Covestro and Rühl Puromer collaborate on direct coating to give components higher performance and design freedom

Direct coating (DC) with polyurethane (PU) coating materials is a new process for the economical and sustainable production of highly functionally integrated and decorative plastic parts
.
Rühl Puromer GmbH and Covestro have joined forces to expand the applicability of this technology for mass production
.

Dr.
Olaf Zöllner, Head of European Application Development at Covestro's Engineering Plastics business entity, explains: "We want to open up the process for widespread use in electric vehicles and autonomous driving, as well as for the manufacture of automotive interiors with multifunctional, seamless integration.
Large area trim for display, operating and ambient lighting elements
.
”

“We also see good opportunities for highly integrated passenger car exterior parts, such as spoilers, radomes, front fenders and pillar trims with touchpads
.
There are also great opportunities in medical, communications and IT technology.
potential
,” added Dr.
Ingo Kleba, Managing Director of Rühl Puromer
.

Virtually unlimited design freedom

The DC process combines two proven technologies in one process: injection molding of thermoplastics and reaction injection molding (RIM) of polyurethane materials
.
First, the thermoplastic carrier is produced in one cavity and then transferred to the second cavity, which expands according to the thickness of the coating
.
There, the polyurethane is injected onto the carrier
.

Polyurethane coatings offer an exceptional degree of freedom in design
.
The surface of the part can be transparent, translucent or colored, it can be highly resistant to chemical agents such as fatty skin creams, or impart a leather-like soft touch
.
Filament-like surface structures can be precisely shaped
.
The highlight area can be directly adjacent to the matte structure
.
Also, there is no need to invest in a separate spray paint
.
Compared to spray painting, there is no paint loss due to overspray and no waste due to wrong painting
.

Scratches that disappear on their own

Covestro has established special PC and PC blend compounds under the Makrolon®, Bayblend® and Makroblend® brands for DC processes, as well as a wide range of solvent-free additive-free modular coating materials - such as special Desmodur® isocyanate crosslinkers and Desmophen® polyols - for PU coatings
.

For DC processes, Rühl also offers the light- and color-stable 2K PU coating combination puroclear® in addition to the conventional products
.
Their RIM cycle time is usually at least equal to that of injection molding
.
The combination includes an "internal demolding function" that has been proven many times in series production
.
Therefore, the mold does not need to be sprayed with a release agent
.
This helps reduce cycle times and simplify the process
.

In addition, a self-healing variant (reflow effect) has been added to the product range
.
Ingo Kleba said: “Shallow scratches on the paint, such as those caused quickly by car keys or stone chips, will disappear on their own over time
.
Components retain their appearance, value and function for longer, which is important for It means maximum suitability for everyday use
.
"

Strong adhesion even after hydrolytic aging

Makrolon® and transparent puroclear® grades stick together firmly on their own
.
There is still room for improvement in adhesion during aging
.
Therefore, the development partners optimized several material combinations and evaluated their adhesive strength in the POSI peel test (ISO 4624)
.
"We can now offer material combinations that meet OEM adhesion specifications, even with prolonged aging in high temperatures and humidity," says Zöllner
.

A flawless, bubble-free surface

Accurate simulation of the PU filling process and warpage is critical for optimal performance of DC parts
.
Covestro has developed suitable tools for this
.
"With a new computational method based on two-phase CFD (computational fluid dynamics) simulations, it is now possible to precisely determine where bubbles form during filling and whether they remain in the part or reach the mold vents
.
This was the case in previous methods.
It is impossible to achieve,
” explains Christoph Bontenackels, who is responsible for the calculation of DC components at Covestro
.
A new calculation tool is also available for simulating part deformation
.
It can be used to find the optimal layer thickness ratio for 3D structures with large wall thickness differences, where warpage and cycle time are minimized
.

Prospect--Integration of Thin Films

The development partners hope to further expand the application scope of the DC process
.
One approach is to integrate thin films into the process to expand the possibilities for functional integration
.
For example, the film can be printed with conductive tracks and circuit diagrams on the back, a prerequisite for backlit, touch-sensitive key functions for operating panels
.