Rapid Prototyping By Injection Molding

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Rapid Prototyping by Injection MoldingShaun D Compton, Charles Thomas, Vipul LotkeUniversity of UtahAbstractThick layered rapid manufacturing, using injection molding techniques, integrates theconcept of layered manufacturing into the molding process. This process takes an STLfile of a part and decomposes it into layers that can be milled on a 3-axis machine. Newfiles are then created of each layer, where upon these layers are used to create moldcavities. Machining code is then written for each mold, and the mold is cut using the 3axis mill. The layers of the part are then molded and combined to form the whole part.In this way, complex parts can be molded without concern for injection molding flaws inthe part or complicated mold design. This technique allows the production of thousandsof complex parts directly from a CAD file in a few days.IntroductionThe goal of Rapid Prototyping by Injection Molding (RPIM) is to produce a medium tolarge quantity of prototypes/parts directly from a CAD file in s short time. In addition, theprocess should be relatively unskilled and automatic. The techniques used in RPIM, layeredmanufacturing, NC machining, and injection molding, are common techniques that have beenaround for a number of years. Each of them have their advantages and disadvantages, and thisprocess is intended to combine the advantages from these techniques while minimizing thedisadvantages.Layered ManufacturingMost rapid prototyping systems currently available employ an additive process for theprototype construction (Thomas, 1995). This means that the part in question is decomposed intolayers, and each individual layer is constructed and "stacked" upon the previous layer, hence, anadditive process. This process is also known as layered manufacturing or solid freeformfabrication (Deckard, 1987). The advantage of creating parts in layers and then stacking them isthat the complexity of the manufacturing process is greatly reduced. One only needs to know thecontour of any specific layer to create that layer. As a result, this process can produce almost anygeometry.One disadvantage of a process such as this is that it is not as accurate as one might wish.An example of the tolerances available with this type of process would be from 8 to12 thousandsof an inch for a Stratasys FDM1650 fused deposition modeling machine (Stratasys , 1996). Theavailable surface finish for this process is also relatively poor. This process is also quite fast atproducing an individual part, but very slow in terms of mass production.607

Numerical Controlled MachiningOne might also consider another type of rapid prototyping to be a subtractive methodsuch as NC machining. In NC machining, the part is simply "cut" out of an existing block ofmaterial; unwanted material is removed form the block to reveal the part. This process isextremely accurate in its tolerances and provides an excellent surface finish. For example, therepeatable tolerance on a Bridgeport Discovery Torq-Cut 22 Vertical CNC machine is O.00015inches (Bridgeport , 1995).The disadvantages to this technique are that it requires a skilled operator to run themachine, with extensive planning and design in the manufacturing process, and it also requires awide range of tooling to produce a wide range of parts. Another disadvantage, perhaps thebiggest, is that some geometries are very difficult to produce or even impossible. For example, acavity the middle of the part would be impossible since there is no way to get to it to removethe material.Injection Moldingnext technique used in this process is injection molding. Injection molding is aprocess through which a mold is created, where upon the mold is injected with a heated polymerto produce the part. This technique is a very fast and efficient means for producing many,identical parts.A disadvantage to this process is that the mold design can be very complicated andexpensive.molding also limits the geometry that can be manufactured. For example,thick walled parts are difficult or impossible to mold.Previous researchers have developed prototyping techniques based on the decompositionof3D parts into thick layers.Weiss and B. Prinz developed an automatedmanufacturing process known as Shape Deposition Manufacturing, or SDM (Weiss, 1998).SDM combines the concepts of molten metal droplet deposition and CNC machining to producecomplex-shaped, multi-material structures. F. Zafar Shaikh and coworkers developed amanual process known as Precision Stratiform Machining;' which decomposes a complex, 3Dpart into layers that can be machined with an NC machine (Shaikh, 1997). The Stratiform process then combines the machined layers using a vacuum furnace brazing process that createsleakbrazed joints between the connecting layers. Cheol H.developed a method knownas Ruled Thick Layered Rapid Prototyping, which incorporates ruled edges on the layerdecomposition to build large, 3D, complex geometries (Chamberlain, 1998).begins with an automated adaptation of the Stratiform process, except that itmachines molds of the thick layers.end product ofRPIM is a collection of thick injectionmoldedinstead of a unitary part as with SDM. The concept of RPIM takes the techniques608

