Six Considerations for Evaluating a Hot Runner System
Details matter when it comes to selecting and integrating a hot runner system. This guide makes that process easier by covering those details thoroughly.
Here are six important items to consider when evaluating a hot runner system:
1. Manifold heaters. For installation, most manufacturers use a press. If the heater contact with the manifold is not uniform, there will be hot spots created in the heater element, which causes premature failure and non-uniform heating of the manifold itself. Also, process capability can be affected especially with some engineering materials, as their viscosity may be more temperature sensitive than commodity resins like polypropylene and polyethylene.
2. Nozzle heaters. Measuring the ohm resistance for each nozzle heater can provide good information when troubleshooting the hot runner system. Too great a variance may prevent uniform temperature profiles and the potential for process variation will increase. Additionally, the life of a heater with less efficiency is reduced because of the required increase in workload.
3. Nozzle tip orifice. Pressure differences in plastic injection molding are defined in the Hagen Poiseuille equation, where (Q) is the flow rate, (l) is the runner length, (n) is the material viscosity and (r) is the radius of the flow channel: ∆P=(8Qln)/(πr4). The equation shows that any slight difference in “r” value, which includes the orifice in a hot runner nozzle and gate tip, is affected to a magnitude of the fourth power. This means that a small change in radius will result in a rather large difference in pressure drop from gate tip to gate tip. The common perception is that if the gate tips are all within tolerance, this will not matter. However, it is important to ask what the acceptable tolerance is along with the gate tip dimensions. Just because they are within tolerance does not mean there will not be a difference in pressure drop.
4. Stack height. It is important to check the uniformity of the stack height across all nozzle tips. If these dimensions vary, then the engagement with the gate will also vary, and these variances could affect process capability because of resulting differences in channel geometry. Ask for the inspection report and consult the manifold supplier for an acceptable tolerance. Revisit the pressure drop equation to see if the tolerance that the supplier provides is acceptable for a given process and set of parts.
5. Flow channels. All gun drill manufacturers will claim variances of 0.001 inch per inch of travel. Check that they can verify that the cross drilling of the flow channels does not leave a step at the intersection of the cross-drilled runners. Refer to the earlier equation again. Check that the gun drill manufacturer scoped and honed the channels. Variations in flow channels could create differences in pressure drop through the regions of a manifold. Again, this refers to the portion of the equation involving the radius (r), which is raised to the fourth power. Steps in the flow channel can create dead spots, which do not flush well when purging and cause color-change issues and material-degradation problems. Also, the steps make it possible for material to sit in the manifold for extended periods, which increases the likelihood for thermal degradation of the polymer. The polymer’s natural flow path around the channel intersections also causes these dead areas. This happens even if the channel intersections are perfectly aligned.
6. System cost. The cost of the system is often one of the most important things to consider, but relying solely on quotes from various suppliers is not the best way to approach the issue. Familiarity with the various options that each hot runner supplier offers is helpful. Another thing to think about is how the channels are manufactured. The manufacturing method may affect color changes and require additional clean-out procedures that expand on traditional purging approaches.
Staying on top of the items presented here will reduce downtime, improve part quality and decrease overall costs.
About the Contributors
David Hoffman and John Blundy
David Hoffman is a senior instructor and development manager for plastics education and training for the American Injection Molding Institute (AIM). John Blundy also contributed to this piece when he served as vice president of business development at Beaumont Technologies Inc..
A rapid heating and cooling technique using pressurized water solves sink, weld line and discoloration problems.
Selecting the right material for specific mold components can help prevent a number of costly issues
One of the most common concerns expressed by molders about hot runner molds is the threat of a leak of molten plastic causing the manifold pocket to fill.