injection molded panel problem 1

[TyWitt]

Hello,
I am relatively new to injection molding and have been greeted with an issue regarding two lines that appear as cracks radiating out from holes molded into our facia panels where we mount switches. They appear to be where the plastic flows around the mounting hold feature in the mold and meet plastic from the other side. It does not appear to adversly affect the strength of the part but is still not desirable.
Do you have any suggestions on cause/corrective action? Thank you in advance for your assistance.

 

[dgowans]

I'm no processing expert, but here's my take:

You're correct in the fact that it is a knit line. There is no getting around this when molten plastic flows around a standing core, but it is not always visible. Knit lines can be problematic from a strength standpoint if they are in areas of interest with regard to part strength, but you say this isn't a concern.

As far as the aesthetics of the part go, you may be able to diminish the visibility of the knit line by gating the part closer to where the knit line forms, thereby increasing the temperature of the melt fronts when they rejoin. Not sure if this is an option without seeing the part.

You may also be able to accomplish the same thing by running the parts at a higher melt temperature. This may cause other problems from a processing standpoint. Getting a process engineer involved would be a good thing.......

 

[TyWitt]

Thanks for the response dgowans. I wondered about raising the temperature myself, but have not evaluated the impact yet. Unfortunately, I don't have a process engineer in house yet for our injection molding.

 

[dgowans]

Depending on what material you're using, and of course the volume you're buying, you may be able to get a tech support engineer from your material supplier in to help with this.

I've worked with people from GE Plastics who have been extremely knowledgeable and more than willing to help. I would be willing to bet that most material suppliers have similar support types.

 

[bvanhiel]

Has this problem always existed with this part? If so, what changed? If the problem has always existed and assuming that the process was optimized to minimize the appearance of knit lines, it may be inherent to the part design and material combination. If you've gotten good parts in the past, then something with the material or precessing has changed.

Ways to fix it: The advice given above is good. Invlolve a process engineer in tweaking the injection process and gate location. You could also remove the pins and drill the holes.

Ways to hide it: Color is an issue with the visibility of knit lines. Metallics are especially bad. Texturing the area can also help mask knit lines.

-b

 

[patprimmer]

Material selection, colour, surface finish, mould temperature, injection speed, mould vents, lubricants, injection pressure, hold up time and pressure all impact on weld or knit lines re appearance and or strength.

The only way to eliminate them entirely is to drill the holes instead of moulding them in.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metron]

Two things spring to mind with one for me.

(1) Does this mould have core pullers?

(2) Does this mould have DME heating?

In regard to the first, slow the core pullers down as low as possible, then increase the speed gradually to the point that the crazing around the inserts have disappeared. (Given of course core pullers are fitted.)

Second point, if DME is fitted, adjust the gate temps to allow maximum flow of material best suited to the mould design. You may have to watch shut-off times at the gates and adjust cooling within the mould. Good venting is critical also.

Moulds where metal inserts are fitted are always subject to many variables, the inserts themselves are not of the same temperature as the incoming plastic to start with, and flow around them is adversely affected.

Good luck with it.

 

[TyWitt]

Thanks to all for your comments. I now beleive that we can put together some experiments that will give us an opportunity to reduce or remove the knit lines around the component mount holes.

 

[smallplaticstelford]

rules in general as i know it,

1- get that material in faster, wham its in there.

2 - still got knit/weld line then slow speeds down try and fill slow around pin area, make shorts if poss without getting part stuck and follow with a high speed at the last few stages or viza versa depends where the pins are in relation to the gate/feed and biuld shot/weight back up.

3 - make sure the tool/die temp is at the correct data according to material manufactuer spec, they ussually about right. go higher temp to get toolmaker out of trouble as they made a stupid design yet again putting the gate way over there, lol,

think logically the material is not melting together around this area why? ussually meens its gone off by the time its got there.

cheers
linden