COLD HOBBING – THE PROCESS
Cold hobbing is a cold forming method, whereby a hardened (master) hob with a very high surface quality (also called a coining die) indents a workpiece (die / mould insert) under constant pressure and at low speed to a predetermined depth. The process is performed below the recrystallisation temperature.
The hob has an outer mould which is mapped as a corresponding hollow mould in the workpiece. Cold hobbing is performed using a special hydraulic press with a lowering speed that can be regulated with the greatest precision, this makes full use of the steel’s natural flow properties.
No reworking or polishing required
With cold hobbing, the inner mould, in particular of workpieces, is made in such a way that no reworking (polishing) is required. Cold hobbing has the following advantages over other methods such as metal-cutting, electrical discharge and electrolytic processing of workpieces:
- Cost effective and economical by reducing the process times, particulary when producing several dies
- High dimension accuracy and geometrical precision of the workpieces
- High surface quality of the inner mould, no subsequent polishing work required
- Maintains the grains structure of the material, which extends the durability of the workpieces
- A multitude of workpieces can be pressed with one hob
The method is ideally suited for repeat high volume runs
Because of its advantages, cold hobbing is used with great success for the production of tools / repeat runs of tools in various industries, such as the plastics industry, both in multiple-cavity moulds for injection machines, but also with specific individual moulds.
There are two types of cold hobbing processes: open-type and closed-type.
Closed-type
When indenting using the closed-type process, the workpiece is set as a rigid body, i.e. clamped in a sufficiently large retaining ring. As the hob presses into the workpiece, the retaining ring directs the flow of material (grain structure) against the directional flow of the hob. Due to the obstruction of this flow and the higher frictional force between the workpiece and the hob, a greater downward compression is required compared to the open-type method with the same press stroke. Whilst the workpiece is being indented, a lower state of stress is generated, which permits a greater hobbing depth than with the open-type process. Thereby not only increasing the accuracy of the hobbed inner contour but also the strain hardening.
Open-type
The exterior wall of the workpiece indented by open-type cold hobbing bulges slightly because the workpiece is only marginally compressed. The open-type process is successfully used for flat engravings.
The hobbing ability depends on the chemical composition
The extent of hobbing of the tool steel depends on the chemical composition of the steel, the annealing treatment and the microstructure attained. Special steels have been developed for plastic injection moulds. They have a good cold hobbing ability, high wear resistance, good polishability and also corrosion resistance.
Cold hobbing is an economical process especially for the production of hollow moulds in tools. The outer mould of a hob can be more easily and accurately machined than the corresponding inner mould. Cold hobbing is particularly economical for producing a high number of tools with the same accuracy.
WHERE QUALITY SETS STANDARDS
We have been producing dies with different mouldings for over 30 years, such as the PCO 1810 and PCO 1881 for the beverage industry, as well as dies for general caps. Thanks to the cold hobbing process, we can manufacture these dies for you in next to no time with a high surface quality and without any reworking. All our dies boast precise dimensions, exact geometry and an extended service life.
