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Emilian Popa Injection Molding Design Guide Manual 2012

Mold Material & Heat Treatment

2- Mold Material

Limiting our scope to a mold for plastic injection molding, steel is the most popular material.

Particularly JIS S50C and S55C are mostly applied because these are standard materials of the mold bases inthe market.

It is important to select right material to satisfy purpose of the mold and its application on the part of end users

If necessary, heat-treating or surface finish must be carried out to satisfy requirements.

Here we will take up materials to be used for main parts of the mold, cavity and core.

2-1 Mold Material

2-1-1 Basics

Normally users specification specifies if the material is of heat-treated (quenched) or not heat-treated (raw) fo

cavity and core material. Note that pre-hardened steel, which is heattreated when supplied but will not be heattreated after machining, is classified as raw steel.

Table 2.1.1.1 shows various steel for plastic mold with bland names. Molds made of asrolled steel and pre-

hardened steel belong to raw type. Pre-hardened steel is heat-treated having 30~40 HRC hardness and yet

having a good machineability. Molds made of prehardened steel are used without heat treatment. Thus the molprocessing is the same as that of as-rolled steel. Cost of a mold is also similar in both cases.

On the other hand, there are two types in quenched type. One is to harden and temper the mold after machininand to finish the mold just by simple polishing. Another is to finish a heat-treated mold with a certain

deformation clearance by a grinder or EDM (Electric discharge machine). The former is used for a mold, whic

does not require high precision but only erosion resistance. Thus the cost is on the same level as raw type. Butthe latter involves time consuming finishing on the hardened steel surface. Thus the cost is much higher than

raw type. Qualified material for mold should satisfy following points

Good machineability.

High abrasion resistance. High corrosion resistance. High toughness. High strength. Homogeneous property without segregation and pin holes. Good heat-treating with less deformation. Good heat conductivity. Reasonable price.

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Emilian Popa Injection Molding Design Guide Manual 2012 Easy procurement.

No single material satisfies all items above. Particularly the extent of abrasion resistancto determine mold life is deeply related to machineability that affects cost of a mold (Fi2-1-1.1).

Major factors for determining mold material in the users specification are number of

injection shot, application and molding material.

2-1-2 Number of shots and mold material

Total number of shots is a product of monthly production volume and mold life.Total number of shots = monthly production volumemold life (month)

Naturally, a mold having less number of shots will be made of cost conscious materialbecause depreciation cost per shot needs to be lowered as much as possible, while amold having high number of shots will be made of life conscious material. Note that inthis case cost means machining cost rather than material cost.

Characteristics and application of popular steel materials for mold for plastic molding isshown below in the order of low durability (Table 2.1.1.1).

2-1-2-1 SC steel

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Emilian Popa Injection Molding Design Guide Manual 2012S50C and S55C are used for material of mold bases available in the market. They arewidely used for cavity and core material of which total shots are less than 100,000.Particularly they are applicable for large molds.

Table 2.1.1.1 shows SC steel under both asrolled steel and pre-hardened steel. It isrecommended to use pre-hardened one for cavity and core due to better abrasionresistance.

2-1-2-2 SCM steel

Generally machineability of SCM steel is not so good comparing with SC steel.Prehardened steel adjusted for better machineability with 28~33HRC hardness is oftenused for cavity and core material, mold base material, mold plates and holders thatrequire hardness to certain extent.

2-1-2-3 AISI-P21 steel

This is a kind of pre-hardened steel, precipitation hardened with 40 HRC hardness,originated to AISI-P21 of US specification. This should stand for 500,000 shots for usualresins.

There are two types. One is a material with improved machinebility, close to S50C andS55C, by adding lead (Pb) and sulfur (S). Another is a material with improvedmachinebility for electric discharge machining, texturing ability and polishing. Apply oneway or another depending upon mold characteristics.

