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Hot Runner Technology OCT 2017

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6 Hot Runner Technology F E AT U R E additional thermal expansion. This reduces the effective radius (r), ultimately restricts the flow and counteracts the intent of the temperature increase (see Figure 1). It is also important to understand that rais- ing local nozzle or manifold temperatures may cause an increase in cycle time since the mate- rial and mold in that region will stay hotter for a longer period. 4. Stack height. It is important to check the uniformity of the stack height across all nozzle tips (see Figure 2). 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 sup- plier provides is acceptable for a given process and set of parts. 5. Flow channels. All gun drill manufactur- ers 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 pres- sure drop through the regions of a manifold. How Much Will a Hot Runner Cost Me? Getting pricing from suppliers is just a start. Also consider these direct and indirect costs. Direct Costs • Hot runner system: number on the quote sheet. • Cost and leadtimes for replacement of spare parts, including tips, heaters, thermocouples and nozzles. • Controller for the hot runner system and associated components and hookups. Indirect Costs • Ease of maintenance, maintenance approach and history of issues. • Residence time and effect on process stability and part quality. • Color changes: time, material and purging compound. • Variation in temperature throughout production, creating additional downtime, scrap and other issues. • Training requirements and demand for higher skill level on proper procedures for maintaining, starting up and shutting down a hot runner system. FIGURE 3: The natural flow path of the polymer through a branched runner and around a corner is shown in red. Potential stagnant locations are indicated in blue. 3 Areas of Stagnant Flow Areas of Stagnant Flow

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