Appliation o solid reeorm abriation proesses or injetion molding low prodution quantities: proess parameters and ejetion ore requirements or SLS inserts M. E. Kinsella 1, B. W. Lilly 2, N. Bhagavatula 2, K. G. Cooper 3 1 Materials and Manuaturing Diretorate, Air Fore Researh Laboratory, WPAFB, OH 45433 2 Departments o Mehanial and Industrial Engineering, The Ohio State University, Columbus, OH 43210 3 Rapid Prototyping, NASA ED34, Marshall Spae Flight Center, AL 35812 Abstrat Studies are underway or the appliation o solid reeorm abriation proesses or mold inserts to be used in thermoplasti injetion molding o low quantities o parts. This work initially ompares a laser sintered insert (LaserForm ST-100) with a steel insert. Models and experiments determine proess parameters, inluding molding latitude, and ejetion ore requirements. Ejetion ore preditions are based on work by Menges, using values or elasti modulus determined rom tensile tests at ejetion temperatures. Similar studies are planned or stereolithography inserts (SL 5170). Introdution Manuaturers who urrently build produts in low volume, suh as aerospae systems, an beneit rom tools that will ost eetively produe low quantities o prodution parts. Injetion molding, whih is typially a very high volume proess, requires signiiant dereases in tooling osts in order to make low quantity prodution easible. The appliation o reeorm abriation tehniques, suh as laser sintering, to build injetion mold inserts is one approah to reduing these tooling osts. Injetion molds or high prodution volumes are traditionally mahined o steel, are very strong, and have good thermal properties. The material properties o tools built using solid reeorm abriation (SFF) vary rom onventional molds (see Table 1) but still may be suitable or injetion molding lower quantities o parts. SFF proesses are also attrative beause they an generate omplex geometries as easily as simple ones, e.g., they an build mold shapes and ooling lines that are impossible to mahine. Seletive Laser Sintering (SLS ) is a good example o a SFF proess that an be used or injetion mold inserts or low prodution volumes. Injetion molding simulations and experiments will be run with thermoplasti materials, irst using a mahined steel mold insert and then using a SLS insert (see Figure 1). The objetive is to determine proess parameters and ejetion ore requirements or the sintered insert and ompare them to those or the steel insert. A modular injetion mold having a steel Master Unit Die mold base will be used, the ore and avity o whih an be removed and replaed with those o other materials. Mahining allowanes were inluded in the design o the inserts so that they an be mahined to it properly into the mold base. The hosen part or this 92
researh is a vented losed-end ylinder, similar to the plasti anisters used to store 35 mm photographi ilm (see Figure 2). Table 1: Properties o steel and SFF mold materials. Proess Mold Material Density Tensile Hardness Condutivity Strength kg/m 3 MPa W/mC Baseline [1] Mahining P-20 Mold Steel 7870 1080 30-35 RC 47.6 @ 204C Rapid Tooling Materials * H-13 Tool Steel 7800 1550 52 RC 25.1 @ 199C 3D Printing Prometal Bronze/iniltrant 8100 406 60 RB 7.35 Laser Sintering 3D Systems Steel, w/opper 3450 33.6 75 ShoreD 1.28 at 40C 0.92 at 150C S. Steel, w/bronze 7700 510 79 RB 49 at 100C as mahined 56 at 200C Laser Generating LENS S. Steel 316 8000 [2] 800 80 RB [2] 15 [2] Plasti Casting CIBA Cerami-illed Epoxy 64 (UFS) 91 ShoreD Stereolithography 3D Sys SL 5170 ured resin 1220 59 85 ShoreD 0.200 *From ompany literature Figure 1: SLS ore and avity insert made with LaserForm ST-100. 93
Figure 2: Canister part. Theory Proess Parameters A typial manuaturing environment in whih very large quantities o injetion molded thermoplasti parts are produed requires high quality produts and minimal yle times. Sine visosity o the thermoplasti melt dereases with inreasing shear rate (see Figure 3), injetion veloities are kept as high as possible to allow the mold to ill quikly and ompletely. Shorter ooling times are also avorable. Visosity (Pa-s) 1400 1200 1000 800 600 400 200 0 Polystyrene 0 1000 2000 3000 4000 5000 6000 Shear Rate (1/s) T=190C T=210C T=225C Figure 3: Visosity vs. shear rate ollowing the power-law model [3]. For the prodution o small quantities, part quality is still very important, but there is less emphasis on minimizing yle times. I yle times an be relaxed, then injetion veloity may be dereased and ooling times may be inreased. This allows or ore and avity inserts o dierent materials, suh as SFF materials. Cores made with some SFF materials may not be able to withstand the pressures and temperatures used with steel molds. A balane must be ound between veloity and visosity, so that the polymer ills the mold ompletely and produes a quality part. 94
