Why You Need Hard Milling in Plastic Injection Mold Making

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 CNC 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.

The importance of the spindle cannot be overlooked because it is the link between the machine tool and the cutter. If the spindle is inadequate, the entire process will suffer greatly.

Do you need special CNC tooling?

Shrink fit tool holders and an HSK interface are mandatory for hard milling. Sure, you can use other methods, but this combination has proven to be very effective. These tool holders are the most accurate available today and they are very easy to use.

The HSK interface is simply the most accurate, secure and stiff type in use today. Failing to use the proper tool holders will shorten tool life dramatically because the chip load will fall on on flute and quickly cause premature wear.

Conclusion

The hard milling of plastic injection molds is becoming a standard method of mold making. There are so many benefits associated that shops who are slow to get onboard will soon be at a serious loss to compete.

With today's sophisticated CNC milling machines, coated carbide end mills, ceramic inserts, advanced software and reasonable prices, hard milling is within the reach of almost any competitive mold shop.

Randy Hough has worked as a plastic injection mold maker since 1978. He heads a group of industry professionals who have a web site dedicated to injection mold making. You can learn the latest trends and techniques by visiting http://www.global-plastic-injection-molding.com

They also have an interactive blog. Learn how to make a plastic injection mold, mold design, about jobs in the plastic industry and more.

"You know Dad, I've been thinkin', one is a lot more than zero!" my son, Thomas, age 5.

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