IMD film forming: hot press thermoforming vs high pressure forming

Where forming sits in the IMD chain

Marabu describes the IMD/FIM process as a combination of printing, deformation, cutting, punching and injection moulding, and notes that moulding printed films involves temperature, gate and tool geometry, cycle times and washout. The film is screen printed (often on the reverse side), dried, formed to the part shape, trimmed or punched, and then placed in the injection mould.

Thermoforming and hot press forming

In thermoforming, radiant heaters heat the film well above its softening temperature (Covestro). According to Covestro, thermoforming allows a greater degree of stretching than HPF and better forming precision, particularly at the corners of a part, and it is well suited to large-format parts and continuous design elements such as wood-grain or carbon patterns.

Heated-press forming works on the same principle with a matched tool: a heating plate softens the film and a cylinder closes the tool, and some machines can also die-cut by changing the tool.

High pressure forming (HPF)

Covestro describes HPF as shaping the film in the region of its softening temperature with compressed air at 50 to 300 bar. Because the film is not heated as far, imprinted symbols, pictograms, letters and numbers remain precisely positioned, and the constant distortion from forming can be compensated by distortion printing. Covestro lists 3D speedometers, fascia and dials among typical HPF parts.

Check the pressure rating of any forming press against the film supplier’s recommendation: some gas forming presses are rated well below the 50–300 bar Covestro quotes for polycarbonate decorative film.

Matte, textured and printed films

• Covestro notes that thermoforming can make matte films appear glossy and that process heat may damage textures; HPF is preferred for matte or textured films.
• Where symbols must line up with buttons, lenses or light windows, plan distortion-compensated artwork and verify it on formed parts.
• Hard-coated films may need their coating cured after forming; follow the film supplier.

Dry the print before forming

Residual solvent in the ink can cause problems later in forming and moulding. For its Mara Mold MPC IMD ink, Marabu recommends drying as completely as possible, using a tunnel dryer with two hot-air zones at 60–75 °C followed by a circulation or cooling zone, then post-tempering the prints for 4 hours at 80 °C. If hardener is added, prints are not stackable straight after the tunnel and should be stored in a drying rack. A rack dryer or louver tunnel oven gives sheets a long, separated dwell; see drying screen-printed sheets.

Choosing a route: a checklist

• Film: PC, PET or other; thickness; matte, gloss or textured; hard-coated or not
• Part: draw depth, corner radii, overall size against the tool area
• Print: symbols that must register to features; distortion compensation needed?
• Ink: forming-grade ink and its drying and post-tempering requirements
• Downstream: trimming or punching, and the injection-moulding resin

LICHAO builds the screen printers that print IMD film: CCD sheet screen printers for cut film and roll-to-roll screen printers for long runs. Send the film, artwork and part drawing for a printing trial.

Related LICHAO equipment

• Fully automatic sheet screen printing machine with CCD alignment
• LC-6080SYJ-C roll-to-roll CCD screen printing machine