Mold-Masters Controllers for reducing hot runner problems and downtime

Editor: Eric Culp

Hot runner systems have become more reliable over the years and the de facto standard for multi cavity moulds, but their complexity makes them susceptible to various reliability issues. Mold-Masters outlines some of the problems and solutions.

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The very thin layer of insulation between the live element and the grounded section can dissipate and lead to “arc over”.
The very thin layer of insulation between the live element and the grounded section can dissipate and lead to “arc over”.
(Source: Mold-Masters)

The most common reasons for potential downtime are plastic leakage, heater failure, thermocouple damage, faulty wiring and long term mechanical wear. Often overlooked are the performance and capabilities of the hot runner controller to minimise hot runner faults and increase reliability.

Improving heater life

With hot runner designs using ever smaller heater elements, the need for ground fault detection and phase angle firing in the controller become essential safety features to protect heaters from “arc over”, i.e. the breakdown of insulation between the live heater element and the grounded outer sheath, typically due to moisture affecting the dielectric properties of the insulation.

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Tube heaters used in modern hot runner probes can be as small as 2 mm diameter (See illustration). The insulation layer, usually magnesium oxide powder compressed in the outside metal sheath, is hydroscopic, with a propensity to absorb moisture from the atmosphere when stored cold. If the insulation becomes damp and the heater is supplied with full 220/240V, with a peak to peak voltage in excess of 600V, there’s a possibility the insulation will break down to allow an arc over spark to damage the heater element. The controller’s ground fault detection system prevents this during the critical soft start period (sometimes called bake out) by constantly measuring the output for any current leaking to ground and automatically reducing the voltage in proportion to the leakage until the heater dries out.

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