Ultramid A3K splay issues

By schiefelbeinr
Published: February 12th, 2013

We are using Ultramid A3K to mold reflectors that get vacuum metalized.  Highly cosmetic parts.  First, is this a bad choice of resin for our application?  Second, every mold and press we run this resin on has a high scrap rate due to splay.  We have checked our drying system and found at least -40 dew point resin in and out of the hopper.  We are not exceeding throughput of the hopper. 
When processing we have been everywhere from extreme low to high as far as recommended processing conditions.  Any help with this splay issue would be greatly appreciated.

The 35%job will need the heat

The 35%job will need the heat profile adjusted, the other should be good. I'm not understanding why your back pressure is so low?! I would expect you would be around 100 psi to 125 or so.
Can you measure the extruder run psi? If so, see what it is running. If my old mind is still sharp enough, I would expect to see your actual around 1000 psi range. Lower and you've too much heat in barrel; Higher- too hot.
So again, where is the splay? At the gate, opposite side at end of fill, between knit lines? Multiple gates? Size and shape of gate? Land length? Cold runner dimensions?
What about nozzle body, tip style, and oriface?
Rick.

I believe on a couple of

I believe on a couple of these molds there are sharp edges. PID reset on nozzle. Barrel temps at 520, nozzle at 505 seems to give us a bit less splay. Using 35% of barrel on one job and 65% on another. Back pressure is 75 psi and screw speed low enough to get back 3-5 seconds before cycle ends. Residence time is roughly 5 minutes. Melt temp is 535. Resin is dried by a Novatec central drying and conveying system.

Why do you have a barrier

Why do you have a barrier screw with mixing elements in a machine that is still "in the wrapper"? I'm not fond of special screws, nozzles, and mixing elements; I feel they require you to give up too much ( like flexibility ) when used, not to mention cost.
Still you didn't state where the splay is ( same spot or does it move ); or type of drier, nozzle body and tip style; nozzle tip oriface and mold oriface. Melt temp and barrel settings would be nice.
Is this resin FR or UV stabilized? Could be shear from screw causing issue. What is speed and stroke of decompression/ suck back? How long ( residence time ) is resin in barrel? Back psi and charge speed? Shot size versus screw size ( residence time ) ?
Does the r melt smoke heavy or pop and crack when purged from nozzle? I've seen old guys test pellet moisture with glass slides and a hot plate. Heat slides, add resin pellets, and mash between slides: huge bubbles could be moisture. How is resin loaded into drier?
Rick.

Why do you have a barrier

Why do you have a barrier screw with mixing elements in a machine that is still "in the wrapper"? I'm not fond of special screws, nozzles, and mixing elements; I feel they require you to give up too much ( like flexibility ) when used, not to mention cost.
Still you didn't state where the splay is ( same spot or does it move ); or type of drier, nozzle body and tip style; nozzle tip oriface and mold oriface. Melt temp and barrel settings would be nice.
Is this resin FR or UV stabilized? Could be shear from screw causing issue. What is speed and stroke of decompression/ suck back? How long ( residence time ) is resin in barrel? Back psi and charge speed? Shot size versus screw size ( residence time ) ?
Does the r melt smoke heavy or pop and crack when purged from nozzle? I've seen old guys test pellet moisture with glass slides and a hot plate. Heat slides, add resin pellets, and mash between slides: huge bubbles could be moisture. How is resin loaded into drier?
Rick.

We have a Xaloy barrier screw

We have a Xaloy barrier screw with a Pulsar mixing head.
The screw tip is a 3 piece free flow. The press is only 2
months old so screw wear is not a problem. We have to use
decompression in order to hold a consistent cushion.
The molds are cold sprue not hot runners. Unfortunately,
we do not have a moisture analyzer, but as I mentioned our
drying system at least -40 dew point in and out of the
hopper. I don't believe it is all moisture related as we
are not exceeding througput. Resin is being consumed in 6-7
hours. Is this enough time to ensure proper drying?

Sorry to hear you are having

Sorry to hear you are having difficulties. A3K must first be dried down to below .05% measured moisture.
.If splay still exists you can try a hump back heat profile
with it hotter in the middle zone and cooler in front and back.
In some extreme demanding applications such as you have
we have built barrier screws from Servtech
and been 100% succesful with this as melt quality is the real
issue.
Can you tell me about your screw designs. My email is
harold.colwell@basf.com. We can start a dialogue and maybe
connect live.

Where is the splay exactly?

Where is the splay exactly? What kind of nozzle bodies and tips are on the machines? Correct nozzle orifice being used? Nozzle heat good? PID of nozzle not causing excessive temp swings?
Is this a hot runner mold? Valve gated? Mold heats not running high or swinging low as press runs?
Is this resin UV or FR treated? Mold running hot enough?
How much decompression stroke and speed are you using? How long between recovery finishes and the mold opens?
Sorry for all the questions, but all these points are relevant to your issue. Last thought, any sharp edges in mold to cause shear induced splay?
Rick.

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