SLS material LaserForm ST-100 has hal the strength o mold steel, but omparable thermal ondutivity. It is expeted that injetion veloity and temperature will have to be hanged to some extent ompared to those o the baseline steel insert. A simulation o injetion ore using a sintered ST-100 insert (Figure 4) shows that the ore an withstand a reasonable injetion veloity. At an injetion pressure o 109 MPa, maximum deletion o the ore at the point o injetion is minimal (0.003 mm). For omparison purposes, a similar model is shown or a stereolithography SL5170 ore. For the same injetion pressure, deletion o the ore is signiiant (0.3 mm). Figure 4: A simulation o stress due to injetion pressure on an ST-100 ore (let) and an SL5170 ore (right). Ejetion Fore Ejetion ores have two primary omponents: opening ores and, more importantly in this ase, release ores. The mold material, the part material, and the proessing onditions are all ators aeting release ores. For sleeve-type parts, the release ore F an be omputed rom the oeiient o rition, the ontat pressure between the part and the ore surae area o the ore A C, as ollows [4]. F = p A R A C R p A, and the For ylindrial sleeves, the part shrinks onto the ore, and stresses are subsequently built up. Immediately upon ejetion, the part reovers. Aording to Menges, the ontat pressure an be estimated rom the ontration o the part diameter or irumerene. The relative hange in diameter is given by d di ( te ) d = C = d where C = relative hange in irumerene d = ore diameter d = inside diameter o sleeve immediately ater ejetion. ( ) i t e 95
Applying Hooke s Law, σ = E ε where σ = stress E = elasti modulus ε = strain. Sine, in this ase, ε = d = C then σ = E T e d ( ) where E ( T e ) = elasti modulus at ejetion temperature. Contat pressure or the sleeve is given by s E( Te ) d m sm p A = σ = r r where s m = wall thikness r = ore radius. The surae area o the ore is AC = d π L where L = ore length. So, with the oeiient o rition, release ore is E( Te ) d sm FR = d π L r Note that the oeiient o rition must be determined at proess onditions during ejetion, and modulus must be determined at ejetion temperature. In this work modulus was measured by tensile testing polymer speimens at temperature, and ejetion ore will be measured experimentally using load ells behind the ejetor pins. Coeiient o rition will then be determined using Menges equation. Future release ores an thereore be alulated or these materials at these proess onditions. Methodology SLS Proess The laser sintering proess used in this researh involves a polymer-oated 420 stainless steel-based powder, known as LaserForm ST-100, and a 3D Systems Vanguard mahine. Speiiations o the Vanguard and material properties o ST-100 are shown in Tables 2 and 3. When the 3-dimensional part is initially built on the Vanguard System, the laser heats the metalli partiles above the glass transition temperature o the polymer oating. The polymer sotens and deorms, then uses with other partiles at eah ontat surae. The temperature is 96
suh that melting o the metal does not our, only visous low o the polymer oating. The metal powder is then bound together by the polymer to orm the green part. Ater the build is omplete, the green part is removed rom the Sinterstation and exess powder is brushed away. A urnae yle ollows in a reduing atmosphere to burn o the polymer, sinter the steel powder, and iniltrate the part with bronze. Iniltration eliminates any voids within the steel, resulting in a ully dense part. [5][6] Table 2: Vanguard System speiiations (3D Systems). Model Number Laser Wavelength Power Beam Diameter Max. San Speed Min. Layer Thikness Build Chamber LC-100 DEOS CO2 Laser 10.6 mirons 100W max at part bed 450 mirons 10,000 mm/se (394 in/se) (0.10 mm) 0.004 in 381w x 330d x 457h mm (15w x 13d x 18h in) Table 3: LaserForm ST-100 