So far steel materials that are not quenched after machining have been introduced.These materials have benefits of easier machining, costs and delivery comparing withquenched type. Currently 40HRC hardness of pre-hardened steel is the hardest, but it isexpected to be 50 HRC hardness in view of recent development in high speed and highprecision machining capability for hard metals.

2-1-2-4 SKD-61

SKD-61 steel is normally used for die-cast mold as tool steel for hot processing. But italso is applicable for plastic mold for relatively large production volume.

Table 2.1.1.1 shows this material under pre-hardened steel with 40 HRC hardness. But

normally raw steel is machined and quenched to 50 HRC hardness after machining. A lifof quenched mold can stand for at least one million shots for usual resins. If conditionsare met, 2~3 times longer life can be expected.

SKD-61 can be nitride to the extent of 0.05mm in depth with 900HV hardness or more. means that nitride layer still exists after finishing as much as 0.01~0.02mm. Thereforenitride SKD-61 is quite effective for a mold that is subject to galling or seizing.

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Emilian Popa Injection Molding Design Guide Manual 20122-1-2-5 SKD-11

SKD-11 steel is normally used for press mold as tool steel for cold processing. But it isalso applicable for mold for plastic with reinforced fiberglass or for mass production.

SKD-11 has high resistance to abrasion. When it is quenched at 58~60 HRC hardness,

SKD-11 can stand for around 5 million shots without special coating on the surface.Weakness may be poor machineability and toughness. Steel suppliers are developingimproved SKD- 11 to cover such weakness.

As Table 2.1.1.1 shows, grains are laid out in dense and homogeneity. Thus powderforging is made available. SKD-11 is applicable for molds that requires mirror polishingand abrasion resistance.

Mold Design (Advance) Page 56

2-1-2-6 Powder metal

This is applicable for a mold for super mass production, super engineering plastics withreinforced fibers, IC, etc.. Similarly to SKD-11, this is made from powder metallurgyprocess. Powder metal is superior to high-speed tool steel (SKH-51) in terms of hardnesand toughness, but the cost is much higher. Therefore this is often used partially in theform of inserts wherever high abrasion resistance is required.

So far we have discussed about typical mold materials in relation with number of shotsrequired for a mold. It is advised to analyze available materials, applications, number ofshots, etc. for mold design. a sample of which is shown in Table 2.1.2.6.1

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2-1-3 Plastic materials and mold materials

Depending upon plastic material that may include, reinforced fibers or additives,requirements for abrasion or corrosion resistance vary on the part of cavity and core.

2-1-3-1 Reinforced plastic

Reinforced plastic with filled material such as fiberglass causes high abrasion on themold. The extent of abrasion is higher if the amount of filled material is greater and thematerial is harder. For example when glass fiber content is more than 30%, the mold lifwill become 10~ 20% of the life otherwise.

Mold materials for anti-abrasion were discussed in the previous section. Be minded thathard steel material may cause chipping due to inferior toughness. It may be necessaryto lower the hardness and compensate it by surface treatment such as PVD.

2-1-3-2 Flame retardant plastic

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Emilian Popa Injection Molding Design Guide Manual 2012Flame retardant plastic that includes halogen (bromine) or fluororesin producescorrosive gas under heat and pressure in the molding. This will shorten the life of amold. In the case of PVC, chlorine gas is generated. Thus you need to select moldmaterial with high corrosion resistance.

2-1-3-2-1 13Cr stainless steel

This is a stainless steel material to include 13% of chromium. This may be called 13Crsteel or SUS420 in JIS. 13Cr SUS is not quite high in corrosion resistance, but beingprehardened steel of Martensite structure it can be used as it is due to its reasonablehardness 33HRC or can be quenched to 50HRC if needed.

Thus 13Cr SUS can be used for a mold to be mirror polished or to be used for fire retardresin or fluororesin.

2-1-3-2-2 SUS 630

This is a precipitation-hardened stainless steel having high corrosion resistance. This issupplied as prehardened steel with 35HRC hardness. SUS 630 stainless steel isapplicable for a mold for highly corrosive resin such as PVC.

