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brentb
24th September 2011, 04:22 AM
What % cavity imbalance is acceptable where you work? Does this vary if it is a close or loose tolerance job? Does this vary with number of cavities? Does this vary if it is a hot or cold runner?

Keep on Molding!

brent

Suhas
27th September 2011, 02:28 PM
Hi Brent,
I think you hit the nail right on the head. All depends on the tolerances. The tighter the tolerances the lesser variation you can accept since it is easy for one to go out of spec when the other is in. If you are molding forks then as long as you do not have flash or shorts you are OK. So a good mold with good shut offs is important here. I am sure you will pick up that piece of steak if the fork was 0.020" out of sepc :)
Thanks,
Suhas

brentb
28th September 2011, 01:49 AM
What fill imbalance 5 do you all accept where you work?

Keep on Molding!

brent

rickbatey
28th September 2011, 06:46 PM
For me, no more than 10%. We run several bezels that are cosmetic critical, but for costs we build family molds with a car set in them (say the B and C-pillar seat belt bezels). I want to open the process window, so I try to stay at 10%. I have seen some molds so far off, we put cavity shut offs in the runner, then ran one set of parts or the other.
Rick.

brentb
4th October 2011, 04:56 AM
I mean % not 5

brentb
4th October 2011, 04:57 AM
Rick,

"Same" parts or family mold?

KOM

brent

rickbatey
4th October 2011, 05:59 PM
Brent,
These were the dreaded family mold. There was not enough adjustments made to the tooling to get the cavity balance close. So after realizing we couldn't make them all, we opted to adjust the cycle time to make one set or the other set as fast as possible.

Suhas
5th October 2011, 02:50 AM
Family molds are always painful. I wish no one had thought about them :) If the tolerances are wide, then having a family mold is OK. I have had a big part and then the smaller part being about 10% of it. They think the consultant has the magic wand! :)
Thanks,
Suhas

NATHAN
27th December 2011, 11:57 AM
5% of flow variation was acceptable for the consumer product.
3% of flow variation was acceptable for the Medical product in which I am working on
2% of flow variation was acceptable for the optical product in which the melt stress distribution is major requirement for eye power readings
1% of flow variation was accepted for the gear products.

With respect to fill of 25%,50%,75% and 90% if you perform the flow balance study the
flow variation was different and shown more variation between the cavities below 90%.

Set the Correct Injection speed from viscosity curve and predict the correct transfer position THEN we can perform the cavity balance test.
I hope that it should be the right way....Try and see.............I have tried..................

Maheshkumar Hiremath
18th January 2012, 11:42 AM
Nathan ,

Can you plesae share in detail ? As I want to have some wrae around 5- 10 % variation . Please share the details to mhiremath@molex.com.

brentb
19th January 2012, 12:55 AM
Nathan ,

Can you plesae share in detail ? As I want to have some wrae around 5- 10 % variation . Please share the details to mhiremath@molex.com.


What is wrae (WRAE)? I hate acronyms!

KOM!

brent

1Aahz
19th January 2012, 03:22 PM
Cavity imbalance in multicavity molds should be less than 5%. Short shots are probably the number one and most costly defect in our industry. Typically a short shot cannot be salvaged except as regrind. The main reason for this seems to be imbalance in the mold. A lot of technicians will fill the mold until the last place on the last part barely fills because if they try to pack it out the first part to fill either flashes or is oversize i.e. out of spec. Then when the viscosity of teh resin shifts to the high side of the tolerance they get shorts. This can lead to multiple process changes because of the technician often does not understand what happened so they move the VPT or increase melt temp or turn up hold pressure each of these of course affects something else and the cycle of tail chasing begins.

The reason I like to do a 50% fill balance check is to determine the true balance of fill of the mold, machine, material combination. Some will only check the balance at 90-95% which does not give them as accurate of analysis. Once we get to that level of fill the pressure has started to redistribute in the melt flow path altering the flow rate in other cavities. For example if i am filling a 16 cavity mold and have a significant imbalance where cavity 1 is the first to fill and cavity 8 is last to fill all others being in between at various stages of fill. As cavity 1 gets close to full the pressure requirement to continue filling rises. This pressure then increases the fill rate on the other cavities.
If the imbalance is due to gate diameter variation it can then be masked if we check the balance at 90-95%. Flow/fill rate will be different from cavity to cavity in the mold if the system is imbalanced. Since fill rate is one of the four plastics variables that determines part quality and consistency it is one we must control.

GBYA,

Aahz