Requirements for selection of materials for injection molding parts

The selection of mold materials is an important issue in mold design and manufacturing, which directly affects the manufacturing process of the mold, the service life of the mold, the molding quality of the plastic parts and the processing cost of the mold.
Because of this, domestic and foreign scholars have developed new mold steels with good performance, processing performance and heat treatment deformation, such as pre-hardened steel, new quenched and tempered steel and maraging steel.
In addition, in order to improve the lifetime of the mold, many research and engineering practices have been done on the surface strengthening treatment of the molded parts, and good results have been achieved.
So far, the above research and development work has been continuously carried out in depth, and the results achieved have been vigorously promoted.
The working conditions of plastic molds generally have to work at 150~200 °C, in addition to the pressure, there is also the effect of temperature.
According to the different conditions and processing methods of plastic molding dies, the basic performance requirements of steel for plastic molds are roughly summarized as follows:

(1) Sufficient surface hardness and wear resistance
The hardness of the plastic mold is usually below 50~60 HRC, and the heat-treated mold should have sufficient surface hardness to ensure the mold has sufficient rigidity.
In the work, the mold is subjected to large compressive stress and friction due to the filling and flowing of the plastic. The mold is required to maintain the accuracy of the shape and the stability of the dimensional accuracy to ensure the mold has a sufficient service life.
The wear resistance of the mold depends on the chemical composition of the steel and the heat treatment hardness, so increasing the hardness of the mold is beneficial to improve its wear resistance.
(2) Good machinability
Use steel grades that are easy to cut and that provide high-precision parts after machining.
For this reason, medium carbon steel and medium carbon alloy steel are most commonly used, which is especially important for large molds.
For parts requiring EDM, the burn hardened layer of the steel is also required to be thin.
(3) Excellent polishing performance
High-quality plastic products require a small roughness value on the surface of the cavity.
Therefore, the working surface of the injection molded part needs to be polished to the mirror surface, Ra≤0.05μm, the hardness of the steel is required to be 35~40 HRC, and the surface is too hard, which makes the polishing difficult.
For this reason, the selected steel requires less material impurities, the microstructure should be uniform and dense, and there is no fiber directionality. There should be no pitting or orange peel defects during polishing.
(4) Good thermal stability
The shape of the parts of the plastic injection mold is often complicated, and it is difficult to process after quenching. Therefore, materials with good thermal stability should be selected as much as possible, so that the linear expansion coefficient of the molded parts after heat treatment is small, the heat treatment deformation is small, and the dimensional change caused by the temperature difference
The rate is small, the metallographic structure and the mold size are stable, which can be reduced or no longer processed, thus ensuring the dimensional accuracy and surface roughness requirements of the mold.
(5) Good wear resistance and fatigue resistance
The injection molded cavity is not only washed by the high pressure plastic melt, but also by the temperature stress of the alternating heat and cold.

The general high carbon alloy steel can be heat treated to obtain high hardness, but the toughness is poor, and it is easy to form surface cracks, which is not suitable.
The selected steel grade should enable the injection mold to reduce the number of times of polishing and repair, and maintain the dimensional accuracy of the cavity for a long time, achieving the service life of mass production.
(6) Corrosion resistance
For some plastic types, such as polyvinyl chloride and flame retardant plastics, steel grades with corrosion resistance must be considered.
In addition, when selecting materials, it is necessary to consider preventing scratches and gluing. If there is relative motion between the two surfaces, try to avoid selecting materials with the same structure. In special cases, one side may be plated or nitride to make the two sides different surface structure.

About JasonMould Industrial Company Limited

Jasonmould is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.

Contact:
Person: James Yuan
Company: JasonMould Industrial Company Limited
Add:  LongGang Village,LongXi Town,BoLuo County,HuiZhou City,GuangDong Province, China
Tel: 86-752-6682869
Email: james@jasonmolding.com

Site: https://www.jasonmolding.com/company/

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