material properties (3D Systems). Density 7.7 g/m 3 ASTM D792 Thermal Condutivity 49 W/m o K @100 o C ASTM E457 56 W/m o K @200 o C ASTM E457 CTE 12.4 ppm/ o C ASTM E831 Tensile Yield Str. (0.2%) 305 MPa ASTM E8 Tensile Strength 510 MPa ASTM E8 Young s Modulus 137 GPa ASTM E8 Elongation 10% ASTM E8 Compression Yld Str (0.2%) 317 MPa ASTM E9 Hardness, Rokwell B 87 As iniltrated ASTM E18 79 As mahined ASTM E18 Modeling and Simulation Mold ill simulations using MoldFlow provide some validation o the mold design and predit what some o the proessing parameters might be or the steel insert. These parameters are a starting point or experimentation and give a reerene rom whih hanges or other ore materials may be determined. For example, Figure 5 shows the ill time or the anister with a HDPE melt temperature o 290 o C, a (steel insert) mold temperature o 104 o C, and an injetion pressure o 123 MPa. Simulations using ANSYS are in proess to determine ontat pressures on the insert ore and to predit required ejetion ores, e.g., see Figure 6. The values or maximum ontat pressure will be used with rition oeiient and surae area in the Menges equation to alulate required ejetion ore. Results o suh simulations will be ompared to experimental data, and a model o ejetion ore will subsequently be reated. 97
Figure 5: MoldFlow image showing anister mold ill time or a steel insert. Figure 6: Contat pressure o the anister on a SL 5170 ore prior to (let) and during (right) ejetion. Tensile Tests Elasti moduli or high density polyethylene (HDPE) and high impat polystyrene (HIPS) used in this researh were measured at various temperatures using ASTM D 638 Standard Test Method or Tensile Properties o Plastis as a guide. The testing apparatus was an Instron model 1322 tensile tester with a tube urnae. An extensometer with a 2-inh gauge and 50 perent strain was used to measure elongation. ASTM Type I (dogbone) speimens o eah thermoplasti material were pulled at room temperature and at ten degree inrements, starting at 30 o C, until no elasti region was deteted. HDPE was tested up through 70 o C, and HIPS was tested up through 60 o C. Results are shown in Figure 7. The values or elasti modulus will be used in ejetion ore alulations. Experiments Injetion molding experiments will be run on a Sumitomo General Injetion Molding Mahine, model SH50M, a horizontal press with a ully hydrauli, 50-ton lamping system. 98
A series o injetions with varying veloity and temperature will be run or eah insert material to determine suitable proessing windows. Values or the ejetion ore as measured by the load ells and anister diameter immediately ater ejetion will be reorded. Using Menges ejetion ore equation and the elasti moduli desribed above, values or oeiient o rition will be determined or eah ore material, and the validity o the equation will be heked. Modulus vs. Temperature 3500 3000 2500 HDPE HIPS Modulus MPa 2000 1500 1000 500 0 20 30 40 50 60 70 80 Temperature deg C Figure 7: Modulus vs. Temperature or HDPE and HIPS. Summary I tooling osts an be greatly dereased, injetion molding beomes a viable proess or prodution o small quantities o parts. One approah to this is the use o SFF proesses or making injetion mold inserts. The researh desribed in this paper will provide useul data on the easibility o this rapid tooling approah. This work will give insight as to the hanges in proessing parameters that must our and the ejetion ores required to aommodate tooling inserts o dierent materials. Reerenes [1] Rubin, I.I. (ed.) 1990, Handbook o Plasti Materials and Tehnology, John Wiley & Sons, In., New York, p. 1411. [2] Matweb Material Property Data, [Online], Available: http://www.matweb.om, various ss316 properties, [Otober 2001]. [3] Fried, J.R. 1995, Polymer Siene and Tehnology, Prentie Hall PTR, New Jersey, p. 394-95. [4] Menges, G., Mihaeli, W., Mohren, P. 2001, How to Make Injetion Molds, Hanser Gardner Publiations, pp. 405-413. 99
[5] Bourell, D.L., Craword, R.H., Marus, H.L., Beaman, J.J., Barlow, J.W. 1994, Seletive Laser Sintering o Metals, Proeedings o the 1994 ASME Winter Annual Meeting, November, 00. 519-528. [6] MAlea, K., Booth, R., Forderhase, P., Lakshminarayan, U. 1995, Materials or Seletive Laser Sintering Proessing, Proeedings o the 27 th International SAMPE Tehnial Conerene, vol. 27, pp. 949-961. 100