Learn About The High Performance Sprue Bushing

For selection of gate diameter it is important to take into consideration the material flow characteristics, shear rate of resin, molding conditions, fill time requirements, gate vestige, wall thickness and configuration of parts to be molded. Situations requiring high injection velocities must be considered when selecting small gate diameters.

High injection rates may require larger gates due to shear heat build up (e.g. high weight thin wall applications). See material manufacturer’s literature for further information regarding material to be molded.

To compensate for nozzle’s growth when heat is applied, the linear expansion of the nozzle (BE) at a given temperature must be added to the nominal “A” dimension. The formula below shows how to figure boring depth (dimension “A” + BE). The tip of the nozzle will now be flush with the cavity line at processing temperature.

Formula for determining this expansion factor is as follows: BE = “A” dimension x 0.00000633 x nozzle set point – 68ºF (assuming the mold is at 68ºF during operation). If mold temperature is different, substitute 68ºF with actual mold temperature.

EXAMPLE: Given a 3.543in “A” dimension, with a set point of 500ºF and mold temperature 68ºF:

BE = 3.543 x 0.00000633 x (500 – 68) = .010

Thus “A” + BE will be 3.553

NOTE: The above information is only given as an example; variations may occur based on mold configurations and cooling factor. In some instances, it may be necessary to obtain an empirical factor.

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