Injection Molding MAURICE L. MACHT, WALTER E. RAHM, AND HAROLD W. PAINE E. 1. du Pont de Nernours 8, Co., Inc., Arlington, N. J.
HE purpose of this paper is to familiarize rubber techT nologists with the art of injection molding, as well as to indicate the potentialities of this relatively new technique. It is essential that we have an understanding of just what is meant by the term “injection molding”, since even recent literature is somewhat confusing (2). The process is similar to the early extrusion method of Hyatt (II), which consisted essentially in the use of a hydraulic or screw extruder to force a plastic of nitrocellulose or of other material, containing solvent, into a die. Later modifications of the Hyatt process involved the use of compositions having volatile solvent which it was necessary to remove by seasoning before the article would be ready for use. The injection process should be differentiated from the earlier techniques, sometimes referred to as pot or transfer molding, which consisted essentially in the use of a pot or well, either separate or built integrally with a die, both of which could be heated so as to plasticize either a thermosetting or thermoplastic material (3). The assembly was heated and the entire charge forced from the pot into the hot die cavity, after which the entire apparatus was chilled (when thermoplastics were used) and the piece or pieces were removed. It should be differentiated also from the so-called extrusion molding which comprises the adaptation of the familiar rubber extruder or tubing machine to the extrusion of forms or shapes which are suitable for use as finish or trim strips as well as in articles which might require a minimum amount of further processing. Today, the term “injection molding” is used to describe the process which consists of the feeding of a granulated solid and, in general, thermoplastic material in a measured quantity into an electrically heated cylinder where it is forced intermittently and progressively forward through the cylinder by a plunger which may be hydraulically, mechanically, or pneumatically actuated. It finally passes through a nozzle or orifice into a tightly locked die, maintained a t a temperature which is sufficiently low to permit the heated plastic to
cool so as to become rigid enough for ejection without losing its shape. Figure 1 shows the difference between this technique and compression molding. The process of injection molding seems so simple that one is led to wonder why its development was so long delayed, for it did not achieve wide popularity until recent years. There are perhaps many reasons for this delay; primarily it was due to the lack of a suitable inexpensive heat-stable plastic, capable of being softened by heat. Lacking such a plastic, the obvious and outstanding advantages of this procedure were not commercially realized. With the advent of heat-stable cellulose acetate the use of the procedure developed almost overnight, for even as late as 1933 i t was thought to be of little consequence (2@, and comprehensive discussions of the method did not appear in the literature until much later (9,3).
Advantages Injection molding offers the following advantages: (a) a high speed of production which is possible because of the rapid press operation as well as the lack of necessity for alternately heating and chilling, or for curing; (b) a low mold cost made possible by the smaller number of cavities required per mold for a given production; ( c ) a greater convenience in setting up the dies, because they are of lighter construction than those used in compression molding, (d) a lower finishing cost after molding, since flash, which is considered inevitable in compression molding, is not formed in a well-made injection mold; (e) a small loss of material, since defective articles and the material forming the channels connecting the cylinder and mold can be r e used; (f) a high thermal efficiency of this system, in which the heating cylinder and chilled molds are maintained a t constant temperatures throughout the cycle. These six factors add up to lower production costs for articles of higher quality. The capacity of the earliest injection molding machines
Papers appearing on pages 563-581 were presented before the Division of Rubber Chemistry at the 1 0 l s t Meeting of the American Chemical Society, St. Louis, M o .
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Figure 1.
Injection and Compression Molding
was so small, they were mechanically so imperfect, and the labor cost per article was so high that commercial use was limited to the manufacture of articles of small cross section and of intricate shape not readily made by compression molding (2%). This early type of injection machine with a capacity of only a few grams could turn out only a single article on each stroke, on a cycle allowing only three strokes per minute and producing 180 articles per hour, and required the undivided attention of an operator. If this same article could be made by compression molding, it could be turned out a t a lower cost; for a ten-cavity compression mold on a cycle of 2 minutes would produce three hundered articles per hour, and one operator could tend two such jobs. The present widespread commercial use of injection molding has been made possible primarily by large increases in the capacity per stroke, by increases in the number of strokes per minute, and by the release of part of the operator’s time for finishing the articles as fast as they are molded. The two latter improvements have resulted from the provision of more effective heating and from the development of completely automatic mechanical operation.
