GB2110598A

GB2110598A - Improvements relating to printing on plastics materials

Info

Publication number: GB2110598A
Application number: GB08229010A
Authority: GB
Inventors: Brian Ralph Barber
Original assignee: NICC Ltd
Current assignee: NICC Ltd
Priority date: 1981-10-10
Filing date: 1982-10-11
Publication date: 1983-06-22

Abstract

Printing by ink-jet onto heat- shrinkable plastics is facilitated by abrading the surface to accept the ink. The abrasion is effected by corona discharge in a narrow gap between a roller and an electrode as the plastics material is drawn around the roller.

Description

SPECIFICATION Improvements relating to printing on plastics materials This invention relates to printing on plastics materials. More specifically it is concerned with marking heat recoverable polymeric material, often in the form of heat shrinkable tubes.

Many printing processes generate or require heat, which is a problem if the workpiece is heat shrinkable. Special measures have to be taken to ensure that it does not reduce in size at the printing stage, so that it is usable at its eventual destination.

According to one aspect of the present invention there is provided a method of printing on plastics materials wherein the printing zone is treated, for example it is abraded, and subsequently printed by an ink-jet.

Printing by ink-jet is a known technique, and its advantage here is that direct contact with the workpiece is avoided. The abrasion may conveniently be by corona discharge, the theory of which will be outlined below, but other roughening techniques may be used, provided excessive heat is not generated. For example, in some circumstances an abrasive wheel may be suitable.

The printing zone may advantageously first be subjected to ultra violet (UV) light, preferably with a wavelength in the range 350---400 nanometres.

The method is particularly applicable to heat recoverable or cross linked polymers. This material may be in web form, and the abrasion is then conveniently carried out as it passes over a roller.

It may be tubular, with the abrasion carried out as it passes in a flattened state over a roller.

Alternatively, the material may be a conduit or cable sheath, already with one or more cores and ready to use, and that may have sufficient inherent stiffness and strength for abrading to be carried out on a length between support rollers.

Referring again to corona discharge, in an electric field ionised particles move about with a speed which increases with the field strength.

Random collisions occur with other charged particles, and this increases the available ions.

When the energy imparted by the field is sufficient, there is an avalanche effect, and the air gap breaks down into an ionized current carrying conductor, manifested by a visible blue corona. As power is increased, this changes colour.

The bombardment and penetration of these ions into the molecular structure at the surface of the material generates chemical change and effectively abrades that surface.

It has been found that losses due to heat dissipation in a roll cover and a cross linked polymer being subjected to corona discharge on that roll only amount to about 0.5% of the true average power of the generator creating the field.

Thus substantially all the generator power is dissipated in the electrical resistance of the air gap after breakdown.

The frequency of oscillation of the ionized particles is of secondary importance, but it has to be chosen so that the transit time for an ion to pass through the air gap with its cloud of charged particles is short in relation to the period of the alternating field. This ensures the ions will arrive at their target, rather than oscillate back and forth. It has been found that generator frequencies below 32 HKz are suitable for air gaps up to 3mm.

The extent of the treatment is dependent on watts per unit area for a given time, and the size of the air gap. However, the effect of the treatment is not linear with time, and it is preferred to concentrate the beam and to have an intense ion bombardment for a short period. This makes best use of generated power.

For a better understanding of the invention reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagram showing units of corona treatment apparatus, and Figure 2 is a diagram for explaining measures necessary with treatment of tubes.

In Figure 1 a generator 1 provides a selected frequency to a step-up and matching transformer 2. This feeds a high voltage to a bar electrode 3 parallel to and spaced closely adjacent an earthed roller 4 with a cover 5. The transformer itself may not be able to provide the correct match, and an inductance coil 6 is provided to cancel the contribution of the roll cover capacitance.

In use, a plastic film 7 wraps around the roller 4 and is drawn continuously through the air gap 8 between the electrode 3 and the cover 5, being there pulled tightly against the cover. Corona discharge from the electrode pits or abrades the surface of the film, which is then ready for ink-jet printing.

As mentioned above, U.V. light treatment may be given to the film upstream of the roller 4.

A plastics tube 9 is treated in the same way, but it must not be in tubular form at the corona discharge zone. Referring to Figure 2, there would then be a secondary air gap 10 within the tube, and this would break down into an ionized current conductor. The inside of the tube would therefore be treated at the expense of the outside. This problem can be cured by using nip rollers either side of the roller 4 and/or evacuating the tube.

The methods of pretreatment described above may also be used to prepare the surfaces of heat recoverable or cross-linked polymers to enhance the application of hot-melt adhesives. Thus from another aspect the invention consists in a method of applying hot-meit adhesive to plastics materials wherein in the application zone is treated, for example by abrasion, and subsequently hot-melt adhesive is applied to the zone.

1. A method of printing on plastics materials wherein the printing zone is treated, for example it is abraded, and subsequently printed by an inkjet.

