GB2118477A - Injection molding a liner onto spool - Google Patents

Abstract

Apparatus is described for injection- molding an insulating liner onto a metal spool for use in an electromagnetic flowmeter. The spool is composed of a cylindrical body 10, having end flanges 11, 12 and a pair of diametrically- opposed circular bosses 13, 14 to receive the meter electrodes. To carry out injection-molding, the spool is supported within a mold which defines a liner cavity 22 conforming to the inner surface of the cylindrical body and to the faces of the end flanges, the liner cavity 22 communicating with cavities 24, 25 conforming to the inner surface of the electrode bosses. The molding material is injected into a sprue 16 which runs along the longitudinal axis of the spool toward the hub 18 of a runner 19 extending radially toward an axi-symmetric ring gate 21 opening into the liner cavity at a point adjacent the boss cavities 24, 25. The molding material passing through the ring gate then flows in opposite directions in the liner cavity 22 to evenly fill this cavity as well as the boss cavities without creating weld lines therein. Shrinkage of the liner in the radial direction of the spool is compensated for by injecting thermosetting material into the resultant gap between the liner and spool body 10. <IMAGE>

Description

SPECIFICATION Technique for injection molding a liner onto spool This invention relates generally to electromagnetic flowmeters, and more particularly to a technique for injection molding a insulating liner onto the inner surface of a metal spool to define a flow conduit for the fluid being metered.

In a magneticflowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively consti tutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.

In order to provide a compact and readily install able elctromagnetic flowmeter whose weight and dimensions are substantially smaller than existing types, the Schmoock patents 4,253,340 and 4,214,477 disclose a highly compact flowmeter which, in spite of its reduced volume and weight, is capable of withstanding high fluid pressures. In the Schmoock flowmeter, use is made of a non magnetic metal spool of high strength whose inner surface is lined with insulating material to define a flow conduit for the fluid to be metered. The spool also serves to withstand fluid pressure as well as the compressive forces to which the meter is subjected by bolts bridging the flanged ends of upstream and downstream pipes between which the unit is inter posed.

Surrounding the Schmoock spool and concentric therewith is a cylindrical housing formed of ferro magnetic material. The housing is provided with annular end plates that are joined to the correspond ing end flanges of the spool to define an inner chamber. Integral with the housing are two magnet cores which are placed at diametrically-opposed positions along an axis which is normal to the longitudinal axis of the housing coils being wound on these cores. A pair of electrodes are mounted on the spool at diametrically-opposed positions along a transverse axis at right angles to the core axis. The inner chamber is filled with a potting compound to encapsulate the electromagnets and the electrodes, the housing serving as a mold for this purpose.

Insulating liner for electromagnetic flowmeters are usually molded of fluorocarbon materials such as PTFE, PFA and FEP. Because fluorocarbons are non-reactive with virtually all corrosive fluids, they have the properties appropriate to liners for flow meters. When injection-molding plastic liners into the body of metal spools of the type included in flowmeters disclosed in the Schmoock patents, ; certain problems are encountered.

One problem which occurs regardless of the nature of the molding material is when the molten thermoplastic material encounters an obstruction in its flow path, such as a core pin or an insert. The molten material is then forced to separate in order to flow around the obstruction; and in that situation, a weld or knot line will be formed where the two flow fronts join on the downstream side of the obstruction in the flow path. Such weld lines create weakened areas in the molded liner. Since the liner is subjected to fluid that may be under high pressure or include abrasive contaminants, the liner in some instances will in time be disrupted in the weakened areas.

Another problem arises in conventional injection molding techniques when use is made of thermoplastic resin molding materials which have reinforcing fibers therein, such as TEFZEL, a fluoropolymer marketed by the DuPont company.

TEFZEL is the trademark covering a family of melt-processable thermoplastics (ETFE) with an outstanding balance of properties. Mechanically, TEF ZEL is exceptionally tough, having an excellent flex life, impact, cut-through and abrasion resistance.

