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Purchasing Basics: Tooling materials

May 23, 2001

6 Min Read
Purchasing Basics: Tooling materials

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Today's technical parts make huge demands on tooling. Selecting the right material is especially essential to the life of long-running and highly complicated molds.
Photo: Gaudlitz 

Ask any moldmaker or molder what the properties of the ideal tool steel would be, and the description might look like this: The material should be free machining to reduce tool costs; capable of a number one finish with 5 minutes of number 120 stoning; as hard as a diamond; capable of flexing 30° for five million cycles without fracture; have 10 times the thermal conductivity of water; and cost less than $.02/cu inch. Because we have yet to find this elusive material, we are stuck with making choices. Every choice affects the well-being of tooling, process, and manufacturer. It also affects the profitability of molders and their customers. 

With today's choices ranging from computer-generated resin or powdered metal tooling to specialty alloys or even pure carbide, everyone from purchasing agents to shop personnel must consider the ramifications of tool hardness and construction. Because the softest tool that will do the job is usually the least expensive to build, here are a few ways you can get the most life from the least tool. 

Rules of Thumb

  • The yield strength of steel varies a little in relation to tensile strength, but may be approximated by taking 80 percent of the tensile strength.

  • Higher hardness of steel improves wear, dent and scratch resistance, and polishability, but lowers machinability and weldability.

  • High sulfur content degrades the stainless qualities and polishability of the steel.

  • Steel cleanliness as specified by the steelmaker refers to the level of impurities as the metal is poured into its ingot mold. To the moldmaker, it's the level of impurities in the surface that is important. Because of the solidification process, mold blocks produced from large ingots have inclusions that are larger and more noticeable than the smaller, more finely dispersed inclusions of a small ingot, even if the two are poured from the same molten metal.

  • Hardness, as a measure of the internal state of stress of the steel, has an adverse effect on weldability, fracture toughness, dimensional stability, and machinability. 

Define Your Parameters 
There are basic answers that should be determined in advance of any specification of tool materials. 

  • Program life. Start by considering the life expectancy of the tool: Know the number of parts per order, the number of separate orders per year, and the number of years in the program. Also, consider the aging your tool will experience because of startup stress. A mold that runs infrequently is more thermally stressed than one that runs continuously. Infrequently used tools, especially those molding high-temperature resins such as PPS, often make fewer parts than expected, thanks to thermal stress.

  • Resin properties. Is the resin abrasive, corrosive, flash prone (at .0001 inch), or in need of a hot mold (more than 250F)? What kind of fill velocity and venting are required? What surface finish is needed for reasonable release and ejection?

    Moldmakers know each increase in tool performance requires an increase in tool strength and precision. This necessitates a harder, or sometimes tougher, tool material, but not every toolmaker has experience with the material you will be using. If you don't have a particular tool material selected, give your tool designer and moldmaker enough information to help them make good decisions.

  • Part design. Next determine what kind and number of shutoffs are required. Is it a speaker grille, a 600-pin connector with four shutoffs per pin, or a bread box with a flat parting line? Each step away from a simple shutoff should be accompanied by a change in hardness or surface coatings (a simple shutoff has no interlocking angles of less than 10°). Additionally, ejection systems, side actions, and objects loaded into the tool (insert molding) influence tool material selection.

  • Productivity issues. What kind of throughput is needed? Are you working toward a critical cycle time? What about preventive maintenance? Is it a way of life or the slogan of the day? Given the program life, does it cost more to trim parts or build a better tool? What does the customer think?

  • Tool maintenance costs. Does your customer pay for maintenance on an as-needed basis, or are you working under a mold warranty agreement? Customers who buy parts from supplier-maintained tooling often pay more for tooling initially because the supplier wants the best tool possible. Are you in a climate where humidity is high or corrosive materials are being run? These may lead you to a high-chromium steel.

Steel grades and moldmaking properties

Grade

Texturing

Nitriding

Machinability

Weldability

Polish-

Carburizing

Flame 

 

 

 

 

 

ability

 

hardening

 

 

 

 

 

 

 

 

1045

good

poor

very good

good

fair

good

good

4140

good

poor

very good

good

fair

good

good

4150 (resulfurized)

poor

poor

excellent

fair

poor

good

good

P-20

good

fair

good

fair

good

good

very good

H13

fair

very good

fair

poor

good

good

excellent

420M

fair

very good

fair

poor

very good

good

excellent

Age hardening

fair

excellent

excellent

excellent

fair

good

poor

Age-hardening

 

very

 

 

 

 

 

stainless

excellent

good

good

excellent

excellent

good

poor

 

Source: A. Finkl & Sons


Life Expectancy 
Here are some guidelines as to the various life expectancies of tooling materials, with limitations listed. 

  • Filled epoxy. Prototyping material for unfilled polymers, 50 to 200 pieces, 1/2 inch minimum core diameter, and 1:1 height-to-diameter ratio. Hardness: less than 3 Rockwell C.

  • Sintered metal. Less than 100,000 part total life, unfilled or lightly filled polymers, maximum part size limited to 4 to 10 sq in. Hardness: less than 50 Rockwell C.

  • QC-7 aluminum. Less than 250,000 part life, easily machined but easily damaged, difficult to weld. Hardness: 5 Rockwell C.

  • P-20 steel. Standard material for unfilled materials and medium life, i.e., one million parts; subject to gate and vent erosion. Hardness: 29 to 36 Rockwell C.

  • H-13 steel. Primary steel for cavity blocks where hardness is required with some ductility; good for high volume in unfilled materials and intermediate volume in loadings of less than 30 percent mineral or glass; typically requires finish machining after heat treat, which adds to costs. Hardness: 43 to 65 Rockwell C.

  • Carbide. Wear resistant, cast material for the highest durability requirements; cracks if improperly machined, long lead time. Hardness: 90 to 92 Rockwell C.

With our thanks . . .
. . . to RC Marketing and A. Finkl & Sons, who supplied the information for this article. 

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