WO1998041377A1

WO1998041377A1 - Continuous mold temperature monitoring system and method for rotational mold

Info

Publication number: WO1998041377A1
Application number: PCT/US1998/005461
Authority: WO
Inventors: Jerry E. Lambert; Smith E. Trent, Iii; Chris E. Ballou
Original assignee: Toter Incorporated
Priority date: 1997-03-20
Filing date: 1998-03-20
Publication date: 1998-09-24

Abstract

A system for continuously monitoring the temperature of a rotational mold (M) mounted to a rotational molding apparatus (10), the apparatus including a base (50), a drive device (11) supported on the base for 360 degree rotation thereon, a mold (M) and a rotatable shaft (13s) extending from the drive device (11) for supporting the mold, the mold being rotatable through a 360 degree axis of rotation with respect to the shaft, and the shaft being rotatable through a 360 degree axis of rotation with respect to the drive device. The system comprises a heat sensor (14) mounted in the mold; a series of first (16), second (26), and third (36) slip ring assemblies, each including a pair of slip rings and a contact device (21, 29, 39), the assemblies maintaining continuous electrical contact during rotation of the mold, the shaft and the drive device; and a control device (40) receiving electrical signals from the heat sensor through the first, second and third slip ring assemblies for continuously monitoring the temperature of the rotational mold. A method for controlling a rotational molding apparatus is also disclosed.

Description

CONTINUOUS MOLD TEMPERATURE MONITORING SYSTEM AND METHOD FOR ROTATIONAL MOLD CROSS REFERENCE TO RELATED APPLICATION

The present application claims benefits under 35 U.S.C. § 119 based U.S. Provisional Application, Serial No. 60/041,113, filed March 20, 1997, the complete disclosure of which is incorporated by reference .

FIELD OF THE INVENTION

The present invention relates generally to rotational molding and, more particularly, to a system and method for continuous monitoring of the internal mold temperature of a rotational mold.

BACKGROUND OF THE INVENTION

Rotational molding is a well known technique for molding hollow plastic containers and other shapes. Rotational molding provides advantages over other molding processes, such as blow-molding or injection molding. For example, rotational molding can produce strong, one- piece, low stress articles that are not weakened by joints or seams. This type of molding also allows relatively complex articles to be constructed. For example, hollow sections of a molded article, such as container rims or handles may be formed in a completely monolithic structure. Single and double walled products may be formed and inserts, such as metallic components, may be more easily integrated into a molded article.

Rotational molding involves the melting of a thermoplastic resin, such as polyethylene or polypropylene, in a heated, biaxially-rotating mold. The shape of the article to be molded generally conforms to the interior shape of the mold. The thermoplastic resin particles melt and puddle in the bottom of the mold. As the mold is rotated simultaneously about two axes, all interior surfaces of the mold rotate through the melted plastic and unmelted particles, causing the melted plastic to coat the interior of the mold. After the plastic has melted and conformed to the mold's interior, the mold is moved to a cooling chamber where it is cooled by air and/or water. After the plastic has fully solidified, the mold is opened and the molded article is removed. Rough edges and unwanted sections of plastic are then trimmed off to give the article its final shape.

A typical hot air rotational molding apparatus is a large piece of equipment, and the bi -axial rotation requires that the apparatus have at least two or three pivot points which allow the mold to be rotated through substantially spherical 360 degree rotations.

The geometry of the rotational molding equipment may vary widely depending primarily, but not necessarily, on size and shape of the article to be formed. However, all types of such equipment include as basic components, a drive device, a shaft rotatable on a primary axis and extending from the drive device, and a mold support carried by the shaft and at least pivotable, if not rotatable, on a secondary axis perpendicular to the primary axis. For example, in turret machines, a plurality of radial arms, defining respective primary axes, extend from a central rotatable drive device and each arm carries the mold support on the projecting end thereof. Rotation of the central drive device results in translation of the arm supported molds through successive treatment stations arranged around the central drive device. In independent arm machines, a plurality of drive devices are independently translatable about a central support column. An arm defining the primary axis extends from each drive device and carries at its end the mold support. Although the independent arm machine is similar to the turret machine in that the drive devices are translatable to carry the supported molds through successive treatment stations, the mold supported by each arm is translatable independently of the other molds . In shuttle machines, the drive device is vehicular in that it may be advanced into and out of various treatment stations which may be arranged in other than a circular relationship. However, the mobile shuttle drive mechanism of the shuttle machine again carries a shaft on a primary axis for supporting a mold on the secondary axis. In clamshell machines, a stationary drive device is used and movable treatment chambers are positioned over the bi-rotational mold and drive device. Finally, in rocking oven machines, the drive device again supports a shaft for rotation on the primary axis. The mold support in this type of machine, however, rocks or oscillates about the secondary axis without undergoing full rotation about the secondary axis. The rocking oven machine is used for forming elongated articles such as canoes, kayaks, lampposts and the like and represents the only type of machine in which the mold does not rotate completely about both the primary and secondary axes .

