I have an 8 cavity plastic injection mold with hot manifold into cold runner.
The outside cavities short shot about 25% less of the inner cavities. What is the best way to balance the outside cavities for uniform mold fill?

What is the runner layout?

originaly i said this was a maifold into cold runner.
this is a 3 plate mold 8 cavity "H" style runner on both sides of cender line of mold. the outside parts are short shot when backing off mold pressure.

Do you have adequate cold wells?

Physics tells us that, at constant temperature (constant melt viscosity), if the flow lengths of all your runners are the same and the runner diameters are all the same, the flow must be balanced.  The only thing that can disrupt this is if your runners are blocked in some way or if there is a temperature variation within your tool.

To balance non-symetrical runner systems, see our website http://www.polytechconsultants.com/balancing.htm

Regards,

Peter P

Why don't you check this article,it may help.

http://www.immnet.com/articlelibrary/archive/getOneArticle.php3?getArtID=1664

It's in the IMM Injection molding magazine

Another thing that can result in imbalanced fill is gate location on the parts.  Some designers orient cavities in the mold without considering differences in wall thickness or gate location.  Even though the runner system may be "balanced" and all cavities cut the same, if the cavities are gated at different locations, they will fill differently (especially if there is a significant difference in part wall thickness at the gate locations).
The easiest thing to do is probably to get a pin grinder and open the runners (and/or gates) to the slow-fill cavities during the short shot process until all cavities are filling equally.  Not very scientific, I know, but effective and simple.

Good luck

I cannot say it enough . . . " VENTING !!!!! ".  Most molds are built with absolutely pathetic venting.  If all other mold features are OK, it usually comes down to venting.  Insideman also made a good point about cold wells.  I have had to use overflow wells on extremely difficult to fill parts. Don't gate into the overflow, just use about 50% more vent than you would ordinarily use, and, naturally, you must also vent the overflow.  

I agree about the vents. If the air is compressed more in one cavity to another, it fills slower. Also, do all vents effectivly go to atmosphere without restriction.
After a small land, vents should be very large so that accumulation of flash, mould deposite etc does not block them.
Another posibility is eneven mould cooling or heating. Do some runners go close to water passages or mould cavities while others do not.
Before you get out your die grinder, do a lot of shorts to various degrees, so you see where the imballance starts. Obviously, take care not to stick the shorts in the mould if at all possible.

Regards
pat

I love vents and can't argue with their necessity, but I think freedesigner has given you the best lead.  We have been using Beaumont's Melt-flipper and it works.  I originally thought it was a crazy idea but it has worked for us.  Of course, you have to pay a royalty to use the idea.  Check the link to IMM.

I just read the artical re melt flipper and it all sounds interesting and feasable, and probably the solution in this case, but I think it pays to check simple solutions first, like hot and cold spots in the mould, effectiveness of vents etc.
Measuring surface temperatures after the mould has been running for a while, and takeing progressive short shots tells us a lot.

I guess the melt flipper is more effective on materials that respond well to a lot to shear, like PP for instance, but not so effective on the likes of nylons. I am certainly interested to find some data regards this

I guess to check if it might be a canditate for melt flip, try very fast injection speed with cold material, then very slow with hot material. I am also interested to know if this test works as an indicator

Regards
pat

Sorry to respond to late in the game, but I just recently visited the forum again.  I appreciate the kind words some have spoken on MeltFlipper technology.  The imbalance in multi-cavity molds can be explained in detail by visiting www.meltflipper.com.

Please visit the site and send me an email if you have any specific questions.  The technology is guaranteed to work on any material as well.  Take care and have fun molding!!

Dave