As computers become more and more prevalent in society it is only natural that the plastics Industry adopt their use as well. As mold shops Integrate computer aided technology in the form of PC-based CAD/CAE and CNC machines the way Plastic molds are built and operated is changing. Unfortunately this change is slow due to the investment involved. This paper will discuss the use of CAD/CAM throughout the entire mold building process. Using computers to design the molded end product is not enough. The mold builders Input during the design stage of the product is invaluable to ensure that a part Is not developed that cannot be molded. Also, the mold builder is then able to design a mold that uses his machines with the most efficiency. In using the mold builder's strengths, the molder gets a better mold and part. The overall human interaction from product design to finished mold is of paramount importance. Engineers and designers must communicate with the men on the shop floor to assure that mistakes are not picked up along the way. Too often this communication simply does not exist. This practice must be changed. Using these methods the independent mold builder essentially becomes a small department of the large mold customer All parties involved benefit as this approach is spread throughout the mold builder's customer base. The mold builder is able to offset the cost of his CNC machines while producing high quality molds. The mold customer is able to enjoy the higher accuracy and "cleaner" designs CNC tends to offer at substantial savings over conventional methods. The advantages that CNC machines have to offer to the plastics industry are limited only by one's imagination. CAE, CAD/CAM, and CIM are very Integral parts of today's manufacturing process. Almost all aspects of plastic molds from the initial design and building to the final molding of the part itself can be directed by computer control. Many independent mold shops have invested the time and capital to establish a long term CNC operation. Justifying the cost of owning and operating CNC equipment is usually the main reason many shops do not make the investment. O.A.R. Tool and Die, Inc. is a twenty-five-man mold shop in Providence, RI This shop has built a CNC department, which has become the primary force in the shop. This paper will address O.A.R.'s chosen path of CAM (computer aided machining) and its potential benefits and shortcomings for the plastics Industry as a whole. In March of 1980 O.A.R. Tool and Die, Inc. bought Its first CNC machine. The degree of difficulty of many parts and complexity of machining methods needed to manufacture them mandated the need for computerized numerical control (CNC). Up to this point in time the company had managed to do very well using conventional methods Including in-house model making, 3-D pantographing, and EDM die sinking. These methods, along with the usual array of mills, lathes, and grinders, helped O.A.R. build a reputation for being able to perform "the impossible". As O.A.R.'s reputation grew, so did its need for machinery that would keep It on the cutting edge. Drawing from experience in conventional 3-D machining, the decision to make the jump to CNC machines was a natural step. Starting with a 2-1/2 axis vertical CNC mill many new machining procedures could be developed. Gone now were many of the old rules of thumb. Many routine tasks such as multiple hole drilling could now be programmed and run automatically. Simple jobs such as filleted corners and angled pockets were now done without the need for rotary tables or other fixtures. As the new CNC department traveled up the learning curve, simple 3-D shapes were introduced. These included examples such as pockets with sloping bottoms and the like. Industry-wide CNC machines were generally perceived as production oriented O.A.R. was showing that CNC's could be used economically for the short run Job as well. Innovative thought is probably the most critical element in utilizing computerized tooling to its fullest potential. To have a CNC machine with all its capability only to use it in the same way as a conventional machine is a waste of material. As the CNC's potential is unlocked new machining practices can be adopted. Such is the case at O.A.R. With the complexity of today's products, tolerances and repeatability from part to part are major concerns. Multiple cavity molds using CNC methods are relatively. easy to achieve. A program is developed for one cavity and is then repeated for however many cavities are needed. High accuracy and fewer mistakes are the norm using these methods. As reliability of the new methods grew so did the CNC machining arsenal. Over the course of a few years the single mill CNC department grew to include two CNC EDM die sinkers, three EDM wire machines, and a 3-axis machining center with a digitizer. Because of the amount of CNC machines now running and the amount of manual programming going on, It was decided to purchase an in-house computer aided design (CAD) system. The PC based system allowed programming to be done directly from a CAD drawing via a post processor. Communications from the PC to CNC were achieved using RS-232 protocol. Floppy disks and paper tape were used as alternative methods. This step of linking CAD directly to programming and CAM virtually eliminated mistakes due to data transfer. If modifications were made to the program at the CNC, the edited program could be repeated back into a drawing. In this way all data would match from CAD to CAM and back to CAD again. In-house CAD proved to be an invaluable resource. The new speed and accuracy with which drawings could be produced and edited greatly benefited all. This new speed even allowed for multiple concept drawings to aid in developing the best choices for manufacturing. With the introduction of PC-based design, O.A.R. truly had achieved a CAD/CAM operation. It has to be noted that it is not only machines that make the system work. The CAD designer and CNC programmer/operator interact with open minds and true team spirit from concept to finished product. Because of this Interaction between office and shop floor, Input and Information is received from many sources. As each Individual's strengths are used, many problems are solved or eliminated before they even start. This cooperation cannot be overemphasized. Thus far this paper has discussed the benefits of CAD/CAM for the manufacturing process of plastic molds. The integrity of data transfer between people and machines and the many different ways this data can be used are well known aspects of CNC machining. But why is the Industry moving toward CNC methods and away from conventional machining practices? Let us evaluate the plastic molds themselves. A mold with a small number of cavities Is usually not too difficult to manufacture. One, two, or four cavity molds typically are not too complex In terms of runners, cooling, ejection, etc. Obviously the complexity of the cavity itself Is a determining factor but overall these molds will generally not present too