During the molding process, it has not been possible to continuously monitor the internal temperature of a mold, so the process is normally controlled by preset time and temperature only, based on trial and error and historical molding data. This leads to inefficiencies in the molding process and variations in the quality of the molded articles. For example, the quality of the rotational molded article is dependent in substantial measure on proper curing of the thermoplastic resin. That curing, in turn, is dependent on heating/cooling temperatures and the duration of time the resin is exposed to such temperatures. The trial-and-error experimentation heretofore required to achieve proper curing for a particular molded product, is both time consuming and waste productive. One of the principal barriers to measuring internal mold temperature has been the inability to obtain a continuous and reliable thermocouple signal from the mold due to the design of the rotating components of such apparatus and the requirement for the 360 degree pivot points. The problem is further compounded by the extreme temperatures used in the molding process, and the exposure to the water in the cooling pr.ocess.

A previous attempt has been made to solve this problem by the University of Belfast in Northern Ireland by using a wireless temperature sensing system. However, the wireless system that was developed can only monitor the internal mold temperature for a limited time, and the temperature sensing mechanism of this system must be removed after two to four cycles of the molding process in order to avoid overheating and destruction of the sensing device. This system is useful for development and troubleshooting cycles, but cannot be directly tied to apparatus controls to create a continuous process monitoring system.

SUMMARY OF THE INVENTION

In view of the above, it is a primary object of the present invention to provide a system and method for accurately monitoring the mold temperature in a rotational molding apparatus. Another object of the invention is to control a rotational molding apparatus in response to the internal mold temperature. It is a further object of the invention to optimize a rotational molding operation by controlling the heating, cooling and removal of the molded article based on internal mold temperature. Another object of the invention is to conserve energy in a rotational molding process and apparatus by precisely monitoring the internal mold temperature, and controlling other apparatus functions as a function of that temperature.

Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims .

To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is a system for continuously monitoring the temperature of a rotational mold mounted to a rotational molding apparatus. The apparatus includes a drive device, a mold and a rotatable shaft extending from the drive device for supporting the mold and defining a primary axis, the shaft being rotatable through 360 degrees axis of rotation about the primary axis, the mold being at least partially rotatable about a secondary axis perpendicular to the primary axis. The system of the invention includes a heat sensor mounted in the mold. At least one slip ring assembly is provided and includes a pair of slip rings and a contact device. A control device is electrically connected to the at least one slip ring assembly. The pair of slip rings is electrically connected to the heat sensor, and the contact device maintains continuous electrical contact between the pair of slip rings and the control device during rotation of the mold. A transmitter is preferably located between the heat sensor and the at least one slip ring assembly for converting electrical signals from the heat sensor and transmitting a converted signal through the at least one slip ring assembly to the control device for continuously monitoring the temperature of the rotational mold.

For rotational molding machines in which the mold is rotatable through 360° about the secondary axis, the system of the invention includes, in addition to the heat sensor mounted in the mold, first and second slip ring assemblies. The first slip ring assembly includes a first pair of first slip rings and a first contact device. One of the first pair of slip rings and the first contact device is electrically connected to the heat sensor, and the other of the pair of first slip rings and the first contact device is mounted to the rotatable shaft for maintaining continuous electrical contact between the pair of first slip rings and the first contact device during rotation of the mold. The invention also includes a second slip ring assembly, including a pair of second slip rings and a second contact device. One of the second slip rings and the second contact device is electrically connected to the first slip ring assembly, and the other of the second slip rings is mounted to the shaft and the second contact device maintains continuous electrical contact between the second slip rings and the second contact device during rotation of the shaft . A control device is electrically connected to the second slip ring assembly, and a transmitter is located between the first and second slip ring assemblies for converting electrical signals from the heat sensor and transmitting converted signals to the control device for continuously monitoring the temperature of the rotational mold.

