Tooling having advantageously located heat transfer channels

Details Tooling having advantageously located heat transfer channels Tooling having advantageously located heat transfer channels

2000-09-01

2002-03-12

Lin, Kuang Y. (Department: 1722)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S122000

Reexamination Certificate

active

06354361

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of tooling manufacture by layered fabrication techniques and more particularly to three dimensional printing of metal and metal/ceramic molds.
BACKGROUND OF THE INVENTION
Metal molds for forming processes such as injection molding, blow molding, die casting, forging, and sheet metal forming are currently made using manufacturing techniques such as machining, EDM, casting and electroforming. (K. Stoeckhert (ed.), “Mold Making Handbook for the Plastics Engineer,” Oxford University Press, New York, N.Y. 1983.) The creation of the tool is a multi-step process involving a variety of manufacturing techniques. The mold is created by removing material from a block of metal, usually a tool material such as tool steel. Typically, a block of annealed tool steel is first rough machined to near-net shape. The near-net shape tool may then be hardened with an appropriate heat treatment cycle to obtain the desired final material properties. Final dimensions are obtained by finish machining, grinding or EDM of the mold pieces. Final finishing may also occur before the hardening step. Selected tool surfaces are then modified as required. Mating surfaces are typically ground to provide adequate sealing. Surfaces which require additional hardness or abrasion resistance can be treated by techniques such as nitriding, boriding, plating or ion implantation. (K. Stoeckhert (ed.), “Mold Making Handbook for the Plastics Engineer,” Oxford University Press, New York, N.Y. 1983.)
Alternate techniques exist for creating the near-net shape tool pieces, such as casting or electroplating. Near-net shape tool ingots made by casting are produced using established casting techniques. After casting, the metal perform must be finished using the additional finish machining or EDM processes described above. Tool performs made by electroforming are produced by electroplating a metal, typically nickel, onto an appropriately shaped mandrel. After plating to sufficient thickness, the tool is removed from the mandrel. Although the face of the tool is completely defined by the electroforming process, other portions of the tool must be created using other processes. A backing material, such as metal-filled epoxy, must be added to the rear of the tool and machined to the appropriate shape before the tool can be used. An alternate method of producing metal tools, the Tartan Tooling method, is described in U.S. Pat. No. 4,431,449 and U.S. Pat. No. 4,455,354. In this method, metal powder is packed around a negative of the shape to be produced and bonded with a polymeric material. The negative can be produced by any convenient means. The bonded powder green part is then fired to remove the polymer and to partially sinter the part. Finally, the porous sintered part is infiltrated with a lower melting point alloy to fill the residual porosity, producing a fully dense metal tool. Tools produced by the Tartan Tooling method have fewer finishing requirements than near-net shape performs made by other processes, but some finishing is usually required.
Metal molds, tools and dies produced by the above techniques must meet a variety of performance requirements. These requirements are determined by the type of forming processes the mold will be used for. Tools used for injection molding, for example, must remove heat from the injected part to cool it and freeze its shape. The transfer of heat away from the molten plastic directly affects part cycle time, dimensional accuracy and material properties. (K.Stoeckhert (ed.), “Mold Making Handbook for the Plastics Engineer,” Oxford University Press, New York, N.Y. 1983.) Molds with poor heat transfer characteristics require longer waiting periods before the plastic part has solidified enough to be ejected without damage, thus increasing cycle time. Molds in which the polymer freezes non-uniformly due to uneven heat removal can result in anisotropic shrinkages across the part, causing part warpage and loss of dimensional control. Additionally, residual stresses are incorporated into the plastic part during uneven cooling, having a detrimental effect on the material properties of the part. Injection molds typically incorporate fluid coolant channels to increase the rate at which heat can be removed from the injected plastic. (R. G. W. Pye, “Injection Mould Design,” 4th ed., Longman Scientific and Technical, Essex, England, 1989.) The coolant channels are incorporated into the mold using traditional machining or EDM techniques. The layout of the coolant channels is dependent on part geometry and the specific limitations of the processes used to create the channels.
Cooling of a tool can be effected in one of two ways. For smaller parts in which a tooling insert mold
1
is used, as shown in
FIG. 1
, the coolant channels
2
are located in the backing plate assembly
3
, which also provides the majority of the mechanical support necessary to resist mold deflection during the injection cycle. (H. Gastrow, “Injection Molds:
102
Proven Designs,” Hanser Publishers, Munich, 1983.) Channels are not directly incorporated into the insert itself because of the additional expense and fabrication time. Also, the size of channels for insert molds may be prohibitively small, making fabrication difficult. For larger, non-insert type molds
4
, shown in
FIG. 2
, the coolant channels
5
are directly incorporated into the mold. In both cases, the location and configuration of the coolant channel layout is a compromise between ideal cooling and practical limitations. For insert molds, the backing plate coolant channel layout is not tailored to a specific insert mold but is designed with a generic layout, and therefore cannot meet the ideal coolant needs of a particular insert geometry. Additionally, the heat flux transferred from the mold insert to the coolant plate must pass through the gap between the insert and plate surfaces. Although this gap is usually filled with a heat conductive grease or other material, the overall heat transfer is lessened. For larger molds
9
, shown in
FIG. 3
, the coolant channels are usually arrays of straight cylindrical holes
10
which are made using standard machining procedures. The channels are incorporated into the mold as an additional mold fabrication step after the mold cavity or core has been defined. The actual layout of the channels is limited by the shape of the cavity or core, in addition to fabrication constraints. The simple straight cylindrical channels cannot follow the complex contours of a typical cavity or core, resulting in uneven cooling of the mold surfaces. Also, the number and placement of the channels cannot be allowed to compromise the mechanical integrity of the mold. Also, rework of coolant channels, as might be required if the initial configuration does not perform adequately, becomes increasingly difficult as more mold material is removed. Improper layout of the channels may require the entire mold to be scraped.
The cylindrical holes are usually created by drilling. The cylindrical shape of the coolant channel is therefore a consequence of the manufacturing technique used to create it and not because it is the ideal shape for heat transfer purposes. The internal surface area of the channel, which directly effects the overall heat transfer from mold metal to coolant, is limited by this requisite cylindrical shape. Increasing channel diameter or the number of channels in the mold are ways to increase the effective channel wall surface area, but these techniques are limited by mold geometry. Other methods of increasing heat transfer which are commonly found in heat exchanger design, such as finned or textured surfaces, are not readily adaptable to mold coolant channels made by conventional means.
Most tool steel alloys are tailored for high strength, hardness and toughness in order to survive millions of injection cycles. Tool steels typically have fair to poor thermal conductivity values compared to other softer tooling alloys, specifically copper alloys, although the copper alloys cann

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