Understanding and controlling taper when laser machining

Taper on the edges of laser-cut material is inevitable. Usually, this taper is oriented such that the laser beam entry point is larger than the exit point (or the bottom of “blind” features).

For the purposes of explaining taper—and due to the space constraints of this column—I’ll limit my discussion to laser-drilled holes.

When hole diameter is specified, it’s usually the exit-side diameter (D in Figure 1). The entry diameter (DE) is determined by the taper angle. This angle generally falls between 2° and 10° and is influenced by a number of factors, including beam delivery, peak power and pulse-repetition rate.

Figure 1: The exit side of a laser-drilled hole (D) is usually the specified diameter, not the entry side (DE). All illustrations courtesy PhotoMachining Inc.

A laser beam can be delivered to the work surface in numerous ways. Three of the most common hole-drilling methods are single shot, percussion and trepanning (Figure 2). All can be performed with either a fixed-beam system or one in which mirror movement is galvanometer-controlled. In practice, though, single shot and percussion are done more often with fixed-beam systems while trepanning tends to be performed with a galvo system.

Figure 2: Common laser-drilling methods are (from left) single shot, percussion and trepanning.

Single-shot drilling is the fastest holemaking method and produces a hole approximately the diameter of the incoming beam. The laser-energy output and material-absorption rate must be such that the material is pierced with a single shot. Taper tends to be more pronounced with single shot because there are no subsequent pulses to “clean” the hole.

Percussion drilling involves application of multiple pulses to pierce the material. It’s slower, but it also produces a rounder hole with less taper than the single-shot method.

With trepanning, the laser beam pierces the workpiece material and is then moved in a circular motion, cutting out the hole. Trepanning, when performed on a galvo system, yields holes with the best circularity and smallest taper.

Besides beam delivery, other factors that affect taper include peak laser power (defined as energy per pulse ÷ pulse width), pulse-repetition rate, number of pulses, assist-gas pressure, focus position and aspect ratio.

Generally, for any kind of laser-machining process, the higher the peak power the better the results. Higher peak power yields a correspondingly lower taper—to a point. (Zero taper is very difficult to achieve.) Therefore, by correlation, higher energy per pulse and shorter pulse widths also tend to lessen taper.

The pulse-repetition rate is the number of pulses delivered to the target per second. Repetition rate doesn’t pertain to single-shot delivery, of course, but it does affect taper with the other two delivery methods. In theory, as long the repetition rate remains low enough that the beam can be moved quickly—relative to the pulse frequency for heat dissipation—the effect on taper should be minimal.

Assist gas can be used with a fixed-beam delivery system. Delivered co-axially with the laser beam, it helps remove plasma and debris from the cutting area and cools the workpiece. Assist gas is not easy to use with galvos unless the scanning field is severely restricted, which would negate a benefit of using galvos in the first place. The type of gas used and the operating pressure it’s applied at have a big impact on laser processing. (This could be the subject of a series of articles, but, suffice it to say, optimizing assist-gas conditions greatly enhances processing and reduces taper.)

For most processes, the focus position is on the top surface of the material. With some processes, however, the focus is on the part’s bottom or middle. And there are many applications that call for a dynamic focus, such as when penetrating thick material.

Aspect ratio also must be considered when discussing laser holemaking. The ratio is found by dividing material thickness (or hole depth, if the hole is blind) by hole diameter. Lasers can be used to make high-aspect-ratio features. Ratios of 10:1 and 20:1 are common, and even 100:1 can be achieved under the right conditions. Generally, as the aspect ratio increases, so does taper. Therefore, smaller holes and/or thicker material mean greater taper.

It should be noted that taper is not always bad. Many parts made by laser processing benefit from taper. Among them are drug-delivery components used in disposable medical devices. Taper creates desirable fluid constriction in the flow direction of these components. In fact, there are optical methods available that introduce negative taper (the entry side of the hole is smaller than exit side). This is accomplished with rotating prisms or offset beam plates.

Hole taper—and by extension, feature taper—are inherent in laser processing. But by controlling the parameters that affect taper, laser operators can control it and even use it to their advantage. µ

About the author: Ronald D. Schaeffer, Ph.D., is CEO of PhotoMachining Inc., a high-precision laser job shop and systems integrator in Pelham, N.H. E-mail: rschaeffer@photomachining.com.