Cold runner systems are an important aspect of mold design. By optimizing cold runner shapes and sizes, important molding processes such as pressure drops, polymer shearing, packing and cooling behavior, gate sealing time, and overall cycle time can be made more efficient.
Cold runners are simply unheated channels that transport molten polymer material from a machine cylinder through the nozzle body and nozzle tip to part that will be molded. The cross-sectional shape of a cold runner influences overall material flow. A small cross-section increases the pressure drop, flow velocity, shear rate, shear stress, and shear heating. A larger cross-section has the inverse effects. Taking this into account, it is important to understand the material properties of the polymer to be molded, specifically viscosity, thermal behavior, and shear sensitivity, so gate and runner sized can be optimized.
When polymer fills a cold runner, the required injection pressure increases. Once the material reaches the gate, it gets compressed inside the cold runner before it continues to flow through the gate. When the gate is smaller, the compression of the polymer is higher, so the shear rates and potential pressure spikes will be higher as well. In addition to small gates, sharp edges inside the cold runner can increase shear rates and potentially lead to shear- induced fill imbalances.
Here are several common runner shapes. It is important to understand how the different shapes and sizes affect material flow, injection pressure requirements, filling velocity, shear rates, and temperature.
Cold runner types and sizes will impact overall polymer processing behavior and the part quality so need to be factored in during the part design phase. CAE simulations can be used to evaluate different runner systems to determine an optimal shape and size for a molded part.
More detailed information on cold runner types and designs is available here. Individual lessons are available to purchase for $40 (unlimited access for 6-months) or with an annual program subscription.