US20160201156A1

US20160201156A1 - Nozzle for cooling vacuum heat treatment furnace

Info

Publication number: US20160201156A1
Application number: US15/073,437
Authority: US
Inventors: Zhizhong Liang
Original assignee: ASSAB Tooling Technology Shanghai Co Ltd
Current assignee: ASSAB Tooling Technology Shanghai Co Ltd
Priority date: 2014-08-29
Filing date: 2016-03-17
Publication date: 2016-07-14
Also published as: EP3034200A1; EP3034200A4; WO2016029713A1; CN105364045A

Abstract

Embodiments of the disclosure are drawn to a system for heat treatment of a mold. The system includes a vacuum heat treatment furnace. The vacuum heat treatment furnace includes a tray for receiving the mold, a plurality of nozzles configured to discharge gas flows and arranged into one or more rows that are evenly distributed around a perimeter of the vacuum heat treatment furnace, and one or more extendible pipes installed on a selection of the nozzles. The extendible pipes are selectively extended to one or more lengths.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of International Application No. PCT/CN2015/078486, filed on May 7, 2015, which claims priority to and benefits of Chinese Patent Application No. 201410437613.1, filed on Aug. 29, 2014. The contents of both of the above-referenced applications are herein incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a vacuum heat treatment system for processing a mold, and more particularly relates to a vacuum heat treatment furnace having one or more cooling nozzles.

BACKGROUND

Die casting molds have become more widely used in applications in various fields in recent years. These applications have promoted the development of ultra-large die casting molds, and die casting molds with more complicated shapes and/or deeper cavities, which lead to higher requirements for the materials and the performance of heat treatment of these molds. At present, die casting molds are generally cooled by using gas-quenching vacuum heat treatment furnace. However, because the mold in the vacuum heat treatment furnace is typically at a distance from the cold gas source when cooling, the gas cannot to be blown to key positions of the mold directly. For example, almost all of the typical vacuum heat treatment systems cannot cool some sections of the mold, such as the bottom and/or root of a cavity of the mold, at a fast cooling speed, which may cause these sections of the mold to have undesirable properties during and/or after the heat treatment. This may cause an initial failure in these sections of the mold during use. For example, the bottom and/or root of the cavity of the mold are key positions for making die casting products, and may affect the appearance and/or quality of die casting products, which may eventually result in early discard of the mold because the appearance and/or properties of the die casting products cannot meet desired requirements. Therefore, there is a need for systems and methods to improve the heat treatment a die casting mold, particularly the bottom and/or root of the cavity of a die casting mold.
Increasing the cooling speed during quenching may improve the heat treatment of the mold. A potential research direction may include improving the heat treatment by increasing the cooling speed of the bottom and/or root of a deep cavity of a die casting mold. Solving this problem may greatly improve the heat treatment of die casting molds having large and/or deep cavities and thus effectively increase the life of these molds. Currently, when a vacuum heat treatment system performs heat treatment on a die casting mold having a deep cavity, the cooling speed of the bottom and/or root of the deep cavity of a die casting mold is slow. This is because the gas ejected from a nozzle of the vacuum heat treatment system is at a distance from the mold during cooling, which may reduce the cooling speed and affects the heat treatment of the mold, particularly of the bottom and/or root of the deep cavity of the mold. Moreover, the mold may have various shapes, such as recess molds, convex molds, and steps. The distances from the nozzle to different positions of the cavity of a recess mold or the root of a convex mold vary, which can cause the cooling speed of different positions of the mold to be uneven. This may cause heterogeneous properties of heat treatment at different positions of the die casting mold, which may even increase the risk of large deformation and cracking of the mold.
As described above, the undesirable heat treatment properties of a die casting mold having a cavity, e.g., a deep cavity, can be caused by uneven cooling speeds at various positions of the mold, e.g., at the bottom and/or root of the cavity. One major cause is that the cold gas source is at a longer distance away from the deep cavity of the die casting. Thus, there is a need for systems and methods to decrease the distance between the cavity and the cold gas source so as to increase the cooling speed of the cavity of the mold.

