Unbalanced part size in family mold

[ggarnier]

Over the years I accepted on faith that trying to have parts of quite different size/geometry together in a family mold is a bad thing.

Recently I began to wonder "Why?". Which part suffers? (The big one/little one/both?) In what way do problems manifest themselves? The most obvious would be incomplete filling in one part - but if the gating and runner geometry were biased strongly in favor of the small part, it would seem that it would fill quickly and then the filling of the larger part would proceed as if the smaller one weren't there.

I'd appreciate any input - I think I have exhausted the patience of the tool maker I have been talking to with my repeated "Why?" (or "Why not?&quot

 

[sbc]

What are your mat'ls? Are the molds cooled?
Are you injection molding plastic, or pouring
lead sinkers?

 

[ggarnier]

Sorry - as Lily Tomlin said: "I should have been more specific".

Injection molding common resins (ABS or Polycarbonate) was the context of my original question.

 

I am a toolmaker that has worked on "unbalanced" molds. You are onto the right direction in determining runner geometry, but runner length, gate size - shape - and position all play important parts in successfully molding from a family mold. Bear in mind that you need to stay within certain guidelines when choosing gate shape, and many molders will even argue that runner shape is important, but I will contend that gate shape and size is more important than runner shape in ABS. Now, the one facet that you need to address is the actual BALANCING of the mold. If you think of the different components being molded as though they were weights on a scale pan with a single point as the fulcrum (this point is the sprue), then you calculate the placement of the component cavities so that the AREA of each component will balance (in PSI developed during the shot) to the sprue in relation to all other components. This way you are not putting unequal pressures into your mold set and thus the clamp of the machine. I have done a number of unbalanced family molds that people have told me are "impossible" to run, and I'm still batting 1000. Such molds require some careful discernment and thought put into them before you ever order the moldbase, but they can be done successfully. Just a closing hint: If you use your own common sense together with the experience of the moldmaker, and stay away from flow analysis "experts", you will have the greatest chance of success. Hope this helps you out. As always "ornery".

 

[ggarnier]

Thanks to "Ornery" for your reply.

Assuming that this "static balance" can be achieved (or that the parts are so small it is not really an issue), are there other problems, like difficulty getting complete fill on all parts? Or excessive cycle times? Or cosmetic issues (burning, etc.)? Or......?

 

Complete fill depends on proper venting, as this is probably the most overlooked aspect of mold building. On an unbalanced mold, in respect to volume as well as area, runner length and gate size and placement will all have an effect on fill. I've used "wells" on long thin parts when being molded in conjunction with grossly under-proportioned "family members". These are simply overflows that are gated back to the part and very well vented so that all gas is allowed to be expelled during the fill stage of the shot. Thereby the parts all fill uniformly. I've sometimes made runners unseemingly lengthy on small volume family members to help balance fill. Burning is almost always associated with insufficient venting, provided all other aspects (resin temp, pressures) are within parameter. As you can see, this is not an exact science, but with a little thought and planning, nearly any group of parts can be made to run well. Hope this helps out.