Design guides for molded plastic asbestos-reinforced products: this article describes the standard design procuedures that were used to form molded plastic products that contained asbestos as fabrics, fibers, or filler. This articles series about the manufacture & use of asbestos-containing products includes detailed information on the production methods, asbestos content, and the identity and use of asbestos-containing materials.
This article series about asbestos plastics & molded materials describes the history, manufacturing process & uses of asbestos plastics and molded materials such as asbestos reinforced handles, the Vanguard rocked nose cone, automobile parts & housings, electronic equipment (radar scanner), asbestos-filled Teflon, rocket motor parts, plastic drop tanks for the Hawker Sea Hawk, and hundreds of other products.
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Figure 9.5. Autoclave or low-pressure curing of asbestos-phenolic is standard. A rubber bag is placed over the lay-up and vacuum-pipes, and sealed to the base plate with chromate paste. - Courtesy The Martin Co. [Click to enlarge any image]
1. Maintain uniform wall thickness.
2. Avoid sharp corners (use liberal radii).
3. Use maximum draft.
4. Avoid undercuts.
5. Consider mold shrinkage in both the direction applicable to the compression load or force and the direction perpendicular to the load.
6. Design to practical dimensional tolerances.
7. Design to avoid warping.
8. Design molds so that material can be removed from the mold in order to compensate for shrinkage of parts.
Any method which has been devised by industry to fabricate plastics, ceramic and cements has been used with asbestos plastics.
The aforementioned rules apply to both metallic and nonmetallic molds. Of principal interest are the steel molds. Mold material depends upon surface desired, production run and complexity of parts.
Tool steel, Kirksite, aluminum Mee hanite and cast iron are common materials. Metallic oxideepoxy molding compounds are also used for the rnanufat of similar types of molds. The various types of materials which are available and are used provide too f. different surface characteristics. In the case of asbestosPh l. compounds, hard chrome plating of the polished Steel molds is desired in order to permit ease of release.
With regard to steel metal dies, it is important to flame harden cutting edges when molds are made With cutting edges. Steel of Rockwell hardness C50 to C70 should be used (1040 or 4140 steel). The 1020 steel should be used for short runs. This type of steel is inexpensive in comparison to the others. When weld joints are to be made in the mold, the 1020 steel will weld easier than the 1040 steel. When designing with guide pins, the pins should be located on the bottom mold.
When fabricating parts at low pressure by the bag molding technique (see Figure 9.5.) or when limited runs are to be made, the plaster type of mold is sometimes Used The Hydrocal * type of plaster is used. This type of material provides for low cost tooling so that prototype and production molds can be made. The material can be Used for very complicated parts. These types of products provide for average compressive strength ranging from 4,000 to 11,000 psi.
* U.S. Gypsum Co.
Compression molds or matched metal die molds are widely used for long production runs and for close tolerance on parts. Molds are heated by steam, hot water or electricity. Dielectric heating is used too. The specific Cure Cycle for the compounds depends upon the type of compound used. The manufacturer's recommendation for the molding compound should be considered. However, trial and error runs are generally required in order to obtain the most desired curing cycle for specific shapes and sizes of parts.
Other methods of fabricating asbestos base molded parts include transfer molds, extrusion molds,* post-forming, autoclave molding, pressure bag molding, vacuum bag molding and spatula application of no pressure-low curing temperature type asbestos-phenolic molding compounds.
* Schneider, E., "The Manufacture of Phenolic Molding Materials by Extrusion Compounding," British Plastics, (Nov., 1958).
In this chapter there are various photographs shown of different parts which have been made up using these various techniques.
In most fabrication plants, the molding compounds or preform tablets made from molding compounds are preheated. High frequency heating has proved to be the ideal way to accomplish preheating. The advantages of preheating are:
1. It is not necessary to first heat the molding compound in the mold.
2. The molding cycle can be shortened.
3. The material is evenly heated and thus the viscosity is equal throughout the bulk of the compound, leading to a better flow.
4. The soft preheated molding compounds do not scratch the mold and at least part of the moisture content can be removed; there is less tendency to form blisters and uniform shrinkage characteristics are permitted.
Generally, preheating temperatures of 140 to 300°F are used. The specific temperature depends upon the thickness of the compound and the hardening velocity of the material.