SUMMARY

Embodiments of the present disclosure are directed to methods, apparatus, and systems for cooling a mold. Various embodiments of the disclosure may include one or more of the following aspects.
One aspect of the present disclosure involves a system for processing a die casting mold. The system may include a vacuum heat treatment furnace comprising: a plurality of nozzles, and one or more extendible pipes, and one or more thermocouples. The die casting mold may include a deep cavity. The plurality of nozzles may be arranged into one or more rows that are evenly distributed around a perimeter of the vacuum heat treatment furnace. A selection of the plurality of nozzles may be installed with the extendible pipe. The length of the extendible pipe may range from 50 mm to 250 mm, and may be adjustable in accordance with a depth of the deep cavity of the die casting mold so that a distance between an open end of the extendible pipe and the interior surface of the cavity may range from 450 mm to 600 mm. The vacuum heat treatment furnace may further include a tray located at a central position inside the vacuum heating furnace, and the die casting mold may be received on the tray. The thermocouples may be placed on the surface of the deep cavity. The extendible pipe may be made of a graphite-based material, and may have a cylindrical tubular shape, which has a circular interior perimeter. The interior of a front half portion of the extendible pipe may have a circular stepped shape, and may be installed onto the open end of one of the nozzles. An exterior wall of a rear portion of the extendible pipe may include a plurality of screw holes for fixing the extendible pipe by fastening bolts or screws through the screw holes.
Another aspect of the present disclosure involves a system for heat treatment of a mold. The system may include a vacuum heat treatment furnace. The vacuum heat treatment furnace may include a tray for receiving the mold, a plurality of nozzles configured to discharge gas flows and arranged into one or more rows that are evenly distributed around a perimeter of the vacuum heat treatment furnace, and one or more extendible pipes installed on a selection of the nozzles. The extendible pipes may be selectively extended to one or more lengths. The mold may be a die casting mold comprising a cavity. The system may further include one or more thermocouples placed on the interior surface of the cavity. The lengths of the extendible pipes may range from 50 mm to 250 mm, such that a distance between an open end of at least one of the extendible pipes and the surface of the mold may range from 450 mm to 600 mm, and/or that a distance between an open end of at least one of the extendible pipes and the interior surface of the cavity may range from 450 mm to 600 mm. The extendible pipes may be selectively extended to lengths such that distances between open ends of the extendible pipes and the surface of different parts of the mold are substantially the same. The extendible pipes may be made of at least one graphite-based material. The extendible pipes may have a shape of a cylindrical tube with a circular interior perimeter. A front half portion of the interior of the extendible pipes may have a circular stepped shape and may be configured to enclose at least part of one of the nozzles. An exterior wall of a rear portion of the extendible pipes may include a plurality of screw holes for fixing the extendible pipes by fastening bolts or screws through the screw holes.
Another aspect of the present disclosure involves a system for heat treatment of a mold. The method may include receiving the mold in a vacuum heat treatment furnace. The vacuum heat treatment furnace may include a tray for receiving the mold, a plurality of nozzles configured to discharge gas flows and arranged into one or more rows that are evenly distributed around a perimeter of the vacuum heat treatment furnace, and one or more extendible pipes installed on a selection of the nozzles. The method may further include selectively extending the extendible pipes to one or more lengths, and discharging the gas flows from the plurality of nozzles to cool the mold. The mold may be a die casting mold comprising a cavity. The method may further include measuring at least one temperature on the interior surface of the cavity using one or more thermocouples placed thereon. The method may further include extending the extendible pipes to lengths ranging from 50 mm to 250 mm. The method may further include extending at least one of the extendible pipes to a length such that the distance between an open end of the extendible pipe and the surface of the mold ranges from 450 mm to 600 mm and/or that the distance between an open end of the extendible pipe and the interior surface of the cavity ranges from 450 mm to 600 mm. The method may further include selectively extending the extendible pipes to lengths such that distances between open ends of the extendible pipes and the surface of different parts of the mold are substantially the same. The method may further include further comprising cooling the different parts of the mold at substantially the same speed.
Additional objects and advantages of the present disclosure will be set forth in part in the following detailed description, and in part will be obvious from the description, or may be learned by practice of the present disclosure. The objects and advantages of the present disclosure will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that the present disclosure is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The present disclosure is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be used as a basis for designing other structures, methods, and systems for carrying out the several purposes of the present disclosure. It is important, therefore, to recognize that the claims should be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the present disclosure, and together with the description, serve to explain the principles of the disclosure.

