Metering pump

Abstract

 A pump is provided which is capable of delivering measured doses of a liquid, the pump comprising a body having a longitudinal passage, and a pair of opposed pistons slidingly received in said longitudinal passage, cross passages in said body which communicate with the longitudinal passage and constitute respectively an inlet port and an outlet port, means for moving the pistons in unison between the inlet port and outlet port while their opposed faces are spaced by a distance defining the liquid dose and means for closing the gap between the pistons while the pistons are in the region of the outlet port so as to deliver a measured dose of liquid through the outlet.

Description

[0001] This invention relates to pumps and in particular is concerned with pumps which are capable of delivering sequential doses of equal volume of a liquid.

[0002] In many manufacturing processes, especially in the pharmaceutical industry, there is a need for pumps which are capable of reliably delivering measured volumes of liquid without requiring adjustment or supervision. It is also a requirement of pumps of this kind that they can be readily cleaned since the liquids which are pumped are often highly chemically reactive and they have a limited pot life. Existing positive piston pumps are capable of accurately metering doses of liquid but employ components which are time consuming to clean. Also, conventional pumps utilize valve mechanisms, such as spring-loaded balls, in which undissolved particles or foreign material can lodge, thereby preventing the proper operation of the pump and causing inaccurate doses to be delivered.

[0003] Peristaltic pumps are also widely used since they have the advantage that no moving parts are in contact with the liquid to be pumped. When a batch has been completed the flexible tube which forms the body of the peristaltic pump can simply be discarded,thereby obviating the need to clean the pump. However, these pumps have the disadvantage that they cannot pump effectively against a positive pressure and the flexible plastic tubing which is used to form the body of the pump tends to be permeable to many reactive or volatile liquids.

[0004] According to the present invention there is provided a valve-less pump which is capable of sequentially delivering measured doses of a liquid, the pump comprising a body having a longitudinal passage and a pair of opposed pistons slidingly received in said longitudinal passage, cross passages in said body which communicate with the longitudinal passage and constitute respectively an inlet port and an outlet port, means for moving the pistons in unison between the inlet port and the outlet port while their opposed faces are spaced by a distance defining the liquid dose and means for closing the gap between the pistons while the pistons are in the region of the outlet port so as to deliver a measured dose of liquid through the outlet.

[0005] Conveniently, the body of the pump consists of a solid block of metal, plastics material or glass, which is formed with a cylindrical longitudinal bore and additionally with longitudinally spaced cross-bores to provide the inlet and outlet ports. The pistons may be cylindrical rods of diameter corresponding to the longitudinal bore in the body and may likewise be formed from a suitable metal, plastics or glass material. Desirably, materials are selected which are resistant to corrosion by the liquids intended to be pumped and provide surfaces which can be readily cleaned. It has been found that polytetrafluoroethylene (PTFE) is an advantageous material for manufacturing components of the pump since it combines low sliding friction with relative inertness to many chemicals and solvents. An effective pump can be manufactured using a block of PTFE for the body of the pump and stainless steel rods for the pistons.

[0006] The body of the pump may be supported on a fixed base and the two pistons mounted on a movable carriage so that sliding movement of the carriage will cause the pistons to slide in the body. Discharge of the liquid from the pump is conveniently achieved by interposing a pneumatic or hydraulic piston and cylinder device between one of the pistons and the carriage.

[0007] The volume of liquid which is to be pumped in each cycle can be controlled by limiting the outward movement of one of the pistons by a predetermined amount. This can be done by providing a proximity switch which senses outward movement of the piston and cylinder device or the pump piston and operates a supply valve for the piston and cylinder device.

[0008] Pumps manufactured in accordance with the invention are particularly suitable for delivering measured amounts of a monomer mixture to a contact lens casting mould. In this application of the invention it is desirable to fill a plurality of molds simultaneously with a measured volume of a polymersiable monomer mixture. A plurality of pumps may be formed as an assembly for this purpose. The individual pump bodies may be supported on a common base and their respective pistons connected to a carriage which is movable relatively to the base so that the pumps simultaneously pump the same measured dose of monomer.

