trouble with overmold cable with PBT

[nemanja]

Hello all,

I start working with overmolding cables for automotive industry. Material that i use i PBT, and i have big problem with "freazeing materilal" in deep cavity. My main problem is that i have wall thicnes of few milimeters and when materilal come to bottom of cavity is allredy cold, and i have constantly line or hole where materila are join together.
When i try with higher speed and preasure, i have problem that mareila come out of cavity and damage cable insulation. Basically i need high speet with low preasure, and that is hard or imossible to achieve. I someone have expirience or some advice how to do that, please hellp.

Regards,
Nemanja.

[rickbatey]

What is the melt and mold temps? What is the melt flow and peak injection pressure? You may be able to raise one or both slightly and get better results, as long you are not at the high end already. One way to get the higher speed is the transfer to hold pressure earlier than you are currently to allow the melt time to stop before leaking out of the cavity.
What is the jacket material of the cable? It could be that the PBT melts too high and you need a resin with a lower melt flow.
Rick.

[Suhas]

Rick made some good suggestions.
You may also have to consider molding a pre-mold first and then overmolding on top of that.
Suhas

[tfrounfe]

I would definately try what Rick has recommended first with changing your transfer position. What type of gating are you using? I have run into some of the same issues with an overmold project. The gate was too close to the wire jacket which caused the jacket to melt. The resolution for me was a combination of moving the gate, getting a thicker jacket on the cable, and some processing adjustments.

[nemanja]

I try temperature range form 200-240C, injection speeds from 20-100cm3/s (this is machine max). I do premolidg with PA, that is only way to do, because i have contacts on other end, and is happens material to exit nad damege contacts, what is very bad. :)
I also try with 1-3 injections speeds, with 2-5 holding preasures values and from 1.5-50 cm3/s holding preassure flow, but resault is always more less same.
Jacket material of cable is FLRYY, what is basically, as i understand, PVC. and higher temperature that can this type of cable can hold is 105C.

[rickbatey]

With a PVC cable jacket, I'm not sure how you'll be able to mold PBT over it. Perhaps a higher melt flow PBT resin will help, but I think you've got other changes to make to solve the issue. Like moving the gate, making the gate larger, possibly adding wall stock (thickness) so you can fill the mold with less pressure.
You never mentioned the actual peak injection pressure!? What is it?
Has this ever ran good? Why the over mold? Is that creating an issue that the next over mold makes even worse? Are you sure the flashing occurs from injection or could it be from pack and hold? Truthfully the methods for fixing non fills on ribs are: faster fill rate, more mold and/or melt temp, thicker wall section for easier filling, larger and/or more gates.
Rick

[nemanja]

I try injections speeds from 20-100cm3/s, preasue peak has been from 350-850bars. Best way for you to see what problem is that you see pictures with explanations. I cant upload here, but i upload to rapidshare. Link for download is:

http://rapidshare.com/files/1375843315/report%2015%20pol.pdf

Next weak my company will open, so i will try some of tips that i get here. Thanks.

Nemanja

[rickbatey]

I suggest you run the water temps lower on the half that is filling and flashing and warmer on the half that is freezing off and poor knit lines.
I also suggest you run parts at various fill rates and shot volumes, without hold pressure. That will show if flashing occurs due to injection or cavity pressure, or could be from insert movement, mold deflection, and loose fit of insert into mold shut offs.
Rick.

[nemanja]

I will try to play with volume widouth hold preasure. I try play with temperatures of mold halfs, and im alredy at upper limit that i can run with curent gear. I even need to lower temperature on hot runner part, becose i have problem with sticking materilal to that part of cavity after cooling time. Also i try to incrise to 2 min, and not hellp.

Nemanja.

[pjhall]

Looks like your getting close in Pictures 5 and 6. Try ramping the injection speeds (fast -slow, fast-slow-fast, slow fast slow) to move that end of fill "Short/burn". Can you add a vent pin at the where the defect is?

I think what you are seeing in picture 4 is the first shot defecting to the side wall of the over mold due to the injection pressure.

Not sure your going to be able to have high injection speed with low injection pressure. The faster you inject the higher your peak pressure will be.

[nemanja]

problem was solved adding some pins for air to go out. there are still sometimes air bubbles inside, but that also depends of premold quality (is hand made ‚‚by feeling‚‚). beside of adding pins, we made also some more changes on tools like screeches on edges of cavitys and so on. in summary we have now good process who is more less stable.


thank you for replays.