Greetings and questions

[110Ton]

Hello and greetings from somewhere in New York.

Have 25 years experience as a process and setup tech... started out as a machinist actually and when the machine shop got one of those "damn plastic machines" I took it upon myself to learn setup and process.


After that I worked for a couple of other dedicated molding companies... all over NY state.

Now here I am, with my own machine shop (two bridgeports, a CNC mill, a lathe and some other toys)... and within the last three months a 110 ton Milicron ACT Electric Molding Machine.

Been working with several shops around the area, taking on "problem child" jobs that require constant tending, too many inserts or epic post-mold nonsense.

Latest project involves a part molded out of Isoplast with long glass fiber. I mold the part, drop it into a bucket of chilled water and let it cool for a half hour then I measure the bore with a drop gauge.

If I have any blowout or blistering in that bore I reject the part. Has to be 100% smooth with no drag on the drop gauge.

Already I'm at 120 seconds cooling time and nearly 190 seconds overall on the cycle. Reject rate is over 50%.

I am making money on this job but I want to see if anyone out there has any insight on this.

My thought is this: My shot sits in the barrel quite a long time. If I set it to sit there screw forward for most of the cycle then recover during the last 20 seconds of the cooling time would that reduce the heat stored in my shot and possibly prevent post-mold blowout and blistering? I know it's possible to set up and run like that but I have no experience doing so.

Any other ideas?

[Jim]

Setting your rotate delay timer to hold the screw forward until the last 20 seconds or so of cooling time is a good idea. Long glass resins don't like to be overcooked. If you are not worried about surface appearance, I would also suggest running very low back pressure and minimum temps. needed. Keep the screw speed as slow as possible; that said, you will still probably have some issues with blistering and blowout, but perhaps your reject rate will come down. Hope this helps and good luck......

J.P.

[110Ton]

On my screen is "extrude delay" which I'm assuming will count down then start my shot recovery.

Finish is only important in that bore, the rest of the part gets finished with a powder coat.

Thanks for the tips Jim, I went the opposite direction with BP in an effort to "mix" the material I'll try backing it off.... same with screw speed.

Reason I got the job is I can hold it around 60% reject rate and still make a profit because it's just me, the woman-unit and an occasional visit from my nephew so I can keep labor-costs low to zero, even "after hours."

[Louis]

Thinking outside of the box here, but can you run that bore undersize and finish bore to size?

[Tom]

Because of the long cooling hold time; resin is being melted in the screw by heat conduction from the barrel. Significant amounts of uncompressed melt are then in the deep sections of the screw. This leads to the potential for air to be trapped in the screw during recovery. That trapped air can lead to surface defects. Increasing the back-pressure setting during recovery can help force the air out the throat rather than into the product. Unfortunately long glass resins usually are recommended to be run with low back-pressure to minimize damage to the glass. Still you may want to try playing with the back-pressure to see if it helps.

[110Ton]

Thinking outside of the box here, but can you run that bore undersize and finish bore to size?

Due to product requirements it has to have this "utterly smooth" finish inside that bore, something that cannot be achieved with machining. Believe me I've tried, other suppliers have tried and we can't make the customer happy that way.

Thanks to Wi Fi and Laptop I am at the molding machine now, I plan to run three batches of 20 parts over the next few hours with various backpressure and extrude delay settings

Batch 1 will be just delay.

Batch 2 will be extrude delay and less backpressure

Batch 3 will be extrude delay and even LESS backpressure.

I'll let everyone know around supper time.

[Tom]

I don't know where you are on back pressure right now, but i would add a higher back pressure trial.

[110Ton]

Right now I am playing with various extrude delay settings at my regular back-pressure setting. I am trying to time it so the screw stops just as the mold begins to open... that way the shot only sits through the 15 seconds of the mold open-eject-close-lock motion.

Haven't looked at the parts quality-wise yet I am still spinning the dials so to speak.

[Tom]

There are two schools of thought here:

Gas from degradation.
Gas from air entrainment in the screw.

Shorter hold times deals with degradation.
Higher back pressure helps to solve entrainment.

[Louis]

Heavily filled materials may require hotter mold temperatures to allow the resin to migrate to the surface.

[110Ton]

I have started measuring parts, and of the 20 I have selected I was able to pass 15.

To repeat this is just the extrude delay setting.

I am currently waiting for the machine to "settle" into it's new backpressure setting... I have reduced it 20%. If I get more fallout I will go in the other direction.

Be reporting back shortly.

[Louis]

OK, We need an iPHONE so that you can web cast in real time. We need pictures

[110Ton]

Maybe for the "HOW MANY INSERTS?!" job that's next on the list. 28 of them. Twenty two molded in brass things each one a different size and the rest are removable cores that have to be pulled out and put back in the mold. Oh and some a$$hat quoted a 45 second cycle time.

They had a robot doing the job at the previous shop but the robot demanded a raise then walked off the job.

But seriously, that's not a bad idea. Ustream.TV is an option it's a free streaming service. Register, set up a channel, plug in a webcam and away you go.


Anyway, I cranked the BP back up when I lost 15/20 parts due to "issues" and now I am getting very very good results.

Final conclusion: Extrude delay, and MORE BP plus an adjustment to screw speed to keep my cycle time.

Thank you for your help.

[Tom]

As one who often tries to provide help, it is gratifying to get feedback on how things worked out. More often than not we do not hear about the outcome.

Hope things continue to go well for you.

[110Ton]

Technology exists these days to get near instant answers to questions if you are willing to learn how to use technology.

My friend with the resin reclaim company is afraid of the internet by contrast. If he doesn't know the answer then there is no answer.

By contrast I sit in my shop (actually now I'm getting a cuppa at the local Dunkn') and find answers to questions that vex me. To me there is no "well since I don't know there is no answer."

Twenty min with Google and a quick signup and I'm up and running here... and I'm getting suggestions on how to improve my process.

Best part is the answer will be here for the next poor bastard that inherits this job.

(actually thinking about accidentally crushing the life out of the tool so they have to build a new tool using modern techniques instead of trying to make a 20 year old mold work but I can't bring myself to commit tooling homicide. )