Polyurethane Tooling and Model Planks: Characteristics and Applications
Architect Louis I. Kahn once said, "Even a brick wants to be something." This is just as true now as rigid polyurethane modeling and tooling-board products become important media in product design and rapid-manufacturing technologies. These synthetic materials are playing an ever-larger part in manufacturing and new-product design.

Like bricks, tooling boards are vital building blocks in creating new things to fill our needs. Advances in polymer chemistry, tool-making technologies and CAD/CAM processes have focused greater attention on these materials.

Historically, designers and tool-makers would craft their products from wood. Due to the increasing scarcity of high-quality wood stock, rising costs and the difficulties and health dangers associated with the use of wood, this practice is changing. While wood has been commonly available and plentiful for centuries, the requirements of 21st-century industry are sidelining its use in tooling and design-modeling.

A natural product, wood often contains inconsistencies and defects (grain effects, knots, wane, checking, splits) affecting performance in cutting and (especially) CNC-machining operations, resulting in longer machine-times and higher costs.

Even special, select-grade wooden pattern boards contain tannins and mold-spores, potential health-hazards released in cutting operations. Wood is also susceptible to dimensional changes due to its affinity for water and its isotropic grain-structure. With designers and tool-makers needing fast processing, tighter tolerances and reliable dimensional stability in prototyping and manufacturing operations, the correct synthetic material often out-performs what Mother Nature provides.

Machineable waxes and modeling clays are also used for design exercises. While they are recyclable, they are impermanent, difficult to paint or finish, and sometimes messy to use. Have a dropped tool inadvertently strike a surface, or have a really warm day in the shop, and important details can be lost. Tooling boards, while not recyclable, are more durable and compatible with CNC processes.

Pioneering work in polyurethane tooling boards was done by Ciba-Geigy in the mid-1980s, with board materials to facilitate design and installation of heat-insulating tiles on the Space Shuttle. Since then, this "space-technology" has spread into almost every business needing to design or manufacture tangible objects from concept-cars to soles for sport shoes.

Today, several manufacturers produce rigid synthetic boards for prototyping, styling and tooling applications. While they are more costly than wood on a per-unit cost basis, the advantages of using tooling board materials lower total project costs to very reasonable levels. Tooling-boards are generally classified by density; encompassing a wide range of options, from medium-density foams to very dense, solid-plastic planks. This allows users to match materials to job requirements while controlling costs.

Some tooling boards are syntactic foams, where very small glass or plastic bubbles are mixed with resin to create a specific-density block. Other boards are expanded foams, where carbon-dioxide gas, a by-product of chemical reactions, makes cells to create a specific density. Both manufacturing methods render consistent boards with no voids, uniform density and predictable processing characteristics. While each tooling-board type has its special use, there are several common advantages that distinguish them from alternatives:

• ­Ease of Machining Because boards are consistent, uniform and grain-free, concerns with cutting-direction, tear-out and warping are few. Faster CNC-machine spindle speeds and material feed-rates are possible, increasing productivity. Manufacturers offer advice on cutters, spindle- speed and feed-rates for their materials.


• ­Shop Safety Most board products create shavings when cut, and many contain anti-static additives for reduced shop dust on the floor and in the air.


• ­Dimensional Stability Boards are closed-cell, and predominantly polyurethane-based, making them relatively insensitive to humidity changes. Thermoset polyurethane polymer has predictable thermal expansion characteristics. Good processors treat their boards to remove residual stresses, eliminating warping.


• Temperature Resistance Most boards are useful up to 150 degrees F, with many usable at temperatures of 210-225 degrees F. Some boards can be processed repeatedly at 250 degrees F.


• Larger Block Sizes available Some manufacturers offer shapes up to 12" thick, and up to 48" x 96" in length and width, greatly reducing glue-up labor in build-ups. This is a tremendous advantage over wood products, especially as fully-loaded model-shop labor costs reach $50.00 per hour and more.


• ­Easily Bonded and Repaired Many manufacturers offer specific adhesives and patch compounds for tooling boards, matched to particular grades.


• ­Easy Surface Finishing Almost all of these products accept a wide-range of coatings and finishes. Small cell-sizes give very good (to excellent) finished surface results. It is possible, with electroless-plating processes, to create polished-metal surfaces on prototypes made with tooling boards and foams.

Each density-range of tooling board has typical applications. None of these suggestions are hard-and-fast rules. Instead, use them as starting points for consideration in using tooling boards in manufacturing operations. Keep in mind: Higher Density = Higher Cost.

• ­High-density boards (>30 lbs. per cubic foot) are used where manufacturing stresses are high, or where exact dimensional tolerances, sharp-corner retention and excellent surface-finish is important. Vacuum-bag tools, thermoforming molds, hydroforming tools, check-fixtures, foundry patterns, trim-fixtures and highly-detailed models or prototypes benefit from use of this grade of tooling board.


• ­Medium-density boards (<30 lbs. per cubic foot) are useful for styling applications, mold-patterns and CNC-program proofing. Fine-detail rendering and surface-finish quality is not as good as boards that are denser, but adequate for many purposes at a reduced cost. Detroit automakers create entire concept cars with tooling- boards in this density range. Forming tools for acrylic plastics, low-rate and prototype thermoforming tools, and molds for low-temperature- curing pre-pregs are also made with these materials.


• ­Low-density boards (<15 lbs. per cubic foot) Really high-density foams, these are used for quick design studies when hand-carving is easier and less expensive than CNC programming and machining. Used for sight models, theme-park characters, architectural studies, or topographical maps, these boards are very low-cost. Automotive design-houses (with large-envelope CNC routers) will sometimes craft prototype car-body styles directly from CAD programs using foam. The table below gives more suggestions for potential tooling-board applications.

  Tooling Board Selection Guide
  Application	Suggested Board Density
  	<15 lbs. cu. ft.		15 to 30 lbs. cu. ft.		30 to 50 lbs. cu. ft.		50 to 80 lbs. cu. ft.
  Design Prototypes	X		X
  Appearance, Styling or “Sight” Models	X		X
  Architectural Models, Topographical Maps	X
  Master Models			X		X		X
  Trims, Jigs and Fixtures			X		X		X
  Thermoforming Tools, Autoclave Tools	X		X		X
  Foundry Patterns					X		X
  Mold Patterns	X		X
  Dimensional Check-fixtures					X		X
  Molds for low-temperature casting (<200° F) seal surfaces before use			X		X		X
  CAD Model Proofs/NC Program Proofs	X		X
  Molds for low-temperature curing pre-pregs (<200° F) seal surfaces before use			X		X		X

So even tooling boards "want to be something" and increasingly, these highly developed model and design materials are gaining their wish. Modern synthetic tooling boards offer significant advantages over wood products, and other media. While more costly to acquire, the over-all system-cost of using tooling boards is less. Savings accrue when durability and performance can be matched to job requirements, and labor inputs for machining, glue-up and finishing can be reduced, all while delivering attractive and functional results.