oflayered manufacturing, NC machining, and injection molding and combines these threeprocesses into a single new process. The following steps outline the operation ofRPIM: 1II1II1II1II1IIDecomposition of a general 3D CAD object into 3 axis machinable layersGeneration of 2 part mold surfaces from layersGeneration of interlocking features to lock adjoining layersGeneration of tool paths to machine mold cavitiesMachining of the cavities into standard mold insertsMolding of the layersAssembly of molded layersDecomposition ofgeneral 3D CAD object into 3 axis machinable layersThe first thing that must be accomplished is to decompose the part into layers that aremachinable by a 3-axis NC machine. In order to accomplish this, the part must be split atintersections where, in the splitting plane, downward facing surfaces meet upward facingsurfaces. In this way all layer surfaces will be seen either from a top view or from a bottom view,allowing the surfaces to be machined with a 3-axis NC machine. This will also allow for a twopart mold.Splitting Surfacefor the laverFigure 1Parting Surface Between LayersAnother consideration to take into account when decomposing each layer is to be awarethat a single layer thickness must not exceed approximately 0.125 inches. This is theapproximate maximum thickness at which any part should be molded do to shrinkage/sinking.Sinking occurs when the part is removed from the mold and begins to cool. If the part is toothick, the internal stressescreated by the coolingprocess will pull the outerwalls inward creating a"sink" in the part (Dym,1987).Figure 2 shows anexample of a die that isvery difficult to injectionmold. This die hasconcave surfaces on all sixsides with further concaveFigure 2Example of Thick Part Decomposition609

indentations totheon each side. Thiswithin the core. All of these factorsthisdifficult, or even 1n"1l1 r"': 'C'1mold. It is therefore split into layers along horizontal planesdownward . . . . .""'. . . .,. .meet upward facing surfaces. This occurs the middle ofA .""""' .Generation of 2 part mold surface from layersThe next phase of the process is to placewithin a mold, ".,. .""'CIIT .,.mold to inject the layers. Duringphase of the process, consideration mustinjection molding constraints. Astandard mold insert will be used forthe injection process, limiting thenumber of layers that will fit intoanyone mold. Allowances mustalso be made to fit runners withinthe mold, which are channels for theheated polymer to reach the partlayers. Ejector pin placement is alsoa factor when placing the layersthe mold.layers must beejected from the mold with theejector pins. This reduces thepossibility of the part sticking in themold cavity.final considerationis for verticalin the mold.the layersvertical surfacesmold relative toimplemented on those surfaces to facilitate the ejectionexample of the die layers within the standard mold inserts.Generation ofInterlocking Features to LockAdjoining LayersInterlocking features are.necessary toreference one layer with another.accomplished in several different ways.way is to create small hemisphereprotrudes fromsurface of one layer, andthen to create a matching cavity of thehemisphere on the adjoining layer. When thelayers are brought together, the hemispherefrom the first layer will go intocavity onthe second layer referencing the two layerstogether. Figure 4 shows how this may beaccomplished.610to

