Choosing a Pad Printing Machine

Pad Printing Press Options and Add-Ons

Most manufacturers provide a variety of add-ons for their presses. I am not talking about handling and feeding systems or pre- and post-treating equipment, but devices that improve print quality.

Pad Cleaning

These systems use adhesive tape dispensed onto a platform. At programmed intervals, the platform is placed under the pad. When the pad comes down onto the tape, any dried ink or debris on the pad surface is removed. The tape platform then retracts and printing continues.

Pad-cleaning systems are used mainly on unmanned machines, although they are sometimes fitted on operator-fed machines. The press operator simply uses the device when needed.

Automatic pad cleaning is generally considered to be a useful addition. But if the substl-ates are clean, ambient conditions are controlled, and the ink is mixed correctly, automatic pad cleaning could be considered a luxury rather than a necessity.

Ink Pumps

The concept is to have a reservoir of ink in a closed container. Ink is pumped from the reservoir into the ink trough, then back again. In some cases, solvent is fed into the reservoir as regulated by automatic viscosity- measurement instrumentation. These devices are notoriously unreliable, so it’s better to mix a specific amount of solvent in at the start of the run and add a controIled amount during the run.

Cleaning these systems is a problem. The reservoir must first be emptied and then the pipes have to be flushed out. Often, spare reservoirs are supplied to speed the changeover. Even if the ink is never changed, the whole assembly must be cleaned out regularly because the ink has a pot life and will tend to "fur up" the equipment.

Two-component inks can only be used with great care and must be changed regularly. Leaving one in for 24 hrs would be a very expensive mistake.

One of the simplest ink-feeding systems Fs a peristattic pump used to take ink to and from the trough. These systems pump ink by compressing the tube carrying it. The advantage that the pump never comes into contact with the ink, only the outside of the tube. Flow rate is infinitely variab[e, and tubing is available that will withstand the aggressive solvents.

Solvent Dispensers

This is a simpler alternative to ink pumps when the volume of ink used is low. These systems dispense a specific amount of solvent at a set interval. Both the amount and time interval can be adjusted to suit the ink and printing conditions. In principle, regularly adding solvent is an exceltent concept. As long as the dispensing system is soundly designed, it will go a long way to reducing one of the major variables in the process.

Use great care in selecting a solvent-dispensing system to ensure that the components will withstand the solvents. Also, the spatula must be modified slightly to ensure that the solvent is mixed in immediately In other systems, a slowly rotating cylinder in the ink tr-ough keeps the ink agitated and the solvent well mixed.

This technique is also used with some closed-cup systems. Great care must be taken In reguiating the flow since the volume of ink is much less, and it is possible to thin the ink too much.

Air Blowers

Here, the goal is to assist solvent evaporation from the ink when it is in the etching, on the pad, or on the substrate. This can be achieved by directing air from either a compressed air line or a hot-air blower. Such blowers can be mounted on the machine but must be used with care and only when ambient conditions demand them.

Static Eliminators

By its very nature, pad printing generates a considerable amount of static electricity. Silicone rubber is an excellent insulator and the continuous mechanical action of the pad can generate many thousands of volts of static charge. Add this to the static charge that is inherent in the moulded plastic substrates - especialIy when they are stored in a plastic bag immediately after molding and removed from the bag just prior to printing - and you have a static cocktai1 that can cause real printing problems. A common effect of static is feathering. Where the ink tries to jump back onto the pad as it lifts away from the substrate.

Before diagnosing static, make sure that the etch depth on the cliche is not too deep or the ink too thick. If static is definitely the culprit, you must eliminate it. You can do this by blowing ionized air over the pad and/or substrate to conduct the static e[ectricity away from the print area. Such static-elimination equipment is basically a series of electrodes in the air stream that are charged with a very high voltage (20,000-40,000 volts). These electrodes emit charged ions that give the air its static-elimination properties.

Static is, to a Large extent, affected by ambient conditions. Very low humidity is the greatest problem. In some print shops, humidifiers are used rather than static eliminators to minimize the effect.

Image recognition

With more companies using unmanned pad-printing presses on automatic lines, the need for image-recognition equipment is increasing. These systems can detect changes in the print of as little as 0.0005 in. (0.012 7 mm). They can also sense colour changes. Image-recognition equipment can be set up to alert operators, reject components, or stop the machine. Costs are continuing to fall. The CD industry uses this technology heavily and is demonstrating its advantages.

Choosing a Pad Printing Machine - Introduction

In the padprinting industry purchasing decisions are often based on asswnptions that are simply nuts - literally.

Manually Operated Pad Printing Presses

Except in cases where pad printing will be done on a very small scale, manual machines are not even entry level anymore. However, some may be useful for testing purposes.

Open Ink Trough Pad Printing Machines

In these original semiautomatic machines, the inking mechanism is mounted above the cliche. Although more modern models ha we better features, contro/lirrg ink condition fluctuations.

Partially Covered Ink Troughs

In these machines, ink is contained in a trough behind the cliche. The ink is pulled forward by a spatula mounted on a horizontal carriage with the doctor-blade assembly.

Sealed Ink Cup Pad Printing Machines

These presses are designed to contain the ink in a cup that is turned upside down and pressed firmly against the cliche, sealing in the ink.

Reciprocating Cliches Pad Printing Machines

In these models, instead of the pad moving back and forth from cliche to substrate, the cliche moves out of the way, while the pad remains stationary.

Rotary Pad Printing Presses

These presses use a rotary drum-type sllicone pad, usually in conjunction with a steel cylinder cllche into which the desfgn is etched.

Total Colour Transfer System

This technique, used almost exclusively in ceramic decoration, combines pad and screen printing.

Carousel Style Pad Printing System

This multicolour pad-printing system uses a cambination of two rotary tables and a rotating pad carousel.

Non-Horizontal Pad Printing

Machines such as this are suited for printing onto surfaces that are vertical or angled.

Computer Numeric Control (CNC)

In presses with computer numeric control (CNC), the substrate is stationary and the pads are programmed to print one image at a time onto the item.

Pad Printing Press Options

Most manufacturers provide a variety of add-ons for their presses.

How to decide what machine to buy

To enable you to judge which press to buy, you will at least need answers to all of the questions shown under Purchasing Considerations.

Pad Printing Machine Purchasing Considerations

Trying to decide which pad-printing machine to buy? Start by answering the following questions.