EP1718419B1

EP1718419B1 - A method for dispensing viscous materials

Info

Publication number: EP1718419B1
Application number: EP05700646.2A
Authority: EP
Inventors: Ture Kindt-Larsen; Jens-Erik Sorensen; Niels Falk
Original assignee: Chempilots AS
Current assignee: Chempilots AS
Priority date: 2004-02-13
Filing date: 2005-02-11
Publication date: 2013-11-06
Anticipated expiration: 2025-02-11
Also published as: WO2005077547A1; ES2436754T3; HK1096898A1; HUE020836T2; EP1718419A1

Description

RELATED APPLICATION INFORMATION

This patent application claims priority of Danish patent application, PA 2004 00220 which was filed 13 February 2004 .

FIELD OF THE INVENTION

The invention relates to the field of dosing or dispensing high viscosity materials with a very high precision and reproducibility. A new method for dispensing accurate portions of highly viscous materials is provided wherein a flowing string of viscous material is metered from a nozzle and when a desired amount has been delivered the string of viscous material connecting nozzle and the delivered material is exposed to one or more heat pulses sufficient to lower the viscosity or the surface tension or both of a small area or cross-section of the string and give a well defined separation between the dispensed portion and the viscous material in the nozzle. In a preferred version the string of high viscosity material is separated from the dosing nozzle with no mechanical contact between the viscous material and the source of heat. In some embodiments, the method of the present invention is particularly useful for heat sensitive and reactive materials with viscosities up to 1,000,000 cps and above. Also provided is a new process for the precision dosing or dispensing of reproducible amounts of high viscosity materials onto surfaces or into moulds or packages. Further apparatus is described for carrying out the new method and process.

BACKGROUND

The accurate dosing or dispensing of viscous materials is a crucial part of many industrial processes. Generally the viscous material is dosed by a metering pump, and the amount of material is controlled by controlling the speed and running time of the pump, the travel length of pistons, or by opening and closing of outlet valves.
The higher the viscosity of the material and the smaller the individual doses, the more difficult it becomes to achieve high precision and reproduceability in the amount of material dispenses.
Various means to improve this situation have been developed. Small plunger pumps have been made in ceramic materials to reduce dimensional changes due to pressure changes during the dosing. Improved valve constructions have been suggested including slide valves or rotating valves.
A special problem arises in volumetric dosing or dispensing from a nozzle onto a surface of, for example, a container or a mould.
During the dosing step the viscous material is forced out of the nozzle as a string of material between the nozzle and the receiving surface. At the end of the dosing step, the flow of material through the nozzle is stopped, for example, by removing the driving pressure or driving force acting on the viscous material.
After the flow of the material through the nozzle is stopped, the viscous material in the string between the nozzle and the receiving surface will continue to flow under the action of gravity and the surface forces and interfacial forces, as defined by the nature of the viscous material and the nozzle material. During this the string may curl or swing or form beads until the string in one place becomes so thin that it breaks in an uncontrollable way under the forces acting upon it.
After breaking of the string, the free surfaces of the viscous material will change into a shape determined primarily by the surface/interfacial forces and the time such forces are allowed to act before a new dosing cycle is started. For example, if the string is broken close to the nozzle, the viscous material may slightly retract into the nozzle due to capillary forces. If the break of the string has happened far from the nozzle, the surface force may draw the viscous material back towards the nozzle where it may spread and wet the outer surface of the nozzle.
At the receiving surface, a swirling string, or a string, which is broken close to the nozzle, may fall onto the receiving surface in any direction and thereby removing control of the spatial distribution of viscous material on the receiving surface. Also a long string falling onto the surface may entrap air bubbles into the viscous material, which in many applications will constitute a major defect.
It will be evident that the lack of control of the cut-off of the viscous material leads to severe problems when the application demands a high precision in the amount of viscous material dosed or dispended. Also the lack of control of where the string of the viscous material is cut-off, leads to other problems such as entrapped air bubbles and uneven spatial distribution of the viscous material dosed or dispended.
Various means to alleviate these problems have been proposed in the past. For example, the cutting means can be moved into the string of viscous material to cut it off at a predetermined position relative to the nozzle. A major drawback of such devices has been that the viscous material will adhere to the cutting surface(s) and the problem of cut-off is moved from the nozzle to the cutting means. Another problem is that viscous material will collect on the cutting means and has to be removed from time to time.
We have now after much experimenting found a new method for high precision dosing and/or dispensing of viscous materials. In some embodiments, the present invention provides a method which allows highly reproducible dosing and/or dispensing of materials having viscosities of up to 1 million centipoises and higher, with a high accuracy and good control of spatial distribution of the material dispensed and with no changes in the properties of the material being dosed. In one version the method is integrated into a new process for performing a clean high precision dosing on an industrial scale.
WO 90/14233 discusses a liquid jet recording apparatus comprises a cavity having at least one exit orifice and means for causing pressure variations within a liquid in the cavity whereby to cause the liquid meniscus at the orifice to oscillate. Means are provided for enabling ejection of a liquid from the oscillating orifice.
US 6352337 discusses a droplet generator that is particularly adapted for generating micro droplets of ink on demand in an inkjet print head having a plurality of nozzles. The droplet generator includes a droplet separator formed from the combination of a droplet assistor and a droplet initiator. The droplet assistor is coupled to ink in each of the nozzles and functions to lower the amount of energy necessary for an ink droplet to form and separate from an ink meniscus extending across the nozzle outlet. The droplet initiator cooperates with the droplet assistor and selectively causes an ink droplet to form and separate from the ink meniscus.

DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the general principle of the method according to the invention. From a nozzle, (10), a viscous material is supplied onto a receiving surface, (11). A string of viscous material, (12), connects the nozzle, (10), and the deposited viscous material, (13). A heating means, (14), is positioned relatively close to the string so that a controlled amount of heat can be supplied to a small surface area or cross section, (15), of the string, (12). If necessary or desired means, (16), (17) and (18), for moving the receiving surface, (11), respectively the heating means, (14), and the nozzle, (10), relatively to each other during the cut-off of the viscous material may be used.
Figures 2a to 2f illustrate a process in which the method is utilized. In figure 2a a receiving surface, (11), is positioned under a nozzle, (10), connected to a metering device (not shown) from which a viscous material can be supplied. In figure 2b the nozzle, (10) by means of positioning device (18), has been moved closer to the receiving surface, (11), to assure that the viscous material, (13), is delivered onto the receiving surface, (11), without air bubbles being entrapped at the interface.
In figure 2c the metering has been stopped, and the distance between the receiving surface, (11), and the nozzle has been increased sufficiently to form a string of viscous material, (12).
In figure 2d the heating means, (14), has been positioned around the string and is supplying a controlled dose of heat towards a narrow surface band, or cross-section, (15), of the string, (12).
In figure 2e the dose of heat has rapidly lowered the viscosity, and the surface tension of the viscous material in the narrow band or cross section, (15), with the- result that the string has necked-down at a well-defined position, (15).
In figure 2f the dose of heat has resulted in an accurate and reproducible separation of the string. (21) is a new surface of viscous material in the nozzle ready for a new process cycle, and (22) is a precise dose of viscous material deposited on the receiving surface, (11). In the figure the heating means, (14), has been returned to a stand-by position so they will not interfere with the first process steps during the next process cycle.
Figures 3a to 3e show the dispensing of a precise amount of a viscous material into a mould cavity, (31). In figure 3a a nozzle, (30), is connected to a supply of a highly viscous reactive composition. (31) is a concave mould part, and (34) is a cold heating means placed in a stand-by position. In figure 3b the mould has been moved up to a position close to the tip of the nozzle, and a portion of highly viscous reactive material, (33), has been deposited onto the surface of the mould cavity.
In figure 3c the mould, (31), has been lowered to form a string, (32), and the heating means, (34), has been moved down to an operating position ready to supply the focused heat. Alternatively, the nozzle can be moved upwards relatively to the mould.
Figure 3d shows the situation just after the separation of the string at the position determined by the heat pulse - optionally assisted or promoted by a slight movement of the mould and/or the nozzle away from each other. (33) Is a precise dose of high viscosity reactive material positioned in the mould cavity, (34) is the now inactive heating means ready to be retracted into a stand-by position?
In figure 3e the mould with the precise dose of material, (33) has been lowered to allow movement into a following process step and the heating means, (34) has been retracted to the stand-by position.
Figure 4 shows a method wherein the means of heating, (44), is placed further away from the mould cavity, (41), and the heat is supplied in the form of one or more pulses of focussed infra-red radiation or as energy in the form of laser light. In the figure (40) is a nozzle through which an amount of a highly viscous material, (43) has been dispensed into the mould, (41), and (45) indicates the cross-section of the material that will be heated by the energy supplied from the heating means, (44), and neck down to afford separation from the material in the nozzle.

