MXPA00005634A - Mold heater startup method - Google Patents

Abstract

A method for mold heater startup and sequencing detects the heater zones associated with greater mass and allows them to heat up before zones of lesser mass, thereby reducing wear of the associated mechanical components and minimizing degradation of plastic material. The zones of smaller mass are kept at minimal temperatures until the zones with greater mass reach a set point. The system is capable of detecting multiple levels of thermal load and provides appropriate sequential startup of the identified thermal loads. The operator may also manually program the system to override certain automated sequences in order to ensure an optimal startup sequence. The sequencing of power application to large heaters also minimizes the peak current draw of the system.

Description

MOLD HEATER IGNITION METHOD Technical Field The present invention relates generally to molds used in injection molding machines and, more particularly, to a method for sequencing the ignition of electric heating elements used in such molds.

BACKGROUND OF THE INVENTION Molds used to form injection molded products often include heating elements to control the temperature of certain portions (zones) of the mold. In particular, heaters are used in "hot runner" systems to provide a controlled temperature flow path to the plastic melt moving from the injection unit of the molding machine to the mold cavity. Such systems typically include a manifold that supplies a number of nozzles each of which supplies melt to a mold cavity. Multiple heaters strategically placed through the mold and / or hot runner system to maintain the proper temperature and thus ensure that a good quality melt is used to form each part. However, given the different restrictions on the number and location of heating elements, as dictated by a particular mold construction (eg, gates, cooling fluid passages, central mechanisms, ejection bolts, etc.), The thermal mass associated with each heater can vary considerably. If the heater zones are not properly sequenced at the ignition, there may be excessive wear associated with the mechanical elements of the system (due to differences in thermal expansion), degradation of the plastic within the mold, lengthening of the mold preparation time, and inadequate total current consumption by the system. In the prior art several ignition systems have been proposed. The simplest method is to turn on all zones simultaneously. The operator of the molding machine implements this method by activating all the heat zones at the same time. The result is that areas with less mass (areas of "rapid temperature rise") reach the desired temperature more quickly, resulting in degradation of the material while the operator is waiting for the other zones to reach the appropriate temperature. In addition, the time difference to reach the temperature can result in excessive wear of the associated sealing elements because the nozzles (and possibly other components of the manifold) have expanded in size and apply pressure on the manifold in one direction while the manifold is still expanding in another direction. An alternative to the simultaneous ignition is the manual sequencing of those heaters. In this method, the operator starts the heater by supplying electric power to the heaters associated with the larger thermal masses (as determined by the operator, the manifolds, for example), allowing them to approach the reference temperature before turn on the heaters in the smaller areas (the nozzles, for example). If there are multiple levels of multiples, the operator can turn on one level, then the next level and, finally, the nozzles. Although manual sequencing may be an improvement over simultaneous ignition, there are still several disadvantages. The operator can simply forget the sequence, turn on all the zones of the heater at about the same time, resulting in the disadvantages of the simultaneous ignition noted above. In particular, the non-uniform thermal expansion causes the fit between the assemblies to be tightened before the manifolds have fully expanded. This causes wear and bonding between adjacent surfaces, which can mean premature failure of the sealing elements between the components of the system. further, the nozzles are usually heated within a few minutes while the manifolds can take between 15 and 30 minutes to reach the desired temperature. This causes the material in the nozzle to degrade, possibly to the point where the material flow is blocked - if this happens, it is necessary to disassemble and clean the system. Manual sequencing also allows the operator to extend the total ignition time considerably beyond what is actually required by being totally conservative in how to implement the sequence. Alternatively, the operator can turn on the manifold heaters and leave the machine to handle other issues. The manifold may actually heat up for some time before the operator returns to turn on the next level of manifold or nozzles. Since this will prolong the time in which the first level is under heat, it will prolong the ignition time. The danger of degradation of the material is also present under these circumstances. To reduce ignition time and material degradation, the operator has to verify the process very closely to determine when each level in the sequencing is heated and when to activate the next level.

