GB2311482A

GB2311482A - Measuring temperature in rotational moulding apparatus

Info

Publication number: GB2311482A
Application number: GB9606517A
Authority: GB
Inventors: Nicholas Grose Henwood
Original assignee: MATRIX POLYMERS Ltd
Current assignee: MATRIX POLYMERS Ltd
Priority date: 1996-03-28
Filing date: 1996-03-28
Publication date: 1997-10-01
Also published as: GB9606517D0

Description

METHOD AND APPARATUS FOR ROTATIONAL MOULDING This invention relates to method and apparatus for rotational moulding.

The invention also relates to a method of optimising a rotational moulding process.

Rotational moulding is a moulding process for producing artides from plastics materials. Rotational moulding is particularly suitable for large articles7 because the moulds are much cheaper than the moulds for other sorts of plastics moulding, such as injection moulding and blow moulding. Large hollow artides such as plastics canoes and plastics wastebins are often made by rotational moulding.

At the start of the rotational moulding process the plastics material, in powder form, is placed in the mould, and the mould is rotated, usually about two perpendicular axes. The mould is then heated to melt the plastics material. The heating is continued at least until the interior of the mould has been evenly coated with the molten plastics material. In general, the heating should be continued for some time after all the plastics material has been melted, because this will improve the properties of the moulded article. Eventually there will come a point where continued heating will result in deterioration of the properties of the moulded artide.

The heating should preferably be stopped at or before this point. After the heating is stopped, the mould is left to cool whilst still rotating; the cooling process is usually accelerated by cooling with water and/or air.

It is well known in the art that there are problems in controlling the rotational moulding process. These problems arise out of the difficulty in knowing when all the plastics material has melted, and the difficulty in knowing how long to heat the melted plastics material in order to get the optimum properties for the moulded artide. For this reason most operators of rotational moulding apparatus usually determine whether or not a particular moulding operation is successful by visually inspecting the moulded artide. Although this visual inspection can identify major flaws, it often does not identify minor flaws. This can be important, because even minor flaws in the moulded artide can lead to premature failure.

Dr. RJ. Crawford and Dr. P.J. Newton, of the Queens University of Belfast, have made considerable advances in the control of rotational moulding. These advances are described, for example, in WO-A-9 105647. Dr. Crawford et al plotted the relationship between the internal temperature of the mould and time, and found that the curve has a number of distinct regions separated by discontinuities. The first discontinuity represented the time at which all the plastics material had melted. The second discontinuity represented the time at which the optimum properties for the moulded artide had been achieved. The third discontinuity represented the time at which the plastics material had been re-solidified and could be removed from the mould.

In WO-A-9 105647, Dr. Crawford et al proposed a method and apparatus for controlling a rotational moulding process which involved using a temperature monitor for measuring the temperature of the air inside the mould, and using a controller for controlling the rotational moulding in response to the measured temperature. The monitoring and control apparatus disclosed in WO-A-9105647 provides a means to produce articles of excellent quality by rotational moulding.

However, this apparatus does have a number of problems. In particular, it is relatively large and is complex to operate. Also, due to the complexity of the apparatus, it is relatively expensive: the cost of the apparatus may be more than the cost of the rest of the mould. Another problem is that the apparatus cannot be used repeatedly - it needs to be cooled down thoroughly after only a few cydes. In addition, the high temperatures used during rotational moulding can effect the reliability of the electronic components. Finally, there is a desire in the industry to retain a degree of manual control of the rotational moulding process, which this apparatus tends to take away.

Broadly, the present invention solves these problems by providing a simple method and apparatus for rotational moulding, in which the temperature of the air inside the mould can be measured quickly and easily, and without the need for any control equipment.

According to one aspect of the invention, there is provided rotational moulding apparatus comprising a mould for receiving a moulding material, and a temperature sensor disposed in the mould, wherein said temperature sensor includes means for recording the maximum temperature to which said sensor has been subjected.

