Optimising substrates for film insert moulding

In the latest in this series Peter Warwick, IMD Project Specialist at Autotype International, explains how to maximise the performance of substrates for the film insert moulding process.

In the previous article in this series we explored how optimum results can be achieved from film insert moulding (FIM) and looked at some of the issues that need to be addressed during each of the four stages of the FIM process.

In this article we highlight the factors that need to be considered in order to optimise the performance of film substrates for the FIM process.

To reiterate, FIM is essentially a form of in-mould decoration (IMD) that provides a means of decorating and manufacturing a wide range of high quality profiled three dimensional plastic components.

Typical applications include fascia panels, lenses, keypads and mouldings, for mobile phones and telecommunications equipment, consumer appliances, electrical goods, medical systems, public information terminals and automotive trim and dashboard panels.

This extremely effective technique offers low unit costs, for both medium and high volume production parts, with the ability to customise components through changes during the initial screen printing stage.

Film is perhaps the most important component part of the FIM process, forming the basic substrate on which all other aspects of the technology depend and forming a tough protective skin over the complete surface of the part.

Today, many of the film substrates designed for FIM have been developed to offer a combination of properties, including the ability to accept intricate graphics, using a range of inks; to be shaped using pressure, heat or vacuum forming systems; and to provide resistance to surface abrasion, physical wear and chemicals, greases and solvents.

In addition, they can be used for extended periods in sunlight, have high levels of transparency, so that backlighting can easily be incorporated and can be surface embossed or textured.

In choosing the best films there are, however, several important factors to consider, predominantly the requirements of the final application so that, for example, chemical or abrasion resistance is incorporated.

To enable the production of quality items with a high definition decorated appearance, such as interior automotive trim and mobile phone fascias, it is important to select one of the latest film substrates where the second surface is compatible with a wide range of graphic inks.

By reverse printing onto the film the graphics are protected by the tough protective skin of the hardcoat so that the image will not be scratched or exposed to damaging solvents or cleaning fluids.

For complex 3D parts the formability of the film is a key element.

In order to prevent surface cracking it is essential to use a specially designed hardcoated film, such as Autoflex HiForm and Autoflex XtraFrom from Autotype.

By selecting the correct product from the extensive portfolio the user can manufacture shallow or deep drawn shapes without surface cracking whilst maintaining the maximum mechanical and physical properties.

Next we must consider the injection moulding process.

There are a number of factors, including the type of resin used, the temperature and pressure of injection, the injection gate design and the distance from gate to ink surface, which will all effect the quality of the parts to be produced.

In general, heated mould tools should be used to reduce injection pressure which results in less stress on printed inks.

In addition, higher tool temperature increases the bond strength between the ink and the moulded resin.

The films used tend to act as insulators, therefore higher tool temperatures may be required for thicker films.

Care should be taken when designing the tool to avoid uneven heating of the tool which may cause warping of the part.

When producing the mould tool it is important to ensure that the wall thickness, especially near the gates, is carefully controlled, as thin walls cause extreme shear stress on the preprinted inks.

Detailed graphics are best kept away from the deep drawn sections and, ideally, gates should be positioned in areas free of ink; if this proves impossible then using a wide fan gate or a gate that produces an even melt-flow across that area of the component, will minimise turbulence and sheer forces in the resin, which result in ink wash.

When selecting films for FIM, it is important to consider ease of handling.

For example, for robotic feed systems you will need a film that does not stick and that does not use a protective laminate that has to be removed prior to insertion into the moulding tool.

Films designed to have good slip properties (eg Autoflex AutoForm G180M) lend themselves well to robotic feeding.

When selecting films for FIM it is of paramount importance that the requirements of the final application are taken into account, for example, that scratch, chemical and solvent resistance are incorporated if specified.

When using FIM for applications such as mobile phone fascias that are likely to be exposed to excessive wear and tear, it is recommended that formable hardcoated polycarbonate films, for instance Autotype's Autoflex XtraForm, are used.

These films have high levels of abrasion resistance and are suitable for extremely high definition printing.

Furthermore, the film substrate should have high levels of transparency, so that backlighting can easily be incorporated, and be able to accommodate textured surface finishes.

Many interior trim parts in cars require simulated wood or carbon fibre effect decoration which usually need high gloss surfaces; it is, therefore, recommended to use a hardcoated film as these can give a deep "French polish" effect, which can be extremely attractive.

Furthermore, the durability of the surface finish produced makes for a long lasting quality component that is scratch and abrasion resistant and enables imagery to be hard-wearing.

The latest generation of materials extend these desirable properties even further.

Autotype's HiForm, for example, has a unique "self healing" hard coat on a polycarbonate film base.

The self healing coating gives further improved scratch resistance, with the surface gradually healing itself over a period of hours to repair light scratches.

The use of high pressure forming techniques allows forming temperatures to be reduced, so that print registration of detailed graphics, logos and text can be maintained even for complex 3D parts.

An understanding of the factors that should be considered at both the product design and production stages, enables the simple selection of the best possible film substrate for the application.

In doing so it can be ensured that the most is made of this exciting and rapidly evolving process that offers considerable opportunities for designers, OEMs and end users alike in a wide range of applications.

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