Rapid Design through Virtual and Physical
Prototyping
Rapid Design through Virtual and Physical
Prototyping
Procedure for Making Epoxy Tooling
Plastic tools can be up to 50% cheaper then conventional tooling and usually take 70% less time to manufacture. Since they are made of plastic, they are light in weight and easy to handle and need no special storage. Building plastic materials are usually epoxies or polyurethane which both set at room temperature. Oven treatment is necessary only when heat resistant tools are to be made. Plastic tools are easy to patch and not brittle. They bond to practically any material when inner support structures for additional tool stiffness is required. They are durable and won't rust and won't warp. They provide quick, easy and inexpensive modification for repair of valuable tools. Before discussing the steps involved in building a plastic tool, model preparation should be taken into account. Models represent the basis on which the tool will be made on.
Model Preparation
Wood .
1. Once the wooden model has been shaped and surface smoothened.
2. Spray a lacquer sealer on the model surface and let it dry completely.
3. If the lacquered model has been prepared long time ago, clean it by wiping it with a lint-free cloth that was moistened in alcohol.
4. Apply a all purpose release agent and rub out thoroughly with a lint-free cloth.
5. Spray a coat of PVA parting agent and let dry completely.
6. Apply an additional all purpose release agent coat, wipe off excess lightly with lint-free cloth and polish.
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Plaster
1. A plaster mold must be sealed completely to eliminate moisture or any chemical leak-through which would break down the release agent. For this reason
2. Spray a lacquer over the model making sure no spots are left unsprayed.
3. Apply two coats of all purpose release agent. Each application should be wiped off and polished while it is still wet, until it is dry and shiny.
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Plastic
1. Remove all foreign matter from surface of model.
2. Apply 3 successive coats of all purpose release agent, lightly polish and buff while still wet until dry and shiny surface is achieved.
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Metal
1. Remove all surface dirt or wash with suitable solvent.
2. Apply 3 to 4 coats of all purpose release agent by lightly polishing and buffing each coat while it is still wet until dry and shiny surface is achieved.
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Plastic Tooling may be divided into 3 general categories
1. Laminated Tools
Laminated tools are made of alternate layers of glass cloth and liquid laminating plastic. After lamination is completed, the liquid plastic solidifies into strong, rigid form. The finished piece has the exact size and shape of the surface from which it was molded. Laminated tools are usually reinforced with, or became a part of a framework. This framework can be made of plastic materials, fabricated steel or Aluminum. The type of framework developed depends on the end use of the tool. These tools are used wherever accuracy, strength and weight are prime considerations. They possess the best dimensional stability of any plastic tools and are widely used for spotting, checking and inspection fixtures, plus many other applications.
Building Procedure
The laminated method consists basically of building up alternate layers of glass cloth and plastic on a form, pattern or model until the desired thickness is obtained. Each layer of 0.013" thick 10-ounce glass cloth, plus the laminating material lays up to approximately 0.20" thickness (i.e. 18 layers = 0.360").
1. "Build-ups" - for parting planes, run-out aprons, dams or retaining boxes. Build-ups are used to confine the plastic lay-up to a certain area. Pattern maker's sheet wax of varying thickness is used for compound curves while thin sheets (1 to 2 layers) of glass cloth laminate are used for sharp radii. Wood or masonite should be utilized for straight sides and large radii. White pine blocks cut and shaped to the correct size also may be used.
2. Thickness Wax - Pattern maker's sheet wax is used to simulate the material thickness in obtaining either the inside or the outside mold line. It is also used for revealing certain areas in checking fixtures and build-ups. It can be applied by painting the model and one side of the sheet wax with shellac. The shellac should be tacky before applying the wax to the model. Sheet wax is also available with adhesive on one side. The wax will also adhere to the model by using a thin layer of petroleum jelly, but extreme care should be taken to insure a uniform coat to eliminate any irregular surface.