In turret and independent arm rotational molding apparatus, the drive device arranged for rotation through a 360 degree angle of rotation about a base or support column. In these types of apparatus, it is also preferred that the system include a third slip ring assembly electrically connected between the second slip ring assembly and the control device. The third slip ring assembly preferably includes a pair of third slip rings and a third contact device, one of the third pair of slip rings and the third contact device being mounted to the drive device, and the other of the third slip rings and the third contact device being arranged on the base for maintaining continuous electrical contact between the third pair of slip rings and the third contact device during rotation of the drive device with respect to the base.

The invention also includes a method for controlling a rotational molding apparatus, comprising the steps of sensing the temperature within the mold using a heat sensing device which generates electrical signals corresponding to the temperature of the mold; transmitting the electrical signals from the heat sensing device to a control device through at least two slip ring assemblies continuously during bi-axial rotation of the mold; and controlling the operation of the rotational molding apparatus in response to the temperatures sensed by the heat sensing device.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description, serve to explain the principles of the invention. In the drawings,

Fig. 1 is a schematic diagram of the rotational molding system of the invention;

Fig. 2 is a perspective view of a mold for a rotational molding apparatus of the invention, showing the temperature probe;

Fig. 3 is a partial cut-away view of the mold supporting end of the shaft of the rotational molding apparatus of Fig. 1;

Fig. 4 is an enlarged view showing the second slip ring assembly of the rotational molding system in Fig. 1; and

Fig. 5 is a an enlarged view of the base showing the third slip ring assembly for one shaft of the molding apparatus .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings . Wherever possible, the same reference characters will be used throughout the drawings to refer to the same or like parts .

In accordance with the invention the system monitors the temperature of a rotational mold mounted to a rotational molding apparatus. The apparatus includes a drive device, a mold and a shaft extending from the drive device for supporting the mold.

The shaft defines a primary axis, and is rotatable through 360 degrees axis of rotation about the primary axis. The mold is at least partially rotatable about a secondary axis perpendicular to the primary axis.

In Fig. 1, an embodiment of the invention is applied to an independent arm rotational molding apparatus generally designated by the reference numeral 10. While this type of machine is particularly challenging in terms of the problem addressed by the present invention, it will be understood from the following description that the system of the invention is applicable to all types of rotational molding machines.

The illustrated apparatus 10 includes a drive device

11, a mold M, and a shaft 13s supported in an arm 13a extending from the drive device 11 to a pair of supports 12 for supporting the mold M and rotatable though 360 degrees of rotation on a secondary axis perpendicular to a primary axis defined by the shaft 13s. The shaft 13s rotates through 360 degrees of rotation with respect to the drive device 11 and about the primary axis. In practice, the independent arm rotational molding apparatus 10 is likely to include a plurality of drive devices 10 and arms 13a, each supporting separate rotational molds. Also, it is to be noted that although a single mold M is mounted to only one of the supports 12 for illustration purposes, it is common practice to mount two or more molds M, at least one to each of the supports

12, to counter balance the inertial loading at the ends of the rotating shaft 13s. However, since both such molds are exposed to the same temperatures during a rotational molding process, temperature monitoring in only one of the molds is applicable to both.

In accordance with the invention, a heat sensor is mounted in the mold. As shown in Fig. 2, a heat sensor 14 is attached to the mold M, and includes a thermal probe (not shown) which extends into the mold M for sensing the temperature. Many types of heat sensors can be used for this purpose. An example of a suitable heat sensor is a type K thermocouple. The probe from the thermocouple can be mounted in the mold M using any suitable fastening means .

The invention includes at least one slip ring assembly, including a pair of slip rings. The pair of slip rings is electrically connected to the heat sensor, and is mounted for rotation with the mold, and the slip ring assembly also includes a contact device in the form of a brush component mounted to the shaft for maintaining continuous electrical contact with the pair of first slip rings. As shown in Fig. 3, a first slip ring assembly 16 includes a pair of first slip rings 17, 18, which are electrically connected to the heat sensor 14 via wires 15. In the illustrated embodiment, wires 15 from the thermocouple are routed to two bands 17, 18 which are fastened to the mold support under a ring gear 19 on the head 20 of the shaft 13s of the rotational molding apparatus 10. Each of the bands 17, 18 is made of a material corresponding to the material of the wires 15 from the thermocouple. The type K thermocouple utilizes chromel and alumel as base materials. The chromel band and the alumel band are isolated from each other and from the rotational molding apparatus 10 using a strip of insulating material, such as Teflon (not shown) . A high temperature isolating paint also may be placed under the insulating material strip to further electrically isolate the slip ring mechanism. The first slip ring assembly 16 must be protected against shorting caused by moisture or physical contact .

