An injection molds that contains more than one cavity of the same shape. Multiple-cavity injection molds have
a symmetrical layout, producing many parts of the same design.
Balanced filling is critical for making china plastic injection molding identical parts, achieving high CPKs, holding tight tolerances, and getting “good” data from design of experiments.
we know for plastic injection moulding about the importance of uniform filling or balanced flow for each cavity of
a multi-cavity mold. Balanced filling is critical for making identical parts, achieving high CPKs, holding tight
tolerances,and getting “good” data from design of experiments (DOEs). Balanced flow is critical for both filling
and packing (filling influences packing).
1. China injection molds with melted plastic.
Unmelted or partially melted plastic can disrupt flow in the runner, gate, or part. This may seem like a minor
issue,yet my experience tells me it is significant. For example, temperature control of the nozzle tip and body
is notoriously problematic, especially with cold runners. Poor screw design may also result in non-uniform melt.
Then there is temperature control of the manifolds and tips in hot-runner systems. How many of you are setting
some “unusual” temperatures to get the hot runner to function?
2. Differential venting among the injection mold cavities or injection mould flow path. Yes cold runners should be vented.Air water vapor, off-gases, or volatiles from the polymer can build pressure in cavities and restrict filling. I demonstrate this at every one of my seminars.When troubleshooting venting issues for fill balance, one trick is to make short shots at 90+% full and 65-80% full to see if the alance changes.
3. Non-uniform cooling of the china injection molds, hot-runner system, or hot tips. Plugged or partially
plugged water lines; water lines too close to the parts or too far away; coolant taking the path of least
resistance;laminar flow;air trapped in a coolant channel; or other equipment coming on and off line can
all wreak havoc with proper cooling.While rarely done, a case can be made for regulating water coolant
flow separately through each channel of your molds.
4. China custom injection mold part design, particularly non-uniform wall thickness. If you have thick and thin sections in the part, the thick section will fill relatively easily compared with the thin section. If gated into the thick section, flow may “hesitate” at the thick-thin junction, causing a seesaw filling pattern. This factor must be considered in “living hinge” applications.
5. Lack of proper velocity control during filling. Too many profiled velocity changes; running the process pressure-limited;or setting up with different fill times—these are big culprits for filling imbalances. Fill time basically establishes shear rate,which in turn establishes viscosity. Varying fill time (shear rate) from shot to shot, run to run, or machine to machine provides a “different” process due to huge changes in viscosity. Change the viscosity and you can the change fill pattern.
6. Custom injection molds gate not all the same size. No problem, as this is difficult to check with a pin
gauge, Relatively few pin gauges measure gate land length.It is difficult and time-consuming to measure land
length, but it’s essential because gate land length establishes pressure drop. To achieve balanced filling. you
need each gate to provide identical pressure drop. There is a reason most mold builders have a rule for
maximum land length.
7. Unbalanced flow path. If flow distance and/or path geometry are not identical, non-uniform filling is inevitable.
8. Laminar flow. This is perhaps most sinister reason on my list, because you can have everything else down
perfectly and still have poor balance in filling. Polymer melt flows in layers (laminar flow) of different velocity,
temperature, shear rate,and viscosity. There are concentric layers within any given flow path—a nozzle, runner,
manifold, gate or part. Shear is the prime determinant for non-uniform viscosity. The melt layer near the wall of
the flow path will see the highest shear,providing the lowest viscosity (easiest flow). The center path has the
lowest (orzero) shear, so its viscosity is the highest (stiffest flow). In the sprue, the concentric layers are
symmetrical,but as soon as you split or branch the flow path, this critical symmetry goes out the window
and you have an imbalance in shear rate across the layers resulting in unbalanced flow of the layers.
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