The basics of selecting permanent mold tooling materials.

This article discusses the decisions that go into selecting permanent mold materials, as well as cast molds' place among other processes.

In every metalcasting process, a precisely constructed mold is the first step in a quality casting. Regardless of how well every other process is controlled, without the correct mold, the casting will ultimately fail or need correction in post-casting, which will drag on a foundry's bottom line.

Unlike most green sand, nobake and investment casting foundries whose moldmaking processes are very much at the heart of their operations, most permanent mold foundries purchase their mold materials from outside sources. Nevertheless, making the right decisions on permanent mold materials are critical in determining a casting's quality, cost-effectiveness and efficiency of production.

In regard to both materials and initial manufacturing method, selection has been left largely to each foundry's personal preference and opinion. Following are issues in selecting iron or steel mold materials, as well as the form by which they are produced.

Tooling for permanent molds have traditionally been produced using a variety of gray and ductile cast irons, bar and forging type steels such as AISI 4140, tool steels such as H-13, and, most recently, maraging steels, which are low-carbon, high strength steels that provide easy welding. Copper-beryllium alloys and water cooling passages are also being used in permanent molds for directional solidification.

The process in which the material is formed must also be specified. When designing a permanent mold in either iron or steel, a net shape casting (produced by traditional foundry methods) or a simple billet shape (continuous cast for iron, or produced by rolling or open die forging for steel) must be chosen as the starting material. This choice has a tremendous influence on the final cost of the mold, mainly due to different machining costs for the two materials.

There is a common perception among foundrymen that the net shape cast starting form of the mold is of lower quality that the billet starting form. Some designers select billets based on their belief that forging provides better internal integrity. This likely stems from the fact that foundrymen know all of the sources of defective castings and thus look skeptically on the process.

In addition, poor risering practices in the past may have resulted in shrinkage cavities revealed during machining of the net shape casting. Also, anecdotal evidence of the number of shots a mold provides in its lifetime may have shown fewer numbers for cast tooling than forged tooling.

The belief that billets provide a longer lifetime over cast tooling, however, is skewed. Billets have traditionally been used to produce molds of simple shapes with simple parting lines rather than complicated castings with offset parting lines. Therefore, the billet-produced molds often have a longer lifetime. This could be a result of the billet's simpler shape, which results in lower and more uniform thermal strains during its lifetime than a more complicated mold.

Mold Requirements

What is it that permanent mold foundrymen should require from a permanent mold? It is reasonable to expect that the mold will be:

* durable and provide a long working life;

* robust and exhibit a high degree of repairability;

* resistant to thermal cracking or heat checking;

* highly resistant to wear, erosion and attack by liquid aluminum;

* economical to machine;

* easily coated and re-coated, and withstand the removal of old coating by blasting;

Finally, the overall economy of production and utilization of the mold must be optimized.

The mold must be durable and give a long working life - What is the number of castings that can reasonably be expected from a mold? The mold will eventually fail by wear or heat checking, but hopefully, only after a long, useful lifetime. While the durability of any mold depends on its complexity and the section sizes of the casting, a typically designed lifetime of a simple H-13 tool steel mold may be 70,000 castings. The mold may be serviceable up to (and past) 120,000 castings. Iron molds are less durable with typical designed lifetimes of 40,000 castings and may be serviceable to 50,000 shots.

The mold must be robust and exhibit a high degree of repairability - The mold is certain to need repair. Some areas of the mold may wear or heat check at a faster rate than other portions. Additionally, molds can be damaged by handling and routine maintenance. All molds must be repaired, and materials choices will have an influence on the difficulty of repair. Steel can be easily welded, while it is more difficult to weld iron. Consequently, many iron molds are repaired by removing the damaged or worn area and restoring the mold by plugging with the appropriate shape.

Resistant to thermal cracking - Thermal cracking, thermal fatigue and heat checking all describe the same mechanism of failure - cracks forming as a result of the thermal cycle and the strains that consequently develop. Iron molds tend to ultimately fail due to cracking. While steel molds can also fail by cracking, they will more often fail through wear.

A net shape casting has an advantage over a billet with regard to cracking because consistent mold wall thicknesses are more easily produced. A consistent mold wall thickness provides more uniformly maintained thermal gradients across the wall of the mold, and thus provides a more uniform state of stress and strain.

Highly resistant to wear, erosion and attack by liquid aluminum - Wear is the principal mechanism by which steel molds fail. After a typical lifetime of 80,000-120,000 castings, molds may wear to the point that the casting produced exceeds original tolerances and will no longer set up properly in fixtures for subsequent machining operations.

While the main source of wear is from removing dead coatings, wear can be minimized by careful blasting and through the use of alloy tool steels such as H-13 that are hard and wear resistant. These materials contain microstructures that have a well-dispersed population of stable carbides in the quenched and tempered conditions, which give the best wear resistance. Tool steels are often used in the fully annealed condition, which produces a microstructure containing a large population of dispersed alloy carbides.

Economical to machine - The cost of machining the mold is the most significant cost of production. The net shape casting yields a tremendous benefit because of significantly lower machining costs.