A more automatic method is the "uniform" ignition, where a computerized system verifies the rate of increase of the temperature of each heating zone in the system. This type of control identifies the areas where the temperature rises rapidly and controls the energy to the heaters to decrease the speed of temperature increase. Basically, the computer allows the areas where the temperature rises more quickly, reaches a certain temperature and then inhibits the entry of additional heat, allowing the areas where the temperature rises more slowly to "settle". This process continues until the temperatures of the different zones reach their point of reference. Despite the more "uniform" thermal expansion of the different zones in this method, material degradation can still occur. For example, even when the nozzles can not reach the reference temperature for a prolonged period of time (while the manifold is "expected" to reach the temperature), they will still be at elevated temperatures during this extended period, resulting in some degradation of the material. In addition, wear will still occur, even when all system components are heating and expanding at approximately the same speed. Although this method reduces some of the wear and degradation problems, it does not eliminate them. This treats all zones of equal heat providing a uniform increase in the heat of all zones. Therefore, I do not really sequence the ignition. It should be noted that the electric heaters used in the systems described are often hygroscopic; that is, they absorb moisture from the air and must be "dried" before all the voltage or energy is applied. If they are not properly dried before applying all the energy, the heaters can be permanently damaged. Although multiple heaters are often built in such a way that moisture absorption is not a problem, nozzle heaters are rarely. This difference suggests that the manifold heaters may not require drying at the start of the ignition procedure, while heaters for the nozzle zones should always be properly dried before applying all the voltage. According to the methods of the prior art, the drying of the nozzle heaters does not start until they are turned on, usually after the manifolds are almost at the desired temperature; this prolongs the ignition time even more. Only the "uniform" method allows drying of all areas during sequencing. Unfortunately, the "uniform" method also allows a significant increase in the heat of the nozzles, resulting in thermal expansion and degradation of the material, as described above.

Description of the Invention Accordingly, an object of the present invention is to provide a method for controlling the activation and sequencing of the zones of the heater resulting in less wear of the sealing elements and effectively preventing the degradation of the plastic melt. due to prolonged exposure to heat. Consistent with the stated objective, the method of the present invention involves detecting the area of the heater that has the highest mass, applying heat to that zone, thereby allowing the mold segment to grow (expand due to temperature increase) without impediment. The heater zones associated with the smaller mass are maintained at minimum temperatures until the larger zones are heated. The system is capable of detecting multiple levels of temperature increase speeds and can provide the sequential ignition of those different types of thermal loads. The operator can also manually program the system to avoid certain automated sequences to ensure a firing sequence that best reduces mechanical wear and prevents degradation of the plastic material. The ignition sequencing method of the mold heater of the present invention verifies the rate of temperature increase in the different zones to determine the relative thermal mass; that is to say, that the greater the mass, the slower the increase in temperature. The method then applies energy to the higher mass areas to control the thermal expansion of the main mold elements in such a way that the wear of the associated mechanical components and sealing elements is minimized. The heater ignition method also minimizes or prevents degradation of the plastic material by reducing the amount of time the material is subjected to high temperatures. Furthermore, the described method serves to minimize the amount of time it takes to prepare the molding system for its operation and to comply at the same time with the previously highlighted advantages for the ignition of the system. Finally, the sequencing of application of energy to large heaters, as taught in the present invention, reduces the peak current consumption of the system, helping to prevent overloads of electric circuits and peak demand loads of utility companies. The apparatuses associated with the described method include suitable microprocessors, analog-to-digital converters and triacs (bi-directional tridgetters or electronic switches) (or other suitable energy switching devices) which are operatively coupled to the corresponding heaters and temperature sensors in each case. zone.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an elevation view, shown partially in section, of a hot runner system for an injection mold, including controlled heating elements according to the method of the present invention. Figure 2 is an enlarged view of a portion of the hot runner system of an injection mold illustrated in Figure 1. Figure 3 is a top view of the one level manifold of a hot runner system for an injection mold illustrated in Figure 1. Figure 4 is a top view of the level manifold two of the hot runner system for an injection mold illustrated in Figure 1. Figure 5 is a schematic representation of a single microprocessor control system which includes the ignition method of the mold heater of the present invention.

