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Composite mold materials muscle into IM applications.

Composite Mold Materials Muscle Into IM Applications

Tartan Tooling is a technique of consolidating metal powders to produce complex cavities and cores for plastics injection molding. The consolidation technique traditionally required an initial master from which duplicates could be rapidly generated; until recently, duplicates were made only in a composite material consisting of stainless Stellite (Stoody Deloro Stellite, Inc.) and copper alloy. However, ongoing R & D has led to a new type A6 tool steel-base composite. This article reviews the Tartan Tool process and presents both Tartan composite materials.

Process

The process is initiated with a female master or model, shown in Fig. 1. To compensate for Tartan Tooling process shrinkage, the master is oversized by 0.8%, or 0.008 in/in, and its parting line and external surfaces contain additional 0.010-in stock, which acts as grind stock for final fitting of the inserts to the mold base. The master may be composed of any rigid material, such as aluminum, wood, or plastic.

A variant to master preparation is shown in Fig. 2 - a male, prepared in the shape of a piece part, is fitted to a metal chase or enclosure to define the outside dimensions of the cavity; these dimensions must include the 0.010-in grind stock mentioned above. This master will be used to cast a precision urethane reverse that will serve as the master depicted in Fig. 1. The casting process has required that male masters be made of metal; they must also be designed 1.0% oversize because the reverse shrinks by 0.2%.

Preparing a proprietary mold from the master is the next step in the process. This mold differs from the complex punch and die set that is used for conventional powder metallurgy in that it is substantially less expensive and can be rapidly produced. It captures with high fidelity the dimensions of the master, which must be carefully machined and inspected because each insert will contain any flaws inadvertently left on the master. Because the master is not subject to heat or pressure during moldmaking, it does not experience the degree of wear that it would in pressure casting or hobbing.

Next, the mold is used to create powder metal inserts, which are sintered in computer-controlled furnaces and combined with a copper alloy. Four inserts can be produced in three weeks; 20 can be produced in four weeks. Starting in the fifth week, larger quantities can be produced at the rate of 30/week. The tolerance of the inserts is [+ or -] 0.001 in/in.

The best surface finish of the inserts is 20 to 25 microinches. The master need not be polished beyond this finish; in fact, a large proportion of the cavities produced to date are not polished beyond this finish, which is quite adequate for many engineering applications. When a superior finish is required, conventional polishing can produce a finish rated 2 or 3 in the SPI Mold Finish Guide.

Because of the difficulty of properly filling the deep, narrow channels of the proprietary mold, length-to-diameter ratios greater than 4:1 are often difficult to mold. Barring this restriction, virtually any detail placed on the master may be duplicated with high fidelity on each cavity.

Stellite/Copper-Tin Alloy

The material originally used in the process was a cobalt-base Stellite material combined with a copper-tin alloy. A sketch of the material's microstructure is shown in Fig. 3. Approximately 70% of this metallic composite consists of Stellite, an alloy well known in the metals industry for its outstanding resistance to corrosion and wear. Stellite comprises cobalt, chromium, tungsten, and carbon, which represent 35%, 21%, 9%, and 1.5%, respectively, of the composite's chemical composition. Dispersed within its cobalt-base solid solution are fine carbides that enhance a hard, Rockwell C50 matrix.

The remaining 30% of the composite consists of a copper-tin alloy, which increases the material's toughness and thermal conductivity. The material's thermal conductivity is similar to that of conventionally machined H-13 die material.

Although Stellite alone exhibits a hardness of Rockwell C55, its combination with a less hard copper alloy produces a composite hardness of C41-43. Field experience indicates that the material's cavity life is similar to that of H-13 material treated to C50-52. This suggests that the Stellite component dictates performance in this application. Further substantiation is provided by laboratory controlled sand abrasion tests, in which the composite exhibits wear resistance similar to that of 440C martensitic stainless steel. A summary of the composite's properties appears in Table 1. [Tabular Data Omitted]

A less obvious advantage of the material is that though its corrosion resistance is similar to that of stainless steel, its thermal conductivity is, as indicated above, similar to that of H-13 tool steel. Thus, because of the relationship of the mold material's thermal conductivity to cycle time, the material can be used to mold corrosive plastics without longer cycle times that are caused by stainless steel's lower thermal conductivity.

The Stellite/copper alloy material is supplied at a hardness of C41-43, with cavity detail to 0.001 in/in. Its proper fit in the mold base requires that its external dimensions be altered through some minor secondary machining operations, which may be readily accomplished by means of conventional alumina grinding wheels such as A60JV. Because of the Stellite/copper-tin alloy's wear-resistant nature, carbide is recommended for milling operations.

A6 Tool Steel/Copper Alloy

The new A6 tool steel/copper alloy was developed to meet the need for a tough, hard alternative tool steel material. It comprises type A6 tool steel, tungsten carbide, and copper alloy, in percentages of 63.8, 2.7, and 33.5, respectively. The base material, type A6 tool steel, is magnetic and thus permits magnetic chuckdown; it contains 0.6% carbon, 2.0% manganese, 1.0% chromium, and 1.3% molybdenum. It was chosen because of its deep air-hardening characteristics and tendency for low and predictable dimensional change due to heat treatment. It is important that little if any dimensional change and distortion occur during hardening, because all of the detail is molded into these cavities. Vacuum heat treatment, followed by gas quenching, has been found to be the optimum austenitizing treatment.