Easily coated and re-coated - Mold coatings have a tremendous influence over the quality of the castings produced. Coatings protect the mold, allow an easy release, control heat flow from the liquid metal to the mold and provide good surface finish.

As mentioned above, old coatings that wear away and/or become "dead" must be removed and reapplied. One of the major difficulties is that portions of the old coating are tenacious and difficult to remove. During the removal process by blasting, the mold may receive its highest degree of abuse and wear. This is an area in which steel molds have a large advantage over iron because of their high hardness, wear resistance and microstructure.

Because the most prominent mode of failure is wear, the number of times a mold is exposed to blasting is the largest indicator of mold materials. Newer, less harsh mold cleaning technologies, such as dry ice cleaning, result in less damage of the mold surface, and may allow foundries more options in selecting mold materials.

Overall economy of production and utilization of the mold must be optimized - Every mold must be produced at the appropriate quality level to adequately serve its intended purpose - to reach its designed lifetime and produce castings of the required dimensions and metallurgical quality. The optimum conditions are unique for each casting.

A jobbing foundry that produces 50,000 castings from a mold set at 5000 castings per year will choose different mold materials than a high production foundry producing up to two million castings over a program lasting several years. A jobbing foundry using a particular mold for only a week at a time, and then cleaning it and putting it into storage, may select a low cost, gray iron mold. An automotive foundry, meanwhile, may use their molds for several weeks at a time, clean them and immediately put them back on, and likely require a higher hardness and wear-resistant material.

Iron Molds

In addition to the fact that iron molds are typically 20% less costly to machine than tool steels, they are also used because of their excellent thermal conductivity. They can also be heat-treated to develop high strengths if necessary. Predominantly used in jobbing foundries, iron molds have a typical lifetime of 40,000-50,000 castings, and are best suited for production runs that require single mold sets or few mold sets.

Steel Molds

Steel molds have about twice the useful lifetime as iron molds. They incur less maintenance cost because they are easier to weld repair. The high strength and high toughness of steel molds make them more resistant to thermal cracking when compared to cast iron. Because of their longer lifetime, in the case of multiple mold sets, fewer molds sets will be required for qualification during a production program. Steel molds can be reliably water-cooled to promote high production and directional solidification.

Steel molds are best suited for long production runs requiring multiple mold sets.

Manufacturing Comparisons

When selecting the billet or net shape casting as the raw material from which to produce the mold, foundrymen must determine which is the most economical. Both iron and steel mold materials are available as billets or as net shape castings from foundries specializing in tooling castings.

With regard to iron billet shapes, the primary advantage of the billet is its graphite flake size uniformity. The continuous cast bar has a chill zone near the surfaces of the bar and will have a consistent microstructure in the remaining portion of the bar.

The net shape iron casting offers lower machining costs because less material must be removed. Net shape iron cast molds have a high degree of castability and high quality castings can be obtained at reasonable costs from a variety of foundries. Net shape castings can also easily produce uniform wall thickness.

In steel, net shape castings have several advantages over steel billet shapes. While steel net shape cast molds offer the same benefits of cast iron, another important feature is its consistent microstructure - regardless of direction. A billet has differing microstructural features depending on the direction in the billet.

Although net shape castings cost more per pound than billets, they may offer a lower starting material cost because of their lower weight. Also, while not playing a large role in a mold's performance, the net-shape cast mold also tends to have rounded inclusion particles rather than the elongated, cigar-like ones found in billets.

Economic Model

To put the selection process into perspective, consider this model. A permanent mold for a 6.8 liter intake manifold requires a billet with the exterior dimension of 36 x 32 x 12 in. per mold half. This billet weighs 3871 lb and at a typical billet cost of $1.80/lb (H-13 tool steel) costs $6968 per mold half. Therefore, the raw starting material for the mold is $13,936.

For the same casting, a net shape cast mold weighs 1675 lb and at a typical cost of $3.25/lb, costs $5443 per mold half, or $10,887 for two halves. This represents a 30% savings in the starting material cost.

Another significant consideration is that 2196 lb of the billet (per mold half) must be converted into chips before a net shape starting material is equivalent to the casting. In this example, the casting clearly has significant advantages.

Net Shape Casting Molds

From an economic standpoint, as the complexity of the mold increases, net shape castings are the most appropriate starting material for permanent mold tooling. This is especially evident when tool steel is the selected mold material and when multiple mold sets for the same casting are in production.

Net shape cast tooling is suited for high-production casting programs, while it is also the material of choice for high-quality molds in single mold sets.

Net shape castings are the most appropriate starting form for tooling because of their ability to easily maintain a consistent wall thickness and help avoid thermally induced cracking.

Tooling designers must not get caught in the perception that cast materials are of lower quality than billet materials. While it may be easier for billet producers to obtain cleaner metal, foundrymen can make castings of equivalent or better quality (depending on what determines the quality level of the part) than that attained in a billet. Net shape castings have a real cost advantage in terms of initial raw material costs and the manufacturing cost of the mold.