From the break down shown above, the cool time is the most significant portion of the cycle and is where the most time can be shaved off the process. Once the cool time is reduced to its minimum, the other components of the cycle are also reduced including the pack time.
The pack time allows material to continue to flow into the cavity through the feed system after the part has been filled during 1st stage. The reason material can continue to flow into the cavity once it's been filled is because the polymer is cooling and shrinking.
As the polymer shrinks, more volume is created and must be filled by more material in order to prevent excessive shrinkage or voids from being created. The packing phase packs the material into the cavity while it shrinks in upon itself.
The material should be packed into the part until the part actually freezes off (which should be the same time the gate freezes off). Once the part and gate are frozen, the pack phase is complete and the cool time can begin.
So how do we know when the part and gate is frozen?
Method for Determining Pack Time On the Molding Floor
The only way to know when the part and gate is frozen is to:
1. Start with a short pack time and weigh the part.
2. Increase the pack time slightly and weigh the part again.
3. Repeat procedure until the part no longer shows a significant increase in weight (suggesting that very little, if any, material can be further packed into the cavity).
This is a good method if the part weight is actually allowed to stabilize at a maximum weight. However, the pack time is often cut short. Because once the part weight comes close to peaking, it is considered sufficiently packed.
The pack time would be increased further if the part weight increased significantly. Any additional pack time minimally increases the part weight yielding very little for the investment of time and time is money.
Unfortunately, this is the most critical portion of the pack phase and it is vital that the part be packed until the part and gate freezes, which means the part weight must be allowed to level off completely!
If the pack time is not long enough, then the part will continue to shrink without additional material being added resulting in the potential for the following issues to occur:
· sink marks
· voids
· non-uniform stress within the part
· non-uniform shrink within the part
· part deformation/warpage
Ironically, these issues are typically not seen immediately on the floor while they are being molded; therefore, the parts are typically deemed acceptable. Due to the nature of plastics, the plastic will relax over time with thermal cycling causing these issues to show up later (typically on the customers dock).
For most parts, these potential issues can have a dramatic effect on part performance and quality.
Using Filling Analysis to Determine Pack Time:
1) A filling analysis can determine exactly when a part is frozen.
2) An analysis can also determine if a feed system is sized properly allowing it to freeze when the part freezes, not before the part freezes.
3) If the feed system is too small, it can be revealed in the analysis as showing high shear rates in the gates or show it freezing off before the part freezes.
4) If the feed system is too large, it will take longer than the part to freeze; therefore, controlling cycle time by requiring more cool time to freeze is necessary.
An analysis can account for every stage in the molding process as well as all of the variables and inputs allowing it to directly correlate to the molding process on the floor.
Due to the importance of the packing phase, it is highly recommended that an analysis be utilized to both a) size the feed system for the part AND b) determine the optimum pack time.
Bozilla Corporation performs these analyses continuously for customers yielding outstanding results.
Contact Bozilla Corporation for your FEA and injection molding troubleshooting needs. We invite you to visit our website at www.BozillaCorporation.com. 