2. A method as claimed in claim 1 , wherein the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements relating to printing on plastics materials This invention relates to printing on plastics materials. More specifically it is concerned with marking heat recoverable polymeric material, often in the form of heat shrinkable tubes. Many printing processes generate or require heat, which is a problem if the workpiece is heat shrinkable. Special measures have to be taken to ensure that it does not reduce in size at the printing stage, so that it is usable at its eventual destination. According to one aspect of the present invention there is provided a method of printing on plastics materials wherein the printing zone is treated, for example it is abraded, and subsequently printed by an ink-jet. Printing by ink-jet is a known technique, and its advantage here is that direct contact with the workpiece is avoided. The abrasion may conveniently be by corona discharge, the theory of which will be outlined below, but other roughening techniques may be used, provided excessive heat is not generated. For example, in some circumstances an abrasive wheel may be suitable. The printing zone may advantageously first be subjected to ultra violet (UV) light, preferably with a wavelength in the range 350---400 nanometres. The method is particularly applicable to heat recoverable or cross linked polymers. This material may be in web form, and the abrasion is then conveniently carried out as it passes over a roller. It may be tubular, with the abrasion carried out as it passes in a flattened state over a roller. Alternatively, the material may be a conduit or cable sheath, already with one or more cores and ready to use, and that may have sufficient inherent stiffness and strength for abrading to be carried out on a length between support rollers. Referring again to corona discharge, in an electric field ionised particles move about with a speed which increases with the field strength. Random collisions occur with other charged particles, and this increases the available ions. When the energy imparted by the field is sufficient, there is an avalanche effect, and the air gap breaks down into an ionized current carrying conductor, manifested by a visible blue corona. As power is increased, this changes colour. The bombardment and penetration of these ions into the molecular structure at the surface of the material generates chemical change and effectively abrades that surface. It has been found that losses due to heat dissipation in a roll cover and a cross linked polymer being subjected to corona discharge on that roll only amount to about 0.5% of the true average power of the generator creating the field. Thus substantially all the generator power is dissipated in the electrical resistance of the air gap after breakdown. The frequency of oscillation of the ionized particles is of secondary importance, but it has to be chosen so that the transit time for an ion to pass through the air gap with its cloud of charged particles is short in relation to the period of the alternating field. This ensures the ions will arrive at their target, rather than oscillate back and forth. It has been found that generator frequencies below 32 HKz are suitable for air gaps up to 3mm. The extent of the treatment is dependent on watts per unit area for a given time, and the size of the air gap. However, the effect of the treatment is not linear with time, and it is preferred to concentrate the beam and to have an intense ion bombardment for a short period. This makes best use of generated power. For a better understanding of the invention reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagram showing units of corona treatment apparatus, and Figure 2 is a diagram for explaining measures necessary with treatment of tubes. In Figure 1 a generator 1 provides a selected frequency to a step-up and matching transformer 2. This feeds a high voltage to a bar electrode 3 parallel to and spaced closely adjacent an earthed roller 4 with a cover 5. The transformer itself may not be able to provide the correct match, and an inductance coil 6 is provided to cancel the contribution of the roll cover capacitance. In use, a plastic film 7 wraps around the roller 4 and is drawn continuously through the air gap 8 between the electrode 3 and the cover 5, being there pulled tightly against the cover. Corona discharge from the electrode pits or abrades the surface of the film, which is then ready for ink-jet printing. As mentioned above, U.V. light treatment may be given to the film upstream of the roller 4. A plastics tube 9 is treated in the same way, but it must not be in tubular form at the corona discharge zone. Referring to Figure 2, there would then be a secondary air gap 10 within the tube, and this would break down into an ionized current conductor. The inside of the tube would therefore be treated at the expense of the outside. This problem can be cured by using nip rollers either side of the roller 4 and/or evacuating the tube. The methods of pretreatment described above may also be used to prepare the surfaces of heat recoverable or cross-linked polymers to enhance the application of hot-melt adhesives. Thus from another aspect the invention consists in a method of applying hot-meit adhesive to plastics materials wherein in the application zone is treated, for example by abrasion, and subsequently hot-melt adhesive is applied to the zone. CLAIMS

2. A method as claimed in claim 1 , wherein the abrasion is caused by corona discharge.

3. A method as claimed in claim 1 or 2, wherein the printing zone is first subjected to U.V. light.

4. A method as claimed in claim 3, wherein the U.V. light has a wavelength in the range 350-400 nanometres.

5. A method as claimed in any preceding claim, wherein the material is a heat recoverable or cross linked polymer.

6. A method as claimed in any preceding claim, wherein the material is in web form and the abrasion is carried out as it passes over a roller.

7. A method as claimed in any one of claims 1 to 5, wherein the material is in tubular form and the abrasion is carried out as it passes in a flattened state over a roller.

8. A method as claimed in any one of claims 1 to 5, wherein the material is a conduit or cable sheath.

9. A method of applying hot-melt adhesive to plastics materials wherein the application zone is treated, for example by abrasion, and subsequently hot-melt adhesive is applied to the zone.