There are many techniques used for the application of adhesives, and each has certain advantages and disadvantages. The choice of technique is governed by the nature of the part and the economics of equipment investment, labor cost, and the cost of the adhesive.
The methods of adhesive application can be broken down into eight basic approaches:
c) Dispensing nozzle
d) Roll Coating
e) Transfer Printing
f) Screen Printing
g) Curtain Coat
h) Application as a solid (film adhesive)
Brush and trowel application are very similar. The adhesive is applied to a surface and spread around with a tool or brush. Advantages of this approach are low capital investment and the ability to cover irregular surfaces. There are several disadvantages, however: high labor content, the possibility of inconsistent coating thickness, and the tendency to use more adhesive than necessary. Certain types of adhesive (hotmelts) are almost impossible to spread by brush or trowel.
In Spray application the adhesive is atomized into fine droplets and deposited onto the surface of the substrate to form a uniform layer. The adhesive is either atomized by air pressure or by pumping at extremely high pressures through a small orifice. Spraying has the advantages of covering large areas quickly, depositing a thin coating, and the fact that it is good for covering irregular surfaces. Most adhesives can be applied by spraying. The drawback to spray application is that only part of the adhesive ends up on the substrate. A portion is atomized and lost into the air, and a portion is deposited on surfaces that adjoin the area being sprayed. This results in high adhesive usage (cost) and extra cost for the cleanup of oversprayed material.
Dispensing nozzles are a very popular method of adhesive application. They can be used for all types of adhesives and take many different forms. The simplest forms are hand-held squeeze bottles and the nozzle at the end of a caulking gun. The most complex are air actuated or electric actuated nozzles. In these nozzles, a needle-type valve blocks the flow of adhesive until it is wanted, then withdraws to allow the adhesive to flow through a fine orifice. The flow rate of the adhesive is controlled by the orifice size, the pressure in the supply line, and in some cases by the position of the needle. Most dispensing nozzles do not contact the part being coated and deposit a bead of adhesive. One variation of this is a nozzle designed to apply a swirl pattern to cover a larger area. Contact nozzles are also used. This style of nozzle is designed to contact the part and extrude a band of adhesive onto the surface. It can be used to apply a thin stripe (or stripes). Air and electric nozzles can be opened and closed rapidly, and are capable of depositing dots or interrupted strips of adhesive. The advantages of nozzle application are the ability to deposit adhesive only where needed, the ability to precisely control the amount of adhesive, and the ease of use in automated equipment. The disadvantage of nozzle application is the inability to cover large areas. It can be done with multiple nozzles or swirl nozzles, but the cost of equipment to cover large areas can become quite high.
Roll coating is used to rapidly apply a controlled coating thickness to a large area. The simplest form of roll coating equipment is a paint roller, but most rollcoaters are designed to apply adhesive to a substrate as it passes through the machine. The adhesive is held in a reservoir and is applied to the surface of the coating roller by one of several methods. The coating roll may dip into the tank, or the adhesive may be pumped from the reservoir and brought into contact with the surface of the coating roll. Once the surface of the coating roll is wetted with adhesive, excess adhesive is removed either by a scraper blade or by another roller, leaving a precise layer of adhesive on the roller to be applied to the part. There are many different configurations of roll coaters with different designs to apply adhesive to sheet goods, continuous webs, and different types of adhesives. Certain types of rollcoaters can be used to print patterns of adhesive in a manner similar to a printing press.
The advantage of rollcoaters are the ability to apply a precisely controlled layer of adhesive without waste, and to apply it quickly. Rollcoating machines are particularly useful for application of adhesive to large surfaces at high speeds.
The disadvantage of rollcoaters are that they only can apply adhesive to materials that are flat, and do not work well on surfaces that are heavily textured or contain bumps or hollows.
Transfer printing is a variation of rollcoating where a pattern pad is run across a roller or pressed against a pad to pick up adhesive, then is used to print the pattern of adhesive onto a part. Most transfer printing is done using rollers, but it can also be done using flat plates. It is an accurate way to deposit a precise pattern of adhesive, and is best suited to the application of a very thin layer. The process is fast, and can be used to print adhesive on items such as envelope flaps and windows.
Disadvantages of transfer printing are that it is best suited to covering only small areas and the fact that the dies and equipment can be expensive.
Screen printing can be used to apply a precisely controlled layer of adhesive to a specific area on a surface. In most cases this technique is used for “spot gluing” or the laydown of a pattern of adhesive. In this method, adhesive is forced through a screen by a squeegee, and the size of the openings in the screen (and screen wire diameter) determine the coating thickness.
The advantage of screen printing are the precise control of the adhesive layer and the ability to deposit adhesive in specific patterns. It is also possible to get into screen printing at a relatively low cost.
The disadvantages are that the process can tend to be labor intensive, although automatic and semi-automatic screen presses can be used, and these require far less labor than manual screening. The equipment cleanup can also be time consuming, and is a drawback.
Curtain Coating is an application method that utilizes a “waterfall” of adhesive or other coatings. The part is passed through a falling curtain of liquid and is covered by a layer of coating or adhesive. The liquid that does not land on the part is collected in a trough and is pumped back up to the point where it flows into a falling curtain. The thickness of coating deposited on the part is dependent on the viscosity of the material (which can influence the ability to form a thin sheet), and the speed at which the part travels through the curtain. Curtain coating is best suited to covering large areas with a relatively heavy coating.
Advantages of curtain coating are the speed of application and uniformity of application. It is also relatively easy to utilize curtain coating units in automated lines since the parts are coated on their top surface, and are fed through the coating section on conveyers.
Disadvantages of curtain coating are the inability to apply certain coatings and adhesives, the high cost of equipment, and the limited ability to apply a thin layer. Not all types of adhesive will run well in a curtain coater – the adhesive must be able to form a thin coherent fluid waterfall in order to be used in this type of equipment.
Application as a solid has become a very popular way to apply adhesives. In most cases the solid adhesive is in the form of a thin film. Some adhesive films contain a carrier sheet made of fibrous material or fabric, while others are unsupported films. Usually the adhesive film is mounted on a sheet of silicone coated release paper. To apply, the adhesive film is bonded to one substrate, the release paper is pulled off, and the part is applied to the second substrate. While most adhesive films are made with pressure sensitive adhesives, films can also be made with heat activated adhesive. Heat activated adhesives do not always require release paper as a carrier. They are carried on one substrate or are placed between the substrates to be bonded then heat is applied to liquefy the adhesive and form the bonds.
Several attractive features of film adhesives are the ease of handling (compared to liquid adhesives), the potential for low capital investment in application equipment, and the versatility to be used for a variety of part shapes and sizes.
The major disadvantage of film adhesives is their high cost. They can cost substantially more than liquid adhesives, in part due to the cost of the silicone release paper carrier.
Even though there are other techniques for applying adhesives, in almost all cases the basic approach will fall into one of the categories listed above. Sometimes combinations of one or more of these methods are used to overcome the drawbacks of a single approach or to tailor the application to the particular characteristics of the part being coated.
No one technique stands out as the best approach for every type of part. For each situation or type of part one technique will usually offer advantages over the other methods, and will prove to be the most cost efficient way to apply adhesive.