Validation of design guidelines for cooling time and runner size

The current global competitive environment does not afford mould makers the luxury of carrying out bench tests and moulding trials to validate their guidelines. They usually rely on “past experience” to establish some of these guidelines. This article discusses how flow simulation software integrated with powerful 3D CAD functionality can be effectively and efficiently applied to validate guidelines for cooling time and runner sizing.

COOLING TIME
A common challenge faced by mould makers is to produce acceptable parts with minimum cycle time. As cooling time accounts for most of the cycle time, reducing the cooling time of a part will considerably reduce the cycle time. However, reducing the cooling time may also result in problems such as excessive part warpage, high part temperature at the time of ejection, etc.

The minimum cooling time required to produce acceptable parts depends on many factors such as part geometry, plastic material, mould material, cooling layout, and melt temperature. Before manufacturing the production tool, the mould maker needs to understand how these factors would simultaneously affect cooling time and part quality. For example, the mould maker may seek to set guidelines on factors such as part thickness, material grade selection, and number of cooling channels. Flow simulation software allows the designer to change these factors and study their influence of these factors on cooling time. The following guidelines for cooling time can be validated using flow simulation.

Part design guidelines
The mould maker can create several part models using the CAD functionality of the flow simulation software or in an external CAD software. A filling, holding and shape analysis can be carried out on each of these models. Simulations results such as pressure, temperature at the end of cooling, warpage, etc. would indicate if the part model would be acceptable or not. Based on these results, the mould maker can optimise the part design to minimise cooling time.

Design changes are carried out very easily in flow simulation software that is integrated with powerful CAD functionality. The integration reduces the steps involved in analysing design changes and also eliminates problems involved in importing and exporting models to and from external CAD software.

Cooling layout guidelines
An array of cooling layouts can be created and analysed inside flow simulation software that has thermal analysis functionality. The cooling layout can be optimised to minimise cycle time based on the results of such a thermal analysis. The effects of cooling layout on part warpage can also be studied by running a thermal, filling, holding and shape analysis. It is also possible to set guidelines on variables such as coolant flow rates, cooling channel diameters, and coolant temperatures to minimise cooling time.

It is quite tiresome to make cooling layout changes and analyse these changes if the mould is designed in an external CAD package. Flow simulation software with integrated mould design capabilities and automatic cooling channel mesh generation functionality greatly reduces the time and effort required to analyse cooling layout changes. In the past it would take several hours or even days to run thermal and warpage analyses. Flow simulation software has become much faster and does not require expensive computers to run analyses. These analyses can now be run in minutes even on standard desktop computers and laptops.

Mould material guidelines
Mould makers quite often use mould materials of varying thermal conductivities to lower cooling time. For example, a copper alloy core insert may be used to extract the heat faster from the part. Flow analysis software allows the user to utilise the CAD model of the mould plates and then change the mould material. To save computer memory usage and to make simulations faster, flow simulation software allows the user to create different size mesh on the mould components. For example, a coarser mesh can be created on a mould plate while using a fine mesh on the mould insert. Using this approach, the mould maker can analyse the effect of mould material; cooling layout and mould insert design on cooling time.

RUNNER sizing
Runner sizing is a very important decision for a mould maker. In hot runners, the mould maker may seek to minimise the runner diameters in order to reduce melt residence time, reduce colour change cycles or allow tighter pitch for drops. In cold runner moulds, smaller runners mean lower material wastage. However, smaller runners in hot and cold runner moulds may lead to excessive pressure drop. Very thick cold runners may also lead to 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 moulds as 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 analyse the effects of these changes on the pressure drop. The user has the option of defining runners on a curve or using the 3D geometry of the runner. Functionalities that automatically detect the runner diameters inside a 3D mould model and automatically create a runner mesh greatly reduce the time required to analyse different runner diameters.

Runner length guidelines
In some cases, mould makers 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 moulds, the length of the runner system to the cavity may be excessively long. Using flow simulation, the mould maker can determine if such designs may lead to excessive pressure drop. If there is excessive pressure drop, the mould maker 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 mould maker often has to make the gate a small as possible. Small gates may lead to excessive pressure drop and excessive shear rates which may degrade the plastic. Using flow simulation, the mould maker can optimise the gate design by analysing different gate geometries under various moulding conditions. The pressure and shear rate results will indicate whether or not the design is acceptable.

Conclusion
An optimised cooling design and runner design can significantly increase the productivity of a mould. By validating the design guidelines, mould makers can reduce or even eliminate the potential problems that may be faced by their customers.

Flow simulation software is a powerful tool that can be used to by mould makers to validate design guidelines and also identify potential problems before the tool is manufactured. Flow simulation software packages are becoming more cost effective and user friendly. New functionalities have been added to make the simulation setup process easier and faster.

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