[0009] Further features and advantages of the present invention will become apparent from the following description of one particular form of the pump which is illustrated in the accompanying drawings.

[0010] In the accompanying drawings, Figure 1 is a side elevation of a pump and operating mechanism in accordance with one embodiment of the invention and,

[0011] Figure 2 is a plan view seen from above of the pump shown in Figure 1.

[0012] Referring to the drawings, the pump comprises a generally rectangular block 1 formed from PTFE having a longitudinal cylindrical bore 2 within which is received a pair of opposed pistons 3 and 4 whose opposed faces when spaced apart provide a chamber 5. The opposite ends of pistons 3 and 4 are received in mountings 6 and 7 respectively and the size of chamber 5 (which will determine the volume of liquid to be dispensed) can be adjusted by altering the amount by which the rod 3 or 4 projects from its respective mounting 6 or 7 or by adjustment of the working stroke of a ram 17 to which mounting 7 is connected.. Valve body 1 is rigidly supported on a plate 8 which is fixed to a base 91 by pillars 9. Mountings 6 and 7, which carry pistons 3 and 4, are supported on a carriage 10 which is mounted for sliding movement on bars 11. Movement of the carriage is effected by ram 12 which, for a precisely accurate movement, is preferably hydraulic. Cross bores 13 and 14 are formed in body 1 and these provide the outlet and inlet ports respectively of the pump. Preferably the inlet port 14 is fed by gravity from a reservoir 15 through a flexible tube 16, since a gravity feed minimises formation of bubbles which would reduce the accuracy of the dose.

[0013] Piston 4 can be moved independently of piston 3 by means of ram 17, which is also preferably hydraulic. Seals, such as O-ring seals, may be pressed into the block 1 at 18 and 19 to prevent seepage of liquid out of the pump between the longitudinal bore and the pistons 3 and 4.

[0014] The pump operates in the following manner. The inlet stroke commences with the piston 3 in the position shown in Figure 1 and the face of piston 4 in contact with it. Piston 4 moves in the right hand direction (as seen in Figure 1) under the influence of ram 17. This causes liquid to be sucked into the pump via inlet 14. Ram 17 continues to move to the right until the desired volume of liquid has been sucked into the pump. Normally, the ram 17 is programmed to move outwardly by a predetermined amount so that a known volume of liquid has been sucked into the pump. Ram 12 is then operated to move the carriage 10, and hence the pistons 3 and 4 in unison (i.e. without altering the relative separation of the opposed faces of pistons 3 and 4), in the direction indicated by arrow A in Figure 1. When thee inner face of piston 3 reaches the left hand side of port 13, ram 12 stops. Then ram 17 operates to move piston 4 towards piston 3, thereby closing the gap between the opposed faces of the two pistons and forcing the entire volume of liquid in chamber 5 out of the pump through outlet 13. Ram 12 then operates again in the reverse direction to return the pistons to their starting position while the pistons remain in contact. This cycle is repeated in a reciprocating fashion to deliver equal measured volumes of liquid sequentially through outlet 13. The movement of rams 12 and 17 can be controlled by proximity switches (e.g. magnetic switches) mounted on the cylinders of the rams which sense the positions of the pistons of the rams and are connected to electro-magnetic valves which control the flow of hydraulic fluid to the rams 12 and 17.

[0015] It will be appreciated that cleaning the pump of the present invention is a simple matter since there are only two moving parts, namely the pistons 3 and 4, and no valves which need to be dismantled.

[0016] Pumps in accordance with the present invention are particularly useful for dispensing measured volumes of monomers into moulds for the manufacture of plastic contact lenses. In such an application, the outlet port 13 of the pump may be fitted with a thin outlet tube 20 which terminates in a hollow needle 21 designed to enter the female mould cavity and deliver a measured volume of monomer into the base of the mould. A suitable mechanism can be provided for advancing unfilled moulds in timed sequence to the outlet from the pump so that each female mould cavity is filled with the same volume of monomer and is then closed and polymerised. The pump of the present invention may be used in conjunction with apparatus for filling and sealing plastic moulds for casting contact lenses which is described in our Patent Application filed concurrently herewith entitled "Process and Apparatus for Casting Lenses" (Application No..............., Attorney's Ref: DCW 102).

