Although different materials are used in the fabrication of molds and tools, selecting the ideal material is based on several factors. Surface quality standards, the requirement of number of components and performance prerequisites all contribute the cost and intricacy of tooling.
Nevertheless, tooling is a key design element of composites and therefore cannot be skipped. High-density, high-temperature epoxy-urethane foam is an economical process for limited-run, high-temperature tooling.
According to Mitch Johnson, Senior Technical Director at General Plastics, potential cure inhibition traditionally hinders the use of foam for tooling. This is mainly because of thermal conductivity problems instead of surface chemistry. Superior quality composite components can be produced by an appropriately designed cure cycle with foam tooling.
Until recently, the low glass transition temperature of most urethane foam formulations limited their use with prepregs to temperatures below 120°C. However, the advent of high-temperature carbon fiber composites and resins increased demand for polyurethane foams capable of operating at elevated process temperatures. This requirement has now been fulfilled, thanks to technological advances.
General Plastics’ LAST-A-FOAM® FR-4700 High-Temperature Tooling Board Series
The LAST-A-FOAM® FR- 4700 high-temperature tooling board line enables limited-run tooling in areas where using metal dies is highly expensive, including pattern making, vacuum forming, high-temperature curing prepregs, and prototyping. Machining the LAST-A- FOAM® FR-4700 tooling boards is much easier, allowing necessary modifications and specification changes rapidly and at a comparatively lower cost.
These qualities make this General Plastics’ material suitable for many different tooling needs, from small quantities to large, monolithic tools. The material’s outstanding adhesive bonding quality makes it an ideal choice for large, mass tools. The cost of low-density foams is much lower than metals, and therefore can be employed to evaluate tooling programs prior to process the metals for tooling.
Figure 1 illustrates the tooling fabrication overview, showing the machining of a FR-4718 high-temperature billet into the desired shape. This is followed by filling, sealing, releasing of the material. After lay-up and curing, the resultant product is released from the mold. This leads to the construction of a backing structure and the creation of a finished tool.
Figure 1. Tooling fabrication overview
Advantages of FR-4700 Tooling Boards
The FR-4700 is composed of the epoxy-hybrid material which provides dimensional stability to polyurethane foams at continuous-use temperatures of up to 177°C and peak temperatures of 200°C. The availability of blocks in large billet sizes to reduce bonding is another benefit. In addition, machining the FR-4700 with standard high-speed steel (HSS) cutting tools is much easier.
Rigid blown foam properties, including density uniformity, porosity, compressive modulus, the glass transition temperature, and the coefficient of thermal expansion (CTE), are based on the desired formulations. Conventional polyurethane foams can be used to make composite components made from prepregs curing at ambient or low temperatures. A low CTE material will be required for high-temperature curing prepregs due to high delta T value of 300°C.
The FR-4718 has a low CTE value, but the material may experience crack formation due to rapid cooling. General Plastics lends hand to users of the FR-4718 to find a suitable cooling rate to alleviate the risk of damage to the foam. Figure 2 presents the modified cooling cycle for soft tools, showing the significance of CTE compensations.
Figure 2. Modified cool down cycle for soft tool
The thermal conductivity of the FR-4718 foam is lower than metals, thus exhibiting very low heat propagation across the thickness of the block. The temperature measurement at five inches below the surface of the foam is represented by the red dashed line, whereas the yellow line represents the measurement at two inches below the surface. The remaining lines are measurements at the foam surface. As can be seen in Figure 2, the rate of CTE does not follow the literature value if the foam has a thicker cross-section. This is applicable to all tooling boards.
Johnson said that since the grow rate of the foam differs from what shown by the temperature ramp up, compensating for a lower CTE value is essential. This ensures that the calculations are accurate and the foam is not overcompensated. High-density polyurethane tooling boards are highly flexible and all of its properties such as thermal resistance and toughness can be tailored to meet individual requirements.
This information has been sourced, reviewed and adapted from materials provided by General Plastics Manufacturing.
For more information on this source, please visit General Plastics Manufacturing.