CA1326118C

CA1326118C - Cooling plugs in thermoplastic pipe forming apparatus

Info

Publication number: CA1326118C
Application number: CA000586528A
Authority: CA
Inventors: Manfred A. A. Lupke
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-12-20
Filing date: 1988-12-20
Publication date: 1994-01-18
Anticipated expiration: 2011-01-18

Abstract

The invention concerns apparatus for molding thermoplastic tubing, especially ribbed and double walled thermoplastic tubing. In such apparatus a sizing plug, which may be of a cooling plug, is often provided to define the inner wall of the tube. According to the invention a pressured gas is expanded isentropically into the cooling plug through a throttle valve so that the expanding gas cools the plug. Suitably the gas is carbon dioxide which may be vented to the atmosphere. The use of a throttle valve for isentropic expansion of gas to cool the plug, is advantageous at least in that the degree of cooling may be controlled through the degree of throttling of the valve and in that gas may be led into the valve at the temperature of the molding process, rather than through cooling lines which may have localised undesirable cooling effect. The invention also includes providing the plug with a porous outer region for the dissemination of lubricant.

Description

The invention relates to apparatus for molding thermoplastic tubing, especially ribbed and double walled thermoplastic tubing in which the ribs may be of helical or annular configuration, or indeed any other configuration.

The apparatus may be of the type in which tubing of thermoplastic material is continuously extruded into a travelling tubular mold tunnel about a mandrel and is confined to the shape of the pipe by a sizing plug downstream of the mandrel. Such a plug may on some occasions, when extrudate does not easily flow fully into the mold, be heated to increase the fluidity of the extrudate so that it may more easily flow into recesses of the mold, for example under the influence of suction from the bases of the recesses. However, more usually the extrudate is provided in sufficiently molten condition to flow fully into the mold. In this case it is necessary to provide a sizing plug to define the inner wall of the pipe so as to confine the extrudate in an appropriate casting cavity.

A simple follower plug defining the inner wall of a tube is described by Chaplain in U.S. Patent No. 4,365,948 issued December 28, 1982. Usually some temperature control is desirable in a sizing plug and Lupke in U.S~
Patent No. 4,545,751 issued October 8, 1985 describes a more sophisticated arrangement. In that arrangement the sizing plug does not define an inner wall of the pipe but is used to distribute air into the inside of corrugated tubing. Pressured air is delivered through a central core coaxial with an extrusion nozzle. Hot peripheral air is drawn off and central cool air is permitted to leave the plug to contact the inside of corrguated pipe, the plug .;~ ~ .

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- 2 - 1326118 being inwardly spaced from the inner pipe wall. Such arrangement is suitable for use where corrugated tubing is involved but the range of temperature in cooling may be limited. The arrangement is less suitable where ribbed - 5 tubing is to be produced since the plug, in that case, should be a plug contacting the inside wall of the pipe to confine it into a casting region. When this is the case, air cannot be released between the plug wall and the inner pipe wall for cooling. A cooling plug suitable for use in the formulation of ribbed pipe is disclosed by Lupke in - hi later U.S. Patent No. 4,555,230 issued N~vember 26, 1985. In that patent Lupke does not attempt to use released cooling air but provides within and near the surface of a follower plug, a helical coil of tubing through which cooling fluid may be passed.

Whether the plug is a cooling plug or a heating plug, the passage of the extruded pipe may be eased by lubricating the plug. The provision of suitable amounts of suitably distributed lubricant has presented problems and is presently the subject of varying techniques.
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However the cooling of the plug has been achieved, there have been certain problems in fine control of the cooling to provide optimum ef$ect at the inner surface of the pipe. Thus, it would be advantageous to provide the high degree of cooling so that the speed of the travelling mold tunnel may be increased with resultant improvement in ~ output.

`~ It has now surprisingly been found that an isentropic expansion of compressed gas may be used to cool a cooling ~- 30 follower plug more efficiently and controllably.
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-~ 3 ~ 1326118 Thus according to the inventiGn there is provided in apparatus for forming seamless thermoplastic pipe in a travelling mold tunnel, a cooling system comprising:
a cooling plug located in the mold tunnel for cooling extruded pipe, the plug being having a surface adapted to contact an inner wall of formed pipe in the mold tunnel, and the plug having a cooling channel therein for passage of a fluid therethrough;
a source of compressed gas;
a passage for said gas between said source and said channel;
a throttle expansion valve located for isentropic cooling expansion of said gas into said channel; and adjustment means to adjust the throttling effected lS by said valve.

