EP0761457A1

EP0761457A1 - Method of screen printing

Info

Publication number: EP0761457A1
Application number: EP96305853A
Authority: EP
Inventors: Jonathan Barraclough; Joseph Mueller
Original assignee: Sericol Ltd; Fujifilm Ink Solutions Ltd
Current assignee: Sericol Ltd; Fujifilm Ink Solutions Ltd
Priority date: 1995-08-31
Filing date: 1996-08-09
Publication date: 1997-03-12

Abstract

A method for screen printing a substrate, the method comprising temporarily holding the substrate to a base during the screen printing, characterised in that the substrate is releasably held to the base by means of a pressure sensitive, hot melt, sprayable adhesive.

Description

This invention relates to printing, more particularly to a method of screen printing in which a textile or other material is supported temporarily on a base.
Screen printing is a process in which ink is forced through open areas of a stencil on to a substrate using a squeegee. The stencil is supported on a taut mesh, usually of nylon or polyester, attached to a rigid frame, usually of aluminium, coated steel or wood. The squeegee usually consists of a flexible, often polyurethane, blade in a rigid holder. Both the profile of the blade and the squeegee pressure can be varied to suit the printing application.
In the screen printing of textiles and other materials it has been the practice to temporarily hold the textile to a support base (platen) of other material such as metal or laminate often having a rubberised surface layer whilst the printing operation is carried out. This is currently done by means of an adhesive applied by an aerosol spray. Aerosol spray adhesives that are currently used are, for example, those sold under the trade names of Flash-Fix, S-Fix Extra and T-Fix Extra and are available from Sericol Ltd. of Broadstairs, Kent, England. These adhesives contain a solvent such as acetone, isohexane or isopropyl alcohol and also a propellant such as propane or butane. The purpose of the solvent is to dissolve the adhesive, which may be a pressure sensitive acrylic adhesive, and to provide suitable rheological properties for application from an aerosol.
With the currently practised method there is some risk of contamination of the working environment, primarily caused by evaporation of the solvent and propellant. There is also a problem of spray particles contaminating machinery and being breathed in by operators.
It would therefore be desirable to reduce this problem for reasons of personal health and safety and for production throughput because the drying of the solvent used in the present technology restricts production speed.
A previous proposal to solve the problem has involved the use of double sided sticky tape but this method, whilst giving benefits in terms of health and safety, has the disadvantage of being slow in application and removal and there is no operator control over the tack level.
It has also been proposed to employ a liquid adhesive, applied by a brush, roller or bulk spray, but this method is also too time consuming because of the slow drying speed on the platen. The adhesive also dries out if the container is left open, causing the brush or roller to become very sticky. Spray nozzles for liquid adhesives are prone to blockage.
The present invention provides a solution to the problem by employing a hot melt, sprayable adhesive, which does not require a solvent or propellant, to hold the substrate to the base.
According to the present invention there is provided a method for screen printing a substrate, the method comprising temporarily holding the substrate to a base during the screen printing, characterised in that the substrate is releasably held to the base by means of a pressure sensitive, hot melt, sprayable adhesive.
The advantage of the present invention is that it provides reduced contamination from solvent and overspray and thereby provides an improved working environment. In addition the adhesive dries almost instantly and provides a base which is immediately useable for the printing process and therefore speeds up production. The risk of the fabric being contaminated with wet adhesive is also eliminated.
The base which provides the support for the substrate may be of metal or a laminate and may have an upper rubberised surface. There may be a plurality of bases each carrying a substrate to be printed (e.g. a carousel screen printing machine).
The substrate material may be a textile or garment, for example, fabric pieces, T-shirts, sweat shirts and the like.
Hot melt, sprayable adhesives are well known adhesives and are available commercially from a number of sources. They usually comprise thermoplastic blends of synthetic polymers with natural and synthetic waxes, resins, plasticiser oils, stabilisers and antioxidants. The adhesive may be pigmented or dyed to produce a coloured adhesive and make the adhesive more visible in use.
Typical hot melt, sprayable adhesives are described in Japan 63061067A and European Patent No 89170A.