FIG. 1 depicts a schematic representation illustrating a cross-sectional view of an exemplary vacuum heat treatment furnace, according to embodiments of the present disclosure.

FIG. 2 depicts a schematic representation illustrating a side view an exemplary nozzle of the vacuum heat treatment furnace of FIG. 1, according to embodiments of the present disclosure.

FIG. 3 depicts a schematic representation illustrating a left side view of the nozzle of FIG. 2.

FIG. 4 depicts a schematic representation illustrating a cross-sectional view of the nozzle of FIG. 2 taken along the line A-A of FIG. 3.

FIG. 5 depicts a schematic representation illustrating a cross-sectional view of an exemplary vacuum heat treatment furnace, in which an exemplary die casting mold is placed on a tray, according to embodiments of the present disclosure.

FIG. 6 depicts a schematic representation illustrating a cross-sectional view of the vacuum heat treatment furnace of FIG. 1 taken along the line A-A of FIG. 5.

FIG. 7 depicts a schematic representation illustrating a cross-sectional view of the vacuum heat treatment furnace of FIG. 1 taken along the line B-B of FIG. 6.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
FIG. 1 depicts a schematic representation illustrating a cross-sectional view of an exemplary vacuum heat treatment system, e.g., a vacuum heat treatment furnace 3. Vacuum heat treatment furnace 3 may have a tubular cylindrical shape. In the exemplary embodiment, vacuum heat treatment furnace 3 includes a plurality of nozzles 1. Nozzle 1 may be tubular and/or cylindrical in shape. Nozzles 1 may be arranged into a plurality of rows, e.g., 12 rows. For example, each row may include 6 nozzles, and vacuum heat treatment furnace 3 may have a total of 72 nozzles. The rows of nozzles 1 may be evenly distributed around the perimeter of vacuum heat treatment furnace 3, for example, spaced at 30° intervals. As shown in FIG. 1, the positions of twelve rows of nozzles 1 are located at 0° to 330° around the perimeter of a cross-section of vacuum heat treatment furnace 3.
Vacuum heat treatment furnace 3 may further include one or more extendible pipes 2. FIG. 2 depicts a schematic representation illustrating a side view of an exemplary embodiment of nozzle 1. As shown in FIG. 2, extendible pipe 2 may be installed on nozzle 1. Extendible pipe 2 may be formed from a suitable type of material that is substantially stable and/or inert at heat-treat temperatures, such as graphite-based materials or graphite-like materials. Extendible pipe 2 may have a generally cylindrical shape. For example, extendible pipe 2 may have a cylindrical tube shape. The exterior and/or interior perimeters of the cylindrical tube may be substantially circular. Extendible pipe 2 may be installed fixedly or removably onto an open end of nozzle 1.
FIG. 3 depicts a schematic representation illustrating a left side view of nozzle 1 installed with extendible pipe 2 of FIG. 2. FIG. 4 depicts a schematic representation illustrating a cross-sectional view of nozzle 1 installed with extendible pipe 2 of FIG. 2 taken along the line A-A of FIG. 3. As shown in FIGS. 2-4, a front half section of the interior of extendible pipe 2 may have a circular stepped shape, which may enclose at least part of nozzle 1. An exterior wall of a rear section of extendible pipe 2 may have a plurality of screw holes 5 for fixing extendible pipe 2 on nozzle 1. For example, a plurality of fasteners may be used to fix extendible pipe 2 to nozzle 1 and/or the length of extendible pipe 2 by any suitable fastening means that mechanically joins or affixes nozzle 1 and extendible pipe 2, such as by fastening screws 6 through screw holes 5, or fastening bolts 6 through screw holes 5 with suitable nuts.
The length of extendible pipe 2 may range from about 50 mm to about 250 mm, and may be selected or adjusted in accordance with the shape of a die casting mold, for example, the depth of a deep cavity of the die casting mold. FIG. 5 depicts a schematic representation illustrating a cross-sectional view of vacuum heat treatment furnace 3. As shown in FIG. 5, an exemplary die casting mold 4 is placed on a tray in vacuum heat treatment furnace 3 for heat treatment. FIG. 6 depicts a schematic representation illustrating a cross-sectional view of vacuum heat treatment furnace 3 taken along the line A-A of FIG. 5. FIG. 7 depicts a schematic representation illustrating a cross-sectional view of vacuum heat treatment furnace 3 taken along the line B-B of FIG. 6.
