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Fargo Flash
3rd December 2013, 09:10 PM
Greetings,
I'm having difficulty with an injection mold. I'm using an Autojector HS-30 machine, 75 durometer PVC mold compound. The finished product is an overmold of a key. The mold is 1.25" x 1" x .25". (four molds per shot)

inj psi is 300
operating temp has been 325 F to 340 F, doesn't seem to matter.

I'm getting dull spots on the surface of the finished molds. Any ideas on how to get a nice clean finish?

Chris
3rd December 2013, 09:42 PM
IM a molder of rigid PVC daily.

What color ?

IS there moisture in it ? don't laugh i have to dry some colors from different manufacturers.

Injection speed ?

What is the compression ratio of screw, and what kind of check ring do you have ?

only 300 PSI of injection pressure, something does not seem right.

Fargo Flash
3rd December 2013, 10:05 PM
Black. Don't think there's moisture. When using other mold cavities, this problem does not exist. It only happens on my key overmold. More than 400psi causes excessive flashing and does not get rid of the dull area. The dull area occurs where the key head is in the mold.

I don't know the compression ratio and I have no idea what a check ring is.

I've tried heating the mold, no change. Heating the key, no change.

I may have to break down and get the cavities stippled to hide the problem.

Suhas
4th December 2013, 04:24 AM
Chris, you are absolutely right about colors and drying. Many people overlook that. Good point.
Fargo Flash, are you saying that the key mold is the only mold you are getting this issue on? When you say 'mold cavities' does that mean you are using the same mold base? with similar runner and gate designs?
Thanks,
Suhas

Fargo Flash
4th December 2013, 01:31 PM
Chris, you are absolutely right about colors and drying. Many people overlook that. Good point.
Fargo Flash, are you saying that the key mold is the only mold you are getting this issue on? When you say 'mold cavities' does that mean you are using the same mold base? with similar runner and gate designs?
Thanks,
Suhas

Yes, and we have five different key mold cavities. The problem only occurs when using the one. The difference between the mold cavities is the customer's emblem. Runner and gate designs are as close to identical as possible. We use a common base that allows for quick change between mold cavities.

Chris
4th December 2013, 01:34 PM
slower injection speed.

What have you done with the barrel temperatures ?

as crazy as it sounds, i have to run the barrel at or close to 400 degrees on my 250 ton toshiba.
Granted, i had this screw made specially for PVC.

so is the dull spot or blemish right around or past(coming from the gate) the emblem ?

Chris

Chris
4th December 2013, 01:38 PM
one more thing,
since it is interchangeable cavities, are you sure there is not a water leak ?

has this tooling ran before, or is it new ?

Fargo Flash
4th December 2013, 02:29 PM
I've played with the nozzle temp quite a bit. 320F to 400F. The dull area decreases a tad on the higher temps but not much.
No water leak. Our cavities are not water cooled.

The tooling is new.
I've had some trouble with flow lines, as you can see in this picture. But the flow lines have been minimized now by knocking down the sharp edges on the gates.

I blacked out the customer emblem in the pic so the big cheese won't get upset.

34

Chris
4th December 2013, 02:49 PM
I've played with the nozzle temp quite a bit. 320F to 400F. The dull area decreases a tad on the higher temps but not much.
No water leak. Our cavities are not water cooled.

The tooling is new.
I've had some trouble with flow lines, as you can see in this picture. But the flow lines have been minimized now by knocking down the sharp edges on the gates.

I blacked out the customer emblem in the pic so the big cheese won't get upset.

34

that is degraded material.

i would knock down the rest of the sharp edges in your runner as well, they look sharp !!

slow down your injection speed, and drop your heats more.

Running any back Pressure ?

Are you running any regrind ? If so, stop and get some virgin material, and try to dry some, I have to dry all of my black material here, at 130 F for 2-3 hours.Per Poly One specs.

Fargo Flash
4th December 2013, 03:02 PM
that's an old pic. The flow lines aren't so apparent anymore.

We don't use any regrind.

I'll give your suggestions a try. thanks for the help.

Chris
4th December 2013, 03:19 PM
Excellent, I'll be waiting for some feedback.

rickbatey
4th December 2013, 05:44 PM
Preheat the key blanks before over molding. The cold key blank is freezing the melt front upon contact. I'd get a hot plate and start testing for proper temp/time to stop defect. I've seen this before when over molding Stainless steel for griddle scrapers.
On other thought, cutting oil or rust preventative on the key blanks. Try cleaning with mold cleaner or alcohol, wait for them to dry, then insert for molding.
Rick.

Fargo Flash
4th December 2013, 08:00 PM
I tried that, Rick. It made no difference.

This morning, however, brought a change to the finished product. The only difference was using a different machine. We went back to the first machine we attempted this mold on. Previously, it brought the same results as the HS-30 (dull area at the key head). But now:

35

I can think of only one thing that may have affected the mold. The temperature has dropped significantly in the last couple of days and with it, the humidity. Maybe it is a water issue as Chris suggested. Is this possible? Could the plant humidity have this much effect on the mold?

Chris
4th December 2013, 08:24 PM
I tried that, Rick. It made no difference.

This morning, however, brought a change to the finished product. The only difference was using a different machine. We went back to the first machine we attempted this mold on. Previously, it brought the same results as the HS-30 (dull area at the key head). But now:

35

I can think of only one thing that may have affected the mold. The temperature has dropped significantly in the last couple of days and with it, the humidity. Maybe it is a water issue as Chris suggested. Is this possible? Could the plant humidity have this much effect on the mold?

not with the humidity with the mold, but with the material, my bet is that the material is absorbing water.

rickbatey
5th December 2013, 06:34 PM
What's the difference in the machines? Screw size, L/D ratio, injecting speeds or psi, compression ratios? Heck are they same clamp type? I have seen three molds that ran perfect in hydraulic clamps but had venting issues in toggle machines. Could you have improper clamping force ( one corner lower than others)?
Rick.

Fargo Flash
16th December 2013, 09:30 PM
We are thinking of buying/making a dryer for our mold material. As the cold temperatures and very low humidity continue, we are seeing near perfect finish on the molded product. Thanks for steering me in the right direction.

Chris
17th December 2013, 01:12 PM
We are thinking of buying/making a dryer for our mold material. As the cold temperatures and very low humidity continue, we are seeing near perfect finish on the molded product. Thanks for steering me in the right direction.
Not a problem :-)

Polyshot
17th December 2013, 02:17 PM
I believe it's definitely a moisture issue. Increasing or decreasing your drying time a bit depending on humidity can make the difference. It's also possible like was previously mentioned that dead spots may be occurring. You should make sure your alignment between the mold sprue and the machine nozzle is correct. Another option is the residence time may be too long, or the shot size may be too small for the machine. It may be necessary to keep the mold on a machine with less injection capacity.

Fargo Flash
18th December 2013, 03:53 PM
What is residence time?

Polyshot
18th December 2013, 05:51 PM
What is residence time?

The amount of time your material is in the system