The glass fiber reinforced compound (Tefzel HT 2004) has even higher tensile and compressive strength, stiffness and creep resistance. Thermally, "Tefzel" has a continuous temperature rating of 1500C,the material being inert to most solvents and chemicals. It is an excellent low-loss dielectric with a uniformity of electrical properties normally absent with otherthermoplastics.

The concern of the present invention is with "Tefzel" or other suitable thermoplastics having reinforcing fibers therein. In molding a fiberreinforced thermoplastic material one must take into account fiber orientation. If the fibers in the molded flowmeter line are aligned in the direction of material flow which is parallel to the longitudinal axis of the meter, this orientation reduces material shrinkage in this direction and thereby prevents the liner from pulling away from the surface of the metal spool. But with conventional injection-molding techniques in which the liner is required to conform to the inner surface of the metal spool which is not purely cylindrical but includes shaped regions, the desired fiber orientation is not realized.

If, as is usually the case with conventional injection-molding techniques for liners, two flow fronts meet at a weld line, the fibers embedded in the molding material will lie parallel to the direction of flow. As a consequence, there will be no fibers extending through the plane of the weld line. And because the plane of the weld line is then devoid of reinforcement, stresses thereafter exerted on the liner will concentrate at the weld line and result in failure.

Summary of the invention In view of the foregoing, the main object of this invention is to provide an improved technique and apparatus for injection molding an insulating liner onto a metal spool to be included in an electro magnetic flowmeter, the lined spool defining a flow conduit for the fluid to be metered.

More particularly, an object of this invention is to provide a mold for carrying out a technique in accordance with the invention, the mold configuration being such as to cause the molding material to fill the cavity defining the liner in a manner avoiding the formation of weld lines that weaken the liner structure.

Also an object of the invention is to provide an injection-molding technique in which use is made of a glass fiber-reinforced ETFE molding material, the technique affording uniform fiber distribution and fiber orientation in the direction of material flow, so that no region of the liner is devoid of reinforcement and the liner is capable of withstanding heavy stresses.

Still another object of the invention is to provide an injection-molding technique for a metal spool having electrode bosses in which the liner is extended into the bosses, making it possible for the electrodes to be thereafter inserted from outside of the spool into the lined bosses which afford a compression seal therefor.

Briefly stated, these objects are attained in a technique for injection-molding an insulating liner onto the surface of a metal spool to be included in an electromagnetic flowmeter in which the fluid to be metered is conducted through the lined spool. The spool is composed of a cylindrical body having end flanges and a pair of diametrically-opposed circular bosses disposed midway between the flanges to receive the meter electrodes. To carry out injectionmolding, the spool is supported within a mold which defines a liner cavity conforming to the inner surface of the cylindrial body and to the faces of the end flanges, the liner cavity communicating with cavities conforming to the inner surface of the electrode bosses.The molding material is injected into a sprue in the mold which runs along the longitudinal axis of the spool toward the hub of a runner extending radially toward an axisymmetric ring gate opening into the liner cavity at a point adjacent the boss cavities. The molding material passing through the ring gate then flows in opposite directions in the liner cavity to evenly fill this cavity as well as the boss cavities without creating weld lines therein.

The above-described injection-molding technique in accordance with the invention tends to reduce shrinkage in the direction of the spool axis and thereby prevent the liner flanges from pulling away, an annular gap may in practice be developed between the liner and the cylindrical spool body.

This gap is created because of the differential shrinkage between the liner and spool body as the thermoplastic material cools and solidified.

As a consequence of this gap, fluid to be metered which flows through the liner may exert a pressure thereon sufficient to cause the liner to expand into the free space. Thus the internal diameter of the liner may be caused to vary as a function of the internal fluid pressure, this resulting in calibration shifts and possibly in liner rupture should the expansion of the liner exceed the physical limits of the material.