much of a problem. As we move into multiple cavity molds with greater than four cavities CNC machines offer simple solutions to potential problems. Reliable repeatability is one advantage a CNC has to offer. Once the CNC program has been established the machine will execute Its instructions without error. Machining multiple cavities takes a fraction of the time as with conventional methods. This repeatability can benefit the mold maker in other ways as well. Machining hazardous materials such as beryllium or multiple graphite electrodes for EDM machining can be done unattended with complete confidence. If the CNC has a multiple tool changer it can take a blank piece of material to finished part with very little effort. The same principles used to directly machine steel cavities can be applied to other facets of the manufacturing process as well. Not only can multiple cavities be machined but multiple ejector pin holes, screw holes, and runners as well. In addition these machines can run twenty-four hours a day, seven days a week if need be. As the machining of a mold becomes easier the time to build it can be cut down. Instead of machining one or two cavities into a block and then inserting these blocks into the mold frame, these cavities can be machined directly into a single steel plate. Pockets and screw holes to hold In cavity blocks are eliminated. Cavities can be placed closer together decreasing the physical size of the mold. Simpler and more efficient cooling circuits can be engineered as there are now not as many obstacles to go around. Better shut-offs can be achieved since there is only one level (the frame plate) to work with at the mold parting line. Using these simple but effective techniques molds with cavities numbering in the hundreds are possible. O.A.R. has built several molds with one hundred to two hundred cavities, all of which ran very successfully with very little start up time. Better cycle times with fewer rejects and less scrap are realized. Another major benefit from CNC is the ability to digitize a part. Many parts come along which are virtually impossible to program. Usually cavities for these parts are machined from 3-D models which are then pantographed. The 3-D digitizer does essentially the same thing only through computer control. As the model is digitized the computer stores all the details of its shape In the form of 3-1) coordinates in a computer file. This file Is later recalled to cut an exact duplicate of the model. Add to this the CNC's ability to scale the file up or down, mirror, or even turn male cores into female cavities and one can generate almost any shape. An advantage that stands out is the ability to cut 3-D parting lines at many locations. All cavities are identical with very little finish work to be done. The digitized file can also be used to help the CAD designer. As a concept is taken from a rough sketch to a finished model many aspects of the concept may change. By the time a model Is finished It may look nothing like what the original drawing and dimensions said it should. Since the digitized file is of the final approved model the CAD designer can take this file and create working drawings directly from it. These drawings would accurately represent the part since they came from the file that cut it. This shows that a customer could present the mold builder with a finished part and an entire set of drawings and a working mold could be built. The entire project could be established from a simple digitized file and the capabilities of CNC. Thus far we have looked at how to simplify the multi-cavity mold. Obviously this cannot always be done. To mold a plastic part can take some pretty fancy machining at times. Cams and Inserts are only two of the many options a mold builder has available to him to accomplish his goal. Once again the CNC's can come to the rescue. Designing the plastic part around the machines available to build It is not an old concept. The CNC's offer a varied array of machining options. A very simple example is the wire EDM'ing of a square pocket and an insert to match. One program cuts. both parts and the female pocket corners 'are virtually square (EDM wire machines typically use OIO_.OI5 diameter wire to machine with). Because of the 3-dimensional aspect of CNC machines the need for fixturing is cut down keeping set-up time down as well. With the mold builder partially involved in product design, costs to produce the part are cut down while a more efficient mold is built. Since the mold builder is very familiar with what each of his machines is capable of doing, his "consulting" role In the development of the mold can be very profitable to the molder as unnecessary machining can usually be eliminated. The less expensive the tooling is for the molder, the less expensive his end product has to be to recover his costs. One final benefit should be noted. It has already been stated that CNC equipment tends to be very expensive. For the small or mid-sized mold shop to own even one piece can be very taxing to the shop's operating budget. At the same time, many large molders have an in-house shop only large enough to maintain their own molding machines. CNC's usually cannot be cost-justified in this environment either. Now suppose the two were to somehow get together. The Independent mold shop will generally have a fairly large customer base. This is common sense as one molder will not always supply 100% of the mold building work necessary to keep a shop going. Using this customer base the Independent mold shop has a very effective way to justify his purchase of CNC. With many customers wanting or needing CNC work the mold builder can keep his CNC machines running almost all the time. Revenues generated make CNC's very cost effective for the mold builder. At the same time the mold customer is also able to reap the benefits of CNC. With the mold builder helping to design the part for ease of machining and outlining the manufacturing process from start to finish, he essentially becomes a sub-department of the mold customer. This one-to-one Interaction helps eliminate mistakes and keeps all involved on the right track. If the Independent mold shop is willing to make the Initial investment it should be fairly easy to establish a strong CNC department relatively quickly. Both the molder and the mold builder are needed to make the system work but in the end, all parties get what they want. This paper has touched briefly on how effective CAD/CAM can be for the small contract shop. Almost every job that goes through O.A.R touches a CNC machine at some point. CAD/CAM has become the back-bone of the shop. How this mold shop makes the system work is by no means the only way available. As stated earlier imagination plays a very important role. There is time and capital investment Involved but O.A.R. has shown that the independent shop with short-run jobs can use CAD/CAM economically, efficiently, and accurately to produce high quality molds for today and tomorrow. |
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