Direct coating process to create a high-quality surface in one step

The trend of electric vehicles, interconnection and autonomous driving is stimulating the emergence of a new design language for automobile manufacturing

The direct coating (DC) process using polyurethane (PU) coating systems is particularly suitable for the manufacture of these components

PU coated parts are created in a mold that combines injection molding and RIM technology

The PU coating system provides a very large degree of freedom for component surface design

Rühl Puromer GmbH and Germany-based polymer manufacturer Covestro are among the pioneers of the direct coating process

Puroclear is a two-component PU coating system based on aliphatic isocyanate launched by Rühl.

The Puroclear product line also includes self-healing products, which have the so-called backflow effect

Covestro has developed special polycarbonate (PC) and PC blends for direct coating processes in the Makrolon, Bayblend and Makroblent product lines, as well as a wide range of modular coating raw materials without solvent additives for PU coatings , Such as: special isocyanate crosslinkers in the Desmodur series and polyols in the Desmophen series

Ensure the bond strength between PU and PC

Generally speaking, the bond strength between PC and PU is extremely challenging, especially under the influence of aging
.
Makrolon and the transparent Puroclear grades have good bonding properties, but development partners Rühl and Covestro have further improved the bonding strength for long-term stability after aging
.
To this end, they optimized a variety of material combinations and evaluated their bond strength through the POSI pull-out test (according to the ISO 4624 standard)
.
The results show that even the samples that have been hydrolyzed and aged during long-term storage in a hot and humid environment meet the requirements of automotive OEM adhesion specifications (Figure 3)
.
Subsequent studies also show that the self-release function of the PU system can also help it complete the expected task-easy release and will not adversely affect the adhesion between the PU and the substrate
.

In addition, both companies are committed to integrating the insert injection molding process (FIM) into the direct coating process
.
Stretched films and decorative printing films not only greatly broaden the possibilities of component design, but are also very important for functional integration
.
For example, they can reflect the sensor key function through printed conductor paths and backlight circuit diagrams (Figure 4)
.
The injection molding step can also be designed as a two-component process
.

 Covestro and Rühl support project partners throughout the development chain of directly coated components through comprehensive application technical support
.
The focus is on process-oriented part design and mold technology, as well as the simulation of direct coating processes and part characteristics
.
Accurate simulation of PU filling and warpage is crucial for large-scale PC trims in automotive interior parts to obtain glass effects and seamlessly integrate lighting elements and touch sensors
.
Parts of this type must have perfect artistic aesthetics and present different PU layer thicknesses
.
Because of their large size and the different shrinkage rates of the PC matrix and the PU system, they are easy to warp and deform
.
In principle, the simulation of PU filling process and warpage should be carried out in the early stage of the project to facilitate the correct design of parts and molds from the beginning, avoiding expensive adjustment costs in the later stage
.


Filling simulation tool

Covestro has developed corresponding calculation and simulation tools
.
A new calculation method based on two-phase CFD (computational fluid dynamics) simulation can be used for filling simulation
.
Compared with the traditional injection molding simulation method, this method regards the air discharged from the mold during the PU filling process as a separate phase rather than a vacuum
.
Compared with the previous traditional simulation method, this method can accurately determine the formation position and movement direction of bubbles according to the structure of the coating and the substrate, and whether they remain in the part or reach the vent (Figure 5)
.

 They also developed a calculation method for warpage simulation
.
This method describes the material properties of the coating system under shrinkage and warpage
.
The PU system shrinks more than the thermoplastic matrix during the molding process
.
Therefore, when designing components, a 3D structure with a larger wall thickness increment should generally be placed on the substrate to maintain a uniform layer thickness and small deformation of the PU system
.
However, this can only be achieved under ideal conditions in most applications
.
Therefore, the simulation helps to find the best layer thickness ratio with the smallest warpage and the shortest cycle time (Figure 6)
.

The automotive industry attaches great importance to the direct coating process

At present, the direct coating process is mainly used in automotive interior parts
.
However, it is also increasingly used in exterior parts, such as pillar trim panels equipped with touch panels that can enter a password to open and close the interior
.
Other potential exterior trim applications include: highly integrated spoiler elements, radomes and front panels (title image) that are reminiscent of radiator grilles
.
The latter is mainly used as a brand positioning design element for electric vehicles regardless of whether the light and sensor body are integrated
.
In general, the automotive industry is increasingly using the direct coating process as an effective alternative to other mass production manufacturing concepts because it provides higher component performance and design freedom
.
At present, medical, equipment, IT and communication technologies have also discovered the advantages of this process
.