3. Apply parting agent to surface of model, dike boards, etc. according to instructions under Model Preparation.
4. Apply desired surface coat.
5. Cut sections of fiberglass (cloth, scrim, mat, etc.) in readiness to fit as patch work over the model. The section should be tailored to fit neatly into corners and over complex contours. Where the model is flat or has simple curvatures, the fiberglass is normally cut into squares measuring 15 x 15 cm to 25 x 25 cm (6" x 6" to 10" x 10"). By utilizing the time span between the surface coat and laminating operation to cut the necessary cloth for the initial layers of the tool and to built up sharp corners, etc., no lost time is involved and the operator is in "touch" with the now critical surface that must be chemically joined with the first layer if glass cloth.
6. Using a "glass paste" mixture, fill in all sharp corners and details. This reduces the possibility of voids or bubbles developing under the first layer of the glass cloth.
7. Prepare the laminating mix according to the appropriate mixing instructions.
8. Using the laminating mix, wet-out the almost tack-free surface. If material can be dented, but does not stick to your finger tip, it is considered "tack-free".
9. Lay-up the first layer of fiberglass sections. Stipple into position by brush or wooden spatula so that the edges butt, the cloth is completely saturated with laminating mix and all entrapped air worked out.
10. Apply second coat of laminating mix and immediately lay-up a second layer of glass cloth sections. Apply as before, but arrange so that the second-layer sections cover the butted joints of the first-layer sections. It is essential to ensure that while the glass cloth is completely wetted out, there is no excess resin. Rolling is a simple, rapid way to eliminate the floating of glass cloth on resin, bring excess resin to the surface and remove entrapped air. Stippling laminate with a short, stiff brush will also give satisfactory results. Ideally a laminate should consist of 60% glass to 40% resin by weight. This is an approximation and is dependent to some extend on the use filled versus unfilled systems.
11. After approximately two layers are applied as described above, larger sections of cloth (up to 1/3 of the entire surface of the tool) may be applied diagonally across the tool surface. Each succeeding layer should be rotated to avoid alignment of the cloth joints. Continue the above process until the desired thickness is attained. Do not stretch the cloth or warping may result when the tool cures or undergoes temperature cycling.
12. When the laminating totals a thickness of 4 to 6 mm (3/16" to ¼ ") allow the laminate skin to cure until at least tack-free.
13. Do not apply more then 12 layers (6 mm -1/4") at any one time since the exothermic heat generated by the plastic may cause excessive shrinkage or warpage of the finished tool. If excessive heat is noted at any time, it is a good rule to stop until the material "sets-up" and has cooled off. If surface is glossy, it should be sanded before next layer is applied. To eliminate hand sending, put down a peel
14. Ply or alternately sprinkle dry, fine aluminum granules, or silica sand on the last layer of laminate while it is still wet. This will result in a rough surface which provides better bonding when laminating is resumed.
15. If a framework is to be added to the laminated tool facing, this should be done immediately after laminating, before removing the plastic tool from the model.
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2. Surface Cast Tools
A surface cast tool usually consists of a metallic core, rough cast to the general shape of the finished tool. This core is suspended over a model of the working surface of the tool and liquid plastic is then cast into the space between the model and the metallic core. Either epoxy of polyurethane may be used. Preliminary preparations are made so that the plastic cast bonds tenaciously to the metallic core, but will part cleanly from the model. The resultant tool will have exact shape and finish quality of the mode;. Generally speaking, surface casts are made against a metal surface such as kirksite, aluminum or steel. Bulk casting made of REN materials are used as cores for surface cast tools. This type of tool is used in some industries for metal forming dies. In the foundry industry, it is used to make patterns of core boxes. The dimensional stability of surface cast tools is somewhat less then that of laminated tools.
Building Procedure
1. The core must be first made, and this is usually produced in the kirksite or aluminum foundry. A rough pattern is made to cast the metallic core. The working face, that is, the face on which the plastic surface is to be cast, is made approximately ½" smaller than the finished surface to allow space for the surface casting plastic. This pattern is then rammed into foundry sand and into the kirksite or aluminum core cast. If a plastic core is required rather then the kirksite or aluminum, refer to the mass casting section.
