WO1996012601A1

WO1996012601A1 - A method of and apparatus for rotational moulding

Info

Publication number: WO1996012601A1
Application number: PCT/GB1995/002492
Authority: WO
Inventors: Roy Crawford; Paul Nugent; Alvin Spence
Original assignee: The Queen's University Of Belfast
Priority date: 1994-10-22
Filing date: 1995-10-23
Publication date: 1996-05-02
Also published as: GB9421596D0; AU3703395A

Abstract

A mould (2) of a rotational moulding apparatus incorporates a vent to which a compressed air line (6) is attached. A temperature sensitive switching device (II) is disposed in the line. When this device (II) reaches a certain predetermined temperature, it 'flips' to allow pressure to flow into the mould. Pressure drop in the line is sensed by transducer and computer link. Instead of using a conventional vent already present in the mould the air line (6) may be connected to an additional vent. In that event a pressure drop in the line is sensed as air moves through and out of the mould.

Description

A METHOD OF AND APPARATUS FOR ROTATIONAL MOULDING

The present invention relates to a method of and apparatus for rotational moulding.

Process control during rotational moulding is made complex by the fact that it uses biaxial rotation in a hot environment. A system such as that

described in European Patent Specification No. 909I4945.2 can be used to transmit temperature data for a period limited by the insulation used to

protect the electronics. A system is required for full-time production which will allow operators (or machines) to control the process at all times. A

continuous cooling system may use water to cool the transmitter and therefore allow continuous use of the electronics. This is complex and may

require changes to machine design.

A simpler, more direct, method is to use the internal air line commonly

found on rotomoulding machines. This passes through the rotating joints and provides an effective path of communication to the mould.

According to one aspect of the present invention there is provided rotational moulding apparatus comprising an oven, a mould disposed inside the oven, means for supporting the mould inside the oven, means for rotating the mould inside the oven about two transverse axes, a fluid line leading from outside the oven into the mould and a temperature sensitive device disposed in the line in the mould operative to influence the fluid pressure in the line in dependence upon the temperature in the mould and thereby to indicate outside the mould the temperature in the mould. According to another aspect of the present invention there is provided a method of indicating the temperature in a mould externally of the mould including the steps of establishing a fluid connection into the mould, sensing the temperature in the mould, restricting or closing the connection inside mould in dependence upon the temperature sensed and measuring fluid pressure variations thereby caused in the connection externally of the mould

in order to indicate the temperature inside the mould.

In a preferred embodiment of the invention, the fluid line is anair pressure line served from a compressed air source providing pressure in the line in the range 5 to I0 psi. The temperature sensitive device may comprise

a bimetallic device such as a disc or strip, low temperature alloys, polymers or other devices. The line may lead into a conventional vent provided in the mould or into a vent additional to that vent. In the latter case the temperature sensitive device will not permit air pressure in the mould to rise

but a change will nevertheless occur when the temperature sensitive device is activated. The device advantageously comprises a temperature sensitive element connected to a member which moves under the influence of the element between a position in which the line is sealed to a position in which the line is open. The seal may comprise an 'O' ring on which the member seats. The member is preferably spring loaded, for example by means of a compression spring, to urge the member against the seat.

In order that the invention may be more clearly understood,

embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure I diagrammatically illustrates rotational moulding apparatus according to the invention.

Figure 2 diagrammatically illustrates a part of the apparatus of figure

I in greater detail,

Figure 3 diagrammatically illustrates an alternative to the arrangement shown in Figure 2, and

Figure 4a and 4b diagrammatically illustrate two modes of operation of the apparatus of Figure I.

Referring to Figure I, the rotational moulding apparatus comprises an oven I inside which is disposed a mould 2. The mould is supported in the

oven on a machine arm 3 for rotation about two mutually perpendicular axes 4 and 5. Axis 5 is the axis about which the arm 3 rotates and axis 4 is the

plate rotation axis about which the mould rotates with respect to the arm 3. An air pressure line 6 leads from a compressed air supply 7 through a

transducer and computer link 8 from outside the oven I into the oven. This line 6 passes through a first rotating joint 9 on the axis 5 and a second rotating joint 10 on the axis 4. Both rotating joints are disposed in or on the machine arm 3.

From the second rotating joint 10, the line 6 enters the mould 2. At or near the mould end of the line 6, a temperature sensitive switching device

II is disposed. Two possible versions of the device II are shown in more detail in Figures 2 and 3 respectively. In the version shown in Figure 2, the device is disposed within the mould. In the version shown in Figure 3, the device extends into the mould. Referring particularly to Figure 2, the device II comprises a body 12 which defines a chamber 13 having upper and lower portions 14 and 15. A shoulder 16 is disposed between portions 14 and 15 and

a sealing ring 17 is disposed on this shoulder. A plunger 18 coacts with this sealing ring 17 to seal off the upper portion 14 from the lower portion 15 of

the chamber 13. The plunger is urged towards the sealing ring 17 by means of a compression spring 19 constrained in the upper portion between the sealing ring and the upper wall 20 of the chamber 13. The plunger 18 has a dependant lower part 21 which extends into portion 15. At the lower end of

this part 21 is disposed a bimetallic disc 22. This disc 22 operates to push the plunger 18 off the sealing ring 17 against the action of the spring 19 permitting air to pass from the line 6, which connects with the chamber 13, through the chamber into the mould.

