Achieve optimum results from film insert moulding

In mould decoration specialist Peter Warwick from Autotype International explains how to achieve optimum results from film insert moulding.

Film insert moulding (FIM) is a form of in mould decoration (IMD) that provides a means of decorating and manufacturing a wide range of profiled or 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.

By comparison with traditional methods of product decoration and manufacture, such as spray painting, laser etching and applique, the FIM process offers excellent benefits to designers, processors and manufacturers.

FIM can significantly reduce the number of production stages and component parts, thereby cutting both production cycle times and cost, while improving the quality, complexity and durability of finished products.

Perhaps as importantly, FIM also offers greater flexibility, in terms of design and manufacturing options for shallow or deeply drawn profiles, enables original and often innovative design concepts to be considered and can be used to produce both short and long production runs without incurring financial penalties.

Although FIM is a highly effective method of producing high quality components there are number of key factors that should be considered at all stages to maximise the performance of the process.

Before looking at these various parameters, it is essential to understand how the FIM process works.

This comprises four key steps: printing, forming, trimming and moulding.

An image is first screen printed onto the underside of a special film, such as Autotype's HiForm or XtraForm harDCoated polycarbonate films; images can be produced either singly or in multiples, for example, approximately 80 mobile phone covers can be incorporated in to a 1.0m2 sheet of film.

Printed sheets are then transferred to a vacuum, pressure or thermoforming press, where they are formed to the exact shape of the required component; the outer hardcoated side of film thus becomes the outer side of the finished component.

The sheet is then trimmed and individual components die-cut to size.

Each is then inserted into a female injection mould cavity, where molten polymer is injected behind the film, bonding the two materials together to create a solid and finished part ready for subsequent product assembly.

The film therefore forms a tough protective skin over the complete surface of the part, which can be shaped extremely accurately, with high definition print.

Screen printing, the initial major stage of the FIM process, is fundamentally used to create the decorative appearance of the product.

When using FIM for applications such as mobile phone fascias that are likely to be exposed to excessive wear and tear, it is possible to use formable hardcoated polycarbonate films, such as Autotype's XtraForm, which has high levels of abrasion resistance.

In addition, to facilitate the creation of textured keys and gloss highlighted areas, special screen printed surface finishes and varnishes, such as Autotype's Aquatex FPH range can be used.

In order to incorporate features such as selective backlighting high opacity inks should be used with the screen printing process to allow only chosen characteristics to be visible from the front of the object.

Finally, to enable the image to withstand the high temperatures and shear stresses imposed during the injection moulding process, specialised screen printing inks should be used such as thermal-cure inks and UV-curable inks (also see the section below on injection moulding).

Forming represents the second major stage in the FIM process, taking the printed flat sheet to create a three dimensional shape, prior to final trimming and injection moulding.

There are four main methods used at this stage: vacuum forming, high pressure forming, hydroforming and matched metal forming.

Choosing the right process for each piece of work is important to ensure that print registration and component quality are retained.

Vacuum forming uses high temperatures to soften the pre-printed sheets, which are then forced into a mould through a combination of vacuum and atmospheric pressure.

The process can produce shallow and deep drawn parts and is capable of handling large area sheets.

However, the high temperatures used can lead to distortions that adversely affect areas requiring high print registration tolerances.

High pressure forming uses lower temperatures in conjunction with high pressure air to mould each sheet.

Although this minimises distortion errors, and can similarly be used for shallow and deep drawn parts alike, the high clamp forces required mean that machines tend to be smaller, so fewer parts can be produced per cycle.

Hydroforming uses hydraulic pressure and a polymer blanket or oil-filled bladder to form film sheets over a metal tool.

The process eliminates the problems of heat distortion but make it difficult to form anything other than shallow drawn shapes.

The final method matched metal forming can be performed with or without the application of heat.

This process can, however, cause damage to the surface of each sheet, especially in deep drawn areas, so it is essential that these are hidden in the final product.

Throughout, one of the main elements for success is the formability of the film.

In order to prevent surface cracking it is essential to use a specially designed film, such as various Autoflex derivatives from Autotype, which are extremely resilient, enabling both shallow and deep drawn shapes to be produced without surface cracking or affecting its mechanical or physical properties.

For products such as mobile phone housings the film substrate should be able to withstand yellowing, which is usually caused by prolonged exposure to sunlight, have high levels of transparency, so that backlighting can easily be incorporated, and be able to accommodate embossed or textured surface finishes.

At this stage the printed film must now be cut for the moulding operation.

Accuracy is key at this stage, especially if the printed image is to closely match the parting line of the mould tool, and this operation requires great care and excellent quality tools.

It is often the case that the cutting tool will require a number of iterations to ensure a perfect match to the mould tool.

The mould stage is vital to the entire process and 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, that should be considered if the exceptional levels of detail and high graphics resolution available through the FIM process are to be realised.

In general, heated mould tools should be used to reduce injection pressure and melt temperature and thus the stresses induced in the final part; it should also be noted that the films used tend to act as an insulator between the mould and the resin and that this can cause uneven heating and cooling and possible warping of the part.

To overcome this issue the temperature on the film side of the mould can be reduced, with guide pins also being used on the film side of the mould to prevent binding when compensating for warp.

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

Detailed graphics should be kept away from deep drawn sections and, ideally, gates should be positioned in areas free of ink; if this proves impossible then use a wide fan gate or a gate that produces an even melt-flow across that area of the component, to minimise turbulence, ink wash and shear effects.

Additionally, to overcome these issues special melt-resistant inks can be used and these generally fall into three categories: thermal-cure inks, UV-curable inks and high melt-resistant solvent soluble polymers inks.

Each ink offers different characteristics that will be appropriate for specific applications.

In general, however, thermal-cure inks resist the melt effectively but can require relatively long curing cycles and do not always provide the best levels of adhesion to the melted polymer resin.

UV-curable inks offer good melt-resistance but may require residual monomers to be baked out, and do not always provide the best degree of opacity.

In addition, some may require adhesion promoting tie coats to enable them to adhere to the resin.

Although melt-resistant solvent based inks can be more difficult to print, they are currently the most widely adopted products in the FIM market.

Film insert moulding is an extremely effective method, offering low unit costs, for both low and high volume production, with the ability easily to customise short run components through changes during the screen print stage.

By following correct procedures during the FIM process you can ensure that the most is made of this highly beneficial and efficient means of decorating and manufacturing three dimensional plastic parts.

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