Shot blocks for use in die casting

Details Shot blocks for use in die casting Shot blocks for use in die casting

2001-06-15

2003-07-15

Lin, Kuang Y. (Department: 1725)

Metal founding

Means to shape metallic material

Pressure shaping means

C164S113000

Reexamination Certificate

active

06591894

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field
The present invention relates to die casting and in particular to new shot blocks for use in die casting and other similar casting operations.
2. Background
Die casting, which is also known as “pressure die casting” and “squeeze casting,” is a well known casting process in which molten metal is forced under high pressure into permanent steel dies. See
Metals Handbook, ©
1985 American Society for Metals, pages 23*32 to 23*41, the disclosure of which is incorporated herein by reference. “Thixoforging” or “thixoforming” are similar processes in which the metal being cast is in a semi-solid state (i.e. a solid/liquid mixture) rather than in purely molten form.
In conventional die casting and thixoforming operations, a piston or like device forces the metal being cast into the die through one or more passageways or “runners” which are connected to a manifold for receiving the pressurized metal. Thus, a conventional die casting machine typically includes a die composed of a cover die half and an ejector die half. The cover die half and the ejector die half mate with one another along a separation surface and together define multiple die cavities. The cover die half is stationary, while the ejector die half is movable so that when a molten charge solidifies, the ejector die half can be moved apart from the cover die half so that the solidified charge in each mold cavity can be removed.
Molten or semi-molten metal to be cast is charged into the die cavities by a charging assembly which includes a pressure cylinder for receiving molten metal from an inlet, a piston movable in the pressure cylinder for forcing the molten metal into the die cavities, and a shot block made from conventional tool steel mounted in or on the cover die half of the die. The shot block defines a manifold or reservoir for receiving molten metal from the pressure cylinder and supplying this molten metal to the die cavities via passageways or “runners” defined in the separation surface between the cover die half and the ejector die half of the die. Flow passageways are normally provided in the shot block for cooling the metal in the reservoir by indirect heat exchange using water, hot oil or other liquid as the cooling medium.
Because molten metal shrinks as it solidifies, it is important that additional amounts of molten metal be continuously supplied at high pressure to the mold cavities until enough metal in these cavities has solidified. To this end, the reservoir in the shot block as well as the runners are normally designed to be large enough so that at least some metal in these locations is still molten when the necessary degree of solidification has been reached in the mold cavities. In actual practice, this often means that the metal in the reservoir (typically referred to as a “biscuit”) will still be molten, or at least partially molten, when the metal in the mold cavities has completely solidified.
Once the metal in the mold cavities has solidified, the mold halves are separated from one another and the solidified castings in these cavities removed for further processing. However, for safety reasons, this cannot be done until the metal in the reservoir of the shot block has also solidified substantially. In this connection, it has been found that the metal in the shot block reservoir, since it is present under high pressure, can actually explode if the mold halves are opened too soon. Therefore, care must be taken to insure that the metal in the shot block reservoir solidifies sufficiently before the mold halves are separated from one another.
In modern industrial practice, it is always desirable to increase efficiency. To this end, commercial die casting machines such as described above are typically operated with as little cycle time as possible. In other words, the time between successive casting cycles is minimized to the greatest extent possible. Unfortunately, the time it takes molten metal in the shot block reservoir to solidify sufficiently represents the constraining factor in achieving shorter cycle times in 25 to 50% of commercial die casting operations.
Accordingly, there is a need for new technology which enables shorter cycle times to be achieved yet still allows the metal biscuit in the reservoir to solidify sufficiently before the mold halves are separated.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been found that the cycle times of die casting and like machines can be considerably shortened, while still allowing the metal biscuits in the shot block reservoirs of such machines to solidify sufficiently, by forming the shot blocks used in such machines from metal or metal alloys having a thermal conductivity of at least about 25 Btu/ft.hr.° F., a Rockwell C hardness of at least about 25 and a 0.2% Yield Strength of at least about 90 ksi.
Thus, the present invention provides a new shot block for use in die casting molten and semi-molten metal parts wherein the shot block is formed from a metal or metal alloy having a thermal conductivity of at least about 25 Btu/ft.hr.° F., a Rockwell C hardness of at least about 25 and a 0.2% Yield Strength of at least about 90 ksi.
In addition, the present invention also provides a new die casting machine including a die, a pressure cylinder for supplying molten or semi-molten metal to the die under pressure and a shot block defining a reservoir for transferring the molten or semi-molten metal received from the pressure cylinder to the die, characterized in that the shot block is made form from a metal or metal alloy having a thermal conductivity of at least about 25 Btu/ft.hr.° F., a Rockwell C hardness of at least about 25 and a 0.2% Yield Strength of at least about 90 ksi.
DETAILED DESCRIPTION
In accordance with the present invention, conventional die casting machines and other like pieces of equipment for charging molten or semi-molten metals into dies under high pressure are equipped with shot blocks made from metals or metal alloys having a thermal conductivity of 25 Btu/ft.hr.° F., a Rockwell C hardness of at least 25 and a 0.2% Yield Strength of at least 90 ksi. Therefore, the shot block in the apparatus described above, rather than being made from H13 tool steel or other conventional alloy, is made from an alloy having this combination of properties.
Properties of Metals Used in Forming Inventive Shot Block
An important feature of the metals or alloys used in making shot block
30
in accordance with the present invention is that they have thermal conductivities of at least about 25 Btu/ft.hr.° F., preferably at least about 37 Btu/ft.hr.° F. Metals or alloys having thermal conductivities of at least about 60 Btu/ft.hr.° F. are more interesting, while metals or alloys having thermal conductivities of at least about 145 Btu/ft.hr.° F. are of special interest. H13 tool steel, which is the material from which shot blocks are typically made, has a thermal conductivity of about 15 Btu/ft.hr.° F., which is about half that of the metals forming the inventive shot blocks or less. Surprisingly, it has been found that this difference allows the inventive shot blocks to provide a more rapid cooling of the metal biscuit in the shot block reservoir, and hence a faster solidification of this metal biscuit, even though the same type and amount of cooling liquid is used to cool the shot block during the casting operation. This, in turn, allows cycle times to be significantly shortened while still maintaining all other structural and operating features of the casting operation the same.
A second important features of the metals and alloys used to form the inventive shot block is that they have a Rockwell C hardness of at least about 25. A common problem associated with conventional shot blocks is that they show significant amounts of surface cracking—i.e., small cracks with an average maximum crack length of about 0.080 inch and a total crack area of about 0.8 in
2
. In accordance with the present invention, it has been found that this problem is substantially eliminated by making the inventiv

Affiliated with

Also associated with

Rating

Shot blocks for use in die casting does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Shot blocks for use in die casting, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Shot blocks for use in die casting will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3039780