The conclusion of this two-part article about the film-insert-molding process focuses on films, processing, and finishing.
By Jim Plamann, Neil Bolding, Peter Warwick
Most assume that the film used in a film-insert-molded part must be the same polymer as the molding resin. While this is a good rule of thumb, it is not an absolute rule. Considerations include melting points of the film and the resin, shrink rates of the film, and resin warping.
The most important issue is to ensure that there is at least a reasonably close match between the melting points of the film and resin. Film that melts at a temperature much lower than that of the molding resin is at risk for melting, especially in areas close to the injection gate. The cooling effect of the tool provides some protection, but serious damage quickly becomes obvious at the point of mismatch as distortion to the print in the gate area. This is easily confused with ink wash, but it will occur even with the best ink when the film itself is melting.
Most of the films used in film-insert molding (FIM) have relatively high melting points and do not cause issues even with high-process-temperature resins like PC. PMMA is a notable exception and must be used with great care with pure PC and even PC/ABS blends. To be safe, it is better to use PMMA films only with PMMA resins.
A mismatch in the other direction can also be a problem. Film that melts at a point that is much higher than the resin’s process temperature makes poor adhesion very likely. This is because adhesion relies on intermingling of the two polymers at the interface. The problem is not necessarily insurmountable, however, because the ink can provide a bridging layer. If the ink adheres well to the film, and the resin adheres well to the ink, then all is well. Remember that there may be part areas that are free of ink, such as lenses. Poor adhesion may be tolerated in these areas, provided the areas are not close to a part edge, but careful testing is required to preclude field failures.
The most important issue is to ensure that there is at least a reasonably close match between the melting points of the film and resin. Film that melts at a temperature much lower than that of the molding resin is at risk for melting, especially in areas close to the injection gate. The cooling effect of the tool provides some protection, but serious damage quickly becomes obvious at the point of mismatch as distortion to the print in the gate area. This is easily confused with ink wash, but it will occur even with the best ink when the film itself is melting.
High-pressure forming (HPF or Niebling forming) Niebling forming (Figure 1) was the first high-pressure process specifically designed for FIM and remains the most popular choice for producing a wider range of small to medium-size parts. The film is preheated to a moderate temperature, usually somewhat above the softening temperature. Hot air is applied to the film under high pressure up to 300 bar, which exerts a lot of force. The larger the area formed, the higher the forces. The engineering required for large sheet sizes to contain the pressure becomes relatively expensive. Large-format machines are becoming available, but most machines on the market are designed around a form platen approximately DIN A4 in size. Cycle time tends to be about 30 seconds, so typically this process has a similar throughput cavity for cavity to injection molding (IM). Processors, therefore, need a similar number of forming machines as IM presses to handle a project.
Forming-tool design
The following guidelines will help you optimize the design for your forming tools.
Maximize part radius. Avoid very small radii on the part. A very small radius at the bottom of a form dramatically increases the draw ratio at this
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