With regard to different methods of curing asbestos filled or reinforced plastics as well as other types of products— the shock-curing technique was developed in England during the period of 1951 through 1953. This procedure permits curing of asbestos-phenolic laminates in approximately onefifth of the time normally required and with a great saving in power.
The electric shock curing theory is believed to involve a current which may serve as a catalyst, or produce an orientation of the molecules. Most of the experiments have been conducted on asbestos-phenolic.
British Patent 737,374 states that the uncured resin must be at least moderately conductive so that it can serve as its own resistance heater. Large-area electrodes in contact with a laminate and insulated from the press platens are powered with line-voltage a.c. There is a sudden rise in the current, followed by a rapid fall to a low value, and the cure is completed.
For example, a laminate has been compressed between polished copper platens at a pressure of approximately 200 psi. A 50-cycle line at 50 V was used which caused the current to rise in a few seconds from .5 to 2.5 amp per sq in. and then to drop to zero of its own accord. The temperature of the cured material reached 160 to 200°C.
Variables exist which require different current conditions. The dampness of the material is important. The damper the material, the lower the required voltage.
The thickness of the material is another variable. The higher the voltage, the more rapid the cure; although, for minimum power consumption there is an optimum voltage. Variations in the frequency of the current between 50 and 500 cps have little effect upon the curing time. Frequencies of 2,000 cps double the curing time required. When direct current is used, difficulties occur.
In order to meet modern high production schedules, commercially available automatic presses are designed to handle molding compounds. The volumetric metering methods are predominantly used for controlling the amount of compound being fed into a mold cavity. The volume method however has been satisfactorily used. New designs have been made which provide for accurate and dependable measurement of compound by weight.
In automatic molding, large production asbestos-phenolic compounds are used for automotive parts (ignition rotors, distributor caps, etc.). Combinations of automatic compression molding, powdered preheating and experimenting by General Electric have resulted in a saving of a minimum $65,000 the first year. Figure 9.6 provides general information concerning this combination.
Preheated charge is in cavities. It has a maximum capacity of 7.5 kw and makes possible production increases of from 30 to 100 per cent or more. It is used here in conjunction with a fully automatic Baker press. - Courtesy General Electric Co., Switch Gear Dept.
Properties & Manufacture of Asbestos-Reinforced Plastic Laminates
This type of product is a separate type from the general powder or compound molding product. It consists of layers of resin-impregnated sheet materials.
With regard to asbestos products, it concerns resin-impregnated felts, cloths and paper which have been pressed into such shapes as flat sheets or tubes, between heated platens or in a mold.
The resulting products have excellent mechanical and electrical properties and find numerous applications in engineering fields.
For the past few years [to the early 1950's - Ed.] there have been very important projects in research and development which have resulted in production of high structural-heat resistant reinforced plastics with the relatively long asbestos fibers. In a laminated structure, the prime function of the resin is to act as binder. Structurally, it plays a secondary role to the reinforcement material.
The resin provides for a means of positioning the reinforcement and interconnects the fibers so that loads can be transferred from one fiber to another. A secondary function of the resin is to stabilize the fiber material. A single unsupported asbestos fiber would collapse under the weight of a very small load. When resin surrounds the fiber, the fiber is stabilized so that it can carry a load up to a point where the asbestos fiber fails.
The resin takes approximately only one to two per cent of the load.
During loading, the resin will fail before the ultimate strength of the laminate is reached.
Figure 9.7. Complete automatic resin content control system for resin-impregnation in horizontal treater. Beta-ray gages are used to measure density of untreated sheet prior to resin dip operation and treated sheet after drying. Courtesy General Electric Co.
The strength of the reinforced plastic is influenced by the degree to which the resin completely surrounds the asbestos fiber. Insufficient wetting by the resin leaves portions of the asbestos fibers unsupported and reduces the strength under loads. Another important factor is the degree of fiberization of the asbestos.
Where resin impregnation treating is used on an extremely large production run, precision equipment can be used to control resin content automatically. In Figure 9.7, automatically controlled squeeze rolls are used to preset resin content in addition to the other general procedures of liquid solids content, etc. The drying ovens range from 75 to 200 ft in length. A beta-ray gage permits uniform and desired resin concentration.
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