[0017] When used for filling moulds for contact lenses a plurality of metering pumps are preferably assembled as a single filling apparatus. This is preferably achieved by mounting a plurality of pump bodies 31,41, side-by-side on the support plate 8 as indicated in Figure 2. The pistons of the pumps may be connected to the carriage 10 and ram 17 so that the pumps operate in synchronism. We have found that up to about 10 to 12 pumps can readily be connected together to form a single filling apparatus. As indicated in Figure 1, the reservoir 15 is provided with an appropriate number of supply nozzles 216, 316 etc, for connection to supply tubes, similar to tube 16, for each of the pumps.

[0018] For ease of filling of the lens moulds, the outlet tubes from the pump outlet ports 20 are conducted to a mould filling station, where mould filling tubes are clustered in a circular array above a table for holding the female moulds. This is shown schematically in Figure 3 in which the table 25 is mounted on a slide so that empty mould halves can be moved beneath the array of filling tubes. After filling each mould half with its measured quantity of monomer mixture, the array of filling tubes is raised and the filled moulds transferred to a mould closing station of the kind described in our above co-pending patent application. It is advantageous for the filling tube to be moved downwardly into the mould (or the mould to be moved upwardly towards the tube) so that its end is just above the bottom of the mould during filling. This avoids any splashing of droplets of monomer up the sides of the mould. Movement of the tube away from the liquid in the mould after filling avoids variations in amount of monomer filled, due to a droplet of monomer adhering to the end of the filling tube.

[0019] It is also advantageous to locate the filling tubes for the mould below the level of the pump and the pump below the level of the reservoir. This is because it facilitates priming of the pump and flushing of air bubbles from the pump and connecting tubes. This operation is conducted by filling the reservoir with monomer mixture and moving the pistons outwardly until both inlet and outlet ports are uncovered. Monomer is then allowed to flow through the pumps and tubes until the monomer emerging from the filling tubes is free from air bubbles. The pistons are then returned to the starting position described above and the controls on the rams which limit the extent of movement of the pumps is restored.

Claims

1. A pump which is capable of sequentially delivering measured doses of a liquid, the pump comprising a body having a longitudinal passage and a pair of opposed pistons slidingly received in said longitudinal passage, cross passages in said body which communicate with the longitudinal passage and constitute respectively an inlet port and an outlet port, means for moving the pistons in unison between the inlet port and outlet port while their opposed faces are spaced by a distance defining the liquid dose and means for closing the gap between the pistons while the pistons are in the region of the outlet port so as to deliver a measured dose of liquid through the outlet.

2. A pump according to claim 1 in which the body of the pump is supported on a base and the two pistons are mounted on a carriage which is movable relatively to the base so as to cause the pistons to slide in unison in the body.

3. A pump according to claim 2 in which the means for closing the gap between the pistons comprises a pneumatic or hydraulic piston and cylinder device acting between one of the pump pistons and the carriage.

4. A pump according to claim 3 in which the volume dispensed by the pump comprises means for limiting the outward movement of the piston and cylinder device to a predetermined amount.

5. A pump according to claim 4 in which the means for limiting the outward movement of the piston and cylinder device comprises a proximity switch which senses outward movement of the piston and operates a supply valve for said piston and cylinder device.

6. A pump according to any one of the preceding claims for filling moulds for casting contact lenses with a measured amount of a polymerisable composition, said pump including a reservoir for holding said composition and a conduit connecting the reservoir to the inlet port of the pump.

7. A pump according to claim 6 in which the outlet port is connected via a conduit to a filling tube for entering a female mould half.

8. An assembly of pumps for simultaneously filling a plurality of contact lens moulds which comprises a plurality of pumps as claimed in any one of claims 1 to 5, the pump bodies being supported side by side on a fixed base so that their longitudinal passages are mutually parallel, the pump pistons being mounted on a common carriage which is movable relatively to the base so that the pistons reciprocate together in the pump bodies and simultaneously deliver a plurality of measured doses from their outlet ports.

9. An assembly according to claim 8 in which the inlet ports of the pumps are connected to a common reservoir for holding a polymerisable composition.

Drawing