The A6 tool steel contains a tungsten carbide dispersion. Fine (1-5 microns in diameter) and uniformly spaced, the carbides add wear resistance to the A6 matrix, but they do not significantly affect secondary machining operations.

The final component of the material is the copper alloy, a continuous or interconnected phase that confers substantially improved heat transfer properties. Tests conducted at the Thermotest Division of Holometrix, Inc., of Cambridge, Mass., have revealed, within the temperature range of 43 [degrees] C to 343 [degrees] C, thermal conductivities in excess of 50% greater than those of conventional H-13 tool steel. Hence, cycle-time reductions are also evident.

Type A6-base composite cavities are supplied at three hardness levels: Rockwells C30-32, C40-42, and C48-50. The C30-32 level is selected when a number of secondary machining operations, such as extensive drilling and tapping, are to be performed. Subsequent to such machining, cavities may be heat-treated to C48-50 with minimal distortion. The C40-42 range is intended for long-run applications that require high toughness and good wear resistance. At this level, the cavities may be conventionally ground and carbide taps may be used.

The Rockwell C48-50 temper is used for long-run applications that require maximum wear resistance and good peening resistance. When supplied in this condition, the material may be conventionally ground, but it cannot be conventionally tapped. A carbide cutter may be used to mill the material.

Cycle Time Testing

Substantial improvement in thermal conductivity of the Tartan materials, in comparison to that of conventional die materials, motivated an evaluation of cycle time. It was reasoned that reducing cycle time with the use of Tartan Tooling could further improve the economics of molding and the quality of plastic parts. A field evaluation was conducted at Contract Design, Inc., of Minneapolis, to compare Tartan tool A6 to conventional H-13 and 420 stainless steels and Tartan stainless Stellite material.

Initially, testing was conducted with the use of a 75-ton Nissei P NC8000 injection molding machine. However, because the Tartan A6's rapid cycle time appeared to push the 75-ton machine to its limit, the tool was moved to a 100-ton JSW.A1-1/2-inch-diameter disk in thicknesses of 0.040,0.080, and 0.120 inch was used; two types of plastics, polypropylene (PP) and GE Noryl, were run.

The following responses were used to judge cycle time: complete fill, freedom from ejector pin punch, and freedom from doming or cupping. Cycle time was adjusted downward until any criterion was violated; the amount of time up to the point just prior to any criterion violation was defined as the minimum cycle time.

Cycle times for the two tests ranged from 5.95 seconds for the 0.040-in Noryl disk molded in Tartan A6 to 30.9 seconds for the 0.120-in PP disk molded in 420 stainless steel. The results of the two tests are shown in Table 2.

Table : TABLE 2: Comparison of Cycle Times for Disks Molded in Tartan A6 Tool Steel/Copper Alloy, H-13, 420, and Tartan Stellite/Copper-Tin Alloy.
 Cycle time, seconds
 Tartan Stellite/
Material thickness (in) A6 H-13 420 copper-tin alloy
PP 0.040 6.95 7.61 8.07 7.47
 0.080 9.81 11.66 13.70 10.02
 0.120 19.05 26.39 30.94 19.42
GE Noryl 0.040 5.95 6.59 7.74 6.37
 0.080 10.48 11.34 11.78 11.05
 0.120 15.81 17.66 19.06 16.04


Computing averages over thicknesses and plastic types yields the following findings:

1. The Tartan Tool A6 composite cavities showed significant cycle time improvements in relation to H-13 and 420 stainless steel cavities, the cycle times of which were 13.3% and 22% longer, respectively, than those of A6.

2. Although the Tartan Tool A6 had shorter cycle times than those of Tartan Stellite/copper alloy material, the difference averaged only 4%. This suggests that in the heat transfer process, the copper alloy is dominant.

3. The Tartan stainless Stellite/copper alloy material had cycle times averaging 18% shorter than those of 420 and 9.5% shorter than those of H-13. Therefore, in applications that require corrosion resistance comparable to that of stainless steel, the Stellite/copper alloy material provides much more rapid cycle times.

Further analysis involved the use of Moldflow's Moldtemp software to cool the mold with different mold materials. It was found that with regard to H-13 tool steel and stainless steel, attempts to eject the parts at the Tartan A6 cycle time resulted in much hotter steel temperatures and larger temperature gradients.

According to the software, to completely freeze PP parts down to the vicat softening point (144 [degrees] C for the material used) takes 0.5 seconds longer with H-13, and 1.6 seconds longer with stainless steel, than it does with A6. Even at these longer times, however, the steel temperature gradient on the parts is still much greater than it is with A6. Large variations in mold surface temperature are undesirable; they are likely to cause uneven cooling of the parts and, hence, differential shrinkage. Thus, even with the longer cycle times of H-13 and 420, equivalent part quality will not be achieved.

PHOTO : FIGURE 1. A female master, the generation of which is the first step in cavity-making.

PHOTO : FIGURE 2. A male master, which is prepared in the shape of a piece part and fitted to a metal chase or enclosure.

PHOTO : FIGURE 3. A typical microstructure, at 1500X, of the cobalt-base Stellite/copper alloy composite.
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Title Annotation:Tooling; injection molding
Author:Terchek, Richard L.
Publication:Plastics Engineering
Date:Apr 1, 1990
Words:1951
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