Mold Design. 12. Mold Materials. Bong-Kee Lee School of Mechanical Engineering Chonnam National University

Transcription

1 12. Mold Materials Bong-Kee Lee Chonnam National University Mold Materials easy toolmaking good performance during production good machining properties ease of hear treatment where hardening is required good toughness and strength polishes and accepts texturing well good resistance to heat and wear good fatigue resistance high thermal conductivity for effective cooling good corrosion resistance

2 Mold Materials no single material will exhibit all these characteristics compromise has to be reached depending on the type of tool design being employed tool life in terms of the quantity of parts required to be produced from the tool molding material being used (e.g., abrasive and corrosive) texturing and polishing requirements whether hardening is required (e.g., for long running tools or for side cores and splits, etc.) whether high thermal conductivity will be required exceptional requirements like the use of very high injection pressure or speeds Mold Materials Common Mold Materials (for most normal applications, steel is used) Application Materials Process preproduction runs epoxy resin casting prototypes low-melting-point alloys, e.g., Kirksite casting and metal spraying small quantities mild steel or low-carbon steel machining medium production runs for straightforward parts medium production runs for technical or highperformance parts mild steel, aluminum, pretoughened alloy tool steels, unhardened alloy tool steels aluminum alloy, pretoughened alloy tool steels, unhardened alloy tool steels machining machining high-speed production hardened alloy tool steels machining and close tolerance work large-volume production hardened alloy tool steels machining

3 Mold Materials Common Mold Materials (texturing, high polished finishes) Application Materials Process low/medium volume aluminum alloy, unhardened and pretoughened alloy steels photo-etching, chemical etching, EDM high volumes pretoughened alloy steels, hardened alloy tool steels photo-etching, chemical etching, EDM high heat transfer beryllium copper casting and machining poor heat transfer ceramic titanium casting and machining industry standard material for the manufacture of mold tools ex. AISI(American Iron and Steel Institute) carbon steels: in general, the higher the level of carbon, the tougher and stronger the steel will be (cf. plain or mild steel with low carbon content) tool steels or alloy tool steels: carbon steels alloyed with other elements to increase their performance

4 plate steel for mold applications Category Uses Standard Comments low-carbon(plain plates, carbon or mild steel) prehardened/pretoughen ed alloy tool steels fully hardening alloy tool steels stainless steel high-carbon-highchrome nitriding steel backplates, support locating rings AISI 1020 used for economy in noncritical applications cavities, punches and cores AISI P20 alloy tool steel used unhardened. can be locally heat-treated high volumes where high quality, strength and hardness are required for cavities and cores used for corrosion resistance to protect against corrosive materials as PVC cavities and cores of high levels of hardness areas of high wear, sliding surfaces, ejector pins and sleeves AISI H13 AISI 420 SS AISI D2 AISI H13 high-quality tool steel. first choice for long-running jobs stainless steels contain up to 18% chromium and hence have worse heat transfer very hard when heat treated. used for small parts. prone to stress cracking extensively used for small sliding parts AISI designated mold steels Types No Comments mild steel AISI 1020 for backplates, support plates, support register rings and ejector bars nickel chrome alloy AISI H13 used unhardened for cavity backing plate where high loads are likely and for medium-quantity jobs for cavities and cores, or for plates likely to suffer bruising, e.g. runner system plates and stripper plates on multiplate tools used unhardened for high-quality, long-running jobs for all cavities, cores and punches hardness value Rc nickel chrome alloy AISI P20 pretoughened steel used for medium-volume works. material can be machined in this state. it is about halfway between the unhardened and hardened grades of H13 in toughness and hardness. used for cavities and cores hardness value Rc

5 AISI designated mold steels (thermal properties) Types No carbon steel alloy steel (tool steels) shock-resisting steels hot-work steel mold steel stainless steel S1 S7 H13 P Thermal conductivity (W/mK) Thermal expansion (10-6 K -1 ) cast steel large mold would be cast from suitable steel because of processibility and/or manufacturing cost (up to about 4 tons per casting, resulting in the finished mold of up to 10 tons in weight) not quite as strong as rolled or forged steels casting-grade steels are selected to give as fine a crystal structure as possible for maximum quality and strength surface defects like pits, blowholes and shrinkage holes long-term damage from thermal shock, which results in surface cracks

6 sand casting process for cast steel molds cast steels for injection molds Types No Properties carbon steel alloy steel heat-resistant steel 501 Welding similar to Welding is readily done by most arc and gas processes. Preheating unnecessary unless parts are very heavy. Welding can be done by arc, resistance and gas processes. To decrease cooling rate and subsequently hardness preheating to above 150 preferably 260 C. Final heating to C restores ductility and relieves stress Can be relatively easily welded with mild-steel filler metal. Welding same as 1330 Welding calls for pre- and postheating and filler metals of the same mechanical properties as the base metal. Stress relieving is desirable especially in repair work. Welding preferably with filler metal of the same chemical analysis. No preheating needed for sections up to 0.5 in. but stress relieving after welding.

