Inconsistent Toughness of Molded Nylon 66 Parts 2

If the pellets are too wet you will see a not so nice surface finish it looks like delamination thin layers of plastic. Near the gate is the best location to look for differences. Sometimes you can peel off the layers. You could do a mfi to check if the polymer is degraded. Regrind could also be a possible cause of mechanical properties detoriation.

We have seen some 'delamination' of some parts, but not within this current issue.

What is an mfi?

melt flow index you will need lab equipment to do it.

jelmerra, thanks for the help and the definition.

I'll check with our usual test lab to see if they can do it.

As you seem to realise, the impact resistance of nylons is heavily depedent on the moisture content. Freshly molded parts have modulus of 3GPa and poor impact resistance due to low water content. The same parts left in normal room conditions will have a modulus around 1.5GPa (i.e. halved) and very good impact resistance. I am not sure what the boiling procedure is or why it's there but I would be sure to check the moisture content of the part being shipped. I heard of a case where it was important to add one or two drops of water to the part for shipping, in the packaging. When the part was molded it was brittle and useless but was perfect by the time it reached the customer as the water had been adsorbed and acted like a plasticiser.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

Also check for consistancy of dried part weights. Wide variation indicates poor processing and possible moisture when molding. Fancier lab tells can actually measure the molecular weight distribution and the amount of degradation. It also could be the molder, in the Far East, is buying off grade or reprocessed material. It is cheaper for them. When in doubt and wanting consistant high quality, buy from a good molder in the US.

Are you suggesting that only the USA can produce good quality parts?

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

By far the most common cause of brittle nylon parts is lack of moisture conditioning of the part.

Correct conditioning can increase impact strength by 300% vs dry as moulded on some grades.

A quick and dirty test for moisture conditioning is to rattle the parts together. If they give a sharp clinky or rattly sound they are dry. If they give a dull thud, they are conditioned.

Other less common reasons for brittle mouldings that should still be considered are:-

Degraded material by moulding with wet graduals.

Degraded material by moulding at to high a melt temperature.

Degraded material by exposing the material to heat for too long either by excessive residence time in the barrel or by drying to long at to high a temperature.

Use of incorrect raw material grade.

Insufficient pack of the mould.

Voids in the moulding.

If the problem is inconsistent moisture conditioning it may be caused by several reasons, which are:-

Dropping parts into water at the machine. The dwell time in the water might vary and the temperature will vary.

Depending on moisture uptake during the shipping time. The parts near the container door will absorb all the moisture coming into the container, thereby acting as a desiccant for the mouldings further from the door.

Immersing in vats of boiling water. Unless the water is agitated, it might have hot and cold spots in the vat. Also boiling water degrades the nylon as it conditions it and on thick mouldings, it swells the surface a lot before the core swells at all, thus causing internal stress. The longer the time taken to moisture condition nylon, the better the final result. It is much better to condition for a longer time at less than 60 deg C.

If the parts are being shipped from China to USA, the time taken should allow for sufficient moisture conditioning during shipping time, so long as the parts are thin and so long as they are in 100% relative humidity (by packing in plastic bags with water added at 3% of weight of nylon), and so long as the temperature averages at least 20 deg C.

There are known relationships between time, temperature, relative humidity and section thickness. I have graphs I can refer to if you supply sufficient detail.

One basic QC test is to weigh the parts. Light parts are not packed or have voids, or a material with a lower SG was used (ie less glass in glass filled grades).

You can measure moisture content by weighing, drying then weighing the parts again. Repeat the process until you get no further weight reduction in weight. For thick parts this can take a considerable time. A dehumidifier can accelerate the process, a vacuum oven can accelerate it furthur and achieve a slightly more accurate result.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

I noticed you say that some batches are brittle and others are OK.

Nylon absorbs moisture from the ambient atmosphere at room temperature. The process is slow, but the nylon eventually reaches its balanced moisture content.

If the older batches are OK but the very new ones are brittle, this might be because the samples have moisture conditioned themselves in storage.

Nylon normally does not need full and even conditioning to overcome brittleness. A conditioned skin of nylon helps a lot as it stops cracks propagating at the surface.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

You can check here for pages that describe how to track down troubles molding nylon and other thermoplastics.



There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell