Tags:Plastic injection molding  Mold Making  Custom Mold

One of the best ways to save time and money in injection mold making is by using hard milling techniques. In fact, if you are not already hard milling, you will soon find yourself at a distinct competitive disadvantage.

Why use hard milling?

To save time

Reduce set ups

Eliminate a great deal of EDM work

Eliminate hand fitting, especially of contoured shut-off surfaces

Produce a surface that is much more true to the CAD model

Eliminate a great deal of stoning and polishing

Move work through the shop more quickly

Eliminate many grinding steps

Shorten delivery dates

Those are some pretty compelling reasons to use hard milling! After all, who doesn't want to achieve any or all of the above benefits?

Do you need specialized CNC milling machines?

Yes, and no. No, you don't have to go out and buy a half million dollar vertical milling machine, in fact, you can do a lot on a basic Bridgeport type of machine. But, if you are serious about hard milling, you should definitely invest in a machine that is designed for this purpose.

You need a CNC milling machine that is rigid enough to withstand the forces generated by the high spindle speeds, rapid feed rates and the vibrations caused during machining. Nowadays there are many high quality machines that are specifically designed for high speed milling and hard milling.

A  milling machine used for hard milling should have

A column and base that is heavy

Box ways with linear roller bearings

A spindle that is core cooled

Ball screws that are dual supported

Thermal stability and structural rigidity

Do you need specialized software?

Again, yes and no. No, you don't need CAD/CAM software that is engineered specifically for hard milling. But your quality will suffer as a consequence.

This is because the requirements for hard milling are different than conventional milling of soft steel. The typical software will cause the cutter to move in a jerky manner, which will shorten tool life dramatically and fail to achieve the desired accuracy and surface finish.

Some other tool path requirements are

The need to control how the cutter enters and exits the cut

The need to maintain a steady, constant chip load

Ensure that the shock conditions for each roughing and finishing pass are maintained

The idea is to produce a surface that is true to the model, dimensionally accurate, has a good surface finish and do it quickly! Sounds like a lot to ask, but it is done everyday by progressive mold making shops around the world. Having the right software is essential.

What about the spindle?

The cutting tool and the tool holder act as one unit. The spindle must be able to protect the integrity of this unit. Therefore, it must be designed for the high speeds that are necessary. Direct drive spindles are called for in hard milling applications. Gear and belt driven spindles are not advised. The control of heat and vibration is also extremely important.