2. The aluminum or kirksite core can be used as it has been cooled to room temperature. The working face is usually cleaned by sand blasting. If the core is not to be used within 24 hours after sand blasting, paint the surface with a REN liquid plastic tooling (laminating or casting type) to prevent oxidation of the surface. When ready to be used, the painted surface should be sanded with denatured alcohol.
3. Prepare the surface of the die model or plaster splash in the same manner as for laminating.
4. Depending upon the surface cast method to be used, set up the core and die model for the casting operation. It is usually advisable to try to core in the mold before the resin is added, especially when the squash method is being considered. This should be done before mixing the raisin and hardener,
5. Check set-up to see if air could be entrapped by the casting plastic as it enters the mold. Drill went holes through the core in these areas or elevate one end of the set-up to eliminate the possibility of any air entrapment. Always have core on top of die model since this will keep any small air bubbles away from the working face of the tool.
6. Mix plastic materials thoroughly according to directions on the can. When pouring into the mold, allow a steady steam of material to flow until the complete cast is finished. Do not change position of pouring as this may entrap air.
7. After proper curing period, tool may be removed from the master.
8. For an alternate method of core construction mold surfaces are lined with a polyethylene sheet. A layer of modeling clay (the clay provides the thickness allowance for the surface casting mixture),1/2 to 1 cm (3/16" to 3/9) thick is applied to the lined mold and draped with a second polyethylene sheet, When draping the second sheet it is not necessary to eliminate slight creases: these give the core surface irregularities which aid bonding to the facing mixture. The sand or washed gravel-loaded mixture (either laminating or surface casting mixture may be used) is trawled and tamped into the clay mold and around vents located where air locks are likely to occur when surface casting mixture is applied to the complete core. The core mixture is cured at room temperature.
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3. Mass Cast Tools
Mass cast tools are usually made of plastic material; however, metal insert are sometimes built into the unit fir added strength at certain stress points. Either epoxy or polyurethane may be used. Mass casting of plastics is usually used where a large tool surface is involved, such as an aircraft stretch block or hamerform. Here a material is required which will provide good physical properties, is relatively inexpensive to use and can be cast in large, thick sections up to 60 cm (24") thick. The latter is possible in plastics tooling with the aid of suitable heat absorbing filler materials. The dimensional stability of a mass cast tool is lower that that of a laminated tool but is still well within the normal tolerances of stretch forming operations.
Building Procedure
Mass casting has the obvious advantage speed and ease of application. It also eliminates the cost of producing and preparing the metal cores needed to surface casting. Mass casting can be poured up to 4 in thick or even thicker if fillers are used to reduce the build up exothermic heat. These advantages emphasize the savings potential of mass casting. There are also some disadvantages, however, and these must be considered before the system is chosen to build a tool. One of these disadvantages is the low strength properties of most mass casting materials. Even the addition of fillers such as sand and gravel, aluminum granules or grain, glass balls, volcanic ash, pumice, cork or ground nut shells do not appreciably increase the physical properties of this type of construction. Another possible disadvantage is the high shrinkage encountered, particularly in heavier castings. This is due to the heat generated when the casting is cured. In the past this shrinkage has made it advisable to built a laminated surface first and then back it up with a casting material, or cast the tool oversize and rework the surface. Recent developments in plastics have greatly both of these objections. Current casting systems offer a considerable increase in physical properties while permitting casting from 2.5 cm to 10 cm (1" to 4") thick, with out generating excessive heat with the accompanying high shrinkage. The preparation of the mold for mass casting is the same as that used for surface casting. Following preparation, the required amount of is poured into the cavity, allowed to cure for the necessary length of time and then pulled from the mold.
References:
Epoxy Plastic Tooling Manual, CIBA-GEIGY Corporation
tprodan@andrew.cmu.edu