white dots in PPMA clear part

By Kammar
Published: February 18th, 2013

Running in dedicated machine and in clean room still we have white dots problem keep on coming, material used is PMMA plexiglas 8N and Altuglas V825T rejection rate due to white dot is nearly 8% are anybody facing same problem please guide what parameter to check to over come this defect.

I wonder if the burn marks

I wonder if the burn marks and the white dots could

be both due to a mite high injection speed?

A slightly slower shot could certainly stop the burn,

letting the gasses get away and the white dots may

be flow stress bubbles.

Not sure, as I have no experience on the material

but easily enough checked.

Bob Burns

Kammar, The 108B extruder psi

The 108B extruder psi looks good, but the 35% shot volume with the 50s cycle is going to make the final adjustments smaller. From this point you will need to finesse the process and keep data on changes and yields so you can decipher better or worse.
I'd like to see the back psi increased 1-2 B, letting the press settle for a couple hours and see what happens. If not worse and some better, then repeat until gone or worse.
As far as smoke line, I think that would be called a burn here. Try to improve all vents in the mold. The mold may not have enough vents or they are too shallow or narrow. Make sure the lip right at the part is draw polished so it is self cleaning, and exhausts into bigger channels until the dump to the atmosphere. Run clamp force as low as possible while getting good parts. Make sure to clean the mold once a shift as it runs. Also when running resins like this you need to make sure someone is waiting on press so that when the heats are up and the mold hot, it's started ASAP! You might even need to lower the fill rate and/or raise switch over on start up for the first 5-10 shots.

Hi, Rick raw material


raw material specified temperature is 240 to 260 degree celcius done 30-30 study found 256 degree celcius,shot percentage is 35%,cycle time 50secs.
when we increased barrel temperature the smoke line defect is increasing at start up.during plasticizing the presure is 108 bar.Back pressure is 10 bar.
Any thing can be done for smoke line.

Hi Rick What's the color for

Hi Rick

What's the color for your PMMA materials ?

We are Screw Barrel manufacturer, if you have any questions or requirements on screw barrel, please feel free to send me email to : export@jinyiscrew.com


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Kammar- you should be able to

Kammar- you should be able to completely eliminate the problem. How much did you increase the heats? What zone or zones did you turn up? What is the shot percentage of the press? Residence time? How long from start up before you see issue? Is this UV or FR treated resin? Can you read pressure on screw motor during plasticizing? What is your back pressure?
I think you will need more barrel heats, most likely from center of barrel forward to nozzle. Where are you versus technical sheet from resin supplier? What is the actual melt temp?
I'm sure we can all get together and make your scrap rare acceptable, just need more time. I've run clear PC before and we only fit down to about 4-5% scrap total. Clear PC is tough and many won't touch it if possible.

Hi, Rick Increased barrel


Increased barrel temperature and taken trial still problem exists but there is a reduction of rejection perccentage from 12% to 8%.Found some fine particles along with regular granules
can dust collectors help out in segregating this.Please guide which type of dust collectors best suited for this applications.
In processing also checked with slow refilling (less rpm) for better homogeneous mixing but couldnt able eliminate.
Can we eliminate 100% of this defect?

Kammar, Gels are basically

Gels are basically unmelted or semi-melted resin. With it being white specks, I'm pretty sure you're to the low end of the temp range for the resin. Could you cut into a part and try to remove white? If so, it might be a pellet when you get to it.
You don't run any other molds, resins or colors in this press fo you? Do you shut it down over the week end? Those could also be where the white is coming from. How do you load the resin into the press?

Are the material dried

Are the material dried advance? this is very important.Pls also try to heat up the material temperature.


Thanks rick I will get back

Thanks rick I will get back to you after trial.I didnt get you about, what exactly the gels.

I think your white dots is

I think your white dots is either gels or unmelted pellets from the injection unit. I would raise the barrel heats slightly on all or the front to middle zones.

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