SUMMARY OF THE INVENTION

The invention relates to a new method for performing and controlling the precision and reproduceability of the dosing or dispensing of high viscosity materials useful in industrial processes.
In a broad sense in some embodiments, the method utilizes a small amount of heat, for example, in the form of one or more heat pulses, to cut-off the string of viscous materials between the nozzle from which the material it is dosed and the dispended portion. The heat pulse is focused so that a well-defined small area or cross section of the string can be heated for a very brief period of time, thereby allowing gravity and surface tension to separate the string in a very controllable, gentle and highly reproducible manner.
In one version the cut-off is performed by contact with a clean heated surface, such as, for example in the form of a wire. The wire may be flat and temperature controlled by pulse heating.
In a preferred version the heating is achieved with no mechanical contact between the viscous material and the source of heat. The heating pulse can be supplied for example, as infrared heat from a heating element close to the string at the time of cut-off. Alternatively, the distance between heat source and string may be larger; and the heating pulse may be supplied, for example, in the form of one or more focused laser beams or jets of hot gas.
In some embodiments, the present invention provides a new process for dispensing and processing accurate portions of highly viscous materials wherein the viscous material is metered from a nozzle said process comprising the following steps:

a) Place a receiving surface in a predetermined position relatively to the dosing nozzle.
b) Start dispensing the viscous material onto the receiving surface. Optionally move the receiving surface and nozzle relatively to each other during the dispensing to maintain a desired relation between the nozzle and receiving surface.
c) Optionally move the receiving surface relative to the nozzle to prepare a string of viscous material connecting the nozzle and the delivered material.
d) After dispensing, supply at least one heat pulse to lower the viscosity, or the surface tension, or both, of a small area or cross-section of the string and give a well defined separation between the dispensed portion and the viscous material in the nozzle at a predetermined position. Optionally, the heating means can be moved from a stand-by position into an optional operational position and optionally moving it back again after the heating.
e) Move the receiving surface with the dispensed material to a holding station or into a new process step for further processing.

The receiving surface may be any flat, convex, concave, or otherwise curved surfaces onto which a precise dose of viscous material is to be accurately placed in a predetermined position.
In a preferred version the receiving surface is a mould cavity for casting a shaped object and the viscous material is a reactive polymer precursor composition and the further processing step is a curing optionally after a forming step In another version the viscous material is a reactive composition comprising a polymeric material plasticized with a monomer.
In another version the receiving surface is a container for packaging the highly viscous material
In some embodiments, the invention further comprises apparatus for carrying out the new method and process.
In a broad sense of the invention the string of viscous material can be cut off by any means capable of heating a cross section or band of the string sufficiently to perform a clean well-defined and reproducible cut-off of the viscous material. In some embodiments, this may be performed with a heated sharp edge or blade moved close to or across the path of the string. To avoid accumulation of viscous material on the heated surface and the resulting decomposition of the material, this sharp edge or blade can be in the form of a thin round or flat wire which is cleaned between cuts or, preferably, is a continuous wire which is moved sidewise between cuts so the cuts are always made with a fresh piece of wire.
In a more preferred version of-the method, the heating can be accomplished with no mechanical contact between the heating means and the string of viscous material.

DETAILED DESCRIPTION OF THE INVENTION:

Definitions

As used in this application, the following definitions may be used to assist in define the invention:

Dosing: a predetermined amount of viscous material that will be dispensed onto a receiving surface.
Dispensing: the action of moving a viscous material through a nozzle, onto a receiving surface.
Precision: highly accurate so as to conform to strict tolerances.
Reproduceability: to produce again with precision.
Highly Viscous Material: any material that is difficult to deliver in precise amounts due to one or more of high viscosity and a tendency to form strings during separation into smaller portions, for example, a viscous material can include a reactive polymer precursor composition with a viscosity greater than 100,000 cps.
Heat pulse: one or more heat cycles from an apparatus that allows the string to be separated between the dispensed portion and the viscous material in the nozzle.
String: a thin threadlike piece of highly viscous material connected between the nozzle, and the dosed material placed onto the receiving surface.
Receiving surface: any flat, convex, concave, or otherwise curved surfaces onto which a precise dose of viscous material is to be placed accurately in a predetermined position.

