Throwback Thursday: Validating Design Guidelines

Let’s take a look back at how flow simulation software integrated with powerful 3-D CAD functionality can be effectively and efficiently applied to validate guidelines for runner sizing and  cooling time.



Runner sizing is a very important decision for a moldmaker, according to VISI by Vero Software. In hot runners, the moldmaker may seek to minimize the runner diameters in order to reduce melt residence time, reduce color change cycles or allow tighter pitch for drops. In cold runner molds, smaller runners mean lower material wastage. However, smaller runners in hot and cold runner molds may lead to excessive pressure drop. Very thick cold runners also may lead to an undesirable increase in cooling time.

Pressure drop of a tool depends on many factors such as runner geometry, plastic material, melt temperature, injection velocity, part geometry, etc. Once the runners are machined, it may prove to be very expensive and sometimes impossible to change these factors to reduce the pressure drop. In hot runner molds it is often required to select the product line. If the wrong product line is selected, the consequences could be catastrophic and may require manufacturing a brand-new expensive hot runner system.

These factors can be easily changed in flow analysis software to determine the pressure drop of the system and identify potential problems. The following guidelines for runner sizing can be established using flow simulation.

Runner Diameter Guidelines. Small changes in runner diameters can considerably affect the pressure drop. Flow analysis software allows the user to change runner diameters and analyze the effects of these changes on the pressure drop. The user has the option of defining runners on a curve or using the 3-D geometry of the runner. Functionalities that automatically detect the runner diameters inside a 3-D mold model and automatically create a runner mesh greatly reduce the time required to analyze different runner diameters.

Runner Length Guidelines. In some cases, moldmakers have to use long hot runner nozzles inside their tools. This problem may be compounded by a valve pin inside a small nozzle bore. In other cases such as in stack molds, the length of the runner system to the cavity may be excessively long. Using flow simulation, the moldmaker can determine if such designs may lead to excessive pressure drop. If there is excessive pressure drop, the moldmaker can make design changes to reduce the pressure drop to an acceptable value. For example, the runner diameters can be increased to compensate for the large runner length.

Gate Geometry Guidelines. Large gates often lead to undesirable gate vestige and the moldmaker often has to make the gate as small as possible. Small gates may lead to excessive pressure drop and excessive shear rates that may degrade the plastic. Using flow simulation, the moldmaker can optimize the gate design by analyzing different gate geometries under various molding conditions. The pressure and shear rate results will indicate whether or not the design is acceptable.

The other half of this article is on validating cooling time with simulation. Click here to read the entire feature. 


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