As shown in Fig. 3, a first contact device 21 is arranged to make continuous contact with the bands 17, 18. The first contact device 21 may be a set of brushes (not shown) , made from the same material as the bands, and mounted on suitable brackets or braces to align the brushes with the bands 17, 18. Each of the brushes may have its own spring-loaded retainer (not shown) to bias the brush against the corresponding bajid 17, 18, as is known to those skilled in the art . The combination of the brushes and the bands creates the slip ring assembly 16 which allows the transfer of the electrical signals from the heat sensor 14 through the rotating joint on the head 20 of each arm 13a of the rotational molding apparatus. Care must be given to comply with all laws governing the principles of thermocouple operation.

In accordance with the invention, a voltage signal generated by the heat sensor is converted to a current signal by a transmitter preferably located and connected to transmit a converted signal from the heat sensor though the slip ring assemblies. In the embodiment illustrated in Fig 1. a signal transmitter T is mounted directly to the shaft 13s outside of a wall W defining a treatment chamber and is electrically connected to the slip ring assembly 16 by lead wires. The transmitter T converts a voltage signal from the heat sensor 14 to a current signal, preferably a 4 - 20 mA signal, which is more effectively conducted through slip ring assemblies than a voltage signal. Although it would be preferred to locate the transmitter T between the heat sensor 14 and the slip ring assembly 16, the current state of the art for such transmitters prevents their use in the extreme environment of the treatment chambers used with rotational molding machines. It has been found in practice, however, that conversion of the voltage signal from the heat sensor 14 to a current signal, even after -lithe voltage signal has passed through one slip ring assembly, provides a more reliable signal for control purposes .

In accordance with the invention, a second slip ring assembly, including a pair of second slip rings, is also provided. The pair of second slip rings is electrically connected to the first slip ring assembly, and is mounted for rotation with the shaft. In the illustrated embodiment of Fig. 4, a second slip ring assembly 26 is constructed like the first slip ring assembly 16. Lead wires 25a connect the transmitter T to the second slip ring assembly 26. Each wire 25a is separated and isolated to prevent grounding or other erroneous readings. As shown in Fig. 4, a pair of slip rings or bands 27, 28 are arranged around the shaft 13s. A corresponding contact device 31 includes sets of brushes 29, mounted to the the bracets 32 fixed to arm 13a, and aligned for continuously contacting the bands 27, 28 to maintain electrical contact between the bands 27, 28 and the brushes 29 during rotation of the shaft 13s about its axis of rotation.

Although the use of first and second slip ring assemblies is important to rotational molding apparatus of the type illustrated, in which the mold M is rotatable through 360 degrees about both the primary and secondary axes of rotation, in rocking oven machines, the mold oscillates though less than 360 degrees. In such machines, it is possible to connect the heat sensor to the shaft rotating on the primary axis by using flexible cable wiring. In this type of machine, therefore, only one slip ring assembly may be required.

The invention also includes a control device electrically connected to the other of the second slip rings. The control device receives electrical signals from the heat sensor through the first and second slip ring assemblies for continuously monitoring the temperature in the rotational mold. As shown in Fig. 1, a machine control device 40 is electrically connected to receive signals from the heat sensor 14 by way of the wires 15, the first slip ring assembly 16, the wires 25, the transmitter T, the wires 25a, the second slip ring assembly 26 and the wires 35. One example of a suitable control device is a programmable logic controller ( PLC) . The PLC provides a wide range of flexibility in controlling the rotational molding apparatus. The PLC can monitor the temperature and the location of the arm in the cycle. This allows the arm 13 to be cycled out based on the temperature of the mold M . The controller 40 can also store data on temperatures in the mold for future analysis.