Accordingly, another object of this invention is to provide a technique for filling in the annular gap created between the injection-molded liner and the body of the spool with a thermosetting resin whereby the liner has an internal diameter which remains constant under varying conditions of fluid pressure, to obviate calibration shifts and possible liner rupture.

In order to fill this annular gap and to bond the liner to the body surfact to prevent a change in the inner diameter of the liner when the liner is subjected to the pressure of a fluid flowing therethrough, the spool body is provided with ports which extend between the exterior and interior surfaces thereof. These ports are sealed during the injectionmolding process by removable plugs. After injection-molding, the plugs are removed and a thermosetting resin is introduced into one of the ports until the gap is filled as evidenced by the fact that the resin begins to rise in the other ports, the resin then being cured to solidify the filler.

Outline of the drawings For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawings, wherein: Figure 1 is an elevational view, partly in section, of the metal spool having a liner therein injectionmolded by a technique in accordance with the invention; and Figure 2 in section, illustrates the metal spool disposed within a mold in which molten plastic material is injected to create the liner for the spool.

Figure 1' is a top view of a metal spool for a flowmeter which includes ports making it possible to carry out a gap-filling technique in accordance with the invention; Figure 2 ' is an end view of the spool, partly in section, Figure 3' is an elevational view of the spool partly in section to expose the liner; and Figure 4' is a sketch illustrating the technique.

Description of the invention The injection-molding technique: Referring now to Figure 1 there is shown a metal spool for inclusion in an electromagnetic flowmeter, onto which a liner has been injection-molded by a technique in accordance with the invention. The spool is constituted by a cylindrical body 10 having flanges 11 and 12 at either end, a pair of bosses 13 and 14 being disposed at diametrically-opposed positions on the spool body midway between the end flanges and being adopted to receive the meter electrodes. The liner is constituted by a cylindrical portion Lc which conforms to body 10, flange portions Lf conforming to the faces of the end flanges 11 and 12 and boss portions Lb conforming to the electrode bosses 13 and 14.

In injection-molding, a thermoplastic molding compound is plasticized in an appropriate heating cylinder, then forced by plunger action through an orifice into relatively cool mold cavities where the material solidifies to the desired shape. The injection molding machine represented by block 15 in Figure 2 is a self-contained unit consisting of various hyd raulic, electrical and mechanical components which are adjustable to various molding requirements.

In practice, the granules of thermoplastic material are loaded into a feed hopper from which a controlled quantity is advanced into the heating cylinder with each complete cycle, the heating cylinder being surrounded by electrical heating coils. Since plastic materials are poor heat conductors, all injectionmolding machines include some form of spreader located in the center of the plasticizing chamber to force the material close to the cylinder wall, thereby to assure uniform heating. To confine the material properly in the mold during the application of the high injection pressure, it is essential that an adequate clamping force be maintained against the mold.

Figure 2 shows the metal spoon symmetrically disposed within a mold in accordance with the invention formed by upper and lower halves. The mold is provided with a diverging or tapered sprue 16 which extends along the longitudinal axis of the spool from a concave inlet 17 into which is inserted the nozzle of the injection-molding machine 15.

Sprue 16 leads into the central hub 18 of a radiallyextending runner 19. Hub 18 is provided with an extension 20 which projects beyond runner 19 and acts as a cold well.

Runner 19 runs radially from hub 18 toward an axi-symmetrical ring orflash gate 21. This gate opens into a liner cavity 22 which conforms to the inner cylindrical surface of spool body 10 and to the faces of flanges 11 and 12. It will be seen that the walls 23 of the mold adjacent the faces of the flanges are corrugated to define a lining overlying the flange faces having corrugations therein constituted by concentric rings to provide a good seal when the flowmeter incorporating the metal spool is compressed between the flanges of the upstream and downstream pipes of the flow line in which the flowmeter is interposed.

The spool liner cavity 22 communicates with a pair of cavities 24 and 25 which conform to the inner surface of the electrode bosses 13 and 14. Ring gate 21 opening into liner cavity 22 at a point adjacent one side of the electrode bosses, the ring gate providing a 360 degree entry into this cavity.