As shown in FIGS. 6 and 7, die casting mold 4 may be a recess mold and/or a convex mold. The positions for placing die casting mold 4 in vacuum heat treatment furnace 3 can be widely different. Thus, to achieve the desirable properties of heat treatment of die casting mold 4, the cooling speed of die casting mold 4 may be accelerated and/or may become substantially even at different positions of die casting mold 4, such as the bottom of the deep cavity, by extending nozzles 1 with extendible pipe 2.
As shown in FIGS. 5-7, a selection of nozzles 1 may be extended by selectively adding, installing, or extending extendible pipes 2 to the selected nozzles 1. For example, a number of nozzles 1 at different positions around vacuum heat treatment furnace 3 may be selected. The installed or extended extendible pipes 2 may have different lengths or may be adjusted to different lengths. The selection of nozzles 1 to be extended and/or the lengths of the extendible pipes 2 may be determined in accordance with the shape and/or installing position of die casting mold 4 in vacuum heat treatment furnace 3, such that the distances between the ends of nozzles 1 and the surfaces of die casting mold 4, including the interior surface of the deep cavity of die casting mold 4, range from about 450 mm to about 600 mm.
As described herein, the open end of nozzle 1 may refer to the end of nozzle 1 having an opening from which cooling gas is ejected, or the open end of extendible pipe 2 installed on nozzle 1 from which cooling gas is ejected. Similarly, the open end of extendible pipe 2 may refer to the end of extendible pipe 2 having an opening from which gas cooling gas is ejected. Therefore, the uneven cooling or the heterogeneous cooling speeds of different parts of die casting mold 4 caused by the different distances between the ends of nozzles 1 and the surface of the different parts of die casting mold 4, such as the deep cavity, is reduced.
Die casting mold 4 may be any type of mold to be heat treated. In some embodiments, the distance between the end of extendible pipe 2 and the surface of die casting mold 4, such as the interior surface of the cavity of die casting mold 4, is adjusted to be between about 450 and about 600 mm by adjusting, e.g., extending or retracting, the length of extendible pipe 2. In such situations, the evenness of cooling and/or the cooling speed of die casting mold 4 may be desirably controlled.
In some embodiments, a method for cooling die casting mold 4 in vacuum heat treatment furnace 3 may include one or more of the following steps. Step 1 may including placing die casting mold 4 on a tray provided in vacuum heat treatment furnace 3. Die casting mold 4 may have a deep cavity, and may be placed on the tray and located at a central position in vacuum heat treatment furnace 3. In some embodiments, one or more thermocouples 7, e.g., about 9 thermocouples, may be placed on the surface of the deep cavity of die casting mold 4 to measure the temperatures of the surface during cooling. Step 2 may include installing or extending extendible pipes 2 to a selection of nozzles 1. Step 2 may further include selectively adjusting the lengths of extendible pipes 2 such that the ends of nozzles 1 and/or extendible pipes 2 may have similar distances to the surfaces of different parts of die casting mold 4. Step 3 may include discharging cold gas flows through the ends of a selection of nozzles 1 and/or extendible pipes 2 during cooling of die casting mold 4. The cold gas flow may be directly discharged to the surface of die casting mold 4, including the interior surface of the deep cavity of die casting mold 4. The cold gas flows then carry amounts of heat away from die casting mold 4. Step 4 may include passing the heated gas flows through a heat exchanger. Step 4 may further include discharging cold gas flows again from the ends of nozzles 1 and/or extendible pipes 2. Steps 1 to 4 may be iterated for a suitable number of times so as to achieve cooling or fast cooling of the surface of die casting mold 4, e.g., the surface of the deep cavity of die casting mold 4, and/or even cooling of various positions of die casting mold 4.

