The choice of mould steel is of great importance if the mould is to function effectively.
The prime function of any mould steel must be that of meeting the service requirements imposed upon it. Moulding requirements vary，e.g. from simple prototype work undertaken on soft aluminium construction moulds, to fully hardened alloy steel volume production moulds moulding to close dimensional tolerances.
In order to avoid costly long-term mistakes, the plastic mould application must be thoroughly investigated and fully understood before a selection decision can be made.
Once decided upon，the selected material should be written into the tooling specification for future procurement purposes.
The use of a tooling specification laying out the mould requirements prior to tool quotation or construction greatly reduces the risk of fundamental mistakes being made at the toolmaking stage.
Mould steel requirements
A mould steel should possess qualities or attributes relevant to the intended application. For general purpose moulding applications, the principal steel attributes are listed and explained below.
(a) High core strength. As a result of the service conditions encountered during injection moulding, i.e. relatively high compressive cyclic loadings, the material core strength is of relevance to the mould designer. The material must be able to withstand high compressive loads without cracking or splitting.
(b) Good wear resistance. Mould tools are subject to considerable wear from many sources, which include:
- (i) the polymer itself;
- (ii) the mould ejection system;
- (iii) the wearing action of shut-off faces;
- (iv) abuse during cycling and shut shots.
Wear resistance may be imparted to a mould tool steel by various means，usually by hardening the material or the addition of property modifying alloying elements. The choice of which method depends on the material in question.
(c) Excellent surface finish. A good serviceable surface finish is of the utmost importance, especially for core and cavity components. The intended material should be capable of sustaining a good long-term surface finish without the additional requirement of polishing between production runs. As with wear resistance，the material’s surface hardness and composition have the greatest influence on its finishing properties.
(d) Dimensional stability. The cyclic loading nature of the injection moulding process subjects the mould materials to considerable levels of stress and elastic deformation. The ideal mould material should possess sufficient strength and durability to resist permanent deformation but sufficient ductility to resist cracking and impact loadings. For this purpose, many grades of mould steel, especially steel alloys, have been developed to fulfil the above requirements.
The term alloy steel describes a steel material which contains other alloying elements in addition to carbon which have been added to deliberately modify the properties of the steel.
Mould tool steels contain quantities of specific elements which combine with the steel to improve the material’s intended application properties.
Typical alloying elements used in mould tool steels and their contributing effects upon the alloy produced are listed below.
|Element||Typical amount (%)||Main effect on properties|
|Aluminium A1||0.5-1.3||Aids nitriding|
|Chromium Cr||0.5-2.0||Increases hardenability and improves corrosion resistance|
|Manganese Ma||0.2-0.4||Acts with sulphur to reduce brittleness|
|Molybdenum Mo||0.1-0.5||Aids heat treatment and improves strength and toughness|
|Nickel Ni||0.3-6.5||Increases hardenability and improves strength and toughness|
|Silicon Si||0.2-2.0||Increases hardenability and limits oxygen in steel making|
|Tungsten W||1.0-18.0||Improves hardness at higher working temperatures|
The alloying elements and their addition percentages vary according to the different steel manufacturers and the intended applications. Choosing a steel type for a mould application is a difficult business, especially for the beginner.
Most steel producers manufacture and supply mould tool steels bearing their own trade names and designation code numbers.
Within the steel industry, various organisations and institutions exist which are responsible for the quality and national standard of steels. Some of the more prominent are:
American Iron and Steel Institute (AISI);
Swedish Standardisering Kommission (ISI);
various European standards institutes，e.g. DIN and BS 970.
Internationally the AISI designation has been adopted and most of the major steel manufacturers give equivalent designation code numbers.
An AISI code reads，for example, AISI D3，where D denotes the class of steel, and 3 denotes the grade of steel within the class. In this case, AISI D3 is a high carbon content, high chromium steel used for mould core or cavity components.
General purpose mould steels
As a result of the demanding requirements of mould steels，alloy steels are commonly used during the construction of mould tools and dies .
***Carburizing is a surface hardening process which involves changing the composition of the steel’s surface layers. The process increases the steel’s surface carbon content from less than 0.2% to about 0.7-0.8%. The resultant steel after quenching treatment has a harder surface layer (e.g. 60 Rc) and a softer, more ductile inner core (e.g. 20 Rc). The depth of the carburized ease layer is dependent on time，temperature and the carburizing medium employed.
Other surface hardening treatments exist which could also be employed，such as nitriding and carbon nitriding. For steels with a carbon content of over 0.4-0.7%, selective surface heat treatments are usually employed, e.g. flame hardening or induction hardening.***
The AISI class designation for mould steels is the letter “P”，for general purpose mould tool applications. The table above shows composition and heat treatment details for AISI P class steels.
The AISI P range of steel grades possesses relatively good machining properties due to the alloy compositions. Machining processes such as spark erosion and all the usual metal cutting processes can be employed during tool manufacture.
As a result of the Ni-Cr content of the steel，a good surface finish is achievable using various polishing and finishing techniques. The steels corrosion resistance is also improved compared with a plain carbon steel.
While the AISI P range of steels represents a reasonable general purpose steel choice for moulding applications, extremes of application requirement will always exist.
In such cases, a more specialised steel should be selected，the properties of which reflect the use application intended. The following examples have been chosen to demonstrate the service conditions in which a specialised steel type/grade may be selected.