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In-Mold Paint (IMP)

September 12, 2017

Romeo RIM was among the first RIM companies to commercialize In-Mold Painting during the 1980’s. “IMP” involves painting the cavity of the tool immediately prior to injecting the Polyurethane (Iso/Poly) material with or without Chopped Glass. During the molding process the paint chemically bonds to the Polyurethane material thereby eliminating the possibility of the paint chipping or flaking off, which can occur with post-applied paint. In-Mold Painted surfaces perform just as well and/or better than post-applied painted surfaces because the chemical adhesion that becomes an integral part of the substrate. The finish part can range from Low gloss to a High Gloss Class-A surface finish. The allows the part to be directly packaged and shipped to the customer for assembly, if no secondary process is needed.

One of the largest costs of painting plastic parts is the prepping, painting and finishing process. This not only includes the cost of the paint and labor, but also overhead costs of additional painting equipment, drying ovens, environmental equipment, etc. Although IMP minimizes these costs, the greatest cost savings are achieved by processing efficiencies that eliminates post-mold painting steps such as cleaning, prepping, sanding, priming and improving first time run yields.

Advantages include:

  • Class “A” High/Low Gloss Finish (UV Stable)
  • Chemically Bonded
  • Excellent Adhesion between the Barrier Coat and the Substrate
  • Achieve any Color Finish
  • Capable of Flexing with the Part with No Results of Cracking or Crazing in the Part Surface
  • Excellent Orange Peel Finish

With the RIM process, it’s possible to apply gel-coats and two-component polyurethane in-mold paints into the mold prior to injection. The injected polyurethane material bonds to the gel-coat or paint during molding, allowing a decorated part to be produced in the mold. This can significantly reduce secondary finishing cost.

In-mold coating technology can be applied to all urethane systems with the exception of DCPD. Similar to a top-coating operation, too little paint and too much paint can have adverse affects on the surface finish of the part. The standard thickness of an in-mold coating in urethane systems is 2.5 mils with a tolerance of 1 mil.

Consider in-mold coatings – special paints sprayed onto the mold surfaces –as an alternative to postmold painting. After spraying, these paints dry for a brief period, so that the injected mixture flows over the semi-dry coating during mold filling. Typically in-mold painting is used for large, relatively simple molds, such as cab roofs and fenders for agricultural combines.

The major difference between our IMC and post applied urethanes is the amount of “accelerator” (drier) and the solvent blend to allow the IMC to cure quickly in the mold.  Below are some common pro’s & con’s of IMP to Post-Applied. 

In-mold Painting Versus Post-mold Painting

Advantages of In-mold Painting

  • Almost perfect adhesion between the coating and the substrate.
  • Because in-mold systems are low in solids they follow the contours of the mold design very well. Therefore, very fine details on the mold will be faithfully reproduced.
  • In certain instances the presence of an in-mold coating can improve the flow characteristics of the polyurethane system resulting in fewer sub-skin defects.
  • Capable of flexing with the part with no results of cracking and crazing in the part surface.
  • Mold spraying is relatively straight-forward and less labor intensive than post-mold painting.
  • Paint usage is relatively low compared with post-mold painting.  

Disadvantages of In-mold Painting

  • Complex mold shapes are difficult to spray with acceptable accuracy or speed.
  • Unlike post-mold painting where a specific area of the factory can be set aside for spray finishing, in -mold painting is generally undertaken on the molding line. However, Romeo RIM’s manufacturing clamps are set-up with all necessary equipment for in-mold painting.
  • If the mass color of the polyurethane system is not a particularly good match for the paint finish, the flash or split line will stand out and may require touching up with a post-mold paint.  (ie – Black substrate with Red topcoat)

Advantages of Post-mold Painting

  • Paint finishes with excellent exterior durability and chemical resistance can be produced.  Achieved through additional topcoats.

Disadvantages of Post-mold Painting

  • Poor adhesion between the coating and substrate is the most common failure area for post-mold paints. In    order to avoid such failure extensive preparation of the substrate is necessary before painting. Such preparation includes removal of all traces of release agents by solvent washing, followed by applying of one or more primer coats.
  • The application of post-mold paints is a skilled operation. Faults in the top coat are all to easily produced (i.e. craters, orange peel, etc.). Operations to correct such faults can be costly in both time and labor. The standard of cleanliness in the spraying area needs to be high in order to avoid contamination by foreign matter on the substrate or wet paint surface.
  • Since most two-component systems use isocyanate prepolymers as a curative, the health risk is somewhat higher than that associated with fully-reacted systems.

If you need information on physical characteristics (ie – chemical resistance) of IMC to Post Paint that information can be supplied.  However as this is done for other customers (Green & Red).  I would need to contact the people responsible for those accounts to use the information.

Regarding Paint Fade between In-Mold Paint (IMP) vs. Post-Applied Paints:  Attached are three forms that show 24 months Florida Exposure.  Two of the documents are IMC coatings that are currently used by Romeo Rim for our customers.  Also attached are results of four post-applied two component urethane paints for that are used on metal and plastic substrates. As you will see, the difference in weathering for IMC vs. post-applied is minimal.  As a general rule, the fade rates are dependant on color.  Meaning clean reds and yellows will not hold color and gloss as well as a white over time.  Please note: Fading will be similar “over time” regardless to IMP or Post-Applied.

Romeorim design guide