Figure 6 is a schematic representation of a modular control system including the ignition method of the mold heater of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION The method of the present invention is typically employed in association with electrical heating elements that are used to provide supplemental heat to components associated with an injection mold. Figure 1 illustrates a hot runner system 10 which is used to transport thermoplastic melt from the injection unit of an injection molding machine to multiple mold cavities. There is a "level one" manifold 12 which receives the flow of plastic melt from the injection unit through a nozzle adapter 14. The plastic melt conveyed through the conduits 16 in the manifold of level one 12 to be received by two "level two" manifolds 18. The two level manifolds 18 also include flow passages 20 for conveying the melt of the manifold of level one 12 to manifold nozzles 22. Each of the nozzles 22 they are in communication with a mold cavity for transporting the received plastic melt in associated flow passage 20 in the level two manifold 18, through a central passage 24 and in the mold cavity.

Since the plastic melt is in a fluid state during the production operation of the injection molding machine, it is not necessary for the connections between the manifolds and the nozzles to be fluid-tight. For this reason, seals 26 are provided at the junction of the passages 16 of the manifold of level one 12 with the passages 20 of the manifold of level two 18. Similarly, sealing rings 28 are provided at the junction of passages 20 of the multiple of the level two 18 with the central passages 24 of the nozzles 22. The hot runner system 10 is provided with multiple heating elements to carry the components (multiple 12, 18 and nozzles 22) at a suitable operating temperature (a switching on initial) and maintain the desired temperature of the plastic melt when it is transported to the mold. Typically, the desired operating temperatures are dictated by the type of plastic material used in a given application. As shown more clearly in Figures 2-4, the manifold of level one 12 has two heating elements 30 that encircle the flow passage 16. Similarly, the level two manifolds have two sets of coil heating elements. 32 for supplying heat very close to the flow passages 20. Alternatively, the manifolds 12, 18 could be equipped with heaters of the standard cartridge type, instead of the elongated coil elements, shown. The nozzles 22 are equipped with heating elements 34 which encircle the main body of the nozzles 22 to ensure proper flow through the central passage 24. The effect that the heaters have on the components of the hot runner system 10 is verified by multiple thermocouples placed to indicate exactly the thermal gradients in the system. In particular, as shown in the drawings, the level one manifold 12 has thermocouples 36, the level two multiples 18 has thermocouples 38 and the nozzles 22 are provided with thermocouples 40. The reference points for the heaters are initially based on the type plastic that is being processed and adjusted, often by trial and error, to achieve a set of conditions that facilitate the continuous operation of the machine by injection molding without "hot spots" that can cause degradation of the material. However, there are several circumstances that require that the operation of the injection molding machine be stopped; for example, mold change, color change, mold maintenance, etc. Obviously, when the heaters are "turned off" the plastic contained in the flow passage 16, 20, 24 solidifies. When it is finally desired to restart the molding process, the material in the flow passages 16, 20, 24 must melt again before the machine can operate to produce parts. The heaters 30, 32, 34 must be controlled so that (a) the thermal expansion of the components is relatively uniform to avoid damaging the seals 26, 28 and (b) the plastic material must not be maintained at temperatures that cause the degrade. These are the main objectives of the ignition method of the heater of the present invention. As illustrated in Figure 5, the method of the present invention can be implemented by a control system that uses a single microprocessor (CPU) to verify and control temperatures in all areas of the heater for a particular mold configuration; this is generally referred to as a multiplexed or multi-cycle control system. Alternatively, the heater ignition method can be included in a modular heater system constructed from multiple single-cycle control modules (see Figure 6) where a communication interface between the modules allows heater zones independently communicate with each other, so that the desired ignition sequence can be implemented. More specifically, in the modular configuration, a network or other communication means is used to allow each zone to share information with other zones to determine the relative thermal characteristics of the zones. Generally speaking, it is likely that the heater zones of a hot runner system will be divided into two, three or more sets of thermal characteristics. Each set becomes a "level" to be included in the ignition sequence. The set with the fastest rate of heat increase characteristics will typically be the zones that include the nozzle heaters 34. Those zones of "faster temperature rise" will be the last to be sequenced in all cases. In hot runner systems where there are multiple heater zone sets of "slow temperature rise" (multiple levels of manifold, for example), it may be necessary for the operator to determine the sequence; that is, which set will be the first to be turned on, which is the next, etc. In addition, even when the method can automatically detect different thermal sets, the system is designed to have enough flexibility to allow the operator to determine which set will be sequenced first. In other words, the operator would be allowed to sequence the zones in such a way that it could possibly prevent the sets and sequences from being determined automatically.