The molten material injected into the mold through sprue 16 is a fiber-reinforced material such as TEFZEL. The liner material flows from the nozzle of the injection machine down the tapered sprue 16 into hub 18, the first "slug" of material being trapped and contained in cold well 20. The molten material then flows evenly in the radial direction through runner 19 toward ring gate 21. Plastic material flows simultaneously through the full 360 degrees of the ring gate into liner cavity 22.

As the molten material exits from ring gate 21, it then flows in the liner cavity 22 toward flange 12 and simultaneously toward flange 13 so that the plastic which proceeds to fill liner cavity 22 flows in opposite directions away from the entry gate. Thus flowing up and down the inner surface of the spool body through the liner cavity are concentric molten cylinders flowing away from each other. The electrode boss cavities are filled evenly in a similar fashion, these cavities being fed from the material flowing past the respective openings.

Because the ring gate is located near the center of the spool body, the reinforcing fibers in the flowing molten plastic material assume an orientation in the direction of material flow, this being parallel to the longitudinal axis of the spool as indicated by the arrows in Figure 2. This orientation reduces material shrinkage in the axis direction and thereby prevents the liner flanges from pulling away.

The gap filling technique Referring now to Figures 1', 2' and 3', there is shown a metal spool for inclusion in an electromagnetic flowmeter onto which a thermoplastic liner has been injection-molded by the technique disclosed in the previous section.

The spool is constituted by a cylindrical body 10 having flanges 11 and 12 at either end. A pair of bosses 13 and 14 is disposed at diametricallyopposed positions in the spool body midway between the end flanges, the bosses being adapted to receive the meter electrodes. The liner is constituted by a cylindrical portion Lc which conforms to the inner surface of spool body 10, flange portions Lf which conform to the faces of the end flanges, and boss portions Lb conforming to the inner surface of electrode bosses 13 and 14.

While ideally the injection-molded liner should abut the inner surface of the spool body, in practice, as pointed previously, a gap is developed therebetween as a result of shrinkage taking place during the course of cooling and solidification.

in order to fill this gap and thereby stabilize the liner, the metal spool is provided with three ports P1, P2 and P3 each sealed by a removable plug 15. As shown in Figure 2', each port or hold extends from the exterior of the spool body to the interior thereof, the port thereby communicating with the annular gap G (see Figure 4') created between the liner portion Lc and the cylindrical spool body 10.

Ports P1 and P2 are located adjacent opposite flanged ends of the spool body on one side thereof, whereas port P3 is located on the opposite side of this spool body in line with port P2. The sealing plugs 15 protrude slightly into the internal diameter of the metal spool to prevent the ports from filling with molten liner material during injection-molding of the liner.

It is to be noted that the body of the metal spool has a slight taper or flare extending from its center to either end, so that the internal diameter of the body is slightly greater at the ends than at the center.

When injection-molding is carried out to fabricate the liner, the port plugs 15 are in place so that the ports do not interfere with injection-molding. After the spool is taken out of the mold, the plugs are removed and the spool is then supported horizontally in the manner shown in Figure 4 with ports P1 and P2 at the top and port P3 at the bottom.

Athermosetting molding material provided by a source 16 is then introduced into lower port P3, the material acting to fill the annular gap G between liner portion Lc and metal spool body 10. The upper ports P1 and P2 serve to permit the escape of air from the gap as the air is displaced by the thermosetting material.

Since the spool body has a double-tapered internal formation, the smaller diameter being at the center, when the operator sees the thermosetting material beginning to rise within the end ports P1 and P2 at the top, he then knows the gap is completely filled and he cuts off further supply. The filler plastic is then permitted to cure and solidify to close the gap and lock the liner to the spool body.

