How does FIM Film-Insert-Molding function?
In the Film Insert Molding (FIM) process finished decorated components are produced by inserting decorated, reshaped and trimmed semi-finished film products into the mold during the injection molding process. In this way components with complex bends can be produced with symbols, transmitted light design and multi-colored two-dimensional decorations (e.g. carbon, wood design) and at the same time with simple decoration change from shot to shot. The following report provides an overview of in-mold decoration and the possible applications of this technology.
Fig 1. FIM: Ford Focus heating/ventilation shutter
- complex-shaped, decorated surface
- extremely flexible change of decoration
- transmitted light design (day/night design)
A wide variety of demands are placed on components made of plastic. The requirements to be met by the surface design today go beyond the customary properties and design desired (e.g. decoration, scratch resistance). For later use, therefore, the components are provided, in many cases, with a surface coating that is abrasion-proof and scratch-resistant, permits a diversity of colors and a high color density and enables two-dimensional decoration (e.g. carbon design) as well as symbols. Because of its flexibility (combination of plastic injection molded part and functional film), FIM offers interesting solution options since the surface of the plastic component is formed by the film in this process. As a result, abrasion resistance, a large depth of color, color diversity, symbols and transmitted light design can be achieved at the same time through the use of coordinated film systems.
In the process of FIM a decorated film is formed and trimmed. The film preform is subsequently placed in the injection mold and overmolded. The figure shows the principle of Film Insert Mold Decoration using an automotive center stack as an example.
The decorated film forms the optical surface on the finished component. This surface can be monochrome, designed with special colors (metallic effects) all the way to multi-colored decorations and additional integration of symbols. Colored films can be used here for simple monochrome applications. Printed films also make it possible to use special colors, multi-colored decorations or symbols. In the case of films printed on the front, the decoration layer is on the outside of the component and must be additionally protected against abrasion by a protective coating in the event of high mechanical stress. With films printed on the back the decoration is protected by the outside transparent film and additionally enables a very high gloss. In the case of additional requirements, such as regarding UV resistance or media resistance, multi-layer films having the required property profile can be used.
Because of the necessary forming of printed film, highly flexible printer's colors that adhere well to the film must be used for the FIM technique. Furthermore, the color must be able to withstand the thermal load and shear applied during FIM, particularly when printing on the back of the film. For the production of components with film printed on the back it is now possible to carry out FIM directly with these films through the development of the screen printing ink Noriphan® HTR. This ink was specially developed for the PC (polycarbonate) and PC/PBT blend films used (Makrofol®, Bayfol®). This was made possible through the use of a special high-temperature-resistant thermoplastic as the basis for the production of the screen printing ink.
After forming, the film must fit into the injection mold with as little play as possible because otherwise subsequent problems with FIM (e.g. wrinkling, surface defects, over molding) can be expected. Therefore, the film shrinkage after forming due to the thermoforming mold design has to be taken into account for the film preform; this shrinkage depends not only on the type and thickness of the film, but also on the forming temperature and the draw ratio, i.e. the geometry of the mold. Cold and hot forming processes can be applied to shape the films.
An important element in shaping films was the development of a method to shape a film below the softening temperature. In this way the printed information symbols are retained at their precise position and/or distortion is constant so it can be compensated for through distortion printing. In the high pressure forming method [HPF] the printed film is abruptly shaped below its softening point through application of compressed air at 50 to 300 bar. In comparison to thermoforming, the attainable degree of forming with pressure forming is less. In the case of decorative moldings for which precisely positioned shaping of the decoration (e.g. wooden decoration) is not important, the thermoforming process is applied. In the thermoforming process the film is heated up by means of infrared heaters and drawn over the mold through the application of a vacuum. During the subsequent cooling process the film is cooled down to room temperature. The temperature of the mold used must be controlled in such a way as to guarantee a reproducible amount of shrinkage in the process and additionally avoid quench marks that may result from molds that are too cold.
Trimming the deep-drawn film at the edges and at recesses is just as important for the production of flawless parts as the forming process. Any projection of the film at the edge of the cavity into which it is inserted inevitably leads to unclean edges of the injection molded part and possibly eventual damage to the sealing edges in the parting line. Excessive shortening of the film results in the overmolded base material becoming visible at the edge of the part. A suitable process for series production is punching as well as stamping. For small batch production or in the optimization phase trimming by means of a milling cutter or abrasive blasting (laser, water) is also possible.
Prior to insertion of the film into the injection mold, dust and impurities are removed from the film surface by blowing with ionized air. The film is inserted by means of a handling unit so as to achieve precisely reproduced and efficient production. The film can be held in place in the injection mold through its geometry. In many cases, however, further measures (e.g. electrostatic charge, mechanical holding) are necessary to improve the retention of the film in the mold. In the event of long flow paths, the use of needle valve nozzles, which are opened according to the filling procedure through cascade control, has proven effective for avoiding wrinkling. In the case of large part dimensions, special consideration must be given to wrinkling, washing out, distortion, delamination and overflow of the melt in the exposed area. As far as wrinkling in the knit line area is concerned, the type of film material, the film thickness and the spacing of the gates have a great influence. Wrinkling can be prevented through suitable film material with a high softening temperature, increased film thickness and optimized flow paths. In general, optimal parts cannot be produced with existing injection molds that are to be used subsequently for in-mold decoration without changes to the mold. Because of the wall thickness reduced by the film thickness and the rheological requirements necessary for film insert mold decoration, at least the gate and the gating system have to be redesigned. Therefore, it is suggested that appropriate measures be taken for FIM right from the mold design phase.
FIM makes it possible to design the appearance of plastic components in a targeted manner. Not only can the decoration (monochrome, multi-colored, integrated symbols, transmitted light design), but also the surface impression (glossy, structured, matte) and the high gloss can be set selectively. There is scarcely another method with which the decoration of the parts can be changed without interrupting production by inserting film with different decorations from shot to shot. Additional properties, such as media resistance and scratch resistance, can be achieved through the use of composite film. FIM decoration is not limited to simple geometries, but is also suitable for components with complex shapes. By covering the surface with a decorated film, recycled material can be utilized as FIM material with this method. There are a wide variety of ways of implementing technically demanding, economically interesting and sensible applications through FIM. The method is very flexible in its application and it permits parts to be generated with specific surface characteristics, such as decoration or lettering, haptics or scratch resistance, in one operation. The prerequisites are suitable, preformed and trimmed film that meets the properties required. Further development of FIM technology in new fields of application, such as large 3D parts, is one of the focal points of process and application development at Bayer MaterialScience.
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