flow marks

[MRFARRAN]

hello,
how can I get rid of the wave marks on the product surface?

photo is attached

thanks and best regards

[Joel JS]

Without knowing the specifics, you can try to increase fill speed, increase mold temp, increase back pressure ......

Joel

[rickbatey]

Those flow marks are typically called ‘tiger striping’ and the only way to get rid of them is to use a resin that is 5+ numbers or so higher in melt flow. I’ve never seen a process setting that could eliminate them short of MFI.
Rick

[MRFARRAN]

I am using polypropylene material with MFI = 20
mold is hot runner
somebody suggested to increase the diameter of gate
what do you think?

[MTUHusky]

Is your hand at the end of fill or at the beginning? It does look like flow hesitation (tiger striping) as Rick pointed out. The causes could be many. What is your shot size and transfer position set to? Do you know if your machine is pressure limiting? You could have your transfer position set too high which would force the machine to pack out the rest which can create marks like that. Your machine could also be set to limiting itself at a certain pressure or it does not have the pressure available to fill the mold under the temp settings you have (or you may need a higher fill pressure machine). Could also be a contaminant raising the viscosity, and there for the pressure to fill.

Fill speed would be the easiest to test. PP is a semi-crystalline material so shearing will have the greatest effect on viscosity. Next, I would raise my barrel temps and mold temps to the high side of the material manufacturers recommendations. The last thing you want to do is change steel without covering all spectrums of the process. With that said increasing the gate size will help with the flow.

Side question, you said you're running a hot runner system, did this problem pop up out of the blue or has it been there since the beginning? Those hot runner systems can get degraded material in the internals and cause all kinds of problems including flow restrictions.

[MRFARRAN]

thank you for your reply

Is your hand at the end of fill or at the beginning? at the end I does look like flow hesitation (tiger striping) as Rick pointed out. The causes could be many. What is your shot size and transfer position set to? I don,t think there is problem with shot size Do you know if your machine is pressure limiting? You could have your transfer position set too high which would force the machine to pack out the rest which can create marks like that. Your machine could also be set to limiting itself at a certain pressure or it does not have the pressure available to fill the mold under the temp settings you have (or you may need a higher fill pressure machine). Could also be a contaminant raising the viscosity, and there for the pressure to fill.

Fill speed would be the easiest to test. PP is a semi-crystalline material so shearing will have the greatest effect on viscosity. Next, I would raise my barrel temps and mold temps to the high side of the material manufacturers recommendations. The last thing you want to do is change steel without covering all spectrums of the process. With that said increasing the gate size will help with the flow.

Side question, you said you're running a hot runner system, did this problem pop up out of the blue or has it been there since the beginning? it has been since the beginning Those hot runner systems can get degraded material in the internals and cause all kinds of problems including flow restrictions.

[Joel JS]

Hey Rick,

Are you suggesting that he switch to resin with a MFI of 25 since he is using a 20 now?

I am curious as I have never heard or that solution before, can you expand a bit, thx!

Joel

[MTUHusky]

Hey Rick,

Are you suggesting that he switch to resin with a MFI of 25 since he is using a 20 now?

I am curious as I have never heard or that solution before, can you expand a bit, thx!

Joel

I think he is suggesting a faster flowing material to try an eliminate the flow hesitation marks.

I'm confused by the flow pattern, it looks like the flow pattern is moving away from the end of fill as if it is race tracking and then back flowing, but I dont shoot a lot of long then tubes like that so I'm not sure if that is normal.

[rickbatey]

I’ve seen this many times mainly with PP for Automotive uses. Oftentimes the resin is 20% talc filled as well. When the melt index drops this is more prominent and of course the customer complains about the parts even though that’s the resin they specified.
Yes I am suggesting that a 25-35 or higher melt flow resin will result in parts without the defect. We’d see it when the MFI fell towards the low end. Lots on the high end never showed the defect.
Rick.

[MRFARRAN]

we have used material with MFI 40. problem 100% solved

thank you all