Another possibility would be to use the ejector pins to create the locking features.could be accomplished by making the ejector pins a little toofor one layer, thereby "'.",.a cavity on that layer; and making the ejector pins a little too short on the adjoining layer,creating a small protrusion.However, in order for thelayers to be positionedcorrectly with respect toeach other, the pins wouldhave to be positioned insuch a manner as to be atthe same place on eachadjoining surface. Figure 5shows the interlocking featurescreated by the ej ector pins onanother example part.Figure 5Interlocking Features Created Using the Ejector PinsGeneration ofTool Paths toMachine Mold Cavities, ,1-.After the layers are inlayed into the mold halves and the interlocking features have been created,tool paths are then generated. Tool paths are the code that tells a certain CNC machine how tomove the cutting tools to create the part that has been designed. Different codes exist fordifferent machines. Therefore, the determination of what code is to be implemented will dependon the machine that will be used for the cutting process.tool paths, in this case, will tell theNC machine to cut the cavities for the each part layer that hascreated.Machining ofthe cavities into standard mold insertsmachining of the cavities willbe done on a 3-axis mill. This means thatthe cutting tool of the mill will only be ableto move in three planes of motion: leftright, forward-backward, and up-down.Each layer of the part has been generatedso that their cavities will be machinablewith this type ofNC machine, reducing thecomplexity of the machining process.Figure 6 shows a simulation of a die cavitybeing machined. As can be seen in thecutting tool can reach each pointon theof the cavity by moving inthe three planes of motion.Figure 6Simulation611h,","". ofMold Cavities

Molding and Stacking the layersThe next phase in the process is to mold the individual layers on a standardmolding machine. Once each layer has been molded, it can then be stackedbonded togetherwith the other layers to form the entire part. The interlocking features willtoadjoining layer, referencing the entire part together.Rapid Prototyping by Injection Molding uses the best features of layered manufacturing,NC machining, and injection molding to produce a medium to large quantity of prototypes/parts.With layered manufacturing, one can produce almost any geometry imaginable, .nrool"I"t.nnroverhanging dimensions and cavities. This process also significantly reducesthe entire part by decomposing it into several layers, each of which are simple to construct. NCmachining, which is very accurate in its dimensioning and surface finish, is used the moldconstruction; and since the design of the mold has been simplified by the decompositionan expert machinist will not have to be employed to operate the NC machine. Injection . . . . . . ,. . . . ". . .is the final technique used which is the perfect process for producing a large quantitysamepart. Therefore, Rapid Prototyping by Injection Molding is the perfect process for creating amedium to large quantity of prototypes/parts. -OWorkFuture work for this process will be its automation. At present, software is beingconstructed that will take anfile of a part, and automatically decompose it into the necessarylayers for the process. It will then generate new STL files for each individual layer. Additionalsoftware will be created to take these new layer files and inlay them into a mold withnecessary locking features, ejector pin locations, and runners for the mold halves. It will thentake these mold halves and generate the tool paths for the NC machine. Additional work can bedone to index the layers, onceto stack them automatically.Bridgeport@, Installation & Maintenance Manual, Discoverywith JI.J,LJlY. ""I-"J,LControl,Vertical Machiningwith Power Drawbar and AutomaticToolchanger, Bridgeport Machines, Inc., 1995.Chamberlain,Van Roosendaal, Mark, Thomas, Charles, "Variable Thicknessedgeslice generation and three-dimensional graphical error visualization," Solid FreeformFabrication Symposium Proceedings, 1998.Deckard, C.J."Solid Freeform Fabrication andPowderNorth American Manufacturing Research Conference Proceedings,May 1987.

Dym, JosephInjection Molds and Molding, A Practical Manual, Second Edition, VanNostrand Reinhold Company Inc., New York, New York, 1987.Shaikh,Zafar, et. al.," Precision Stratiform Machining - 100 Day engine project",Prototyping International, 1997.An Introduction to Rapid Prototyping, Schroff Development Corp., Mission,Thomas, C.Kansas, 1995Weiss,Prinz, B. and coworkers, Journal ofManufacturing Science and Engineering,Transactions of the ASME v 120 n 3, ASME Fairfield, New Jersey, August, 1998.613

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Injection Molding nexttechnique used in this process is injection molding. Injection molding is a process through which a mold is created, where upon the mold is injected with a heated polymer to produce the part. This technique is a very fast

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