What Hardness For Plastic Molds?


  How hard should the plastic mold be? Since the first […]

  How hard should the plastic mold be? Since the first attempt to exchange plastic mold design ideas, the storm in the teacup has been raging.


  For a molder, 48 Rockwell "C" is enough; for another 54 years old, and another one does not accept anything under 60 years old.


  In use, the mold hardness for economic operation is as low as 85 Rockwell "B" and as high as 65 Rockwell "C".


  The industry is consistent on one point-completely different in cavity hardness.


  So, if it is possible to successfully operate with a die-soft mold, why bother to harden it?


  To answer this question, heat treatment is performed to develop the most ideal properties in steel.


  Hardened surfaces will be polished better than soft surfaces, and are less likely to be damaged by scratches or normal wear.


  In places prone to friction and wear, such as thimble, compression cavity wells, sliders, etc., hardened steel must be used to minimize wear or tear.


  In addition to increasing surface hardness, heat treatment can often improve core properties.


  Mild steel has a tendency to crush or hob under molding pressure, especially during compression molding, heat treatment gives the mold the required strength.


  Obviously, the maximum hardness is not a necessary condition for a good cavity. Instead, the cavity should be heat treated to obtain maximum toughness and sufficient surface hardness.


  With this in mind, let us examine the various materials used to make the cavity.


  The first material that may be used is mild steel. This is carburized and hardened.


  Although the results are satisfactory in many cases, this material is not good compared to available modern tool steels.


  Carburizing gives a hard shell, 60 to 62 Rockwell "C", but the strength of the core is slightly higher than before heat treatment.


  In addition to hobbing steel, alloy carburized steel, such as SAE 4615, 3120 or 3312, has usually replaced ordinary carbon steel for carburizing.


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


  As before, the surface hardness is usually 60 Rockwell "C" or higher.


  Gear hobbing steel requires higher core strength, so steel such as SAE 3110 is used for this purpose.


  A steel company is developing an oil-hardened steel profile that is easy to hobbing, but is not ready to put it on the market.


  Oil and air hardened steel are usually used for the cavity.


  Depending on the heat treatment and the steel, these may vary from 48 Rockwell "C" to 65. Steel that has been hardened has its maximum hardness in the quenched state.


  However, since the maximum hardness is not the decisive factor, the steel is further heated or stretched to obtain the desired characteristics-maximum core strength.


  The necessary hardness reduction will depend entirely on the steel and can be anything.


  For example, a popular oil-hardened steel is hardened to 64-66 Rockwell "C" when quenched.


  By stretching at 425° F, maximum toughness can be produced with little loss of hardness.


  Additional tempering will make the steel tougher, but only at the expense of hardness.


  When processed in the normal way (e.g. SAE 6145), some oil-hardened steels will harden to a maximum of 48-50 Rockwell "C".


  However, steel can be cyanidized and quenched in oil.


  This gives a surface hardness of 58-60 Rockwell "C", while the core has the required strength.


  This processing also produces minimal distortion. The success rate of beryllium copper castings as cavity materials is getting higher and higher.


  They can be hardened to 38-42 Rockwell "C" and polished well.


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


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


  Choose a steel whose normal heat treatment will provide the characteristics required for the job.

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