Also according to the invention is provided a method for forming seamless thermoplastic pipe in a travelling mold tunnel, comprising extruding a parison into the tunnel to form a pipe in a mold face of the tunnel, and cooling the thus formed pipe by a cooling plug located within the mold t~nnel, said plug having a surface contacting an inner wall of the formed pipe~ in which the plug is cooled by isentropically expanding pressurized gas through a throttle expansion valve into the plug and adjusting the opening of the throttle valve to control the flow of gas and hence the degree of cooling.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:
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Figure 1 is a sectional view of an extrusion nozzle of apparatus for molding thermoplastic pipe, showing part of ,!

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. ;. ' ' ' ' . ' .~: . ' ,.` ~ ' . . ' ~`' ' ' ' -` 13261 18 - the mold tunnel and the cooling plug;

Figure 2 is a plan of one cooling system for the cooling plug;

Figure 3 shows the configuration of one cooling plug and throttle valve which ma~ be used in the invention;

Figure 4 shows another configuration of cooling plug and throttle valve which may be used in the invention; and Figure 5 shows an arrangement using consecutive cooling plugs~
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Referring to the drawings and more particularly to Figure 1 thereof, the apparatus comprises a pair of complementary upper and lower mold assemblies. Each mold assembly compri~es an endless chain of articulately interconnected ~ mold blocks 16 The mold blocks 16 may be such as to mold pipe of any desired configuration. For example, mold blocks 16 may be such as to mold annularly ribbed pipe or helically ribbed - pipe, double walled pipe, or other configuration.
Generally, however, the inner wall of the pipe will be smooth.

The mold assemblies may be operatively positioned to locate an extrusion head being operatively coupled to the nozzle of an extrusion machine, which may be of conventional form.
If required, the mold assemblies can be moved away from the extrusion head to provide access to the extrusion head.

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~ 5 ~ 1326118 The extrusion head ~ comprises an axially extending tubular portion 26 which is surrounded in spaced relationship thereto by a tubular member 27, one end portion of which screw-threadedly supports an outer member 28 of an corrugate annular extrusion nozzle 24. The tubular portion 26 carries an inner member 29 of the extrusion head having a frusto-conical form which terminates in annular orifice 31 and which communicates with the annular space 32 between the pipe 27 and the ; 10 p~rtion 26. This annular space 32, in turn, communicates with the output of the extrusion machine (not shown) passing the thermoplastic material, such as PVC to the extrusion nozzle 24.

Downstream of the extrusion nozzle 24 the extrusion head 23 carries, on an extension of tubular portion 26, a generally cylindrical forming plug 46. The plug 46 is effective to define the inner wall of pipe formed by thermoplastic material leaving the extrusion nozzle 24 to be molded by mold blocks 16. In particular, plug 46 holds thermoplastic material in troughs 36 of the mold blocks 16 to form annular ribs on the outer surface of the resulting pipe.

Cooling of plug 46 may be by isentropic expansion of compressed gas, for example, carbon dioxide, from a source ~ through a supply line 52 which extends through a central core 53 of tubular portion 26. The supply line 52 leads into a throttle expansion valve 54 which may be of . any suitable type to allow the gas to expand therethrough . to cool plug 46. For example, throttle valve 54 may be a needle valve or a plunger valve or any other suitable type of valve.

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Adjustment means may be provided to adjust the opening of valve 54. These adjustment means may be manual screw down means such as tap 56 for the plunger valve shown in Figure 2. More conveniently however the adjustment means may be thermostatically controlled in response to the temperature at the surface of plug 46. Thus, when plug 46 is too cool for optimum cooling of the inner surface of the pipe, the adjustment means will tend to close throttle valve 54 and when the temperature of plug 46 rises the valve 54 will be correspondingly opened.

~;-`. The interior of the cooling plug ~ should have at least a channel for expanded cooled gas near its surface or it may be hollow~ Suitably baffles 57 are arranged to enhance cooling or, helical coil 60 or other channels may be used.

When the gas to be expanded to cool plug 46 is carbon dioxide, it is possible to vent the gas directly to the atmosphere as, for example, at vent 55. It may be convenient to provide some fan fed chimney or other venting arrangement to inhibit build up of carbon dioxide in the work place but such arrangement will be dependent upon the concentration of carbon dioxide produced or any regulations concerning its dispersal. In many cases it may be convenient to use a recirculation system such as ` that illustrated in Figure 2. In this case, expanded gas from cooling plug 46 is returned through return line 66 through core 53 of tubular portion 26. The expanded gas is then passed to a compressor 65 before return to a - holding tank 64. An additional control on the cooling expansion of gas into plug 46 may be through automatic or manual adjustment of an exhsust v~lve 61.