The implementation of the method of the present invention begins with all zones being activated in a dry mode; that is to say, voltage (or energy) very low. Using the feedback network generated by appropriate sensors during the initial part of the dry mode, the "slow" and "fast" heater zones are identified. More specifically, a higher current draw for a particular area during drying would indicate that there are high power or energy heaters in this area that would be associated with a higher thermal mass. Conversely, a lower current draw for a particular zone indicates smaller heaters associated with less thermal mass. Alternatively, the determination of the slow and fast heater zones can be made more directly by calculating the speed of the temperature in the different zones during the drying; the higher the speed, the "faster" the area. Once the drying of the "slower" zone is complete, a predetermined voltage is applied to bring the zone to its programmed set point. Depending on the construction of the mold, voltage can be applied to multiple zones otherwise adverse expansion will occur. In any case, the application of a low voltage (drying) is maintained for the "fast" zones until the "slow" zones have reached all their reference temperatures. The application of low voltage means that there will only be a minimum temperature increase in the "fast" zones to prevent undesirable thermal expansion and degradation of the material. If there are only two levels to be sequenced, nozzle heater zones will be released from low voltage (drying) when a single set of multiples reaches a predetermined temperature (often the lower end of the alarm or proportional band of the control) . With the multiple levels of multiples, the manifold of the second level will be released from drying when the first level reaches the predetermined temperature; in this way the process continues until all levels of the multiple reach the desired reference point. Finally, the heaters of the nozzles are activated to raise their temperature. Preferably, the control of the system would allow the operator to input the predetermined temperatures (reference point) that activate the ignition of the next level or set of heater zone. Although this sequence for the ignition of the heater means that the manifolds will maintain the temperature for some time before the nozzles are heated, the degradation of the material in the manifolds is negligible. The manifolds have large flow channels and more thermal mass that allows heat to be distributed without adverse effects. In contrast, if the material in the small holes of the nozzles is maintained at a high temperature for a long time, degradation is likely. If desired, the method of the present invention could be used together with the "Moisture Heater and Drying Apparatus Moisture Detection" described in U.S. Patent No. 5,039,842 to (a) prevent the application of a high voltage or energy to the heater if a large amount of leakage current to ground was detected and / or (b) interrupting the power to the heater if the earth leakage current exceeded a pre-set maximum and can not be corrected with the application of a low voltage. In the preferred embodiment, microprocessors are used to determine the thermal properties of the zone quickly and automatically. Optionally, the operator could enter the thermal ratio of the zones, if known, into the control system manually via a keyboard, a touch screen or other means known in the art. However, in a modular control system (as generally shown in Figure 6), the modules can be programmed using switches or physical bridges to provide the identification of the desired thermal level.

