow to evaluate the part defects. if the tool has enough cooling to make part. We can provide engineering service to help you resolve it, please contact me if you would like to .

Any luck on this?...... sounds like it's moisture related if you are inconsistent, as another user pointed to. Try to put parts in 3 different "Lots" ... First lot put parts in bags sealed tight, and second set, parts add 1/2 cup water and also put in bags, and for last lot just keep them out in the atmosphere. Check these at 10 min, 1 hour, 4 hours, 24 hours, 48 hours, and see your results with different amount of moisture coming back into the parts....... 2nd step, may need to anneal parts.... (but, is your mold temp warm enough as JayDub stated?)...... (So, annealing is heating back up, and cooling down, can find better definition and specifics on internet for your material).... and with annealing, parts will fully crystallize and shrink and stay more uniform from there.

Good luck
Joel

Hi
Besides all suggestions that partners already give you, I will recommend to check mold temperature during your trials, not just the reading in the MTC display, also check real temperature in cavity and cores, to ensure you are using same conditions on cooling temperature every trial , other wise you could get different results,
try to use the correct hopper/ dryer size, dry the amount of resin that you will use according to shot size its important .

we had a similar issue, not exactly same, but some issue with PA resin, we has been receiving the resin in Gaylord of 1500lbs, material handler open the Gaylord and full the dryer of resin ,, the parts appearance has variations between each running also some dimensions, we request to resin supplier to change the resin packing from Gaylord of 1500lbs to a 25kg pre dryed bags, after that we control the hopper size and our issues was solved, by the way you can find the application "calculation tools" make by Suhas in playstore, you can download for free and you can make different calculations (clamping force, hopper dryer size, shot size estimator, water flow etc.. its nice tool
regards