GB2349355A

GB2349355A - Infrared radiation heating process

Info

Publication number: GB2349355A
Application number: GB9909710A
Authority: GB
Inventors: Richard Anthony Rudd Little
Original assignee: Jenton International Ltd
Current assignee: Jenton International Ltd
Priority date: 1999-04-27
Filing date: 1999-04-27
Publication date: 2000-11-01
Also published as: GB9909710D0

Abstract

A process for joining, sealing and/or encapsulating an object comprising the steps of, providing a quantity of solid thermoplastics adhesive (8), positioning the solid adhesive on the object, melting the adhesive by applying heat of a predetermined magnitude and for a predetermined duration by irradiation with infra-red radiation having a wavelength in the range from 700nm to 1500nm, the source of infra-red radiation (2) being located remotely from the object and the infra-red energy being transmitted to a position close to the object via an optical fibre or liquid filled light guide (6), the heat being applied without directly contacting the object or the adhesive. The adhesive 8 may be applied to the object in powdered form or in a pre-shaped form. The hot melt adhesive 8 is typically based on polyamides, polyethylene. EVA, polypropylene or polybutyl materials. The adhesive is typically melted in the range 150 {C to 180 {C.

Description

PROCESS FOR JOINING SEALING AND/OR ENCAPSULATING AN OBJECT This invention relates to processes for joining, sealing and/or encapsulating an object.

According to a first aspect of the invention there is provided a process for joining, sealing and/or encapsulating an object comprising the steps of, providing a quantity of solid thermoplastics adhesive, positioning the solid adhesive on the object, melting the adhesive by applying heat of a predetermined magnitude and for a predetermined duration by irradiation with infra-red radiation having a wavelength in the range from 700nm to 1500nm, the source of infra-red radiation being located remotely from the object and the infra-red energy being transmitted to a position close to the object via an optical fibre or liquid filled light guide, the heat being applied without directly contacting the object or the adhesive.

According to a second aspect of the invention there is provided a process for joining two objects together comprising the steps of applying heated adhesive to at least one of the objects, allowing the adhesive to cool, and subsequently joining the objects according to the process of the first aspect.

Embodiments of a joining, sealing and/or encapsulation process in accordance with the invention will now be described by way of example with reference to Figure 1 which is a schematic cross section of apparatus for applying heat to the adhesive. It is known to join objects using a meltable adhesive (commonly termed a"hot melt adhesive"). This invention is applicable to the field of hot melt adhesives.

Typically hot melt adhesives are thermoplastics materials. Typically they are based on polyamides, polyethylene, EVA, polypropylene or polybutyl materials. In use, the material is melted using temperatures typically in the range of 150 C to 180 C. The material is then able to wet objects to be joined. Once the objects are in position, the adhesive can be allowed to cool and typically begins to form a bond between the objects at temperatures of 90 to 100 C.

Hot melt adhesive systems have several advantages; in particular, no solvents are required in use and they are relatively cheap. Furthermore, the material can be reworked and thus it is possible to recycle the adhesive.

Conventionally, the adhesive is applied by first heating a quantity of the adhesive to beyond its melting point. The adhesive may then be distributed in its liquid form by means of pipes and finally applied using a nozzle. However, since the adhesive solidifies quickly (in some cases in as little as 5 seconds), it is necessary to heat the pipes and nozzles (applicators) to ensure that the adhesive does not solidify until it has reached the point where objects must be joined together. Thus, in order to maintain a controlled viscosity, calibrated heaters are used throughout the distribution network of pipes and applicators. This is costly. Also, in order to minimise the distribution problem, it is common to have bulky apparatus which performs the initial heating and also storing of the stock of solid material, located close to the point of application. This presents problems and increases costs when designing and operating a production line.

An additional problem with the conventional application process is that of"stringing". When melted, hot melt adhesives are still relatively viscous and have high surface tension values. Thus it is difficult to dispense very small quantities and/or accurately place the materials since the applicator and object to which the adhesive has been applied often becomes connected by a"string"of fluid adhesive which becomes dragged across the object to which the adhesive is being applied. Also, it is difficult to apply a thin layer of the adhesive since a thin layer solidifies very quickly when it comes into contact with a relatively cooler object to which the adhesive is applied.

It therefore does not flow properly when applied in thin layers.