Referring to Figure 3, the alternative form of device II comprises a body 12 disposed partly outside and partly inside the mould. In other respects the construction is very much the same as that of the device of

Figure 2 except that the dimensions and disposition of the chamber 13 differ. Also the air pressure outlet 23 is to the side of the chamber 13 in Figure 2 and at the base of the chamber in Figure 3. In both figures 2 and 3

equivalent parts bear the same reference numerals. The above described arrangements operate in the following manner.

The arrangement would usually first be calibrated using an accurate electronic measuring device such as is described in European Patent Specification No.90914945.2. In the calibration procedure, the actuation temperature required for the switch device II is determined. This temperature is then used to select a temperature sensitive component (in this case the bimetallic disc 22) which will operate at this temperature. The component wilt usually be selected from a range of components which will allow activation over a range between I40°C and 300°C depending on the

moulding process. As indicated earlier, the drop in pressure in the line 6 due to activation of the bimetallic disc at the particular temperature can be sensed by the transducer and computer link 8 and used to allow the apparatus to automatically control the moulding cycle independent of operator or machine variables. Two modes of operation may be used as illustrated in Figures 4a and 4b.

The device II may be pressurised from the compressed air line 6 to the

required pressure (e.g. 5 to I0 psi) and connected to the mould in the same manner as a normal vent as shown in Figure 4a. When the set temperature is reached inside the mould 2, the device II will flip and the pressure is

allowed to pass into the mould 2. The device will give a 'blip' in the pressure line due to a temporary drop in pressure as the mould is pressurised. The transducer and computer 8 attached to the pressure line 6 will detect the change and activate the next stage of the process. This process could be

used to remove bubbles from the part and reduce cycle times. Alternatively, the device II may be pressurised from the compressed air line 6 to the required pressure (e.g. 5 to I0 psi) and connected to the mould 2 alongside a normal vent as shown in Figure 4b. When the set temperature is reached inside the mould 2, the device will flip and the pressure is allowed to pass into the mould 2. Since the mould cannot become pressurised, the device will produce a drop in pressure on the line 6 as the air passes out through the mould 2. The transducer and computer

8 attached to the pressure line 6 will detect the drop and activate the next stage of the process. In both cases the device II is auto resetting (or normal) so that it can be caused in the next cycle.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention. For example, the device II may

comprise low temperature alloys , polymers or other devices.

Claims

CLAIMS I. A rotational moulding apparatus comprising an oven, a mould disposed inside the oven, means for supporting the mould inside the oven, means for rotating the mould inside the oven about two transverse axes, a fluid line leading from outside the oven into the mould and a temperature sensitive device disposed in the line in the mould operative to influence the fluid pressure in the line in dependence upon the temperature in the mould and thereby to indicate outside the mould the temperature in the mould.

2. A rotational moulding apparatus as claimed in claim I, in which the

fluid line is an air pressure line adapted in operation to be connected to a compressed air source.

3. A rotational moulding apparatus as claimed in claim I or 2, in which the temperature sensitive device comprises a bimetallic device such as a disc

or strip.

4. A rotational moulding apparatus as claimed in claim 3, in which the temperature sensitive device comprises low temperature alloys.

5. A rotational moulding apparatus as claimed in claim 4, in which the

temperature sensitive device comprises polymers.

6. A rotational moulding apparatus as claimed in any preceding claim, in which the fluid line leads to a conventional vent in the mould.

7. A rotational moulding apparatus as claimed in any of claims I to 5, in

which the fluid line lead to a vent additional to a conventional vent provided

in the mould.

8. A rotational moulding as claimed in any preceding claim, in which the temperature sensitive device comprises a temperature sensitive element connected to a member which is operative to move under the influence of the element between a position in which the tine is sealed to a position in which the line is open.

9. A rotational moulding as claimed in claim 8, in which the seal comprises an 'O' ring on which the member seats.

10. A rotational moulding as claimed in claim 8 or 9, in which the member

is spring loaded to urge it into a sealing position. II. A method of indicating the temperature in a mould externally of the mould including the steps of establishing a fluid connection into the mould, sensing the temperature in the mould, restricting or closing the connection inside mould in dependence upon the temperature sensed and measuring fluid pressure variations thereby caused in the connection externally of the mould in order to indicate the temperature inside the mould.

12. A method of indicating the temperature in a mould as claimed in claim

11. in which air is supplied from a source of compressed air through the fluid

line to a conventional vent in the mould.

13. A method of indicating the temperature in a mould as claimed in claim 11 in which air is supplied from a source of compressed air through the fluid

line to a vent in the mould specially provided for the purpose.