2-1-3-2-2 SUS 630

This is a precipitation-hardened stainless steel having high corrosion resistance. This issupplied as prehardened steel with 35HRC hardness. SUS 630 stainless steel isapplicable for a mold for highly corrosive resin such as PVC.

2-1-3-2-3 Transparent resin

Cavity and core need to be mirror polished when transparent resins such as GPPS, AS,PITAC,PMMA, PC, etc. are molded. Particularly for photoproducts such as optical discs orlenses, high grade of transparency is required.

Although JIS provides no specific standard for mold steel for plastic injection molding,special steel suppliers made such standard available for our application. Referring tosuch references select appropriate steel material to be mirror polished for transparentresin molding. Improved materials are often processed by special smelting processingsuch as vacuum process which brings about homogeneous and dense grain structure

with minimum segregation and pin holes so as to assure a mold to satisfy with precisetranscriptioncapability. 13Cr stainless steel for such purpose is normally made by vacuum process tosuit precision mirror polishing.

2-1-3-2-4 Thin products

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Emilian Popa Injection Molding Design Guide Manual 2012Steel material for thin core or fine core is required to be with high rigidity and hightoughness, particularly when injection is made from one side only. For such application,Maraging steel to include 18% Ni is recommended.

Maraging steel is supplied as solution treated condition and is to be hardened to 53HRCthrough age hardening. This material is often used for thin wall core, mirror polishedcore, and ejector pin with thin wall or small diameter.

2-1-4 Other mold materials

As explained so far, steel material is most balanced in properties as mold material. Thuit is widely used. Other materials than steel are being introduced for particularapplications.

2-1-4-1 Aluminum alloy

Mold for extremely small production volume is not necessary is of steel material. Moldmade of aluminum alloy can stand for 20,000~30,000 shots. You may extend the lifeeven more by hardening the mold surface with alumite processing. But be aware thataluminum alloy is always subject to damage on its surface because of soft material bynature.

Benefits gained from this material must be low cost, short delivery and improved cycletime due to high thermal conductivity.

2-1-4-2 Copper Alloys

Beryllium copper (BeCu) is a typical copper alloy used for copper alloy mold for plasticmolding. This material can be improved in abrasion resistance through age hardening.

Advantage of copper alloy is its high thermal conductivity, while disadvantage must beits high cost. Therefore copper alloy is used for inserts to remove heat from hot spots.

Application to a whole cavity is limited to pressure casting and precision casting, whichwill be explained afterward (Fig. 2-1-4-2.1).

In processing BeCu by EDM, be equipped with partial ventilation facility due togeneration of toxic gas. As polar consumption is high, processing BeCu by EDM is bettebe avoided.

BeCu has limitation for corrosion resistance, but it can be improved by electro lessnickelplating on the surface as much as 0.01mm. In this way it will be also improved inabrasion resistance.

2-1-4-3 Tungsten carbide alloy

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Tungsten carbide alloy consists of tungsten carbide (WC), cobalt (Co) and nickel (Ni).Tungsten carbide alloy with more of cobalt, which has high transverse strength, is usedfor mold applied to disc mold (CD), mold for highly reinforced resin, IC mold, etc..

Strength of carbide alloy mold is its high abrasion resistance, while weakness is its highcost. Thus this material should be used just as inserts to the more extent than BeCu. In

order to cover its small transverse strength, which is a half of steel, it is recommendedto apply shrink fitting wherever applicable. Also be aware that its thermal expansioncoefficient is different from that of steel.

Therefore pay attention to fitting accuracy when it is used as an insert for hightemperature application.

2-2 Heat Treatment and Coating

Physical properties of steel such as tensile strength, hardness, elongation, etc. vary inaccordance with amount of carbon contents. To a greater extent, heat treatment willinfluence to physical properties. We can say that good steel characteristics can berealized depending upon how the steel is heat-treated. In these days, not only heattreatment but also surface hardening process such as PVD is applied on mold to satisfyexpected longer life of molds or requirements from engineering plastic molding.

