What hardness for plastic molds?

How hard should a plastic mold be? A tempest in a teacup has been raging since the first attempt to swap ideas on plastic mold design.

To one molder, 48 Rockwell “C” is sufficient; to another, 54, and yet another will accept nothing under 60.

There are in service, operating economically, molds with a hardness as low as 85 Rockwell “B” and as high as 65 Rockwell “C”.

The industry is unanimous on the one point—to completely disagree on cavity hardness.

If it is possible, then, to operate successfully with dead soft molds, why bother with hardening at all?

To answer this question, heat treating is done to develop the maximum desirable properties in the steel.

The hardened surface will take a much better polish than a soft surface and is much less susceptible to damage by scratching or normal wear.

In places where frictional wear is apt to occur, as in ejector pins, wells of compression cavities, slides, etc., it is imperative that hardened steel be used to minimize galling or tearing.

Besides increasing the surface hardness, heat treating usually improves the core properties.

Soft steel has a tendency to crush or hob under the pressure of molding, especially in compression molding, and heat treatment gives the mold the required strength.

Obviously maximum hardness is not a requisite for good cavities. Rather a cavity should be heat treated for maximum toughness and the surface hardness will be adequate.

In the light of this, let us examine the various materials used in the manufacture of cavities.

Probably the first material used was mild steel. This was carburized and hardened.

While the results were satisfactory in many cases, the material was not good when compared to the modern tool steels available.

The carburizing gave a hard case, 60 to 62 Rockwell “C”, but the core had little more strength than before the heat treatment.

Alloy carburizing steels, such as SAE 4615, 3120 or 3312, have generally replaced the plain carbon steel for carburizing, except in the case of hobbing steels.

With proper heat treatment the tensile strength of the core may run as high as 130,000 pounds per square inch in some of these alloy steels.

The surface hardness, as before, is usually 60 Rockwell “C” or over.

The need for greater core strength in hobbing steels has resulted in the use of such steels as SAE 3110 for this purpose.

One steel company is developing an oil-hardening type steel that can be readily hobbed, but this is not yet ready for the market.

Oil and air hardening steels are in general use for cavities.

These may vary from 48 Rockwell “C” to 65, depending on the heat treatment and steel. Steels that harden all the way through have their maximum hardness in the as quenched condition.

However, since the maximum hardness is not the determining factor, the steel is further heated or drawn to give the desired property— maximum core strength.

The amount of reduction in hardness necessary will depend entirely on the steel and may be anything.

For example, one popular oil hardening steel hardens to 64-66 Rockwell “C” as quenched.

The chart below shows how the hardness falls as the tempering temperature rises:

[one_half] Tempering
Temperature
As-quenched
250° F
300° F
350° F
400° F
450° F
500° F
600° F
700° F[/one_half]

[one_half_last]Rockwell “C”
Hardness
64-66
63-65
63-65
62-64
61-63
59-62
58-61
55-57
51-53 [/one_half_last]

 

By drawing at 425° F a maximum toughness is produced with little loss of hardness.

Additional tempering will toughen the steel but only at the expense of hardness.

Some oil hardening steels will harden to a maximum of 48-50 Rockwell “C” when treated in a normal fashion (SAE 6145 for example).

However, the steel can be cyanided and quenched in oil.

This gives a surface hardness of 58-60 Rockwell “C” while the core has the desired strength.

This treatment also produces minimum distortion. Beryllium copper castings have been meeting increased success as a cavity material.

They can be hardened to 38-42 Rockwell “C” and take a good polish.

In conclusion, the analysis of a steel and the heat treatment used will determine the proper hardness for a given steel.

While it is desirable to have a hard cavity, this property should not be obtained at the expense of toughness.

Select a steel whose normal heat treatment will give the desired characteristics for the job.