Material Stock Temperature

[jnewmanco1]

How critical or important is the material's stock temperature when it goes into the feed throat of the molding machine?
For instance, if your loader on top of the injection unit is rather large, and a large amount of material is outside the drier for extended
amount of time, can it be a source of process inconsistencies?

In my case, I have a clear polycarbonate part with an unscrewing rack that seems to start out fine.
After running for awhile I start getting intermittent voids, always in the same, thicker area of the part. We have checked all waterflow, venting, material moisture, etc, and everything looks like it would be ok.
But the problem persists.

I'm looking at all possible causes for the issue and one thing that occurs is the almost 2 hours that the material is sitting in the loader and magnet box before actually being fed into the injection unit.
The material at the feed throat still passes the moisture analysis but they are actually cold at this point. Could this stock temperature reduction lead to something like the voids I am fighting?
Thanks.
James

[Joel JS]

Hi James,

I doubt it....but easy to verify, just drain the cold, and refill with hot mat'l, while the machine is running.

Perhaps try reducing rear barrel temps as the material's residence time and it's affect on viscosity/melt temp/shrinkage may be playing a role here.

What is the % of shot size in the barrel?

Joel

[chrisprocess]

Hi I've found material temp going in the feed throat does make a difference - specifically in terms of melt flow. And PC is certainly no exception (I'd say more critical). Whether it's contributing to your voids is hard to say for sure.

Easy test though, look at your injection pressure differences from the cold vs warm material. I'll bet you will see the cold uses more injection pressure.

Drying it sufficiently will not only get rid of moisture, it will heat up the inside of the pellet which will make achieving a homogeneous mix much easier. Also less shear involved this way.
If you must run it slightly cold, I'd try increasing the rear barrel temps to get as much heat to the system earlier on.

[jnewmanco1]

Thanks Joel. I'm using a bit over 55% of my .95 oz barrel.
Thank you too Chris. I will check this out next time the mold goes into the press.

[CP Yu]

Dried material stay in the hopper too long is not good and will affected the plasticising & molding stability.
Also material have high possibility absorbing moisture again.
If without autoloading system, use smaller hopper as possible.
If have autoloading system, reduce the suction time as possible.

[monu96]

Thanks for sharing the useful post.

[iamplwirtz]

I had a similar issue while running some Isoplast material a few years back on older Van Dorn machines. Especially in summer. It actually was the machines getting hot. I had to adjust the pressure as the day went on and the machines got hotter. I'd suggest that if the material is is cold at feed throat that you switch to a smaller loader if possible.

[rickbatey]

You can make a piece to go into the hopper out of sheet metal and cut the hopper volume down to reduce time in the hopper. I’ve done this many times over the years.
Now you may be seeing this due to cold pellets or gels in the melt. Watch your screw motor feed pressure as the job runs. I bet it starts to slowly rise as the job runs. Clear sign that the barrel temps are too low due to having to melt the resin.
Rick.