Importance of heat treatment is sometimes overlooked because we normallysubcontract heat treatment to outside vendors without being involved. But it isimportant to understand basics of heat treatment and surface coating to be able tospecify appropriate processing to satisfy objectives of the mold.

2-2-1 Heat treatment

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Let us review basics of heat treatment.

2-2-1-1 Basics of heat treatment

Steel changes in atomic sequence due to allotropic transformation and structure due tosolid solution and separation of carbide in steel under thermal influence. Heat treatmen

is to utilize such changes in characteristics of steel material.

2-2-1-1-1 Allotropic transformation

Pure iron has a form of iron of body-centered cubic structure up to 911C, and transforms to iron of face-centered cubic structure from 911C to 1392C. When iro transforms to iron, the volume shrinks. The reverse transformation causes expansion (Fig. 2-2-1-1-1.1). In the case of steel, transformation temperature and structure varysubstantially depending upon carbon contents. This relationship is given in a graphknown as Iron-carbon equilibrium chart. You may refer it to textbook or handbooksupplied by steel manufacturer. A sample is shown in Fig. 2-2-1-1-1.2.

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2-2-1-2 Heat-treating Method

Important point for heat treatment lies in how to heat and how to cool. When heating,temperature is the important factor, while in cooling the cooling speed is the importantfactor.

Heating

Heating rate: Heating should be done slowly except for surface quenching. The rate of30 minuets per one inch for rising ambient to designated temperature is well acceptedstandard. Simultaneous temperature rising from surface to the center is ideal.

Heating temperature: Tempering and annealing are carried out at lower than A1,transformation temperature (727). Complete annealing and quenching are carried outat A3 transformation temperature (or A1) + 50. In the case of alloy tool steel, oftenused for mold, the temperature is 800 ~ 880 for SKS steel and 950 ~ 1050 for SKDsteel taking account of influence of alloy elements. Temperature is determined referringto technical data from steel suppliers and JIS as well.

Cooling

Cooling rate: Basic is to anneal slowly and quench fast. But low carbon steel requiresfast annealing and certain steel can be quenched under slow cooling rate. Particularlyinfluence of cooling rate varies substantially for alloy tool steel. Thus you should refer

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Emilian Popa Injection Molding Design Guide Manual 2012specific transformation curve (TTT curve or S curve) given in hand book or catalogsupplied by steel suppliers for appropriate cooling rate.

Cooling range: Referring to Fig. 2-2-1-2.1, steel with poor quenching characteristic showsnose of S curve in a short time, while steel with good quenching characteristic shows thnose in the late stage. In quenching, cooling rate should be controlled in a way thattemperature up to Ms point (Martensite point) should stay out of the nose in question. I

short, cooling rate should be controlled to cool fast from heated point to Mspoint and to cool slowly after Ms point to assure homogeneous Martensite.

2-2-1-3 Quenching and tempering

The purpose of annealing and normalizing is to soften the steel, to relieve internal strainor to improve internal structure. While, in the case of mold, quenching and temperingare conducted to improve hardness, strength and abrasion resistance.

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Quenching is a process in which heated steel in Austenite temperature is changed toMartensite grain structure by being cooled quickly. As explained, once steel is heated toquenching temperature, iron and carbide changes to iron solid solution (austenite) with shrinkage, and in cooling process, solid solution changes to solid solution with

expansion.

High carbon steel or high alloy steel, which is often used for mold, tends to leaveaustenite structure in martensite structure. Important point to assure dimensionalstability of mold is how to minimize retained austenite content through appropriateheat-treating.

Tempering is done immediately after quenching at lower temperature than A1,transformation point (727C). In the tempering, low tempering is done at 150C ~ 200Cand high tempering is done at 400 ~ 650C.

