Improving Five-Axis Mold Finishing Operations

Injection molds are known for tight tolerances, intricate features and complex geometries including a large number of planes and connecting faces such as fillets. At the same time, requirements on surface quality are extremely high. According to Peter Brambs, Principal Engineer of Innovation at CAD/CAM company Open Mind, conventional milling strategies using ball nose cutters or bullnose end mills produce excellent surface finishes, but are not very efficient. In contrast, using face mills is highly productive, but does not lead to the desired surface quality. In my interview he explains how Open Mind’s five-axis tangent plane machining strategy can achieve time savings of up to 90 percent.

Open Mind has spent a lot of time on developing a highly-efficient method for finishing planes. Why?

We found there is enormous potential for savings when it comes to finishing operations of molds. Ball nose cutters are often used for finishing operations on workpieces such as molds with complex surfaces and hard to reach planes. However, while this process results in excellent surface quality, it takes a very long time. In particular, ball mills are used for line-by-line milling, since the faces here are either too large or hard to reach using traditional 2D strategies. At the same time, the tool path is not sufficient to reach the outer faces for top milling. Since problems arise relating to the required surface tolerances when finishing with more efficient large face cutters, up to 70 per cent of ball milling programs produce simple prismatic molds. This means there is enormous potential for savings.  

You say, the machining strategy you have developed can decrease machining time of up to 90 percent in finishing operations. How?

Our method essentially consists of two elements: the five-axis tangent plane machining strategy and the conical barrel cutter, a new tool that has also been developed by Open Mind. The idea behind the technology is to facilitate the perfect interplay between these two elements. The five-axis tangent plane machining strategy allows you to finish all components that feature hard-to-reach planes very efficiently and free of collisions. Additionally, the barrel cutter is generally superior to the ball mill when it comes to finishing connecting radii and planes.

What makes the barrel cutter superior?

Barrel cutters have a large radius. We have enlarged the radius even more for the conical barrel cutter. The special tool geometry ensures that the conical barrel cutter is very stable. A section of the circumference is mapped at the cutter, making it possible to achieve very large radii from 250 to 1500 millimeters at the tool. The large barrel radius smooths small axial deviations of the machine, preventing markings at the transition. The tangential tilt angle ensures constant tool engagement and constant machining forces. This in turn ensures a very high level of repetition accuracy. Moreover, thanks to its special geometry, the conical barrel cutter incorporates the advantages of ball mills and barrel cutters in a single tool. user can carry out tangential milling and machining of adjacent areas without needing to switch tools.

How does the five-axis tangent plane machining strategy complement these advantages?

The perfect interplay between tool and strategy allows you to fully exploit the potential of these tools. It’s possible to use five-axis tools with axial cutting depths from 3 to 8 mm instead of only 0.2 to 0.4 mm while achieving the same or better surface quality. The five-axis tangent plane machining strategy harnesses all of the advantages of the new tool and makes it possible to achieve the optimal tilt angle against the plane being machined. Intelligent automated functions ensure optimum tool orientation and fit. As a result, the user enormously reduces manufacturing times, achieves a high level of surface quality, increases tool life and uses fewer tools. 

Do you have an example?

Yes. Trials on a mold plate for a large injection mold featuring numerous planes showed a decrease in machining time from 16 to two hours. What is important in this process is to keep the cutting conditions constant and stable. The five-axis tangent plane machining strategy enables a high degree of precision that far exceeds what is possible when machining with a ball mill. The measured roughness values are often five to ten times lower than the measured roughness values achieved by machining with ball mills. 

Is there a steep learning curve to implement this machining strategy?

Creating the NC programs is very easy and convenient. All the user needs to do is select the faces to be machined, and the toolpaths are then generated automatically and fully checked for collisions. The CAM system implements hyperMill five-axis simultaneous strategies for 2D manufacturing tasks that appear to be simple, which means even the deepest pocket walls can be milled extremely efficiently. For planes that do not need to be machined simultaneously, the faces are distributed strategically in order to allocate some of them with fixed tilt angles. This is a completely new and extremely simple method of programming steep and flat planes.