EXAMPLE 1

Comparison of Cooling Exemplary Die Casting Molds Having a Recess in an Exemplary Vacuum Heat Treatment Furnace With and Without Extendible Pipes 2

Testing data were obtained for cooling two exemplary die casting molds 4, both of which had a recess and were heated to be at substantially the same temperature using the same technology before cooling. The testing data, i.e., cooling speed (° C./min), are summarized in Table 1 below.

TABLE 1

Comparison of testing data for cooling die casting molds in vacuum
heat treatment furnace 3 with and without extendible pipes 2.

	The mold was placed	The mold was placed at
	at the center of the tray,	the center of the tray, and
	and extendible pipes	one extendible pipe 2
	2 were not extended	was extended by 200 mm
	Temperature range:	Temperature range:
	(1020-540° C.)	(1020-540° C.)
	cooling speed ° C./min	cooling speed ° C./min

B2: On the end surface	65.56	90.77
of the opening of the
recess
B3: The upper portion	29.56	38.77
of the bottom of the
recess
B4: The middle portion	34.33	57.85
of the bottom of the
recess
B5: The lower portion	23.78	40.00
of the bottom of the
recess
B6: An upper portion	48.00	67.08
in the recess 100 mm
from the end surface
of the opening
B7: A middle portion	44.00	62.92
in the recess 100 mm
from the end surface
of the opening
B8: A lower portion	43.22	69.69
in the recess 100 mm
from the end surface
of the opening
B9: A middle portion	47.33	64.92
in the recess 100 mm
from the end surface
of the opening

It can be seen from Table 1 above, after an exemplary nozzle 1 located at the opening of the recess of one die casting mold 4 was installed with an exemplary extendible pipe 2, the cooling speed of various positions of the recess of die casting mold 4 was increased. For example, the cooling speed of the surface of the recess of die casting mode 4 was generally increased by 50 percent. These testing data show that the cooling speed at the recess of the deep cavity of die casting mold 4 was increased and thus the heat treatment of die casting mold 4 was improved. The problem of different cooling speeds of the surface of the mold can be solved by installing extendible pipes 2 to a selection of nozzles 1 at different positions.
It can be seen from above example that the cooling speed of die casting mold 4, e.g., the cooling speed of the surface of the cavity of die casting mold 4, can be desirably increased by using extendible pipe 2.
The vacuum heat treatment furnace 3 described in the present application may be generally applied to various types of molds. For example, besides die casting mold 4 having a recess or a deep cavity as described above, die casting mold 4 whose thickness is not uniform, such as a mold having substantial thickness differences between different portions, can also be cooled using vacuum heat treatment furnace 3. For example, one or more extendible pipes 2 may be used to cool the thicker portions of the mold, so as to achieve comparable or similar cooling speed as the thinner portions of the mold. This may reduce the risk of deformation and cracking of the mold and improve the heat treatment performance of the material of the mold, such as by obtaining desirable properties of the mold after the heat treatment.
Based on the above example and manufacturing practice, the present disclosure describe systems and methods that solve the problem of partial slow cooling and heterogeneous cooling of any suitable type of mold during heat treatment, e.g., gas quenching, which may improve the heat treatment performance of the mold and reduce the probability of cracking during gas quenching, and may extend the life of the mold.
The many features and advantages of the present disclosure are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the present disclosure that fall within the true spirit and scope of the present disclosure. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the present disclosure to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the present disclosure.
Moreover, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be used as a basis for designing other structures, methods, and systems for carrying out the several purposes of the present disclosure. Accordingly, the claims are not to be considered as limited by the foregoing description.

