Black-to-Black Laser Transmission Welding

Black-to-Black Laser Transmission Welding

Laser plastic welding, also known as through transmission welding, is quite simple in its essence (see Basic Process Video). Laser radiation is shot through an upper transmissive plastic part and the energy is absorbed by the lower layer, creating heat at the interface and melting both parts.

However, the process does not always seem so intuitive. For instance when welding colored materials, specifically ones with opaque upper layers. The Basic Process Video, linked above, is somewhat misleading in this instance. For the sake of simplicity the video displays the upper layer to be transparent optically or clear to the human eye. This is perfectly possible, many thermoplastic in their natural state do appear clear, however, this does not tell the whole story.

Laser weldable materials do not have to have specific optical characteristics, only specific characteristics in how they interact with laser/infrared radiation. It is important that we distinguish the difference between optical transparency and infrared transparency. Optical light, light in the visual spectrum, ranges from about 400nm to 700nm. Alternatively, light energy used for laser plastic welding typical falls in the infrared or near infrared range from around 800nm to roughly 1,000nm

When it comes to finding materials suitable with laser plastic welding it is only required to find an upper material that is transmissive to the laser radiation in the infrared or near-infrared spectrum and a lower layer that absorbes the same wavelength. The majority of thermoplastics have transmissive properties somewhere in the range of 800nm to 1,000nm.

The point to make is that it is possible to have plastic that appears opaque, be transmissive to laser radiation.

Pigmentation

Most thermoplastics have natural visual transparency as well as infrared transmissivity. Pigmentation and dies are used to give plastics colors and opacity. Some of these color additives will change the laser transmissive behavior of the plastic, specifically carbon black. Carbon black is excellent for absorbing laser radiation and it also happens to be the most widely used additive in the plastics world for coloring thermoplastics black.

For this reason carbon black is also the most commonly used additive for the lower absorbing layer. But, then how can the upper layer also appear black, yet transmit laser radiation? The answer is in alternative dies.

The gear sensor below is a perfect example where carbon black is used for the lower absorbing layer and other pigments are used to create a black appearance on the upper layer. More specifically, dark red and green dies are mixed into the upper layer plastic, creating a black/opaque look, but still allowing for laser radiation to pass through.

Opaque cover and opaque body

Opaque cover and opaque body

Crossover

Naturally there will be some crossover with visual light interaction and laser radiation interaction. The two ranges are found right next to one another on the electromagnetic spectrum. Regardless of what pigments or dies are used there will be some affect on the transmissivity of the laser energy, the question is to what degree?

Electromagnetic Spectrum Image

Electromagnetic Spectrum

Where carbon black is a great absorber, able to absorb laser radiation effectively even when doped at small percentages, other black or color additives tend to have far less impact on absorption. Only about 5% of the laser radiation is required to transmit to get a successful weld; a higher rate is ideal. The dark red and green pigments used in the gear sensor above, although blocking some laser radiation allowed enough through to get a good weld.

In fact, black-to-black welds are relatively easy to accomplish. The only easier weld being a carbon black doped lower layer with a natural, un-doped upper layer. For information on other color options and weld difficulty please see this article, How Does Color Affect the Welding Process?.

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