Simple Manufacture Of Molded Parts
Rifeng® PMI foam allows relatively simple production of molded parts by cold or hot forming. The most appropriate method in each case can be selected in accordance with the extent of forming, the panel thickness, the Rifeng® PMI foam grade, and the subsequent processing.
Rifeng® PMI foam can be thermoformed between 165°C-205°C. The sheets can be heated in an oven or with infrared heaters. The duration depends on the sheet thickness (approximately 1mm/1min).
Since the low heat capacity of the foam and the sheets’ surface area multiplied due to the vast cut-out closed cell, the sheet will cool rapidly. During the transfer from the heating cabinet to the forming device, cooling of the sheet must be kept at a minimum. To minimize the rapid cooling effect and satisfy the thermoforming process temperature requirements, a protective cover on the heated foam sheet is required. Cotton cloth, breather, glass fabric or silicone rubber can be used as a protective cover.
For shaping core materials, the changes of curve are trying to be deformed greatly in only one dimension or to be deformed slightly in two dimensions. Radome as an example, the radome surface is undevelopable, so it needs to be divided into 6-8 fan-shaped with the same shape, and be thermoformed respectively before bonding into a complete radome core material.
The high heat resistance of the mold during thermoforming is not required. There for, wood, polyester, epoxy resin or fiberglass molds are acceptable. Once the formed part has cooled to 100°C, it may be removed from the forming mold. By the use of foam’s thermoforming technology, there are three main molding methods, compression molding, vacuum forming and “Cold-deforming-heat-shaping”. The former two methods require the foam to be heated before forming. The latter one requires the foam to be thermoformed after cold forming.
If using the compression molding process, it’s required that the design dimension is processed by machining after thermoforming the foam sheet, while the precise positioning during the processing is relatively complicated to implement. Compare with cold forming, the compression molding deformation could be designed to be a little larger. According to the different usage of molds, it could be implemented by both-sided hardened mold, single-sided mold, one-sided hardened mold with one-sided soft mold.
1).Both-sided hardened mold: this design has the advantages on accurately controlling the thickness of the foam sheet during the process, and the deforming sheet could be developable or undevelopable curves. The disadvantage needs to make male and female molds.
2).Single-sided hardened mold: if the foam’s thickness or deformation is quite large, generally, could be thermoformed by a single-sided mold with the use of press machine. However, it’s difficult to control the changes in sheet’s thickness direction. The deformed curve of the foam is an undevelopable spherical surface, and the sheet is deformed in both directions.
3).one-sided hardened mold with one-sided soft mold: its disadvantage is similar with the single-sided hardened molding. But compare with the former one, the deformation of the sheet is usually in one direction, that is, the deformed sheet is a developable surface.
Vacuum forming also uses the design of one-sided hardened mold with one-sided soft mold. However, comparing with the one-sided hardened mold with one-sided soft mold as the described above, vacuum forming is able to thermoform the undevelopable curve by means of vacuum-sorbing force. It’s required that the design dimension to be processed by machining after thermoforming the foam sheet. But compare with the single-sided molding, the deformation is rather constricted due to the vacuum-sorbing fore’s restriction.
Machining the foam sheet into the precise developable dimension, and then fix it on a die, by means of a vacuum, heated in an oven to the forming temperature for 1 hour, and slowly cooled to the normal temperature after removing it from the oven. The advantage of this method is that the thermoforming process can be well controlled. And the post-machining process after thermoforming is not required, if the positioning accurately. While the cold deformation has a certain restriction, that is, cannot deform too much, especially in an undevelopable case.
What but is worth reminding, as a rare event, due to the characteristics of foam’s production process, there will be as mall degree of warpage which occurs in the finished sheet after place for a period of time. This is the result of dual functions of moisture absorption and internal stresses release, but not the defect of the product. The process is reversible and would not affect the product’s quality. The specific shaping process is as follows:
1). Place the foam horizontally and distribute evenly the weights on top like steel plate, aluminum plate, etc. until the foam is resiliently deformed to the flat state at room temperature. The weights must be placed evenly and must not be concentrated in one area. Otherwise, there will be pits on the surface of foam after shaping.
2). Put the foam together with the weights into the air circulation oven up to 200°C (1°C/1 min) for 8-10 hrs (depending on the thickness of the shape). Once the oven has cooled to the room temperature, they can be removed from it.