Three Principles for Selecting Plastic Molds

A. Working Requirements of Molds
1. Wear Resistance 
When the blank is deformed in the mold cavity, it flows and slides along the cavity surface, causing severe friction between the cavity surface and the blank, resulting in mold failures due to wear. So the wear resistance of material is one of the most basic and important properties of a mold. Hardness is the main factor affecting wear resistance. Under normal circumstances, the higher the hardness of mold parts is, the less the wear becomes and the better the wear resistance is. In addition, the wear resistance is also related to the type, quantity, shape, size and distribution of carbides in the material.

2. Obdurability
Most molds have to work under terrible conditions, and some often bear great impact loads which will result in brittle fracture. In order to prevent the sudden brittle fracture happening on mold parts, the mold must have higher strength and toughness. The toughness of the mold mainly depends on the carbon content, grain size and tissue state of the material. 

3. Fatigue Fracture Resistance
In the working process of the mold, it often causes fatigue fracture under the impact of the cyclic stress. The forms of fatigue fracture are: little energy repeated impact fatigue fracture, tensile fatigue fracture, contacted fatigue fracture and bending fatigue fracture. The fatigue fracture property of the mold mainly depends on its strength, toughness, hardness, and the content of inclusions in the material.
 
4. High-temperature Resistance
When the mold is working under high temperature, it will make the hardness and strength of the mold weakened, resulting in early wear or plastic deformation and final failure of the mold. Therefore, the mold material should have good tempering resistance to guarantee the high hardness and strength of the mold when it is working under the working temperature.
 
5. Hotness and Coldness Fatigue Resistance 
Some molds are kept working in the process of repeated heating and cooling, causing the cavity surface to crack and peal under the effects of tension and stress, the friction to increase, the plastic deformation to be blocked and the size of the accuracy to be reduced, finally leading to mold failure. Hot-cold fatigue is one of the main forms of hot working die failures. Thus, molds should have good resistance to hotness and coldness.

6. Corrosion Resistance
Some molds such as plastic molds, when they are working, due to the presence of chlorine, fluorine and other elements in the plastic, they will decompose HCI, HF and other strong corrosive gases after being heated, corroding the surface of mold cavity, increasing its surface roughness and accelerating the wear failure.

B. The Process Performance Requirements of Molds
Mold manufacturing generally has to go through several processes like forging, cutting, heat treatment, etc. In order to ensure the quality of the mold and reduce the production cost, the material should have the properties of good malleability, machinability, hardenability, hardenability and grindability. It should also have advantages of small oxidation, low decarburization sensitivity, tendency of quenching deformation as well as cracking.
 
 1. Malleability
Have a low deformation resistance to hot forging, a good plasticity property, a wide forging temperature range, a low tendency of forging cracking and precipitation of mesh carbide.
 
2. Machinability
 Have a large amount of cutting, a low tool consumption and low surface roughness.
 
3. Oxidation and Decarburization Sensitivity
Have good anti-oxidation performance, a slow process of decarburization, a low tendency of resulting in pits and is insensitive to heating medium when being heated under high temperature.

4. Hardenability
Have high and uniform surface hardness or get a deeper hardened layer after quenching.
 
5. Tendency of Quenching Deformation and Cracking
Have a small change of volume, warpage shape, slight distortion, and low tendency of abnormal deformation, low cracking sensitivity and no sensitivity of quenching temperature and the shapes of the workpieces during the process of conventional quenching. 
    
6. Grindability
Have less grinding wheel loss, low sensitivity to grinding wheel quality as well as cooling conditions and small possibility of grinding cracks.
 
C. Economic Requirements of Molds
In the selection of molds, we must consider the principle of economy and reduce manufacturing costs as much as possible. Therefore, under the premise of meeting the service performance, we first choose molds of lower prices, for example, using carbon steels instead of alloy steels and using domestic materials instead of imported materials. In addition, we should also consider the production and supply situation of the market when we select molds and try to choose as few kinds of steel as possible, which can also be easily purchased in the market.