Pressure result

The Pressure result is generated from a Fill analysis, and shows the pressure distribution through the flow path inside the mold at the time the result was written.

Pressure results derivation discusses the color distribution of this result. At the beginning of filling, the pressure is zero (or 1 atm, in the absolute pressure scale) everywhere in the mold. The pressure at a specific location starts to increase only after the melt front reaches that location. The pressure continues to increase as the melt front moves past, due to the increasing flow length between this specific location and the melt front.

Note: For compression-type molding processes, this result is plotted displayed on the deformed mesh as the default setting. With this default setting, if you click through the time steps you can watch the polymer as it is compressed into the part shape. To turn it off, so that all you see is the result on the part, right-click the result, select Properties and uncheck Display on deformed mesh.

The pressure difference from one location to another is the force that pushes the polymer melt to flow during filling. The pressure gradient is the pressure difference divided by the distance between two locations. Polymer always moves in the direction of the negative pressure gradient, from higher pressure to lower pressure. (This is analogous to water flowing from higher elevations to lower elevations). Thus, the maximum pressure always occurs at the polymer injection locations and the minimum pressure occurs at the melt front during the filling stage.

The magnitude of the pressure (or pressure gradient) depends on the resistance of the polymer in the mold, because polymer with high viscosity requires more pressure to fill the cavity. Restricted areas in the mold, such as thin sections or small runners, and long flow lengths also require a larger pressure gradient and thus higher pressure to fill.

Note: The Pressure result is an intermediate result, meaning its animation by default is through time and the scale by default is the minimum to maximum of the entire range of the result.

Using this result

Normally the maximum injection pressure at the nozzle is about 140 MPa (20,000 psi). We recommend having a maximum pressure of 100 MPa (14,500 psi) for the mold (part and feed system) and 70 MPa (10,000 psi) maximum for the part. There are many molding machines with higher pressure capacities. If you don't know what the pressure capacity is, assume it is 140 MPa.

If the pressure capacity of the molding machine is known, use no more than about 75% of the pressure capacity for a design guide for the entire mold, and 50% for just the part.

The default molding machine for the simulation has a 180 MPa pressure limit. You can use the pressure limitations two ways. Assume the injection molding for making the parts has a pressure limit of 140 MPa. You can run an analysis with the default 180 MPa limit and review the pressure requirements of the mold. It should not be over about 100 MPa, as a design guide line. If the analysis pressure is 120 MPa, the pressure should be reduced. You could also set the molding machine limit to 100 MPa and run the analysis. You would be simulating a pressure limited process. The filling phase should be controlled by velocity, not pressure. Therefore the process should be fixed.

Note: The typical maximum hydraulic pressure of an injection molding machine ram is around 14 MPa. When the polymer is injected and is forced into the nozzle, there is a pressure intensification factor of between 8 -15 due to the smaller area of the ram compared to the hydraulic cylinder moving the ram. Therefore, the pressure available at the nozzle is normally between 110 MPa and 210 MPa. 140MPa is around the average.

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