How to Select the Mold Steel?

The product will distort and maybe shatter as a result of residual stress left in it from the stretching and shearing of molecules during plastic molding. During production, Mold Steel will also develop residual stress, this residual stress will cause the mold to distort or possibly shatter.

Mold Steel must meet certain specifications to order to be used in injection molds, and the choice of steel will have a significant impact on the precision, processability, and life of the mold. The precise requirements rely on the design of the mold, its lifespan, the type of plastic used to make the molded object, its aesthetic needs, and its dimensional precision. The following criteria will be the general requirements.

1. Easy to buy.

2. Good machining performance.

3. Easy to polish, beautiful processing surface.

4. Good strength, toughness, and wear resistance.

5. No internal defects such as sand holes, material selection.

6. Easy heat treatment, small thermal deformation.

7. Good heat resistance, low thermal expansion coefficient.

8. Good welding performance.

9. Corrosion resistance to heat-sensitive plastics and steel.

When the dimensional accuracy is very high, heat-treated materials or hot-rolled die steels with small stress and deformation should be used. If the production batch of plastic parts is large, the steel used for mold-forming parts also requires quenching.

Mold Steel: Mirror Polishability

In the injection molding mold, the surface roughness of the mold cavity directly affects the roughness of the product surface. Mirror polishing is required for the core surface of the transparent cavity, and the polishability of the mold material becomes the main problem at this time. In terms of polishing performance, it must be done

• No sand hole

• No scratches

• Few non-metallic objects

• Uniform organization (no segregation)

• Fine grain size (no coarse structure)

• Appropriate hardness

• Appropriate mechanical properties

Mirror polishing of mold materials must also pay attention to the following polishing techniques:

• Abrasive paste (diamond paste) with high polishing ability must be used

• The polishing force must be uniform and reasonable. The uneven polishing force will produce an orange peel-like surface.

• Due to electrode processing and wire cutting processing, the abnormal metamorphic layer needs to be removed by sandpaper, and then ground with abrasive paste.

7uhn injection mold parts processing can be roughly divided into machining (such as turning, ordinary milling, CNC milling, grinding, pliers, engraving, polishing, etc), internal processing (such as electrode processing, wire cutting processing, etc), and casting processing, mold steel the selection must take into account both the service life of the mold and the processing of the mold. Considering the mold material from the cutting process, it is best to use a low-hardness material that can be processed by conventional machining during cutting, but this material is difficult to guarantee the life of the mold and the accuracy of the cavity.

Therefore, in actual work, pre-hardened (tempered and tempered) mold steel with good machinability is often used as the inner mold insert of the mold. For precision molds and long-life molds, the surface of the core cavity after pre-hardening mold steel can be nitrided to improve the wear resistance of the core.

Mold Steel: Corrosion Resistance

Plastics such as polyvinyl chloride (PVC), halogen-based compounds containing bromine and chlorine, flame-retardant resins of phosphorus compounds, ABS after adding sulfur, polyoxymethylene, low-foaming resins containing chlorine and carbon monoxide, etc. Exudes a corrosive effect on the mold, and the molded parts for molding these plastic products must be made of stainless steel.

Corrosion-resistant mold steels include 420, 2316, S136, and PAK90. In addition, beryllium copper is also a good corrosion-resistant material.

In addition, to make the mold anti-corrosion, chrome plating can also be used in the cavity, but hard chrome plating has the following disadvantages.

• Weak corrosion resistance to corrosive gases

• The electroplating layer will have pinholes or cracks unique to the electroplating layer, and the chrome plating layer is easy to peel off after a long time of injection molding.