How to Cut Setup Time, Extend Gate Life With the Right Machining Strategy

Microcut honing technology is a user-friendly, economical solution for optimizing the function of small bores (diameters from 0.015 millimeter to 8 millimeters), especially for hard and difficult-to-machine materials. This technology enables the user to (a) produce the shape (roundness and cylindricity) and diameter within the narrowest tolerance ranges, (b) improve damaged surface structures (e.g., caused by spark erosion or hardening) via “cold” microchip machining and (c) compress the bore surface (inherent compressive stress).

The technology offers two significant advantages over conventional methods — milling, wire EDM and jig grinding — to machine needle guides and valve gates in hot runner systems:

1. A cost-effective, easily controllable process

The investment for a microcut honing machine is relatively small compared to conventional processing machines for bore machining. There is also savings potential in its ease of use because minimal training is required to operate the machine.

There is no need for time-consuming machine setup or tool dressing.  Moreover, the machining process is robust and repeatable and the system requires minimal space (1 square meter) and energy consumption (2.5 volt-amps at 400 volts).

2. Quality

The critical tolerances for hot runner applications are diameter, roundness and surface roughness. For example, surface finish and roundness <0.001 millimeter must be achieved to yield diameter tolerances of 0.001 millimeter. The one-piece honing tools are pre-calibrated to the target diameter to reproduce these results without any problems.

Below are the tolerances microcut honing can achieve.  

Microcut Honing Tolerance
Diameter Range	0.25 (0.01")  – 8 mm (0.31") Bonded Grits
Diameter tolerance	±0.5 µm (0.00002")
Roundness	<0.2 µm (0.00001")
Cylindricity	< 1 µm (0.00004")
Parallelism	<0.4 µm (0.00002")
Surface finish	Rz: 0.5 µm (0.00002") (Ra: 0.05 µm) (0.000002") (N2)
Cycle time	Depends on the material, allowance, bore length
Part geometries	Rotationally and non-rotationally symmetrical
Materials	PCD, sapphire, ceramics, carbide, steel, powder metallurgical steel, various metal alloys, glass, various plastics

 

Benefits for Needle Valve Nozzles

An important application in moldmaking is needle valve nozzles (Figure 1). The goal is to achieve an ideal sealing function between the needle and the needle valve nozzle (gate) with the needle still able to move. To respect that is a minimal defined mating clearance required. With the processing technology of microcut honing the best possible shape (roundness, cylindricity), surface and an exact dimension of the bore can be achieved.

Figure 2 compares the roundness measurement of a jig ground or milled bore and a bore after microcut honing. Due to the geometrically precise bore with microcut honing, the diameter of the needle can be defined more accurately. This results in burr-free cast-on from the very first shot and an extended gate lifetime. Microcut honing can be used on new or existing molds to retrofit needle valve nozzles.

Benefits for Needle Guides

The advantages of a very cylindrical bore with an optimized surface are also evident in needle guides with reduced wear on the needle and bore that enables the hot runner systems to be used on the machine for longer, reducing downtime and mold maintenance. 

Unique innovative machining tools and the intelligent, force-sensitive control of the machining process offer an easily controllable and robust process with minimum dispersion. The actual machining is carried out with tools coated with diamond or CBN grit.

Microcut honing can precisely, efficiently and economically produce bores below 8 millimeters in diameter. The system, which can be configured in various sizes, offers advantages for bores with cross holes or clearances.