The hot melt, sprayable adhesive for use in the present invention should have a reasonable open time during which it has the necessary tack to hold the substrate, e.g. T-shirt, on to the base in the screen printing process. The hot melt, sprayable adhesive needs to have a tack level suitable for releasably holding the substrate to the base. The hot melt sprayable adhesive should also have a tack level that will allow reuse of the adhesive for further substrates. The tack level should be low enough for the substrate to be peeled off the base after printing without causing any distortion or damage to the fabric. It is desirable that the adhesive can be removed from the base to avoid build up of residue, and any residue on the substrate should not adversely affect it in any way. A person skilled in the art of screen printing will be able to easily determine whether a particular adhesive is suitable for use in the present invention. Open time is the time taken for an adhesive to revert back to its solid and non-tacky state after it has been applied to the base in a molten state. The open time for an adhesive suitable for use in the present invention is preferably greater than 2-3 minutes, and even more preferably greater than 6 minutes. The tack level of an adhesive is temperature dependent. In use the tack of the hot melt, sprayable adhesive will be reduced both due to open time and fibres and lint that will stick to the adhesive.
The adhesive should have a melting point in the range from 70 to 250°C, preferably in the range from about 110 to 220°C.
The adhesive should be compatible with the substrate, e.g. the same adhesive may not be suitable for both sweat shirts and fine gauze fabrics. Tack should be retained throughout the printing process, which may include flash cure of each colour prior to printing with an additional colour or freezing of each printed ink layer with liquid nitrogen prior to printing of subsequent colours.
One particular advantage of the adhesive of the present invention over the use of an aerosol adhesive is that during flash curing, which involves partially drying the printing ink with heat and/or IR radiation, the hot melt, sprayable adhesive is warmed and it retains its adhesive tack.
The adhesive is applied to the substrate by hot melt spraying. This method involves heating the adhesive to about 110 to 220°C so that it becomes molten and then spraying the molten adhesive on to the support base.
The spraying is carried out by use of a spray gun, for example, Hot Shot Adhesive gun available from Sericol Limited, which uses compressed air at up to 10 bar, and ideally from 5 to 7 bar. Power HB 600 spay melt, a spray gun available from Power Adhesives Limited, is also suitable for use in the present invention. This gun uses compressed air at up to 8 bar, heated to 70 to 250°C. Other hot melt spray equipment can also be used.
The spraying is conveniently carried out using a hand held spray gun provided with an electrically heated compartment for the adhesive and means for supplying air under pressure to the molten adhesive to assist the spraying. Instead of the gun being hand held the adhesive may be automatically applied in a preset pattern using an automatic spray gun or guns or other suitable applicators.
The Hot Shot Adhesive gun uses cores of adhesive. The cores of adhesive are placed in the gun barrel. Part of the core at the firing end of the gun barrel is heated by an electrical component so that it is in a molten sprayable state. The other cores are not initially heated by the electrical component so they remain in a solid state. The cores in the solid state form a piston to help force the molten adhesive through a nozzle and out of the gun. The cores at the firing end of the gun barrel are melted on demand and the gun barrel refilled with solid cores. The nozzle in the gun needs to be capable of spraying the hot molten adhesive from the gun. The nozzle preferably has a diameter of form 0.8mm to 1.5mm, and more preferably a diameter form 1 mm to 1.2mm.
Details of the Hot Shot Adhesive gun can be found in patent no. US-A-5,375,766. Details of the adhesive cores can be found in FR 239 3044 and DE 282 3898.
The spray gun should be adjusted so that the hot melt spray is even and fine and the glue lays as flat as possible on the base.
After spraying the adhesive on to the base, the substrate is brought into contact with the base by hand, by the press operator, and pressure is applied to smooth out any wrinkles in the substrate.
The screen printing operation is effected by forcing ink through the open areas of a stencil on to the substrate in known manner.
After printing, the substrate is removed from the base by peeling off by hand.
The base may then be reused in some cases without the need for reapplication of adhesive.
After the base has been used several times, the adhesive may become contaminated with fibres and it may be necessary to reapply the hot melt sprayable adhesive, and this can be carried out as described above.
According to another aspect of the invention there is provided a method of preparing a base material to receive a textile or similar material and releasably bond thereto so that the textile may be screen printed, which method comprises applying to the base a hot melt, sprayable adhesive.

Claims

A method for screen printing a substrate, the method comprising temporarily holding the substrate to a base during the screen printing, characterised in that the substrate is releasably held to the base by means of a pressure sensitive, hot melt, sprayable adhesive.
A method for screen printing substrates, the method comprising the steps of:
a) spraying a hot melt adhesive on to a base,

b) temporarily attaching a substrate to the base,

c) screen printing the substrate,

d) removing the printed substrate from the base,

e) if necessary, respraying hot melt adhesive on to the base, and

f) continuing from step b).
The method claimed in Claims 1 or 2, wherein the adhesive is applied to the base by the use of a spray gun.
The method claimed in Claim 3, wherein the adhesive is heated to a temperature in the range 70 to 250°C, preferably 110 to 220°C, and sprayed on to the base.
The method claimed in Claims 3 or 4, wherein the adhesive is heated electrically in a compartment of the spray gun.