It is important to understand that the accuracy of the temperature measurement is not usually critical. Thus the maximum recorded temperature may only be within 5"C or 10 C of the actual maximum temperature to which it has been subjected. Of course, if necessary the maximum recorded temperature may be accurate to within 1 C, or less.

Advantageously, the temperature sensor comprises a support, and a temperature sensitive material on said support, wherein said temperature sensitive material includes a plurality of sections, each section being such as to change colour in response to a temperature of a certain level. For example, the temperature sensitive material could indude 10 sections, the first section changing colour in response to a temperature of 170"C, and the subsequent sections changing colour in response to a successively higher temperatures, in 5"C increments, up to 215"C for the tenth section. A temperature monitor of this form would be able to provide a record of the maximum temperature, to the nearest 5"C, provided that the maximum temperature was within the range 170"C to 215"C. It will be appreciated that the number of temperature responsive sections can be increased or decreased in order to widen or reduce the breadth of the temperature sensitive range, and/or in order to alter accuracy of the temperature measurement.

It will be appreciated that the maximum temperature to which the temperature sensor has been subjected is not necessarily as high as the maximum temperature within the mould; this will depend upon where the temperature sensor is disposed within the mould. The temperature sensor is usually used to record the temperature of the air within the mould, but it is possible to use it to record the temperature of the mouldable material within the mould.

Preferably the mould includes an aperture therein. This aperture allows the pressure within the mould to remain at about atmospheric pressure throughout the moulding process. The aperture will form a hole in the moulded artide, but this is not a problem for most artides: either the hole can be disposed at a part of the article that is to be cut away, or it can be disposed at a part of the artide that is not normally visible.

It is desirable that the support of the temperature sensor is elongate and extends through the aperture in the mould into the interior thereof. Means can be provided to removably secure the support to the exterior of the mould; typically the securing means would be a form of clip.

In some circumstances it is desirable to carry out the rotational moulding above atmospheric pressure, in which case an aperture cannot be provided in the apparatus. In this case, other means can be provided to hold the sensor in the mould.

In the preferred form of the invention the temperature inside the mould is permanently and irreversibly recorded on the temperature sensitive material. In order to provide a degree of re-usability, it is preferred that the temperature sensitive material is removably secured to the support. Thus, the temperature sensor can be reused by removing the used temperature sensitive material, and replacing it with an unused temperature sensitive material.

invention can be provided with all the usual equipment, including: means to rotate the mould about at least one axis, preferably about two axes; an oven to provide the heat; and an air or water cooling system. The moulding material will typically be a synthetic plastics material, such as LMDPE (linear medium density polyethylene).

According to another aspect of the invention there is provided a method of forming an artide by rotational moulding, comprising heating a mould containing a mouldable material, rotating the mould about at least one axis, measuring the temperature within the mould with a temperature sensor, and recording the maximum temperature to which said temperature sensor is subjected in the mould.

According to another aspect of the invention there is provided a method of optimising a rotational moulding process comprising the steps of: (a) heating and rotating a mould containing mouldable material for a pre-determined amount of time to form a moulded artide; (b) during step (a), sensing the maximum temperature in part of the mould using a temperature sensor having means to record the maximum temperature to which said temperature sensor is subjected during step (a); (c) measuring a physical property of said moulded artide; (d) repeating steps (a) to (c), using different pre-selected heating times in step (a), in order to determine a relationship between the temperature measured in step (b) and the physical property measured in step (c); and (e) using the relationship derived in step (d) to determine the heating time and the internal mould temperature necessary to achieve a desired value for the physical property.

Once the heating time has been determined in step (e), the rotational moulding apparatus can be used to produce the moulded articles by heating for the time determined in step (a). It is not now absolutely essential to measure the temperature inside the mould when each moulded article is produced, because the desired temperature can be reached by carrying out the heating for heating time.

