Family Moulds and Balance.

Family molds are those tools /moulds that have multiple impressions that are not of the same design. The balance of fill in these tools is very important, especially if part fit and function depend on critical dimensions.  In most cases, processors know they have a problem but since the tool made acceptable parts why change, though due to this, the process is severely limited. In some cases, cycle time is sacrificed.

As mentioned, the family tool, is one that has different parts in it of varying sizes and weights. Possibly, different wall thickness can be found but hopefully not. In determining the balance of the mould, the first step is to know the full part weight of the parts producing.  This can be accomplished because they all fill normally and or by blocking off cavities and shooting individually. At this point we now set the tool up with no pack pressure, transfer by position and see how short the parts are relative to each other. Than by adjusting the position transfer we produce parts that are shorter and shorter. This than well show what is going on. Be sure to save the sets of the parts as you produce them.

What you now have is a visual picture of what is going on. However, what is needed is the percentage short to full by weight.  At this time the parts are weighed, the short shot weight is compared to the full weight by part.  This than gives the actual data of what happens in filling the parts.  It may be that a part is 90% full and one is 75% full but due to part configuration, they look about the same percentage visually. Nevertheless, by weighing the parts there now is real data.  The other data that should be kept at this point would be the pressure at transfer and possibly even pictures of the parts. This than gives the pressure to reach the fill at those points and if continued this could be used to develop a pressure drop which has been discussed before.

Now that it is known, what is happening with the fill process, decisions can be made based on the findings.  If all parts fill within approximately 10% than possibly all is okay. However, if the range is approximately 25 % or greater than a problem exist.  Most people would change the gate dimensions, but gates should only be changed as a last resort. The Diameter of the runner is what needs to be examined. The diameter is what creates the various pressure drops and it is hoped most of the gates are freezing at approximately the same time. (Determining gate freeze has been previously discussed) Thus, adjusting runner diameter would allow for pressure and flow of material reaching the cavities at the same time. The pressure drop through individual cavities combined with the above process gives the best picture and solution to achieving a balanced fill.

The largest variation seen to date is that of filling four parts (smallest in tool) and yet the material hasn't filled the runner to the gate for the other four parts (largest in tool). As you could imagine the pressure differential was huge, but all the customer needed were full parts. Yet, it was up to the processor to fix a tooling issue, resulting in a longer cycle time and smaller process window.
 

Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers Division
ASHLAND Distribution