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, ' ~ ' ' This method also is very advantageous for small diameter rib or double wall pipe as this system does not require insulated cooling pipes through the extruder die which will free~e-up and stop flowing. The duct 52 conveying the compressed gas to ~he cooling plug may be of the same temperature of the die, because only at the point of the gas expansion will it create the temperature drop.

It is conventional to lubricate the surface of at least cooling plug 46 allow for easy travel of the formed pipe thereover.

The improvements to the cooling plug makes it more and more important to also improve the friction co-efficient.
It has previously been suggested to cover a sizing plug with polytetraflouroethylene as, for example, Teflon (Trade Mark), chrome plating it, tapering it to a smaller diameter towards the downstream end so as to avoid shrinkage of the pipe on to the plug. None of these methods is wholly satisfactory.

Lubrication may, however, be led in through lubricant line 62 through core 53 of tubular portion 26.
` Conveniently as shown in Figure 1, the plug 46 is provided with a porous portion 63, at least at or near its upstream end but preferably covering the entire . surface of plug 46 as shown. The porous material may ~ 25 suitably be sintered material allowing seepage of oil or- other lubricant therethrough. Lubricant supply line 62 may open through an annular or other shape accumulator chamber 48. This lubricant has the additional benefit that it will enhance the conductivity and therefore increase the production speed into this porous region 63 which extends around the surface of plug 46 and into the `:

- 8 - ' 13261 18 -~ body thereof to allow even distribution of lubricant throughout the region 48 and at the surface thereof.

Figure 5 shows a modification in which two plugs are used.
; Pressurized gas in line 52 is isentropically partially ~ 5 expanded into plug 46 through throttle valve 54. As it - passes through plug 46, it heats up and may be further isentropically expanded by throttle valve 59 into plug 47.
Thereafter, it is vented to atmoshpere ~as shown) by valve 61. Alternatively, it may be recycled by a return line ~` 10 and compressor as shown in Figure 2.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In apparatus for forming seamless thermoplastic pipe in a travelling mold tunnel, a cooling system comprising:
a cooling plug located in the mold tunnel for cooling extruded pipe, the plug being having a surface adapted to contact an inner wall of formed pipe in the mold tunnel, and the plug having a cooling channel therein for passage of a fluid therethrough;
a source of compressed gas;
a passage for said gas between said source and said channel;
a throttle expansion valve located for isentropic cooling expansion of said gas into said channel; and adjustment means to adjust the throttling effected by said valve.

2. A cooling system as claimed in Claim 1, in which said passage is located axially through an elongate extrusion nozzle and mandrel for extrudate, and in which the throttle valve is located substantially level with or downstream of an extrusion orifice for extrudate.

3. A cooling system as claimed in Claim 2, in which the adjustment means is manually operable.

4. A cooling system as claimed in Claim 2, in which the adjustment means automatically adjusts the opening of the throttle valve as a function of the temperature of the surface of the cooling plug.

5. A cooling system as claimed in Claim 2, in which the source of compressed gas is a source of compressed carbon dioxide.

6. A cooling system as claimed in Claim 2, in which recirculation means are provided to recirculate and compress expanded gas vented from the cooling plug.

7. A cooling system as claimed in Claim 5, in which the cooling channel of the plug vents to the atmosphere.

8. A cooling system as claimed in Claim 1, in which an outer part of the cooling plug is porous and a lubricant supply line communicates with the porous outer part.

9. A cooling system as claimed in claim 8, in which the lubricant supply line communicates with the porous outer part through an accumulator chamber having one inner face formed by an inner surface of the porous outer part.

10. A method for forming seamless thermoplastic pipe in a travelling mold tunnel, comprising extruding a parison into the tunnel to form a pipe in a mold face of the tunnel, and cooling the thus formed pipe by a cooling plug located within the mold tunnel, said plug having a surface contacting an inner wall of the formed pipe, in which the plug is cooled by isentropically expanding pressurized gas through a throttle expansion valve into the plug and adjusting the opening of the throttle valve to control the flow of gas and hence the degree of cooling.

11. A method as claimed in Claim 10, in which the compressed gas is provided through a supply line from a source compressed gas, the supply line being located axially through an elongate extrusion nozzle.