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tips & techniques - Using Moldflow Plastics Advisers 5.0

By Murali Anna-Reddy, Moldflow Corporation

Technique to model a direct sprue, gate or hot sprue

For cylindrical or symmetrical shaped parts (boxes, cups, helmets, etc.) that are molded in single-cavity molds, a sprue gate is preferred. Here, the part is gated directly from the sprue. If the sprue is cold, then there is no gate and the sprue directly connects to the part. However, with a hot sprue, a small gate is used to facilitate automatic degating. When modeling these types of gates, the key is to understand that Mold Adviser requires a sprue and a gate as the minimum for a feed system. Since there is no gate in the case of a cold sprue, you will need to model a portion of the sprue as gate entity. Say, for example, that your cold sprue is three inches long. Then, you could model 2.5 inch as sprue and the remaining 0.5 inch as a gate. Use the taper angle to calculate the starting and ending diameters of the sprue and the gate. Here is a step by step procedure to model these two gates.

Import the part model and set the model type to Single Cavity.
Orient the part such that the sprue is in the positive Z-axis. Next, select the injection location.
Select Parting Plane and position the parting plane a small distance above the injection point. The distance between the parting plane and the injection point will be the length of your gate.
Select Specify Mold Dimensions . Set the 'A Plate' thickness to the length of the sprue.
Select Edit Runner Defaults . Set the dimensions of the sprue and gate.
Select Sprue Location . Click on the middle of the injection location, and click OK. This will create the sprue directly above the injection location.
On the Runner Generation dialog, select Generate Sprue and Gates, and click OK.

This should complete the creation of the direct sprue gate or the hot sprue. Examples of each are shown below.

Sprue Gate (Cold Sprue)	Sprue Gate (Hot Sprue)

Technique to model a hot drop runner system

The process of modeling a hot drop runner system is outlined below using a simple example of a single-cavity mold. However, the same procedure can be applied to multi-cavity and family molds, as well as to modeling 3-plate cold runner systems.

Import part model and set the model type to Single Cavity.
Orient the part such that the sprue is in the positive Z-axis. Next, select the injection location.
Select Parting Plane and position the parting plane a small distance above the injection point. The distance between the parting plane and the injection point will be the length of your gate. Typically, with hot drops and 3-plate cold runner molds, a restricted gate called a Pin Point Gate is used for automatic degating purposes. The gate dimensions are typically very small compared to other types of gates.
Select Specify Mold Dimensions . Set the A Plate thickness to the length of the sprue. Turn on the Floating Plate, and set its thickness to the length of the hot drop.
Select Edit Runner Defaults . Set the dimensions of the sprue, runner and gate.
Select Sprue Location . Click on the middle of the injection location, and click OK. This will create the sprue directly above the injection location.
On the Runner Generation dialog, select Generate Sprue, and click OK.
Select Design Runners . This will allow you to create runners and gates manually. Click the bottom of the sprue. The Runner Creation dialog pops up. Select Create Drop and the hot drop will be created automatically using the dimensions specified earlier.
Only the gate is left to be created. Select Display Beam Elements to display the runner as a beam element. This is quite useful, especially if the runner is large and obstructs the view. Similarly, select Display Cavities to turn off the display of the cavities. This too can be helpful during the modeling of gates.
Now select Design Runners again and click on the end of the hot drop and then on the injection cone. The gate will be created automatically between the hot runner and the injection point.

This will complete the creation of a single-cavity mold with a hot runner system. An example of such a model is shown below. As mentioned earlier, the same procedure could be applied for creating hot runners for multi-cavity and family molds as well as 3-plate cold runner systems. Examples of such molds are shown below.


Single-cavity mold with a hot runner system	Family mold with a hot runner system	Single-cavity mold with a 3-plate cold runner system

Techniques to model fan, film, ring, or diaphram gates

Using the geometry tools provided in the Moldflow Mold Adviser module, you can very easily model tunnel, cashew, and tab gates. However, when it comes to modeling fan, film, ring, and diaphram gates, the geometry modeling tools fall short. The limitations arise from being unable either to accurately represent the volume of plastic in the gate region or to accurately model the flow of the plastic from the gate into the cavity. The common aspect of all these types of gates is that the gates are very wide (sometimes extending the entire width or circumference of the part). The larger gate width spreads and slows the melt as it enters the cavity. If your object is to simulate only the effect of the gate on the plastic flow, you can do so by modeling several injection points along the edge where the gate connects to the part. In this case, you need not model the runners and gate, and the analysis would be done in the Part Only mode.

On the other hand, if you would like to design the entire sprue, runner, and gate system, then our suggestion is to model these types of gates in your CAD system. The STL file you would export from the CAD system should contain both the part and the gate geometry. The sprue and runners would later be added inside the Mold Adviser.

The example shown below features a part model with a fan gate. The two modeling scenarios explained above are shown in the accompanying images.


Several injection points used to represent plastic flow through the fan gate	Model with the part and fan gate designed in the CAD system	Sprue and runner added to the fan gate model to complete the runner system