With the preferred embodiment, sets of heater zones presented on a computer screen or other appropriate reading provide visual feedback to the operator. The operator would then be able to modify the allocation of the set for each zone, if desired. The operator would also be able to enter this information before performing an initial ignition of the control system. Finally, in the preferred embodiment, the control system would allow the operator to keep the ratio of the thermal mass obtained and / or programmed from the heater in a storage device such as a hard / flexible disk, or memory devices in a solid state, such as a RAM, EPROM, EAROM or instant ROM supported by a battery. The same storage device could also store the reference temperature used to determine when to turn on the sequence of subsequent levels.

Industrial Applicability As described, the present invention provides a method for controlling the activation and sequencing of heater zones in the molds used in injection molding machines, which results in less wear of the sealing elements and effectively prevents degradation of the plastic melt due to prolonged exposure to heat. The system is capable of detecting multiple levels of temperature increase speeds and can provide the sequential ignition of those different types of thermal loads. In addition, the method described serves to minimize the period of time it takes to prepare the molding system for the operation and at the same time fulfill the advantages noted above for the ignition of the system. Finally, the sequencing of the application of energy to large heaters, as taught by the present invention, reduces the peak current consumption of the system, helping to prevent overloads of electrical circuits and peak demand loads of power supply companies. The apparatus associated with the method described includes suitable microprocessors, analog-to-digital converters and triacs (or other suitable energy switching devices) that are operatively coupled to the corresponding heaters and temperature sensors in each zone. Although the invention has been illustrated and described in some detail according to the preferred embodiment, there is no intention to limit the invention to such details. On the contrary, it is intended to cover all modifications, alterations and equivalents that fall within the scope of the appended claims. For example, depending on the construction of the mold, a single zone may include more than one heater or different heater configurations. In addition, other systems or mechanisms may be used to control the supply of electric power to the heaters. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (3)

1. CLAIMS Having described the invention as above, the claim contained in the following claims 1 is claimed as property. In an injection molding system it includes a mold with a hot runner system for supplying molten plastic to a cavity within a mold and a plurality of molds. heating elements associated with the mold and runner system, a method for bringing the mold and runner system to a desired operating temperature, characterized in that it comprises the steps of: (a) placing the heaters within the mold and runner system for divide the mass of the mold and runner system into multiple zones, (b) place temperature sensors within each zone to provide temperature-related feedback within the associated zones at any point in time, (c) apply a controlled current to the heating elements within each zone during a defined period of time, (d) verify the temperature of each zone using the feedback of the temperature sensors; (e) determining the rate of temperature increase for each zone based on the change in temperature produced by the controlled current and therefore identifying the relative thermal mass associated with each zone; (f) establish an ignition sequence for the zones based on the relative thermal mass determined in step (e), so that the heaters in the zones with the highest thermal mass will be activated to heat the associated mass and the system of mold and runner to a predetermined reference point before heaters in the area that have less thermal mass are activated.
2. 2. In an injection molding system that includes a mold with a hot runner system for supplying molten plastic to a cavity within the mold and a plurality of heating elements associated with the mold and runner system, a method for carrying the mold and runner system at a desired operating temperature, characterized in that it comprises the steps of: (a) placing the heaters within the mold and runner system to divide the mass of the mold and runner system into multiple zones, so that the potential heater output is proportional to the mass of the associated zone, (b) place temperature sensors within each zone to provide temperature-related feedback within the associated zones at any point in time, (c) apply a controlled vortex to the heating elements within each zone for a defined period of time, (d) verify the resulting current supplied to the heaters in each zone; (e) determine the relative thermal mass associated with each zone based on the amount of current supplied to each zone, and (f) establish an ignition sequence for the zones on the bath of the relative thermal mass determined in step (e), so that heaters in areas that have higher thermal mass will be activated to heat the associated mass and the mold system and runner to a predetermined reference point before the heaters in the area that have less thermal mass be activated. The method according to claim 1 or 2, characterized in that it comprises the steps of: (g) adjusting the ignition sequence established in step (f) based on the operator input to optimize the firing sequence of the heater .