The invention in various exemplary versions and embodiments will now be described in more details by referring to the drawings. Figure 1 illustrates the general principle of the method according to the invention. From a nozzle, (10), a viscous material is supplied onto a receiving surface, (11). A string of viscous material, (12), connects the nozzle, (10), and the deposited viscous material, (13). A heating means, (14), is positioned relatively close to the string so a controlled amount of heat can be supplied to a small surface area or cross section, (15), of the string, (12). As necessary or desired means, (16), (17) and (18), for moving the receiving surface, (11), respectively the heating means, (14), and the nozzle, (10), relatively to each other during the cut-off of the viscous material may be used.
The receiving surface may be any surface of any geometry and of any material onto which it is desired to position a precise amount of a material with a high viscosity. The receiving surface may be a part of an individual item each item receiving one or a small number of portions of highly viscous material, such as individual moulds, or packing containers, or the receiving surface may be part of a continuous band.
The means (16), (17), and (18) may be any form of mechanical, electric, hydraulic, or pneumatic actuators capable or moving the individual parts with the necessary precision to accomplish the positioning of a precise positioning of a precise portion of the highly viscous material.
In some versions of the process they may not be necessary
Figures 2a to 2f illustrate a process in which the present invention is utilized. In figure 2a a receiving surface, (11), is positioned under a nozzle, (10), connected to a metering device (not shown) from which a viscous material can be supplied. In figure 2b the nozzle (10) by means of position device (18) has been moved closer to the receiving surface (11). Alternatively the receiving surface, (11), may be moved closer to the nozzle, (10), to assure that the viscous material, (13), is delivered onto the receiving surface, (11), without air bubbles being entrapped at the interface.
In figure 2c the metering has been stopped, and the distance between the receiving surface, (11), and the nozzle has been increased sufficiently to form a string (12) of viscous material.
Figure 2d shows the heating means (14) positioned around the string and supplying a controlled dose of heat towards a narrow surface band, or cross section, (15), of the string, (12).
The dose of heat will rapidly lower the viscosity, and the surface tension of the viscous material in the narrow band or cross section, (15), with the result that the string will neck-down at a well-defined position (15) as shown in figure 2e.
This will rapidly result in an accurate and reproducible separation of the string, as shown in figure 2f, where (21) is a new surface of viscous material in the nozzle ready for a new process cycle, and (22) is a precise dose of viscous material deposited on the receiving surface, (11). In the figure 2f the heating means, (14), has been returned to a stand-by position so it will not interfere with the first process steps during the next process cycle.
Figures 3a to 3e show the dispensing of a precise amount of a viscous material into a mould cavity (31).
The viscous material may be a reactive composition comprising one or more crosslinkable prepolymer in a solvent or diluent. Due to the high reactivity of such materials, they are unstable and cannot be handled or processed at elevated temperatures. At acceptable temperatures, such as 10 to 50 degC, they have extremely high viscosity and are difficult to dispense in precise reproducible portions. With such materials the method and processes according to the invention are particularly advantageous.
In figure 3 a a nozzle, (30), is connected to a supply of a highly viscous reactive composition (not shown). (31) is a concave mould part, and (34) is a cold heating means placed in a stand-by position. In figure 3b the mould has been moved up to a position close to the tip of the nozzle, and a portion of highly viscous reactive material, (33), has been deposited onto the surface of the mould.
In figure 3c the mould, (31), has been lowered to form a string, (32), and the heating means, (34), moved down to an operating position ready to supply the focused heat. Alternatively, in some embodiments, the nozzle can be moved upwards relatively to the mould to form the string.
Figure 3d shows the situation just after the separation of the string at the position determined by the heat pulse - optionally assisted or promoted by a slight movement of the mould and/or the nozzle away from each other. (33) is a precise portion of high viscosity material positioned in the mould cavity, and (34) is the now inactive heating means ready to be retracted into a stand-by position.
In figure 3e the mould has been lowered to allow movement into a following process step and the heating means has been retracted to the stand-by position.
The mould contains an accurate amount of highly viscous reactive material, which has been placed very precisely in the mould and with full control over the concentric distribution of material.
Figure 4 shows a method wherein the means of heating, (44), is placed further away from the mould cavity, (41), and the heat is supplied in the form of one or more pulses of focussed infra-red radiation or as energy in the form of laser light. In the figure (40) is a nozzle through which an amount of a highly viscous material, (43) has been dispensed into the mould, (41), and (45) indicates the cross-section of the material that will be heated by the energy supplied from the heating means, (44), and neck down to afford separation from the material in the nozzle.
It should be understood that the inclusion of means for controlling the positions of the nozzle, the receiving surface and the heating means relative to each other during the process gives many advantages to the process, and a great flexibility in optimising process conditions to the specific properties of the viscous material and receiving surface.
For example, the movement of the nozzle and receiving surface relatively to each other allows a deposition of viscous material with the nozzle very close to the receiving surface.
During the metering the distance may be controlled to give a desired wetting of the receiving surface or controlling the wetting of the nozzle tip.
The formation and/or control of the string dimensions before the cutting can also be controlled by changing the distance between the nozzle and the receiving surface, and finally a slight elongation of the string during or after the heat pulse has been supplied can give a faster necking-down and separation.
The means for controlling the positions of the nozzle, the receiving surface and the heating means relative to each other may be any form of mechanical, electric, hydraulic, or pneumatic actuators capable or moving the individual parts with the necessary precision to accomplish the positioning of a precise positioning of a accurate portion of the highly viscous material.
In some versions of the process they may not be necessary.
In processes where the entrapment of air bubbles is of no concern it is also within the invention to cut off a precise portion of high viscosity material by use of one or more heat pulses before contact is made between the high viscosity material and the receiving surface.
The viscous material may be any material that is difficult to deliver in precise amounts due to one or more of: high viscosity, and a tendency to form stable strings during separation into smaller portions. This problem is often found in microstuctured fluids and fluids containing polymers of high molecular weight and/or comprising a high concentration of polymer. Such complex fluids manifest an extensional viscosity effect which dramatically increases the lifetime of a fluid thread undergoing capillary break-up In a pinching thread, elastic and viscous forces resists the effect of surface tension and control the "necking" and slows down dramatically the pinch-off process leading to "tackiness", and "stringiness". When such fluids must be delivered in precise portions for further processing or for packaging the method of the invention can be used with advantage particularly if the material is temperature sensitive or contain a temperature sensitive additive and must be handled at low temperatures. Some examples of such high viscosity temperature sensitive materials can include solutions of polymers comprising thermosensitive pharmaceutically active ingredients.
A preferred group of high viscosity materials are those that contain reactive groups that can utilized for curing of the material in a later process step following the precise dosing of the material using the method of the invention. The curing reaction may be by condensation or by addition.
A particularly preferred group of the high viscosity materials are reactive polymer precursor compositions comprising one or more crosslinkable prepolymer in a solvent or diluent. Such materials are for example used in the moulding of contact lenses and are well known in the art. Due to the high reactivity of such materials, they are unstable and cannot be handled or processed at elevated temperatures. At acceptable temperatures, such as 10 to 50 deg C, they have extremely high viscosity and are difficult to dispense in precise reproducible portions.

