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November 26, 1999

4 Min Read
Tools and runners designed for MIM

Molds for MIM have much in common with their plastic counterparts, but being aware of several key distinctions before designing a MIM tool can mean the difference between success and failure. According to Karl Frank Hens of Thermat Precision Technology, a feedstock supplier and molder, these design changes are based on the inherent characteristics of MIM feedstocks and as-molded parts. Specifically, Hens offers the following guidelines for tool and runner designs.

Low Green Strength
In its green state, a MIM part can withstand a high degree of compressive stress. However, green parts have low strength and limited elastic properties, so any tensile, torsional, or shear stress can cause distortion, cracking, and residual stress. To prevent these problems, tools should include the following characteristics:

  • Front half ejection. This is required unless slides hold a part positively in the back half during mold opening.

  • Uniform ejection/core pulls for entire part. It is not recommended to eject a part on end points only and rely on the rest of the part to be pulled out of the cavity. Uniform ejection (using a large number of ejector pins or blades) will ensure the entire part is ejected without exposure to tensile, shear, or torsional stresses.

  • Excellent mold polish. Specify a fine diamond paste on all surfaces perpendicular to the parting line.

  • Draft wherever possible. MIM feedstock has a high filler content, roughly 65 to 70 percent metal powder by volume. Metallic filler is incompressible and closely packed, so parts typically exhibit very little shrinkage as they solidify. As a result, use a maximum amount of draft wherever component geometry permits.

High Filler Content
The flow of a liquid MIM feedstock is analogous to tiny pebbles in water flowing at high speed. Binders typically have densities of 1 g/cc, while the metal particles are much denser at 8 g/cc. Parts fill in the .2- to .6-second range, producing shear rates of several hundred thousand 1/sec in gates and thin cross sections. As a result, the metal particles will stack up at every direction change in the flow pattern until pushed by the binder. Because they are heavier, and have greater inertia, metal particles require more energy to be set into motion.

These dynamics can cause metal/binder separation, which leads to part distortion during sintering. Any area with too much binder shrinks more, while metal-rich areas shrink less. To avoid these effects, tools, gates, and runners should include the following design parameters:

  • Smooth transitions and minimal abrupt direction reversals. For corners, use diverter pins and smooth radiuses to guide the flow (Figure 1). Transitions to gates should be blended to avoid sponge effects (Figure 2), in which a metal-rich area squeezes the binder into the cavity.

  • Gating that injects along cavity walls. To avoid metal/binder separation, do not shoot melt directly against a wall or core pin (Figure 3). Instead, shoot melt along the wall to form a good melt front and build plug flow.

Shear-thinning Effects
As with polymers, shear rate affects viscosity. Any slowdown of the melt front causes significant thickening due to loss of shear rate. To guard against this event, runner systems should use cross-sectional matching for subrunners. A runner split or Y (Figure 4) should be designed to match the flow volume of the incoming melt stream with the combined flow in each subrunner.

Low Viscosity Binder
Most MIM feedstocks use low molecular weight polymers with good flow properties to allow high filler content and minimize residual stress during subsequent sintering stages. These low-viscosity binders are naturally prone to flash. To avoid flash problems, follow these two guidelines:

  • Vent depths of .0005 inch or less. MIM molds require extensive venting with wide vents, but they should be no more than .0005 inch deep.

  • Tight mold inserts and ejector pins/blades. Because of the intricate nature of many MIM components, it is not unusual to have multiple slides, core pulls, strippers, and numerous ejector pins and blades. Make sure that all of these mold inserts are built to an accuracy of .0002 inch.

High-Conductivity Feedstock
Metal particles in the feedstock readily transfer heat to the mold because of their conductivity, cooling the feedstock prematurely. To reduce this effect, tools should include the following design points:

  • Large gates. Include generous gating into the thickest section in the cavity.

  • Generously sized runners. These should be used to support packing the thickest cross sections.

  • Full-round runners. To get melt to the cavity without losing heat, full-round runners should be used.

Abrasive Feedstock
Metal powder fillers at 65 to 70 percent volume tend to be abrasive, particularly in thin sections such as gates. For dimensional stability, all tooling should be built from hardened steels, typically in the 60 to 62 Rockwell C hardness range. To design a tool that will withstand abrasion requires following two rules:

  • Choose materials carefully. Gate blocks and cavities are typically made in D-2, critical slides in M-2, A-2, or CPM10V, and everything else in titanium-nitride-coated H-13.

  • Include sufficient cavity lead-ins and interlocks. The brittle nature of hardened tool steels requires perfect positioning of shutoffs on the tool.

Contact information
Thermat Precision Technology
Corry, PA
Thomas Roche
Phone: (814) 665-8437
Fax: (814) 664-989

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