The thermosetting plastic used as a filler is preferably one in the epoxy family which cures at room temperature to prevent differential shrinkage from occurring during the cure cycle, for this would result in the formation of additional gaps. A preferred epoxy resin for this purpose is a two component epoxy "Eccoseal W-1 9" marketed by Emerson and Cuming of Canton, Ma.

Claims (15)

1. Apparatus for injection-molding an insulating liner onto the surface of a metal spool to be included in an electromagnetic flowmeter wherein fluid to be metered is conducted through the lined spool the spool being constituted by a cylindrical body having flanges at either end and a pair of diametricallyopposed circular bosses midway between the end flanges to receive the meter electrodes, said apparatus comprising:: (a) a mold adapted to receive the spool to define therewith a liner cavity conforming to the inner surface of the cylinrical body and to the faces of the end flanges, the liner cavity communicating with cavities conforming to the inner surface of the electrode bosses, said mold further including a sprue extending from an inlet along the longitudinal axis of the spool toward the central hub of a runner extending radially from the hub to an annular flash gate opening into the liner cavity communicating with cavities conforming to the inner surface of the electrode bosses, said mold further including a sprue extending from an inlet along the longitudinal axis ofthe spool toward the central hub of a runner extending radially from the hub to an annularflash gate opening into the liner cavity at a point adjacent the boss cavities, said annular gate being coaxial with said longitudinal axis and being axisymmetric with the sprue; and (b) means to inject molten molding material containing reinforcing fibers which are randomly oriented into the inlet to cause the material to flow through the sprue into the hub from which it flows through the runner and the axisymmetric (ring) flash gate into the liner cavity wherein the material flows in opposite directions from the gate toward the flanges at either end of the spoil body to fill the liner cavity and also the boss cavities, said flow being at a rate causing the reinforcing fibers to assume an orientation in the direction of flow whereby the fibers embedded in the molded in the liner are parallel to the spool.

2. Apparatus as set forth in claim 1 wherein said sprue is tapered outwardly from the inlet to the central hub;

3. Apparatus as set forth in claim 2 wherein said hub extends beyond said runner into a cold well.

4. Apparatus as set forth in claim 1 wherein the wall of the mold which defines the liner cavity in the region of the end flanges has a corrugated formation to create corrugated flange linings.

5. Apparatus as set forth in claim 1 wherein said molding material is a fluorocarbon containing reinforcing fibers.

6. Apparatus as set forth in claim 5 wherein said material is ETFE;

7. Apparatus as set forth in claim 6 wherein said fibers are formed of glass.

8. A technique for stabilizing an insulating liner formed in the manner set forth in claim 1 by injection-molding a thermoplastic material onto the inner surface of a metal spool having a cylindrical body provided with end flanges, a gap being created between the liner and the body due to differential shrinkage as the injection-molded liner cools and solidifies, the technique comprising the steps: (a) supporting the lined spool in a horizontal position; (b)introducing a thermosetting resin into the gap through a port in the spool body to fill the gap therewith; and (c) curing the resin to form a filler in the gap to thereby maintain the internal diameter of the liner at a constant value regardless of varying fluid pressure.

9. A technique as set forth in claim 8 wherein said resin is curable at room temperature.

10. A technique as set forth in claim 8 wherein said spool is provided with first and second ports placed on one side of the body adjacent the end flanges thereof and a third port on the opposite side of the body, said spool being supported with the third port on the bottom through which the resin is introduced, the presence of resin in the first and third ports being indicative of the filled state.

11. A technique as set forth in claim 9 wherein said resin is a two-component epoxy.

12. Apparatus substantially as herein particularly described with reference to and as illustrated in the accompaying drawings.

13. A method of injection molding a insulating liner onto the surface of a spool substantially as herein particularly described with reference to the accompanying drawings.

14. A spool provided with an insulating liner using apparatus as claimed in any of claims 1 to 7 and 12 or the method as claimed in any of claims 8 to 11 and 13.

15. A spool provided with an insulating liner substantially as herein particularly described with reference to and as illustrated in the accompangying drawings.