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Driving the Development of Film-Insert Molding

(October 2013) posted on Fri Oct 25, 2013

Find out how FIM functions in a variety of high-end applications and determine whether it’s a fit for your business.


By Neil Bolding

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A typical film for FIM has a thickness between 125-250 μm. When drawn by a factor of three, even the thinnest material still has a thickness of more than 40 μm. This is usually enough to retain mechanical integrity in the pre-form. It is not, therefore, necessary to specify excessive film thicknesses of 375 μm or more. This can provide a large cost saving. Film thickness can be important, however, when it is necessary to hide witness marks from cut-outs in the molding, where tactile dome switches are to be incorporated into the part, or where unsupported windows are required. These are all specialized applications and require great care in part and process design, and such proposals must be approved throughout the supply chain to avoid unforeseen issues.

In general, the structural integrity of the preformed part determines the minimum film thickness you can use. Film materials vary in their rigidity. Polyester film has the highest rigidity and can therefore be used in the thinnest gauges. For some shallow-draw applications, PET films as thin as 50 μm have been used. The remaining materials have broadly similar rigidity and are normally used in thicknesses of 175 or 250 μm. Deep, wet-look effects usually require a thicker film, but 250 μm is normally sufficient to get the required effect.

Printing equipment
Good control of the screen-printing process is essential to ensure reproducibility of print thickness and edge definition. High-quality equipment capable of handling thin, plastic substrates and holding tight registration (±0.2 mm) is essential to ensure success. For most films, manual and automatic presses are acceptable. Cylinder and roll-fed presses must include the ability to remove print-side laminates at the unwind station, as virtually all films are supplied with a protective second-surface laminate. A good starting point for the squeegee is 80 durometer.

Environment
Humidity in process and storage areas for the product should be maintained at below 55% RH. Very low humidity may cause problems with static electricity so an ideal humidity range between 50-55% RH is recommended. This is a tight tolerance. Most FIM applications require a very high level of cosmetic performance. Use of a cleanroom specified and maintained to at least ISO7 standard is highly recommended.


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