SELLENIS LIMITED INNOVATION IN INDUSTRIAL INKJET TECHNOLOGY

EHT OR HV FAULTS/TRIPS – WHEN?

One of the most troublesome and most occurring CIJ printer faults that contributes to downtime and

repeated operator intervention is EHT fault or sometimes called HV trip.

This fault usually and often occurs as a result of a short circuit of one or both (if applicable) of the live

EHT plates to a lower potential (ground) .

This short maybe as a result of a gradual or sudden formation of a conductive path between the live

plates and a lower potential or ground.

CIJ electronics/firmware/software implement a HV short circuit detection or more precisely a method

for detecting a drop, a decrease in EHT voltage below a predetermined level for a set short period of

time (milliseconds).

EHT OR HV FAULTS/TRIPS – HOW?

If the EHT voltage drops below a given level for longer than a certain set time the CIJ printer system

generates a fault and switch off the HV supply to the deflector plates.

This is to prevent charged drops being deflected randomly and uncontrollably onto other critical

printhead parts and degrade the print further.

It is also important for safety purposes to cut off the HV supply to the deflector under these

circumstances.

The fault & switch off of EHT occur only after a drop in voltage for a minimum period of time to ensure

the printer system distinguishes between a real problem and an instantaneous drop in EHT due to

electrical noise (false alarm)

EHT OR HV FAULTS/TRIPS – WHY?

There are several root causes for the occurrence of EHT fault or trip. These can be

classified into two areas:

a. Design robustness

b. Operating conditions

EHT trip as a result of a printhead design that is not robust enough

Non-robust design specification of deflector plates shape, form and surface finish

Non-robust design specification of deflector plates spacing or inter-gap

Non-robust design specification of deflector plates mounting geometry, shape, surface finish and

location

Non-robust printhead system design (ie designing the printhead taking into account all the

interractions of the deflector plates and EHT voltage with other components of the printhead during

operation.)

EHT OR HV FAULTS/TRIPS – WHY?

EHT TRIP as a result of operating conditions

EHT trip sensitivity setting

EHT voltage setting [too high or too low for the operating parameters such as raster in use, drop

velocity (or pressure setting), charge voltage setting, modulation voltage setting)

Operating temperature (room temperature; the higher this is the more likely EHT trip in the case of a

non-robust design)

Moisture/humid /wet operating environments such as in drink bottling , steam laden air applications

plants, food canning plants and the like)

Inks formulation quality that leads to the creation of microsatellite droplets

Ink splash back from gutter

EHT OR HV FAULTS/TRIPS- HOW TO SOLVE?

EHT Robust Design

The designer needs to design the whole printhead not just the deflector plates taking into account the interactions

between the various printhead components. During operation./

The design needs to take into account and design for the various operating conditions listed previously including possible

manufacturing and assembly variations.

EHT TRIP consideration of operating conditions

EHT trip sensitivity setting – understand this setting when applicable and ensure it is adequately set

EHT & Drop velocity voltage settings :

• Ensure EHT voltage is not set too high for the raster in use so as not to deflect drops into the defector plate. This is particularly relevant when printing 2, 3 or 4 lines or large fonts

• Ensure pressure or drop velocity is not too low for the raster in use so as not to deflect drops into the deflector plate. This is particularly relevant when printing 2, 3 or 4 lines or large fonts

• Ensure EHT voltage is not set too low so as the least deflected drop does not clear the gutter edge. This is particularly relevant when printing 2, 3 or 4 lines or large fonts

Ensure EHT voltage is not set too low so as the least deflected drop does not clear the gutter edge. This is particularly relevant when printing 2, 3 or 4 lines or large fonts

EHT OR HV FAULTS/TRIPS- HOW TO SOLVE?

EHT TRIP: Consideration of operating conditions

Moisture/humid/wet operating environments

• Use of a positive DRY air system.

• A positive air system only such as those supplied by a standard air pump will probably not be

effective and might even increase the occurrence of EHT trips.

Ink splash back from gutter

• Investigate ink quality and stability

• Adjust jet into gutter

• Printhead design robustness

• Microsatellite droplets

• Consider changing ink or nozzle

Sellenis engineers have the expertise and the know how in this field. This ensures that Sellenis

designs of printhead and CIJ components and assemblies are robust incorporating optimum geometry,

shape, surface finish and optimum system configuration to operate reliably under varying conditions of

operations and environment.

Sellenis Limited INNOVATION IN INDUSTRIAL INKJET TECHNOLOGY

CONTACT SELLENIS

Sellenis Limited

Unit 1, Saxon Way

Melbourn, South Cambridge

SG8 6DN, England, UK

Tel +44 (0) 1763 269554

Email info@sellenis.com

Web Sellenis.com