Claims

What is claimed is:

1. A system for processing a die casting mold, comprising:

a vacuum heat treatment furnace, comprising:

a plurality of nozzles, and

one or more extendible pipes, and

one or more thermocouples,

wherein the die casting mold comprises a deep cavity,

wherein the plurality of nozzles are arranged into one or more rows that are evenly distributed around a perimeter of the vacuum heat treatment furnace,

wherein a selection of the plurality of nozzles are installed with the extendible pipe,

wherein the length of the extendible pipe ranges from 50 mm to 250 mm, and is adjustable in accordance with a depth of the deep cavity of the die casting mold so that a distance between an open end of the extendible pipe and the interior surface of the cavity ranges from 450 mm to 600 mm,

wherein the vacuum heat treatment furnace further comprises a tray located at a central position inside the vacuum heat treatment furnace, and the die casting mold is received on the tray, and

wherein the thermocouples are placed on the surface of the deep cavity.

2. The system of claim 1,

wherein the extendible pipe is made of a graphite-based material, and has a cylindrical tubular shape, which has a circular interior perimeter,

wherein the interior of a front half portion of the extendible pipe has a circular stepped shape, and is installed onto the open end of one of the nozzles,

wherein an exterior wall of a rear portion of the extendible pipe comprises a plurality of screw holes for fixing the extendible pipe by fastening bolts or screws through the screw holes.

3. A system for heat treatment of a mold, comprising:

a vacuum heat treatment furnace comprising a tray for receiving the mold;

a plurality of nozzles configured to discharge gas flows and arranged into one or more rows that are evenly distributed around a perimeter of the vacuum heat treatment furnace; and

one or more extendible pipes installed on a selection of the nozzles;

wherein the extendible pipes are selectively extended to one or more lengths.

4. The system of claim 3, wherein the mold is a die casting mold comprising a cavity.

5. The system of claim 4, further comprising one or more thermocouples placed on the interior surface of the cavity.

6. The system of claim 3, wherein the lengths of the extendible pipes range from 50 mm to 250 mm.

7. The system of claim 4, wherein a distance between an open end of at least one of the extendible pipes and the surface of the mold ranges from 450 mm to 600 mm.

8. The system of claim 4, wherein a distance between an open end of at least one of the extendible pipes and the interior surface of the cavity ranges from 450 mm to 600 mm.

9. The system of claim 4, wherein the extendible pipes are selectively extended to lengths such that distances between open ends of the extendible pipes and the surface of different parts of the mold are substantially the same.

10. The system of claim 3, wherein the extendible pipes are made of at least one graphite-based material.

11. The system of claim 3, wherein the extendible pipes have a shape of a cylindrical tube with a circular interior perimeter.

12. The system of claim 11, wherein a front half portion of the interior of the extendible pipes has a circular stepped shape and is configured to enclose at least part of one of the nozzles.

13. The system of claim 3, wherein an exterior wall of a rear portion of the extendible pipes comprises a plurality of screw holes for fixing the extendible pipes by fastening bolts or screws through the screw holes.

14. A method for heat treatment of a mold, comprising:

receiving the mold in a vacuum heat treatment furnace, the vacuum heat treatment furnace comprising:

a tray for receiving the mold;

one or more extendible pipes installed on a selection of the nozzles;

selectively extending the extendible pipes to one or more lengths, and discharging the gas flows from the plurality of nozzles to cool the mold.

15. The method of claim 14, wherein the mold is a die casting mold comprising a cavity.

16. The method of claim 15, further comprising measuring at least one temperature on the interior surface of the cavity using one or more thermocouples placed thereon.

17. The method of claim 14, further comprising extending the extendible pipes to lengths ranging from 50 mm to 250 mm.

18. The method of claim 14, further comprising extending at least one of the extendible pipes to a length such that the distance between an open end of the extendible pipe and the surface of the mold ranges from 450 mm to 600 mm.

19. The method of claim 14, further comprising selectively extending the extendible pipes to lengths such that distances between open ends of the extendible pipes and the surface of different parts of the mold are substantially the same.

20. The method of claim 19, further comprising cooling different parts of the mold at substantially the same speed.