EXAMPLES:

A poly-hydroxyethylmethacarylate, which has been functionalised with methacrylate groups, is mixed with PEG 200 and a photo initiator to make a reactive polymeric casting composition.
The casting composition is placed in reservoir connected to metering device comprising a numerical volumetric dispenser typeNVD2 with control system type CLNVD from ATM Automation AG, CH-2076 Gals, in Switzerland.
The metering set-up includes heating means for close control of the temperature of the casting composition. A mould fixture with means for accurately positioning and movement of mould cavities under the outlet nozzle is also provided.
A number of dispensing experiments are made where the casting composition is dosed into mould cavities, and the weight of each dose is determined. Table 1 Comparative Example

Dose number Weight of dose milligrams.

1 42,25

2 42,45

3 42,32

4 42,65

5 42,42

6 42,31

7 44,34

8 43,68

9 42,56

10 42,60

Average 42,75

Max. -Min. 2,09

STD 0,6902
It will appear from the results that the variation in weight from sample to sample is high. It is also observed that the dispensed material is sometimes placed off centre.
The set-up is now modified according to the invention by including a ring-shaped heating element with an inner diameter slightly larger than the nozzle. The heating element is mounted on an actuator in the form of a step-motor, which can rapidly move the heating element coaxially with the nozzle and place it in precisely in a desired position along the axis of the nozzle and the string. The heating element is in the form of a flat thin resistance wire and has a very small thermal mass.
The heating wire material has a linear temperature/resistance ratio, and its temperature can be controlled by measuring the resistance of the heating element as the temperature changes. This is done by a Resistron Controller from Ropex Industrie Elektronik GmbH, D-7120 Bietigheim-Bissingen, Germany. This controller measures Voltage and Current 50 times/second, and the Voltage is then automatically adjusted by phase control to achieve the desired temperature. This allows a very rapid temperature to be reached and held constantly with high precision. Due to the minute thermal mass of the heating element, a very rapid heating to the control temperature is achieved when the current is switched on and a rapid cooling is achieved when the current is switched off.
In the experiment the temperature of the wire during the heat pulse is set at 200°C ± 2°C.
The dispensing cycle is as follows. At the start of the cycle the heating element is placed in a stand-by position around the nozzle:

1. a polystyrene mould cavity is placed in a fixture below the nozzle,
2. the mould cavity is moved upwards until it nearly touches the nozzle,
3. the dosing pump draws the viscous material from the reservoir into the pump chamber,
4. the dosing pump forces the viscous material out of the nozzle and onto the mould cavity at a rate of 8 µl/sec,
5. The pump stops, and the mould cavity is lowered to form a string. The heating element is moved downwards to a position around the string just below the nozzle,
6. the heating is switched on for 1 second to give a heat pulse to the narrow section of the string positioned inside the heating element,
7. due to this slight heating the string rapidly necks-down and breaks,
8. the mould is moved down to allow removal for further processing steps, and the heating element is moved up to its stand-by position.

The whole cycle is done in 6 seconds. The weight of the material dispensed is given in tables 2. It can be seen that by using the method of the invention, the highly viscous material can be dosed with a much-improved precision. In addition the dispensed material is placed precisely in the centre of the mould. Table 2 Dosing according to the Invention

Dose number Weight of dose milligram

1 30,99

2 30,79

3 31,00

4 30,74

5 30,89

6 31,01

7 30,98

8 30,85

9 30,95

10 30,90

Average 30,91

Max. -Min. 0,26

STD 0,093333

Claims

A method for dispensing accurate portions of highly viscous materials, the method comprising the steps of:
metering the viscous material from a nozzle (10) and at least until a desired amount has been delivered to a receiving surface (11) and a string (12) of viscous material connecting nozzle and the delivered material (13) is formed;
characterized by exposing the string of viscous material to one or more heat pulses sufficient to lower at least one of: the viscosity and the surface tension of a small area of the string and provide separation between the dispensed portion and the viscous material in the nozzle.
The method of claim 1 wherein the string is formed during the metering operation.
The method of claim 1 wherein the string is formed after the metering operation.
The method of claims 1, 2 or 3 wherein the string is formed by increasing the distance between the nozzle and the receiving surface.
The method of claim 1 wherein the string is put under tension during the heating by increasing the distance between the nozzle and the receiving surface
The method of claim 1 wherein the heat is applied with no mechanical contact between the viscous material and the source of heat
The method of claim 6 wherein the heat is supplied in the form of infrared heat.
The method of claim 6 wherein the heat is supplied in the form of one or more beams of laser energy.
The method of claims 1 or 6 wherein the heat is supplied to one side of the string.
The method of claims 1 or 6 wherein the heat is supplied to a narrow band around the string.
The method of claims 1 to 10 wherein the viscous material comprises a reactive polymer precursor composition.
The method of claim 11 wherein the reactive polymer precursor composition is a contact lens casting composition.
The method of claim 1 wherein the small area of the string comprises a cross section.
An apparatus for carrying out the method of claim 1, the apparatus comprising;
a dispensing nozzle (10) for dispensing the string (12) of highly viscous material onto a receiving surface (11);
a metering apparatus operatively attached to the dispensing nozzle for metering a desired amount of highly viscous material dispensed as the string from the dispensing nozzle; and characterized by
a source of heat (14) located in near proximity to the string of highly viscous material,
wherein the heat is of sufficient magnitude to lower the surface tension and viscosity of a small area of the string of highly viscous material.
The apparatus of claim 14 wherein the source of the heat comprises a device generating pulses of focused infrared radiation.
The apparatus of claim 14 wherein the source of the heat comprises a device generating laser light energy.
The apparatus of claim 14 wherein the source of the heat comprises a resistance wire.
The apparatus of claim 14 wherein the source of the heat comprises a continuous wire and the apparatus further comprises a means for moving the wire relative to the string of highly viscous material so that a small area of a subsequent metered dose of the highly viscous material can be heated with a portion of the wire not previously in contact with the highly viscous material.
The apparatus of claim 14 wherein the small area of the string of the highly viscous material comprises a cross section of the material.