The cycle of these new processes is very short and may promote the use of composite sandwich structures in certain areas, currently in the field of metal or common plastics.
Thermoforming
Although the foam has a certain strength and rigidity at normal temperature, it will become soft after heating. In this way, it can be shaped into any shape. After the temperature is lowered back to room temperature, the foam will harden again and maintain its shape. In general, the thermoforming process can be subdivided into three stages: heating, shaping, and cooling. Depending on cycle time, required accuracy and the complexity of the shape to be molded, it can be thermally conductive (eg Teflon coated heating plates), thermal radiation (eg infrared heaters, ceramic plate heaters, halogen heaters) or thermal convection The foam is heated by means such as an oven. Usually, a mold is required during the molding process. Some molds consist of upper and lower parts, the upper part is pressed together; some molds have only the lower part, plus a device that presses the foam (such as vacuum bag, sandbag). aIREX® T90 and T92 are the two simplest and easy to handle foams. They can be thermoformed into a variety of complex shapes with little or no rebound. After applying processing methods such as compression molding and stamping thermoforming, the shaping of aIREX® T92 has been fully demonstrated in laboratory tests.
Molding
After heating the foam plate to 170 ° C in a convection oven, the foam plate can be pressed into the mold. After a brief cooling, the formed sample can be demolded, the surface is uniform, and no shape bounce occurs. The partial foam material becomes dense due to being squeezed, and the foam structure is also more deformed.
Stamping thermoforming
This is a sheet molding process that uses a mechanical impact force to press a thermoplastic sheet into a mold. The combination of thermoformable aIREX® PET foam core materials and state-of-the-art production facilities provides a new production method for sandwich components with short cycle times and consistent quality. In order to control the actual thickness within the error tolerance range, it is preferable to use a closed mold for thermoforming. AIREX® T92.100 samples with a thickness of 10 mm (0.4 in) were produced in cooperation with Plastika BalumagaG in Horchdorf, Switzerland. The 800 x 500 mm (31.5 x 19.7 in.) foam board was clamped to the open area by a clamp frame and heated simultaneously from both sides to a specified temperature using two outdoor halogen lamps. Since the light intensity at each location can be adjusted separately, each part of the foam board can be heated evenly. After heating at 170 ° C (338 ° F) for 30 seconds, the halogen lamp was turned over and the upper and lower punching tools were heated at 80 ° C (176 ° F) for 3 seconds. Cool down for 20 seconds before removing the parts.
Resins as composite matrix can be divided into two broad categories: thermoset and thermoplastic. The thermosetting resin is usually in a liquid state, and after being mixed with the catalyst, a chemical reaction occurs to become a solid. Thermoplastic resins have a molecular structure that can be repeatedly melted and reused. For this reason, the production of thermoplastic sandwich materials is usually faster than thermoset materials (the latter has a longer production cycle), making it possible to use thermoplastic materials in large quantities. Compared with thermosetting plastics, the recycling of thermoplastic materials is easier; if the temperature is higher than the melting point of the matrix, different parts of the thermoplastic composite can be separated.
In addition, the thermoplastic material can be self-bonded without the need to additionally add a binder such as glue or resin.
Individual heating panel
The soldering iron, infrared or halogen heater can heat the surface layer to a very high temperature without affecting the core material, enough to bond the substrate to the panel. This production process makes it possible to combine a high temperature skin substrate with a low temperature resistant sandwich material. The heated surface layer must be quickly integrated into the lower temperature foam. In practice, automated handling is essential; the surface must remain hot until it is pressed against the foam. Only when the relevant technical parameters are controlled within the fair range, the surface material and the sandwich material can achieve the desired bonding state, which is also the decisive factor determining whether the finished surface is smooth and beautiful. Usually, the foam itself does not need to be heated separately. The heat absorbed by the foam from the surface layer is sufficient to thermoform it. The difference in thickness of the foam and the desired shape can impose some limitations on the application. Tests have shown that when the surface layer is heated to a melting point of 230 ° C (446 ° F) with a soldering iron, the surface PBT substrate begins to soften. After a short assembly, several layers of 130 x 50 mm (5.1 x 2 inches) samples were pressed for 20 seconds and then fused, and then cooled for approximately the same amount of time.
The process takes very short time (eg 90 seconds for small parts) and, in addition, saves material costs by eliminating the need for special adhesives or resins.
Molded heating creep type
In the process of molding heating and creeping, the panel and the sandwich material are first placed in the mold and then placed in a convection oven for heating. Since the pressure contribution of the mold is constant, the surface layer and the core are slowly deformed as they are gradually heated to the desired temperature. The melting temperature of the thermoplastic panel substrate must be below the stable temperature of the foam core material. In order to ensure proper bonding of the panel and the core, the upper and lower ends of the mold must be applied with sufficient pressure. This requires the core material to have sufficient resistance to pressure at the specified temperature. This compressive capacity of the sandwich material plays a pivotal role in the ability of the thermoplastic product to achieve a high quality appearance. After the sandwich structure is pressed into the closed mold for a few minutes, the part can be cooled and demolded. For this process with aIREX® T92, the optimum temperature is 170-180 ° C (338-356 ° F). Within this temperature range, thermoplastic panels and fabric panels with a PP matrix are available. This process requires a longer processing cycle than the aforementioned panel heating process. Figure 2 shows the car rear spoiler sample.
Thermal bonding of sandwich composite panels
For all composite sandwich structures, a matrix or other bonding agent is necessary to bond the surface to the sandwich material. This adhesive layer or resin matrix not only increases the weight and cost, but also prolongs the production cycle and also emits harmful gases. In addition, they also hinder the recycling of materials.
It is generally believed that molten glue is the best thermoplastic composite bonding. As with soldering, the contact surfaces of the two parts are first melted and then cooled under pressure to firmly bond the two together. Similar to hot melt bonding, the surface of the PET foam is heated to about 280 ° C (536 ° F), beyond its melting point, to form a PET adhesion layer, which bonds the panel to the core. The following experiment confirmed the possibility of making an aluminum-PET-foam sandwich structure without adding any binder throughout the mixing process. In the experiment, the aluminum plate was heated to 290 ° C (554 ° F) by a soldering iron and then pressed to both sides of the lower temperature aIREX® T92. The aluminum plate was not subjected to any pretreatment except for simple sanding. During the low pressure forming stage, the aluminum sheet must be kept at a constant temperature to form a PET melt layer between the foam and the aluminum skin. It can be heated for a few seconds to liquefy the foam of about 1.5 mm into the PET melt. The mixture will be cooled to room temperature under pressure for a few seconds. After peeling off the foam surface of the core material, you can see excellent adhesion.
Compared with the traditional production method of sandwich structure, the advantage of this new method is that the production process cycle is extremely short. This makes mass production possible. In addition, since this simplification process does not require the use of an adhesive, it is light in weight, saves money, and does not require pretreatment of the surface layer, has better impact resistance and is easier to recycle. From the point of view of industrial production, all non-interrupted heating, extrusion and cooling processes can be used for all panel materials with thermal conductivity around 270 ° C (518 ° F).
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