Mold-Masters Controllers for reducing hot runner problems and downtime
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.
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.
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.
Time proportioning heating
Basic controllers and integrated machine controls for hot runners typically offer only time proportioning heater output control, which means that even at power output values of 1% or less, during bake out, the controller supplies short bursts of full mains voltage, increasing the possibility of arc over in the heater element. More advanced controllers employ phase angle firing during soft start to manage the dispersal of any potential moisture using reduced voltage and ground fault detection. Mold-Masters controllers have ground fault detection and phase angle firing soft start, enabling the company to offer a 10-year warranty of the heater circuits.
Phase angle firing
The raw power used for mains voltage heaters in Europe is typically 220/240V AC (alternating current), 50Hz. In other words, voltage swings from plus and minus at a frequency of 50 times a second. Graphically it can be represented by a continuous sine wave.
The most basic method of percentage control of AC power is by adjusting the ratio of on-time to the off-time to achieve the amount of power required, or time proportioning control. Whereas this system requires simpler low speed multiplexed control electronics, there are major disadvantages with regard to response times and the full mains voltage pulses applied during start up.
An alternative method of output power adjustment – phase angle firing – is achieved by delaying the switch on point to part way through each half cycle of the AC mains power. High performance hot runner controllers with dedicated high speed CPU electronics can make control decisions and alter the switch on time during each half cycle of the mains, with a control resolution suitable for effective control measured in microseconds.
This has two advantages. First of all the output power is absolutely correct every half cycle= 10 milliseconds. Secondly, for output values below 50% the voltage is reduced, which is the main factor related to heater arc over. At 1% output the voltage is measured sub 20 V, where arc over due to dampness is not possible.
Small diameter thermocouples typically used in hot runner systems are easily damaged or pinched during mould disassembly. More advanced controllers have functions to diagnose thermocouple problems and overcome faults using various strategies. Common faults are cross or reversed wiring, open circuit, or potentially the worst case, pinching at a point away from the heated zone, in which case a runaway situation can cause serious damage. State-of-the-art controllers incorporate a safety feature which detects the lack of correlation between power output and temperature rise and cuts the power with a relay before any damage is done. Slave mode can be employed if a thermocouple becomes an open circuit, or unreliable in duplicating the power output of any adjacent zone. There are also various auto slave features available that allow for seamless operation in the event of a thermocouple fault without resulting in downtime.
Moreover, additional wiring diagnostics software can detect various heater and thermocouple faults as well as keep a record of heater performance that can be used for comparison on tests at a later date.
A basic control system that allows probes, due to their low mass, to reach set point well ahead of the manifolds, can clamp down the expanding assembly, which can cause long term wear, increasing the possibility of leakage in the future. Depending on the hot runner design, various modes available can either bring all zones to the set point together, manifolds first, then probes – or a sequential process that heats pre-defined groups of zones, one group at a time. Detecting a hidden hot runner leak in the early stages can save substantial downtime and repair costs. Controllers such as the Mold-Masters M2 feature plastic leak detection. When leaks occur, plastic is released into the air gap insulation between the hot runner assembly and the cold cavity plate of the mould.
Plastic leak detection
When leaks happens, additional power is required to hold the set point because more heat is transferred to the water-cooled mould plate. The controller also applies more power to compensate for the heat loss and the resulting rise can be detected by the computer, which can trigger an alarm.
Mold-Masters said it has recently released an addition to its M1 range of hot runner temperature controllers. The M1-24 model, for up to 24 x 15A zones, is positioned between two earlier cabinet sizes for 12 or 48 zone capability, offering a very compact and economical modular controller with many advanced features, the supplier explained.
This mid-range controller houses plug-in six-zone 15A control cards with low voltage soft start, current measurement and ground fault detection as standard for heater protection.
A decade of protection
The company said it offers a 10-year warranty for the heater circuits of a complete hot half and controller package. The colour touch screen console clicks into various viewing angles or packs away vertically for protection. The supplier said the software is primarily designed for simplicity of use but with many advanced functions available such as individual or group setting of set points, alarm limits, boost and standby values, wiring diagnostics, temperature and power output graph, automatic follow start up modes, multi language support, password control option, tool store memory and USB file transfer. An optional trolley base compliments the package. The M1 controller range and Mold-Masters’ advanced control technology are said to have a proven track record for accurate control and robust reliability in service.