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It is preferable to use a 1st stage fill which provides a ~90% filled part. This provides a good representation of how the part will perform during production.

If you have a product which require a profiled 1st stage injection, it may be a useful exercise to perform such a test at each stage in the profile. This will help you ensure the final fill balance is optimal.

For more blog entries on cavity imbalance, please feel free to review the following:

Correcting for Cavity Imbalance…

Adjusting Hot Runner Systems…

-Andy

Actually, the focus should not be on the machine inputs, but the actual process outputs. this is a little more complicated, but below are some examples:

Generally speaking, the feed zone of the screw should be at least 1.5 times the total injection stroke. This allows the material in front of the feedthroat to properly convey to the transition zone with a low likelihood of blow-back.

When processing materials which tend to melt better with conductive heating such as ABS, PP, and Nylon, the feed zones are often longer to allow more material heating to take place before reaching the transition (compression) zone. An ABS screw, for instance, typically has a feed zone approximately half the overall length of the screw.

-Andy

Basically, hydrolysis is the process of breaking the water molecule into a positive hydrogen (H+) atom and a negative hydroxide (OH-) molecule. These powerful ions can react with the atoms of many different polymer chains. When these ions attack the polymer chains, they often cause the chains to breakdown, resulting in degradation of the polymer. In other cases, these ions attach to the polymer chains and interfere with the inter-molecular forces between the chains. In either case, the hydrolysis causes a reduction of the polymer strength and often affects the appearance of the molded part.

-Andy

Although the intent of these systems is to decompress the hot runner system to eliminate gate drool, it will introduce variability. Since the carriage movement is not precisely controlled, the amount and rate of displacement both forward and back is not consistent from shot to shot.

Basically, the most effective and consistent method of decompression is to use screw suckback, or retract, after recovery. As a result, the volume of material entering the mold during 1st stage fill is likely to be more inconsistent when this is used. Additionally, you are relying on your maintenance staff to keep the barrel and platens perfectly aligned. When this is not the case, premature wear, leakage, and/or breakage will occur.

After suckback, a shut-off nozzle can be used to maintain a negative pressure in the hot runner system as well as keep the material in the barrel between shots. As shut-off nozzle technology is getting better, the applications which can benefit from a shut-off nozzle.

If you machines already have these nozzle systems, minimize the use of the sprue break and always ensure accurate alignment of the nozzle and and linearity of the carriage movement.

-Andy