A few important considerations in quenching and tempering mold will be explainedbelow.

1. To regard quenching and tempering as one process

Tempering must be done immediately after quenching. You should never skip temperinnor temper after elapsed time. Even if quenching temperature is happened to be bit lowfor a material which is not hardened by tempering, you should carry out tempering withlow temperature around 100C. In this way toughness will be improved without losinghardness. Sometimes tempering is conducted at low temperature at 180C intending toimprove toughness knowing some sacrifice in losing hardness.

Normally it is advised to quench at austenite temperature and then to temper at 400 ~600C to assure intended hardness.

2. To temper at high temperature

It is advised to apply high temperature (400 ~ 650C) for tempering mold for not onlyhigh temperature molding for thermosetting resin or super engineering plastics but alsofor usual thermo plastic resins. High temperature tempering can minimize remainingaustenite structure to cause dimensional deflection as time elapses, and can minimize

deflection due to heat treatment when surface hardening such as PVD is conducted.

When high temperature tempering is conducted on many kinds of alloy tool steel,hardness can be improved at 500C due to improved conversion to martensite grainstructure (Fig. 2-2-1-3.1). Refer to catalogs and handbooks supplied by tool steelmanufactures for further details.

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3. Subzero treatment for long life precision mold

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Emilian Popa Injection Molding Design Guide Manual 2012Problem incurred from remaining austenite can be solved by high temperaturetempering for usual molds. If a mold is for high precision to be used at high temperature(100 or higher) for long period of time, subzero treatment is recommended.

Subzero treatment is conducted immediately after quenching when austenite is notstabilized. It is to hold the mold for certain time length at minus 100, and to temper ahigh temperature afterward. Subzero treatment minimizes retained austenite structure

thus assures to minimize deformation after elapsed time, and in additionimproveshardness (Fig. 2-2-1-3.2).

4. To apply steel with high quenching characteristic

Mass effect is a phenomenon in that cooling rate at the center of material cannot be asfast as the surface so that the quenched hardness cannot be attained in the center ofthick material. Mass effect is associated with thickness of the material to be quenchedand quenching characteristic of the material. You need not too much concern aboutmass effect on the mold for plastic molding because quenched hardness is not requiredin thecenter the mold wall usually. However be minded in this respect if cavity and core areodd shaped so that quenching of a material with poor quenching characteristic mayinvite cracks or deformation due to mass effect.

2-2-1-4 Vacuum heat treatment

Vacuum heat treatment is conducted in a vacuum environment of a certain vacuumrate. For quenching, usually 10-2 ~ 10-5 tore (mmHg) of vacuum rate is applied. The

vacuum furnace is made to vacuum condition before heating. This is to take oxygen outto protect the mold from oxidization. And then the furnace is heated by adding nitrogen0.5 torr to minimize evaporation of steel element. Characteristics of vacuum heattreatment are as follows.

Shiny surface can be attained without oxidization influence.Deformation can be minimized through proper installation of a product in the furnace

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Emilian Popa Injection Molding Design Guide Manual 2012Automated heat treatment reduces manpower overhead.Working environment is clean and comfortable because the furnace is insulated.There is no environmental issue like salt bath furnace.Cost of facility is high.

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2-2-2 Surface treatment

Limiting our scope to surface hardening, various processes are classified as shown inTable 2.2.2.1. Below explained is some of the surface hardening processing often usedfor the mold of plastic molding. In order to improve surface hardening quality, somecombined processes are applied.

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2-2-2-1 Gas Nitriding

An object is heated in the atmosphere of nitride gas such as ammonia gas (NH3) to bediffusionpermeated by carbon and nitrogen. Characteristics of this process can be summarizedas follows:

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Emilian Popa Injection Molding Design Guide Manual 2012 Any small surface of any kind such as internal surface of small hole can be hardened.

Assuming proper tempering at higher temperature than nitride temperature is conducteddeformation is small due to processing in rather low temperature (500 for alloy tool steel).

