PMMA Products: Reducing Scrap Rate

[orion]

Hello All,

We are currently working on reduction of scrap rate on our PMMA products so I would ask you if you have PMMA process in your injection molding plant following questions:

what is drying equipment you are using in your plant, what is specification of the moisture entering injection molding process, any recommendation of drying process, what is scrap rate ( qty scrap/qty manufactured *100)?

Any recommendation on handling and using this material (PMMA) would be helpful.

What are the most frequent defects and causes of these defects?


Thank you.

[rickbatey]

Some lessons from an old who molded clear parts.

1- Dedicate the press to clear or white parts only, preferably PMMA, PA, or even PC resins.
2- On shut down, bank heats down but never turn them off. Especially on weekends.
3- Watch screw pressure when molding parts. You'd like to see ~1500-1800 psi on screw motor.
4- Use full internal taper nozzle bodies and tips. Anything else and you'll see cold slugs and burnt resin.
5- Your best water temp controllers on press. You'll never survive high temp range swings.
6- Drier must hold 2-3 hours of resin. Be careful to not degrade in drier.
7- Scrap rates will be 3-10% until everyone gets good at molding these parts. Then you may get below 3%.
8- Small hoppers on press but be careful to not convey resin too fast as poor piping plus high velocities equals more fines to end up as specks.
9- Thick sections will be milky out of the mold because they haven't completely cooled. You can drop them into a tote of clean water.
10- Keep them running via robots or some other means. Waiting on operators mean scrap.

Rick.

[Suhas]

Rick,
That is an impressive list! Thanks. May be it will go into the next edition of the book!
Thanks a ton!
Suhas

[orion]

Some lessons from an old who molded clear parts.

1- Dedicate the press to clear or white parts only, preferably PMMA, PA, or even PC resins.
2- On shut down, bank heats down but never turn them off. Especially on weekends.
3- Watch screw pressure when molding parts. You'd like to see ~1500-1800 psi on screw motor.
4- Use full internal taper nozzle bodies and tips. Anything else and you'll see cold slugs and burnt resin.
5- Your best water temp controllers on press. You'll never survive high temp range swings.
6- Drier must hold 2-3 hours of resin. Be careful to not degrade in drier.
7- Scrap rates will be 3-10% until everyone gets good at molding these parts. Then you may get below 3%.
8- Small hoppers on press but be careful to not convey resin too fast as poor piping plus high velocities equals more fines to end up as specks.
9- Thick sections will be milky out of the mold because they haven't completely cooled. You can drop them into a tote of clean water.
10- Keep them running via robots or some other means. Waiting on operators mean scrap.

Rick.

Dear Rick,

Thank you for thorough suggestions. It is more than I expected.

My comments and questions:

1. We do.
2. We shut down presses on Sundays. So we will try to work continuously 7 days and to see is there reduced scrap rate.
4. Could you please give more info on this point. I can not fully understand it?
5. We'll check.
6. We dry 4 hours. Sometimes more.
7. Our is over 20 %.
8. I checked. There are lot of fines in the raw material PMMA and in hoppers on the injection molding press!! We have long PMMA transportation hose. We have lot of defect - scratches , maybe it is coming from fines?
9. Could you please give more info on this point. I can not fully understand it?
10 We operate with robots.

Thanks again.

[3NGLENN1889]

Can you specify what kind of defects you're getting? I've never worked much with PMMA but knowing the defect will help zero in on the solution. G.P.

[JayDub]

"There are lot of fines in the raw material PMMA and in hoppers on the injection molding press!! We have long PMMA transportation hose. We have lot of defect - scratches , maybe it is coming from fines?"

Fines tend to burn and cause black specks, not scratches. Scratches come from part handling. Are your scratches always in the same location(s)? If so you can trace them to locations on the tool (which should be polished to a mirror finish) or the robot tooling. Check that you are not over-packing.

[rickbatey]

165
See pic from PPE illustrating a full internal taper nozzle tip.
Thick sections are areas +3 mm in thickness and out of the mold will be hazy, white or milky, unable to see through. That is due to the resin not being completely cooled.
So the scratches tend to come from three sources. 1- Cold slugs/gels/poorly melted pellets. As they are injected into the mold the scratch the mold surface as they are pushed across the polished surface. 2-Handling via hand, robot or how you deposit onto table or conveyor belts. 3-Parting line flash or sprue strings. Many times these molds will have small flash due to the heavy packing pressure. These flakes build up in the parting line and fall into the cavity or onto part causing scratches. Strings create scratches when getting pushed around the cavity by the melt front.
Rick.

[orion]

165
See pic from PPE illustrating a full internal taper nozzle tip.
Thick sections are areas +3 mm in thickness and out of the mold will be hazy, white or milky, unable to see through. That is due to the resin not being completely cooled.
So the scratches tend to come from three sources. 1- Cold slugs/gels/poorly melted pellets. As they are injected into the mold the scratch the mold surface as they are pushed across the polished surface. 2-Handling via hand, robot or how you deposit onto table or conveyor belts. 3-Parting line flash or sprue strings. Many times these molds will have small flash due to the heavy packing pressure. These flakes build up in the parting line and fall into the cavity or onto part causing scratches. Strings create scratches when getting pushed around the cavity by the melt front.
Rick.



Hello,

The defects are:

1. Scratches - are always at different locations, not coming from the issue with mold ( regarding defects coming from the robot no 2 from your list, i am sure we do not have it because of robot gripper handling, but for other causes 1 and 3 it is definitely possible, the question is how we can reduce this causes ?)

2. Degradation.

Orrion

[rickbatey]

You need to check actual screw rotation pressure on the press. Then you'll truly know if the barrel settings are high enough to eliminate tge gels and unmelted pellets. With the flakes you could rig up an air blast to clear the mold every cycle. Make absolutely sure the air is clean, dry, and filtered. You might even be better off with an ionizing air blast. You're sure the sprue bushing isn't stringing when mold opens?
Rick.