However, there are occasions when the correct temperature may not be reached, or may be exceeded, due to the timing being mis-set, or due to other chance factors. For this reason it is a highly preferred feature of the invention to measure the temperature with a temperature sensor during each moulding operation. This will enable the operator to confirm that the correct temperature was reached during each moulding operation, and will enable the operator to have greater confidence that there are no flaws in the moulded artide.

The physical property measured in step (d) is preferably impact strength.

However, other physical properties may be measured as well or instead. Another physical property that may usefully be measured is brittleness.

Reference is now made to the accompanying drawings, in which: Figure 1 is a cross-sectional view of part of an embodiment of rotational moulding apparatus according to the invention; Figure 2 is a perspective view of part of the embodiment of rotational moulding apparatus shown in Figure 1, on an enlarged scale; Figure 3 is a plan view of a strip of temperature sensitive material for use with the apparatus according to the invention, on an enlarged scale; and Figure 4 is a graph of impact strength and mould temperature vs. cook time for use in a method of optimising a rotational moulding process.

Referring specifically to Figures 1 and 2, there is shown a part of a rotational moulding apparatus, which is generally designated 10. The apparatus 10 includes a mould 12 that has an internal shape corresponding to the external shape of the article to be moulded.

The mould 12 has a vent tube 14 disposed in an aperture on the mould 12. The vent tube 14 provides an air passageway 16, which enables the air pressure within the mould 12 to be maintained close to atmospheric during heating and cooling of the mould.

A temperature sensor generally designated 18 is disposed in the passageway 16, with a part thereof projecting into the mould 12, and a part thereof projecting out of the mould 12. The temperature sensor 18 comprises an elongate support 20 for supporting two strips 22 and 24 of a temperature sensitive material.

Positional adjustment means in the form of an adjustable sleeve 26 and adjustment screw 28 is disposed about part of the support 20. The adjustment screw 28 enables the adjustment sleeve 26 to slide axially relative to the support 20. The positional adjustment means allows for adjustment of the position of the strips 22 and 24 within the mould 12. Usually, the strips 22 and 24 would be disposed substantially centrally within the mould 12.

A circular screw clip 30 is provided to secure the temperature sensor 18 to the vent tube 14. The screw clip 30 is secured to the sleeve 28, and is removably secured to the vent tube 14.

Figure 3 shows the strip 22 on an enlarged scale. The strip 22 comprises a plurality of heat sensitive sections 22a to 22e. Each section 22a to 22e comprises a material that changes colour in response to a specific temperature level. Thus, the section 22a changes colour at a temperature of 170"C, the section 22b changes colour at a temperature of 175"C, the section 22c changes colour at a temperature of 180"C, the section 22d changes colour at a temperature of 185"C, and the section 22e changes colour at a temperature of 1900C. The strip 24 comprises five sections that change colour in response to temperatures in the range 19S"C to 215"C. The colour change is permanent and irreversible. Thus, after being heated it is possible, within limits, to determine the maximum temperature to which the sections 22 and 24 have been subjected. If none of the sections has changed colour, then the temperature never rose above 170"C. If all of the sections have changed colour then the temperature rose above 215"C. If some of the sections have changed colour, then the maximum temperature can be determined to the nearest 5"C. Of course, the number of the sections can be changed, and the level of temperature sensitivity can be changed, in order to increase or decrease the working temperature range, and in order to increase or decrease the accuracy of the temperature measurement.

The temperature sensitive strips 22 and 24 are readily available. One suitable strip is sold by RS Components Limited of Corby under the code 555-421.

A method of optimising a rotational moulding process will now be described with reference to Figure 4. The rotational moulding apparatus 10 is used to carry out a series of runs each lasting a different length of time. Each run is carried out using the same grade of material- in this case LMDPE. In addition the oven temperature is the same during each run - this represents the maximum temperature that the mould could reach if it were heated indefinitely. In practice, the moulding operation is stopped before all the air within the mould reaches this temperature.