Does not influence surface roughness.

All steel except stainless steel can be treated, particularly effective for SCM steel, SKD-61steel and prehardened steel of precipitation hardened type.

There is no environmental problem as salt bath Nitriding.

White layer (or layer), hard and brittle composite, is formed on the surface. But this canbe minimized by controlling temperature and nitrogen concentration precisely.

In view of various characteristics above, a mold can be gas-nitride after finishing or before finalfinishing. In the case of precision mold, gas Nitriding is conducted before final finish by leavingfinishing margin 0.01 ~ 0.02 mm. Nitride depths is at most 0.05 mm even for SKD-61. Thereforeamount of margin for finishing should be limited. Gas Nitriding is effective against galling andseizing. Therefore you can apply this surface hardening not only for cavity and core but also forsliding surface in the mold components.

Gas Nitriding is difficult to apply on stainless steel, as explained, because its surface is made ofstable oxidized steel. But ionized Nitriding to apply glow discharge under low-pressure gas canmake stainless steel nitride.

2-2-2-2 Ion Plating

Ion plating is a kind of physical vapor deposition (PVD). This is a surface treatment method toionize vaporized coating elements such as carbide and nitride and to deposit them on thesurface of an object with negative voltage (Fig. 2-2-2-2.1).

PVD includes vacuum vapor deposition and spattering. But they are usually not applicable for

surface hardening.

Characteristics of ion plating are as follows:

Deformation is extremely small because temperature under treatment is only 300 ~500C. Be minded tempering should be conducted at the higher temperature.

Film thickness 1 ~ 4 m is given evenly.

Super hard coating HV 2000 ~ 3000 can be attained by applying Tin and TiCN.

No influence to surface roughness.

Applicable to all kinds of steel.

Work environment is favorable because all activities are carried out in the vacuumchamber.

Generally adhesive strength is not as high as CVD. The adhesive strength is muchaffected by surface condition and treatment temperature.

Coating on the surface such as internal surface of small hole, on which deposit is unlikelymade, is rather difficult.

In view of above characteristics, this method is applicable to cavity with flat shape and smoothsurface, core and core insert with simple shape. But you need to check carefully items , and.

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Referring to , ion plating is applicable to aluminum alloy and copper ally as well. But adhesivestrength is not so high because hardness of such material is not hard enough. Thus it isrecommended to limit this application to steel having hardness more than 50 HRC. Ion platingcan be combined with gas Nitriding for better surface hardening. In this case white layer shouldbe removed by shot pining in order to assure adhesive strength in ion plating.

The surface condition before ion plating processing should be metallurgically active. This can besaid to the surface when electro discharge machining is conducted. Ion bombardmentprocessing is a popular method in this respect by bombarding ionized argon gas on the objectsurface before ion plating is processed.

2-2-2-3 Other surface treatments

Traditionally hard chrome plating and electro less nickel plating have been widely applied.Treatment temperature for hard chrome plating is as low as 45 ~ 65 . Hard chrome plating is low cost but with good abrasion resistance, mold separation and corrosion resistance. It hasbeen widely applied to mold, which is not complex in shape, or IC mold. Comparing with PVD,plated film is thick, 0.01 ~ 0.03 mm, and in addition there will be a build up at edges or corners

Thus you need to evaluate the usage carefully before application. Table 2.2.2.3.1 illustratescharacteristics of hard chrome plating and electro less nickel-plating for your reference.

Surface treatment by spattering, which is a dry type instead of a wet type in hard chromeplating, is highlighted in these days. This has better adhesion and even film thickness, butcannot be applied to small area such as internal surface of fine hole. With regard to coated film

CVD, the same dry type as spattering, is far better in adhesive strength, but itsweakness lies in deformation to precision parts due to high temperature treatment.

In this respect it should be worth attention that CVD in low temperature treatment or plasmaCVD is under development. Such new technology should provide you with a new insight for asuperior surface treatment.

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