MOULD MATERIAL SELECTION FOR INJECTION MOULD:
It is well known that without proper mould material, proper tooling cannot be achieved. Wide ranges of mould materials are used for fabrication of moulds & dies for plastics.
Mould material selection should be carried out based on the requirements from Product / Mould designer, mould maker & molder.
Steel Requirements:
1. Product: Surface Finish, Photo Etching, Effect of Moulding Material, Production quantity.
2. Mould: Economy, Standard Sizes.
3. Mould maker: Free From Defects, Easy to machine & Polish, Stable in Heat treatment, Suitable for EDM / etching.
4. Moulder: Wear Resistance, Compressive Strength, Corrosion resistance, Thermal conductivity.
Product Design Requirements: A part from functional performance of the component, the moulding is required to meet the high standards of surface finish as well as dimensional tolerance over long production run. These requirements will meet only if we have good product design, good mould design, good mould making and proper selection of tool steels for the mould.
Mould Design Requirements: The mould designer can economize the tooling of the mould by selecting standard steel grades, standard steel sizes and standard machined plated. He along with the mould maker shares the responsibility of producing a mould, which will give reliable and economical production of the part visualized by the product designer. He has to see that the mould maker fabricates the mold as easily and economically as possible. This largely depends on specifying best mould steel and optimum hardness required for different mould parts. Selection of standard parts such as guide pillars, guide bushes, ejector pins etc., & standard mould bases along with other machined plates considerably reduces time and cost as these parts are now a days available at far more competitive price. This in turn helps in better planning, prompt mould delivery as well as in minimizing initial machining cost and material losses.
Mould Making Requirements: A substantial part of the total mould cost is that incurred in machining of the mould. Hence, one must see the mould making process is as straight forward as possible. A mould maker will see that the steel he is going to use has following properties.
Materials are free from any manufacturing defects
1. Good machinability
2. Good polishability
3. Good hardenability
4. Good dimensional stability
Moulder Requirements: A processor expects certain features in the mould such as:
1. Uniform and high rate of production
2. Uniform moulding quality
3. Longer mould life
4. Low mould maintenance cost
5. Lowest possible production cost
6. Easy components & material replacement
It can be summarized that the selection of tool steels for mould not only depends on the product/Mould design but a considerable thinking has to be done from mould maker and processor point of view. If all these factors are given through weightage, then only a mould can be manufactured and processed economically and effectively.
Essential Properties of Mould Materials:-
1. Excellent Machinability: The economic importance of machinability is very great indeed. Roughly 30 % of the total cost of a mould is accounted for by the machining costs.
2. High Compressive Strength Combined with Sufficient Toughness: In injection mould, it is estimated that a locking force of 1 ton per 3 - 4 cm2 (0.465-0.620sq.in.) of projected surface is required. When large items are being injection moulded the locking force, therefore, will be very great indeed. There is always a risk of indentation of the parting lines, but the risk decreases with increasing compressive strength on the part of the tool steel use.
High compressive strength is also desirable in view of the fact that the tools are liable to mechanical damage in course of transportation and installation. In some cases, the tools are nitrided to prevent the cavity from becoming scored or otherwise spoiled.
The compressive strength of the steels can be improved through nitriding.
3. Capacity for heat treatment without problems: A small change in hardening is unavoidable. It is nevertheless possible to limit the changes (warpage) through slow and even heating to the hardening temperature and by choosing a low hardening temperature and a suitable quenching medium.
The most satisfactory solution to the problem is to use hardened and tempered steels where no further heat treatment is necessary.
4. VG Good resistance to heat and wear: In order to improve the strength of the plastic product, an addition of glass fibre, asbestos, wood fibre, etc., is frequently made. These substances have an abrasive effect on parting lines, runner gates and inlet nozzles.
For such products, it can therefore be advisable to choose particularly wear-resistant tool steel.
5. High thermal conductivity: Every plastic manufacturer is anxious to maintain as high a production pace as possible. The limitation lies in the ability of the mould to conduct heat away from the plastic item. In this context, the thermal conductivity of the material is obviously important, but so is heat transfer between tool and coolant and heat transport through the coolant.
