Runner Balancing in SW Plastics – A Primer by Tony Botting

by GoEngineer

A family mold has cavities for components of an assembly that need to fit together after ejection from the mold. Each of the cavities for filling with hot liquid plastic can have different volumes.  Part of a mold designer’s responsibility is to get each of the cavities to finish filling with plastic at the same time.  If each cavity does not finish filling at the same time, they will not “pack” for the same amount of time, and they may cool at different rates and thus shrink by different amounts.  If one part shrinks more than another, they may not fit together properly.  Fortunately, SolidWorks Plastics comes to the rescue. It has an optimization routine that can help to “balance” the channel sizes used to direct the flow of liquid plastic to each cavity in the mold.

The CAD model image shows component parts that are connected to each other with channels for routing the flow of hot liquid plastic.  The entrance for the plastic starts at the injection “gate” at the top of a “sprue”, and is distributed to the component part cavities through “runners”, and again through a gate for each cavity. Note this representation does not show the mold, but only the cavities that are inside the mold.  So, you are seeing the final shape of the component parts after the mold cavities are filled with plastic, allowed to cool and solidify, and ejected from the mold. After ejection, the channeling system is trimmed off and the parts can be assembled.

Cavities inside the mold.

SolidWorks Plastics can help the designer predict the filling time of the cavities directly using the CAD model shown above. A filling time prediction is shown in the image below: this is a “snap-shot” in time and captures a moment during the flow of liquid plastic. The colors on the legend indicate time in seconds.  You can see the sprue and runners, now colored blue, indicate the liquid passed their position at time values near zero seconds. The green and yellow colors indicate the liquid plastic passed those positions later in time, but before one second. Finally, the “flow front” of the liquid plastic is colored red and shows its position at one second. The light tan-colored areas represent unfilled portions of the cavity.

Time to Fill plot from SW Plastics

You can infer the cavity on the right has not completed filling at one second, while the cavity on the left appears to be filled.  The plastic injection will continue until the cavity on the right is filled.  Next there is a holding period to allow what is called “packing”: injection pressure is maintained for a short time.  Following that, the pressure is terminated and the new parts are allowed to cool prior to opening the mold.  During the holding time for packing, it is obvious the cavity on the left tends to get “packed” for a longer time than the cavity on the right.  This can be a cause for the two component parts to shrink by different amounts.

One solution is to perform what is called “Runner Balancing”. There is a built-in automatic optimization routine in SolidWorks Plastics Premium that will adjust the sizes of the runners to compensate for the different volumes, and cause all components to complete the filling stage simultaneously.

The images below show runner tube diameters on the CAD model both before and after activating the Runner Balancing optimization.  This particular case appears rather simple, but the software can do this on very complex systems automatically. Conducting this optimization manually (by trial and error) would take a very long time.

Runner Diameters Before Runner Balancing

Runner Diameters After Runner Balancing

The next image shows the time to fill the cavities with liquid plastic using the new “balanced” runner system. You can see the liquid plastic flow front fills both components, all the way, at the same time.

Time to Fill After Runner Balancing

Since the manufacture of a mold can potentially cost tens of thousands of dollars, it is obviously prudent to use computer software to optimize the mold cavity design.   Fortunately, SolidWorks Plastics Premium can do this optimization and provide accurate estimates of all phases of the injection molding process:  from the injection stage for filling, then packing, next cooling and finally shrinkage estimates can all be simulated prior to committing to a mold design for fabrication.

Anthony Botting – Contributor

 

 

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