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How to choose mold steel?

There are some basic principle in choosing mold steel.

A. Mold steel must meet its work condition.

1. abrasive resistance.

When plastic flows in mold cavity, there will be intensive friction between plastic and the cavity surface, and further more affect mold quality. So abrasive resistance can be said a basic and the most important quality of mold steel.

Hardness is the key element of abrasive resistance. In common sense, the higher HRC of the steel, the less friction there will be, and the better abrasive resistance. And carbon compounds variety, quantity, form, volume and distribution in steel will also affect abrasive resistance.

2. obdurability.

Molds are always put into bad working environment, and some should bear severe strike, and result in break up. In order to avoid the break up, molds must have good obdurability quality.

Mold obdurability is determined by carbon content, grain size and structure of steel..

3. fatigue fracture quality.

During working, molds under circulation force can be caused of fatigue crack, which include strike fatigue fracture, extension fatigue fracture, contact fatigue fracture, and bending fatigue fracture.

The fatigue fracture quality is mainly determined by strength, toughness, hardness of mould base and other mineral in mould steel.

4. High temperature durability.

When have mould works in high temperature, its hardness and strength will decrease, and so resulted in wear out or plastic distortion. As a result, mould steel should have the quality of high temperature durability to ensure mould can be put in high temperature environment with high hardness and strength.

5. Hot and cold fatigue resistance.

When have mould works in transfer of hot and cold, cavity surface will be crack or peel off, and so increase friction and block up plastic formation, and further affect products accuracy. Hot and cold fatigue is a key feature for hot mould to become ineffective. Hot mould should have good hot and cold fatigue resistance quality..

6. Corrosion resistance.

If some chlorine and fluorine maybe in the plastic material, when this kind of plastic is injected in mould with high temperature, corrosion gas like HCI, HF will erode cavity surface, and result in more friction..

B. Mould steel must meet with processing technology.

General mould processing procedures include forging, cutting, and heat treatment. In order to ensure mould quality and reduce cost, mould steel should have the qualities of forgeability, machinability, hardening, and easy-to-cut.

1. Forgeability.

Mould steel should have the qualities of low hot forge deformation resistance, easy to mold, wide range of forging temperature, no tendency of crack and carbon.

2. Annealing.

Wide range of spheroidizing temperature, low annealing hardness and fluctuate in a small range.

3. Easy-to-cut.

Cutting is used frequently in processing. Easy to cut steel can decrease cutting tool lose.

4. Oxygenation resistance and decarburization sensitivity.

When heated in high temperature, mould steel should be oxygenation resistant, decarburization sensitive.

5. Hardening.

High hardness after quenching.

6. hardenability.

Deeper hardening can be achieved after quenching, so that hardening can be carried out with common hardening material.

7. hardening deformation and cracking.

With common quenching, mould steel will be of small size, little deformation, no abnormal deformation, crack resistance and non sensitive to quench temperature and working condition.

8. Machinability.

When processed with grinding wheel, mould steel would not affect grinding wheel, and is not sensitive with grinding wheel quality, no worn out or crack.

C. Mould steel should meet with economic demands.

When choosing mould steel, economy must be considered to reduce cost as much as possible. Under the condition that mould steel can meet all work conditions, low cost steel should be the No. 1 choice.


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