In accordance with the present invention, instead of applying the adhesive in liquid form, the adhesive is applied in solid form. For example, in the form of a solid block or pellets or in powdered form. This does away with the need for complicated and expensive supply piping and heaters. In order to join objects using the adhesive, heat is applied at an appropriate part of the manufacturing process of the object. Thus, at the point where the adhesive is used, it is only necessary to supply heat rather than heat and melted adhesive.

Typically, the heat is applied using a source of infra-red radiation. The optimum wavelengths are between 700 and 1500 nm and the glue may be adapted by including, for example, black particles, to absorb particular wavelengths of infra-red radiation more efficiently.

The use of a radiant source of heat avoids the stringing problems associated with the prior art. Thus it is possible to obtain precise and (if desired) small dosages of the order of 1 micro litre. This opens up new applications for hot melt adhesive systems such as, for example, fixing and/or encapsulation of electrical components on carriers such as printed circuit boards. Or sealing holes or porous materials.

As described above, the adhesive may be supplied in solid blocks. These may, for example, be machined or moulded to particular shapes which assist in the manufacture of the objects being joined. The shape may for example help to locate the solid glue on the object prior to the application of heat.

Alternatively, the adhesive may be preceding-applied using conventional applicators at a separate location. Thus application may occur remotely from use of the glue. This is advantageous since not only can the cost and inconvenience of the prior art application method be minimised, but the application can occur in a relatively dirty environment whereas the use may be made in a relatively clean environment, for example, in an electronic assembly process.

After the"preceding-application", the adhesive is re-worked by heating as described above.

The heat may be applied using a source of radiant energy (for example a quartzhalogen bulb) located at a remote end of a flexible light guide or optical fibre with the infra-red energy being applied to the adhesive using the other end of the light guide.

An exemplorary arrangement is shown schematically in Figure 1. A quartz-halogen bulb 2 radiates infra-red energy at wavelengths greater than 800 nm. The energy is focused by a gold-coated reflector 4 onto a light guide (preferably a flexible light guide) 6. The light guide may be up to 2 metres in length. At the other end of the light guide, the infra-red radiation impinges on a quantity of hot melt adhesive 8. The size of the beam where it impinges the adhesive is of the order of 1 Omm in diameter. This size may be achieved or varied using converging or diverging lens systems between the end of light guide 6 and the adhesive 8.

The duration and magnitude of the applied heat is controlled to achieve a desired degree of wetting/flowing of the solid adhesive. These characteristics may be varied using a shutter and/or iris arrangement inserted in the light path between the bulb 2 and the adhesive 6. Melting of the adhesive may, for example, be performed in two steps; a first partial melt (low heat input) followed by precise positioning or re-positioning of the objects to be joined and then a second full melt to secure the objects together.

Claims

CLAIMS 1. A process for joining, sealing and/or encapsulating an object comprising the steps of, providing a quantity of solid thermoplastics adhesive, positioning the solid adhesive on the object, melting the adhesive by applying heat of a predetermined magnitude and for a predetermined duration by irradiation with infra-red radiation having a wavelength in the range from 700nm to 1500nm, the source of infrared radiation being located remotely from the object and the infra red energy being transmitted to a position close to the object via an optical fibre or liquid filled light guide, the heat being applied without directly contacting the object or the adhesive.
2. A process according to claim 1, wherein the heat is applied in a spot of less than 10mm diameter.
3. A process according to claim 1 or claim 2 wherein the total power of the applied heat is less than 25W.
4. A process according to any preceding claim, wherein the step of providing a quantity of solid adhesive is performed at a different location from the remaining steps.
5. A process according to any preceding claim, including the step of performing precise positional adjustment of the object relative to another object before applying heat.
6. A process according to any preceding claim, wherein the quantity of adhesive is of the order of 1 micro litre.
7. A process according to any preceding claim, wherein the adhesive is provided in powdered form.
8. A process according to any of claims 1 to 6, wherein the adhesive is provided in a pre-shaped form which is located by and/or engaged by the object.
9. A process according to claim 8, wherein the adhesive is pre-shaped by a moulding and/or machining process.
10. A process for joining two objects together comprising the steps of applying heated adhesive to at least one of the objects, allowing the adhesive to cool, and subsequently joining the objects according to the process of any preceding claim.
11. An article formed from at least two objects joined by the process of any preceding claim.
12. An object encapsulated using the process of any of claims 1 to 9.
13. An object sealed using the process of any of claims 1 to 9.
14. A process as described herein with reference to the drawing.