HyboFOAM® Foam Thermoforming
HyboFOAM® can undergo plastic deformation between 190°C and 210°C. The heating process can be carried out in an oven or with infrared heating equipment. The holding time is determined based on the thickness of the foam sheet (approximately 1 minute per mm).
Due to the low specific heat capacity of the foam and the significant increase in surface area caused by the numerous cut cells, the temperature of the foam surface can drop rapidly. Therefore, during the period from removal from the oven to shaping, the exposure time should be minimized. The HyboFOAM® sheets must be protected to prevent excessive temperature drop, ensuring that the required temperature for thermoforming is maintained. Protective materials can include cotton cloth, breathable felt, glass fabric, or silicone rubber fabric.
For core materials being formed, it is best to ensure that curved deformations occur primarily in one dimension or with small deformations in two dimensions. For example, when forming a radome, due to the non-expandable curvature of the radome, it should be divided into 6–8 identical wedge-shaped sections. After thermoforming each section, these pieces can be bonded to form a complete radome core.
The heat resistance requirement for molds used in thermoforming is not very high. Therefore, molds made of wood, polyester, epoxy resin, or fiberglass are sufficient. Once the shaped components have cooled to below 100°C, they can be removed from the mold.
Thermoforming processes using foam primarily include mold pressure forming, vacuum pressure forming, and “cold deformation with heat setting”. The first two processes require foam to be heated first before forming, while the latter involves forming the foam first and then heating it for setting.
Molded Compression Forming
When using the compression molding process, the foam sheet, after thermoforming, must be mechanically post-processed to the design dimensions, which complicates precise positioning during post-processing. Compared to cold deformation and heat setting processes, compression molding allows for larger deformations to be designed. Depending on the mold used, it can be implemented through two-sided hard molds, single-sided molding, or one hard mold and one soft mold.
1.Two-sided Hard Mold:
This mold design allows for accurate control of the foam sheet’s thickness during the forming process. The resulting sheet can either be a deployable curved surface or a non-deployable curved surface. The downside is that it requires the creation of both male and female molds.
2.Single-sided Hard Mold:
Typically used for thicker foam sheets or larger deformations, the single-sided mold allows for thermoforming using a press machine. However, the change in thickness in the foam sheet’s direction is difficult to control during molding. As shown in Figure 3.28, the resulting deformation produces a non-deployable spherical surface with deformation in both directions. While similar to the single-sided mold compression molding process, the deformation in this case is usually along one direction, allowing the sheet to maintain a deployable curved surface, as shown in Figure 3.29.
Vacuum-assisted Thermoforming
The vacuum-assisted thermoforming process also uses a one hard mold, one soft mold design. However, compared to the single-sided mold compression molding process described above, this method utilizes vacuum suction to thermoform non-deployable curved surfaces (see Figure 3.30). After thermoforming, the foam sheet requires mechanical post-processing to the design dimensions. Due to the limitations of vacuum suction, the deformation in this process is relatively restricted compared to single-sided mold compression molding.
Cold Deformation and Heat Setting
In this method, the foam sheet is first cut to the precise unfolded dimensions for the curved surface. The foam is then vacuum-fixed onto the mold and heated to the thermal deformation temperature in an oven. The temperature is held for 1 hour, then gradually cooled to room temperature. The advantage of this method is that it offers excellent control over the thermoforming process. If the positioning is accurate, no post-shaping is required after forming. However, the cold deformation capacity of the foam is limited and cannot be too large, especially for forming non-deployable curved surfaces.
It is worth noting that, as a rare occurrence, due to the characteristics of the foam's production process, the finished foam sheet may undergo slight warping after being stored for a period. This is a result of both moisture absorption and internal stress release, not a defect in the product. This process is reversible and does not affect product quality. The specific reshaping process is as follows:
Place the foam sheet horizontally, and evenly distribute weights such as steel plates or aluminum plates on top until the foam sheet elastically deforms and flattens at room temperature. Care should be taken to distribute the weight evenly, ensuring no concentration in any specific area, as this could create dents on the foam surface after reshaping.
Place the foam and the weight load in an air-circulating heating furnace. Using a 1°C per minute temperature increase, heat to 200°C and maintain for 8–10 hours (depending on the thickness of the foam sheet). Once the temperature reaches 200°C, allow the foam to cool in the furnace to room temperature before removal.