6. Ability to resist corrosion: Practically all moulds used for injection moulding come into contact with cooling water. Some plastics are also corrosive to the mould material. Certain plastics generate corrosive products such as hydrochloric acid from PVC plastics, acetic acids from acetate plastics and water from amino plastics. PVC plastics are well known for their tendency to produce hydrochloric acid when heated to high temperatures.
This type of corrosion attacks the surface of the mould. If the mould is then polished to high gloss, the corrosive attack necessitates expensive repolishing.
7. Good polishability: Polishing is a time-consuming and costly process. The result of polishing will depend in the first instance on the polishing technique used.
The polishability of steel depends on the homogeneity of the steel and on the type, distribution and size of slag inclusions. Hard, large slag particles are particularly troublesome. Polishability is also highly dependent on the hardness level and heat treatment of the material.
To receive the highest purity, the steel should be vacuum degassed or electro-slag refining (ESR).
Factors Governing The Choice of Mould Materials:
1. Length of Production Run
2. Injection Pressure Required
3. Type of Moulding Material
4. Dimensional Accuracy & Intricacy
5. Method of Manufacture
COST CONSIDERATIONS:
STEEL COST THE TIP OF THE ICEBERG
One of the major decision that can favorable influence overall mould performance and long term maintenance costs is to specify the best possible mould steel for the job. The cost of the tool steel in a mould usually represent only 10 - 15 % of the total cost, and offer savings made on buying cheaper mould steels turn out to be false encouragement.
Through Hardened Steels Applications
1. Moulds with long production runs
2. To resist abrasion from certain moulding materials
3. To counter high closing/injection pressures
Advantages
1. These steels are available in soft annealed & stress relieved condition.
2. Also available in hardened and tempered to the required harness (about 48-60 HRC)
3. These are used for core and cavity inserts
4. Better wear resistance, resistance to deformation and indentation & better polishability.
5. Improved wear resistance is important when filled or reinforced plastic materials are to be used.
6. Resistance to deformation & indention in the cavity, gate areas and parting lines helps in maintaining proper part quality.
Corrosion Resistant Mould Steels
Application
When a mould is likely to be exposed to a corrosive atmosphere, stainless steel is strongly recommended. The higher initial cost of the steel will be less than the cost involved in repolishing or replating of mould parts affected by corrosion.
Pre Hardened Steels
Applications
1. Large Moulds
2. Moulds with lower wear resistance requirement
3. High strength holder plates
4. Moulds with moderate production run
Advantages
1. These steels are available in the hardened and tempered condition having hardness about 25-30 HRC.
2. No heat treatment is necessary before the mould is put into use.
3. Flame hardening or nitriding can increase the surface hardness.
MATERIALS AND TREATMENTS FOR MOULD PARTS AT A GLANCE”
Service requirement, fabrication requirement & economical requirement of various products are different from one another. These input requirements are to be clearly understood before the selection of right material.
There are some common properties required in all parts such as good strength, less dimensional changes during Heat Resistance treatments, good machinability, good polishability etc.,
INJECTION MOULD:
The core and cavity inserts are the most important parts, which has to withstand various stresses continuously. There are various materials used for core and cavity inserts and the proper selection is a somewhat difficult task, since it decides the performance and life of the mould, along with other factors.
Steels Used For Different Mould Parts:
1. Constructional Steel- Used for mould bases & structural
2. Components such as blocks, spacers, ejector pins, support
3. Pillars, Backing plates & locating rings. Etc.,
Material for Guide Pillar, Bushes, Ejector Pins:-
EN-18, EN-19, EN-24, EN-32
PRE HARDENED STEELS:
AISI: P-20
PX-4 (JAPAN)
IMPAX SUPREME (ASSAB UDDEHOLEM)
BUDERUS 2311 ISO-BM (BUDERUS )
MOULD MATERIALS EQUIVALENTS:
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