The first run lasts for 6 minutes, and the run time is successively increased in one minute intervals up to a maximum of 18 minutes. During each run the maximum air temperature within the mould is measured with a temperature sensor 18; the strips 22 and 24 are renewed after each run. The impact strength of the moulded artide produced during each run is measured using a method specified by the USbased Association of Rotational Molders. This information is used to form the graph shown in Figure 4.

From Figure 4 it can be established that the maximum impact strength was obtained using a heating time of 14 minutes, and that the temperature at this point was about 225"C. A heating time of 12 minutes provides good impact strength and corresponds to a temperature of about 200"C. It will be dear that provided the heating of the mould is carried out for 12 to 14 minutes, the moulded article formed should have good impact strength. This corresponds to an air temperature at the centre of the mould in the range 200-225"C. The temperature of the air in the mould can be measured during each moulding operation, and provided that the temperature is in the range 200-225"C, the operator can be confident that an article of good quality has been moulded.

It will be appreciated that many modifications may be made to the invention described above. For example the temperature sensor may be made as a single integral article (still with a removable temperature sensitive material). In addition, a purpose made fixing point for the temperature sensor may be provided on the mould, instead of using the vent tube.

Claims

CLAIMS:

1. Rotational moulding apparatus comprising a mould for receiving a moulding material, and a temperature sensor disposed in the mould, wherein said temperature sensor includes means for recording, to within 10 C, the maximum temperature to which said sensor has been subjected.

2. Apparatus according to claim 1, wherein the temperature sensor comprises a support and a temperature sensitive material on said support.

3. Apparatus according to claim 2, wherein said temperature sensitive material includes a plurality of sections, each section being such as to change colour in response to a temperature of a certain level.

4. Apparatus according to claim 3, wherein the temperature sensitive material includes ten sections, the first section changing colour in response to a temperature of 1700C , and the subsequent sections changing colour in response to successively higher temperatures, in 50C increments, up to 2 150C for the tenth section.

5. Apparatus according to any one of claims 2 to 4, wherein means is provided to removably secure the support to the exterior of the mould.

6. Apparatus according to any one of claims 2 to 5, wherein the maximum temperature is permanently and irreversibly recorded on the temperature sensitive material.

7. Apparatus according to any one of claims 2 to 6, wherein the temperature sensitive material is removably secured to the support.

8. Apparatus according to any preceding claim, wherein the support of the temperature sensor is elongate.

9. Apparatus according to claim 8, wherein the mould includes an aperture therein, and the support of the temperature sensor extends through the aperture into the interior of the mould.

10. A method of forming an article by rotational moulding, comprising heating a mould containing a mouldable material, rotating the mould about at least one axis, measuring the temperature within the mould with a temperature sensor, and recording to within 10 C the maximum temperature to which said temperature sensor is subjected in the mould.

11. A method according to claim 10, wherein said temperature sensor comprises a support having a temperature sensitive material thereon, and said maximum temperature is permanently and irreversibly recorded on said temperature sensitive material.

12. A method of optimising a rotational moulding process comprising the steps of: (a) heating and rotating a mould containing mouldable material for a predetermined amount of time to form a moulded article; (b) during step (a), sensing to within 10 C the maximum temperature in part of the mould using a temperature sensor having means to record the maximum temperature to which said temperature sensor is subjected during step (a), (c) measuring a physical property of said moulded article; (d) repeating steps (a) to (c), using different pre-selected heating times in step (a), in order to determine a relationship between the temperature measured in step (b) and the physical property measured in step (c); and (e) using the relationship derived in step (d) to determine the heating time and the internal mould temperature necessary to achieve a desired value for the physical property.

13. A method according to claim 12, wherein the physical property measure in step (d) is impact strength..

14. Rotational moulding apparatus substantially as herein described with reference to and as shown in the accompanying drawings.

15. A method of forming an article by rotational moulding substantially as herein described with reference to and as shown in the accompanying drawings.

16. A method of optimising a rotational moulding process substantially as herein described with reference to and as shown in the accompanying drawings.