The illustrated independent arm rotational molding apparatus also includes a drive device on which the base of the apparatus rotates through a 360 degree angle of rotation. In this arrangement, the system includes a third slip ring assembly electrically mounted between the second slip ring assembly and the control device. The third slip ring assembly is constructed similar to the first and second slip ring assemblies. In the illustrated embodiment of Fig 5, a third slip ring assembly 36 includes a pair of third slip rings or bands 37, 38 mounted to a base 50 or center pivot post of the rotational molding apparatus 10. A third contact device 39 , such as a set of brushes, moves with the drive device 11 while maintaining electrical contact with the bands 37, 38. Lead wires 35 from the second slip ring assembly 26 connect to the brushes 39 of the third slip ring assembly 36. The third slip ring assembly 36 is then connected via lead wires 45 to the control device 40.

The invention provides significant benefits in allowing a rotational molding apparatus to be controlled to index the arms 13 out of an oven and into a cooling station when the temperature of the mold M reaches a specified level. Other functions also can be controlled in response to the varying temperature ranges, including, but not limited to air cooling, water mist cooling, and water spray cooling. All of these can help to mold an optimal article with a minimum of warpage, and provide ease of part removal from the mold. The invention provides an ability to monitor and trigger appropriate apparatus functions when the article being molded is at its optimal temperature. This also allows for optimization of material characteristics by using statistical data to improve impact resistance and other physical characteristics. For example, overheating of some materials may cause a breakdown or destruction of the physical characteristics of the material . Precise temperature monitoring allows this problem to be avoided or minimized.

Obviously, this system has numerous benefits, including energy savings, operational efficiency improvements, shorter cycle times, increased production, and improved quality of molded articles.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

What is claimed is :

1. A system for continuously monitoring the temperature of a rotational mold mounted to a rotational molding apparatus, the apparatus including a drive device, a mold and a rotatable shaft extending from the drive device for supporting the mold and defining a primary axis, the shaft being rotatable through 360 degrees axis of rotation about the primary axis, the mold being at least partially rotatable about a secondary axis perpendicular to the primary axis, the system comprising: a heat sensor mounted in the mold; at least one slip ring assembly including a pair of slip rings and a contact device; a control device electrically connected to the at least one slip ring assembly, the pair of slip rings being electrically connected to the heat sensor, and the contact device maintaining continuous electrical contact between the pair of slip rings and the control device during rotation of the mold; and a transmitter for converting and transmitting a signal generated by the heat sensor through the at least one slip ring assembly to the control device for continuously monitoring the temperature of the rotational mold.

2. The system of claim 1, wherein the signal generated by the heat sensor is a voltage signal, and the transmitter converts the voltage signal to a current signal .

3. The system of claim 1 including: a first slip ring assembly, including a pair of first slip rings and a first contact device, one of the pair of first slip rings and the first contact device being electrically connected to the heat sensor, and the other of the pair of first slip rings and the first contact device being connected for maintaining continuous electrical contact between the pair of first slip rings and the first contact device during rotation of the mold; and a second slip ring assembly, including a pair of second slip rings and a second contact device, one of the second slip rings and the second contact device being electrically connected to the first slip ring assembly, and the other of the second slip rings and the second contact device being connected for maintaining continuous electrical contact between the second slip rings and the second contact device during rotation of the arm, the transmitter being electrically connected between the first slip ring assembly and the second slip ring assembly, and the control device being electrically connected to the second slip ring assembly.

4. The system of claim 3, wherein the rotational molding apparatus also includes a base, the drive device being supported on the base for translation through a 360 degree angle of rotation, and the system also includes a third slip ring assembly electrically mounted between the second slip ring assembly and the control device, including a pair of third slip rings and a third contact device, one of the third pair of slip rings and the third contact device being mounted to the drive device, and the other of the third slip rings and the third contact device being arranged on the base for maintaining continuous electrical contact between the third pair of slip rings and the third contact device during rotation of the drive device with respect to the base.

5. A method for controlling a rotational molding apparatus having a rotational mold, comprising the steps of: sensing the temperature within the mold using a heat sensing device which generates electrical signals corresponding to the temperature of the mold; transmitting the electrical signals from the heat sensing device to a control device through at least one slip ring assembly continuously during bi-axial rotation of the mold; and controlling the operation of the rotational molding apparatus in response to the temperatures sensed by the heat sensing device.

6. The method of claim 5, wherein the electrical signal from the heat sensor is a voltage signal, and including the step of converting the voltage signal to a current signal .