Experimentally determining the packing time

Everyone knows packing the parts out is important.  But in the quest for shorter more optimized cycle times have you found the minimum and most efficient packing time?  Here’s how you do it:  start with a short packing time and weigh all the parts in a shot but not the runner.  You’ll need an accurate scale because the weight differences you’ll see will be small.  If the parts are lightweight weigh at least five shots at this setting.  Write down the weight of the parts.  Increase the packing time by a second and repeat the process.  Continue to do so until you have four one-second increases in packing time where there is no longer an increase in weight.  Now draw a curve plotting the packing time against the part weight.  It should look something like this:

The part weight will increase with packing time then level off.  When the curve has leveled, the gate has frozen off and no more packing can occur.  Ideally, where the curve breaks would be the setting for your packing timer.  However gate freeze off is a function of material temperature, gate size, and mold temperature.  Your setting should have a slight safety margin to cope with the minor variations in mold and material temperature.  Your packing time should be about a second after the curve levels off.

Note this has nothing to do with the packing pressure.  If your part has sinks, voids, or cosmetic problems that can be solved with packing pressure they all must be solved within your experimentally determined time.  If they can’t, you’ll need to open up the gate(s) and repeat this experiment again because the gate will now take longer to freeze off.
 

Contact me with questions put in the subject line QUESTIONS.
Bill Tobin, WJT Associates, E-mail: bill4012@hotmail.com