7 for cold-working STD11 (Steel Tool Die) KS (cf. JIS: SKD61) general purposes, wear-resistance, uniform hardness better performance for press dies DC53 enhanced hardness and toughness during high-tempering general purposes and precision dies punches, dies, precision molds, etc. for hot-working STD61 (Steel Tool Die) generally used for hot-working (<600 C) precision dies and heat-treated dies good endurance for thermal impact and thermal fatigue high thermal expansion

8 for plastic molds NAK55 good machinability and excellent surface easy polishing after mechanical machining or EDM for precision molds due to low deformation precision injection molds, rubber molds NAK80 enhanced properties comparing with NAK55 excellent polishing properties good surfaces after EDM used (i) only for the specific purpose within a steel mold base or (ii) as the main mold base material aluminum alloys zinc alloys beryllium-copper alloys bismuth-tin alloys epoxy resin

9 aluminum alloys advantages costs less than steel has good machineability (5-10 times faster than steel) (for EDM techniques, 6 times faster than steel) shows minimal distortion from machining owing to a special heat treatment during production has an excellent thermal conductivity, which promotes rapid and efficient heat removal from the mold shows less weight may be chrome plated or anodized to reduce wear and corrosion can be polished and etched in the same way as steel aluminum alloys disadvantages has low elastic modulus, which is only 30% of steel cannot achieve the same levels of hardness as steel is mechanically weaker than steel, resulting in the thick plate (around 40% greater than steel) wear is greater and the material bruises more easily increasing use of hybrid aluminum-steel mold steel for highwear areas and aluminum for less critical areas and high rates of mold cooling

10 aluminum alloys Chemical composition No. Composition (wt.-%) Cu Cs Fe Mg Mn Si Ti Zn Others (Ti+Zr) =0.25 Physical properties No. Density Yield strength Tensile strength Elastic modulus Thermal expansion Thermal conductivity kg/dm 3 MPa MPa GPa 10-6 K -1 W/mK (0-100 C) aluminum alloys Coating process for aluminum and the obtained properties Process Layer thickness (μm) Hardness (HV) chrome plating nickel plating anodizing PVD coating

11 zinc alloys have poor mechanical strength and are unsuitable for production tooling often used for prototype tooling have a low casting temperature of around C and therefore well suited to casting resulting castings reproduce intricate pattern detail extremely well, giving a smooth nonporous surface that ultimately produces good-quality plastic moldings also suitable for cold hobbing to impart undercut forms into the cavity insert Zamak, Kirksite zinc alloys Density (kg/m 3 ) Melting point ( C) Shrinkage (%) Thermal expansion (10-6 K -1 ) Tensile strength (MPa) Elongation (% in 50mm) Brinell hardness (BH) Comp. strength (MPa) Shear strength (MPa) Zamak Kirksite A Kayem Kayem very low

12 beryllium-copper alloys pure copper is used in mold tools only as a heat exchanger, usually inside core pins excellent thermal conductivity beryllium-copper alloy is a much stronger, tougher material and is used for cavity and punch applications beryllium content ~ mechanical properties & thermal properties heat treatment or ion implantation techniques to enhance a surface hardness casting, machining, hobbing, chromium and nickel plating highly suitable for cavity inserts where large amounts of heat have to be removed beryllium-copper alloys (I) solution heat treated (II) cold drawn (III) solution heat treated and age hardened (IV) cold drawn and age hardened Types C C C C C density (kg/m 3 ) therm. conductivity (W/mK) therm. expansion (10-6 K -1 ) specific heat capacity (J/kgK) electric resistivity (10-8 Ωm) elastic modulus (GPa) tensile strength (MPa) (I) (II) (III) (IV) yield strength (MPa) (I (II) (III) (IV) elongation (%) (I) (II) (III) 3-10 (IV) 2-5 (I) (II) (III) (IV) (I) (II) (III) (IV) (I) (II) (III) 4-10 (IV) 2-5 (I) (II) (III) (IV) (I) (II) (III) (IV) (I) (II) (III) (IV) 10-20

13 bismuth-tin alloys commonly known as Cerro alloys only for prototype moldings low melting points, ranging from 40 to 180 C conventional casting techniques special-purpose spray gun, which is used to spray a coating of the alloy onto a master to form a cavity bismuth-tin alloys (Cerro alloys) for injection molds Cerrotru Cerrocast density (kg/dm 3 ) melting point (range) ( C) specific heat (kj/kg) thermal expansion (10-6 K -1 ) thermal conductivity (W/mK) Brinell hardness (BH) tensile strength (MPa) elongation (slow loading) (%) composition % Bi %Sn

14 epoxy resin cheap and quick method of making cavity inserts from suitable master forms low mechanical strength and poor thermal conductivity prototype works resin + hardener + finely divided aluminum powder (as an additive to enhance heat conduction) mixing and evacuation casting onto a master loading into a mold