Help please, Milky Bloom/stain

Hi All,

I am hoping somebody can help me to determine the root cause of the Milky bloom issue we experience in one of our new products and we are running out of options/ideas to explore.

Below is some information regarding the part/mold/material:

• The Part is Fender flare(X4)
• We can see this stain/bloom in all 4 tools
• Surface finish of all tools with naked eye looks fine
• Material: PP+EP-TD20 (with UV additive for automotive exterior application)
• MFI: 31
• 3 hot tips with Sequential gating
• wall thickness confirmed to the Cad OK
• tool made as per Moldflow analysis
• part has a grain with 30micron depth
• Trialled with different melt temperature, injection speed, pressure, holding time, cooling time and combination of them: similar result
• (it seems long holding and cooling time disguised it, but the bloom is still there)
• Trialled with dried material: no improvement
• Trialled with similar grade PP with different mineral and UV content, the problem is still there
• Trialled with pure PP copolymer with Black masterbatch, no improvement
• Trialled in a different machine (same result )
• Trial with warm Tool, cold tool didn't help
• Short-run experiment shows material flows uniformly.
• Different sequential gate timing did not help

Note:
1- The locations of the bloom changes with different setups and trials
2-This milky bloom is not visible in the covered space light, only visible on sunlight in a certain direction which picked up by our customer. (See attached photo please, sorry I can't share the whole part photos as the product has not launched officially yet)

Do you have any idea what could be wrong with our process/tooling/material?

Thanks,
Saeed

I looks like pressure marks. please try to switch off holding pressure and inject to 90% of filling. If the issue will be still present, please delay the gate close to defect (only for trial)

please post also Picture of oposite side of part. Best one photo with A and B side

Thanks Pilot, we have already done them. no luck :(
I attached a photo of B-Surface.

It looks like you are shooting into sub-gated cold runner from valve gate. I think the gates are too small and shearing the melt to the point that the talc is blooming onto the surface. Sequencing valve gates with cold runner requires pin point timing as resin pushed into the cold runner may freeze or begin to causing sluggish reaction to the valve gate trying to get the resin flowing.
I think you need larger gates and I’d even try shooting mold with all gates open start to finish to see how the part looks. Are you closing the first gate after the 2nd or 3rd one opens? If not could be localized over packing as well as excessive shear heating.
Rick.

Hi,
Ricks idea is realy good. What about the temperture of melt. please check, if the hot runner heating is OK. (measure temp on the tip and compare Amps with similar heaters. The materiál can be too hot. I see a lot of tools with wromg thermocouples. Please check also setting of right type of thc on hot runner controlers (J,K,L,PT100 etc)

Thanks Rick,
We have massive cross-sectional area through large underhung tab gates. No melt shear through these gates. We’ve tried all the gate sequencing options imaginable as well. We’ve also looked at the effect of cavity pressure (localized overpacking) and tried many varients of pressure /time combinations as well. No Luck
I attached a photo of the gate.

Hi Saeed, could you please post a new (next) photo of bloom area and Picture of injection diagram (injection pressure + screw speed/ position)? thanks

What is the width and depth of the tab gate? What is the part thickness at the gate? I think your gates should be much wider and the depth adjusted to create a better melt flow in the cavity. I also would add a cross cut rib to the cold runner, done with a ball end mill, to stop melt front tumble and acceleration when filling the cavity. We added it to a tool running Xenoy once and a Delrin job again for poor surface right as the melt started to fill the cavity.
What is the residence time and shot utilization? Might need some seriously staggered barrel heats to make the parts acceptable.

Saeed,

I agree with Rick on verifying the barrel utilization. Make sure you don't have too long of a residence time with the material.

Have you tried running different tool temperatures for A side and B Side. For example when I worked in Automotive we would run the A side (A surface of part) hot a hotter temperature to bring the scratch and mar package to the surface. Sometimes these values may be 20 to 30 Deg F hotter on the A side versus the B side or vice versus depending on where the "A" surface of the part was located. If you have the capability try running one side hotter than the other for a trial and then reverse that and see if doing either of those makes a difference. Keep us updated!

-Robert

Hey Rick - trying to understand your comment on adding a cross cut rib to a cold runner to help melt front tumble/acceleration. Have a colored delrin product that we are struggling to get a good surface finish on (variation in gloss, flow lines, wrinkles, as well as coloring issues).