The forerunner of today’s high-speed automatic injection molding machines was first developed in Germany about fifteen years ago. The early work was done by one concern, using a simple, manually operated cam and lever rig which had a capacity of only a few grams. Their next step involved the use of air pressure for powering the stock cylinder, which was increased in size and was reputed to have a capacity of 2 ounces. Meanwhile, experimental work on hydraulic machines of the same general type had been started both in England and in our laboratories a t Arlington, N. J., and there were rumors of similar work elsewhere throughout the United States. During 1931 hand-operated pneumatic machines were imported to this country and were found to have a p proximately half the capacity claimed for them by their German makers. At the same time the articles made on such a machine were of inferior quality, primarily as a result of lack of uniform heating of material passing through the cylinder. This obvious defect was reduced, if not eliminated, through the use of a so-called breaker (1.2) or pineapple located in the front portion of the cylinder, whose function was to break or distribute the flow of stock so that the heat would have to transfer only through a thin surface layer rather than to the center of a thick plug or core. Thus the adaptation of a principle established by one of the pioneers in the plastics industry, J. W. Hyatt, played an important part in bringing this new technique into commercial utility. The manually operated pneumatic machines were soon followed by automatically controlled pneumatic as well as mechanical (3) machines which could be operated on cycles as fast as four and eight charges per minute, respectively. They could handle from 15 to 25 grams per charge. These early machines were small in capacity, high in cost, and lacking in the ruggedness of design necessary to withstand the pace of American industry. Consequently a number of American manufacturers designed and built machines which were larger and sturdy enough to stand 24-hour operation. These new machines can today be operated as rapidly as twelve cycles a minute, although the highest speeds are not in great favor since there are too many possibilities of spoiled work or accidental damage to dies if an article sticks in a die cavity or if any of the mechanism fails to function properly. They have capacities as high as 16 ounces per shot for a single heating cylinder. Other machines having multiple cylinders are used in the production of moldings weighing as much as 36 ounces. While very short cycles at these large capacities are possible, it has not in general been found feasible to operate machines of highest capacity a t maximum speed on articles of heavy cross section, because the thermal conductivity of current plastics is such that
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Figure 2.
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Injection Press of The Hydraulic Press Manufacturing Company
appreciable time is required for the chilling or setting of the piece after the material is forced into the die. Consequently the shortness of the cycle is frequently limited by the thickness of cross section of the article rather than by cylinder capacity. The temperatures established as best for a given job are maintained either by direct contact with band heaters, by induction type heaters, by the circulation of heated oil, or in some cases by means of integral coring through which steam can be passed. The use of accurate control instruments has played an important part both in shortening the molding cycle and in improving the quality of the articles produced. Pressures on material in the heating cylinder may be of the order of 12,000 to 30,000 pounds per square inch, depending upon the type of plastic as well as the size and shape of the article being molded. Material cylinder temperatures similarly may be maintained as low as 265" or as high as 500' F. or more. I n practice, small thin articles are molded a t rates of four to eight cycles with occasional production items running to twelve per minute, while very large thick pieces may require from 1to 3 minutes per cycle. The temperature of the mold is of primary importance. Means should be provided for circulating either cold, warm, or hot water through the mold throughout the molding operation. Regardless of the exact temperature, i t is desirable to maintain it fairly constant. Recently, extremely thick articles such as hairbrush backs with thicknesses as great as one inch have been successfully molded commercially through the use of what would have heretofore been considered excessively high die temperatures. The mold is suitably clamped or locked between platens which, on some of the commercial machines, are 36 X 48 inches in size and must resist the internal pressure over the projected molding area without yielding or opening. Locking pressures available on commercial presses range between 75 and 500 tons, depending on the size of the machine.
Typical Commercial Machines Today, as a result of further closely cooperative develop ment work between machine manufacturers and custom
Figure 3.
Injection Press of D e Mattia Machine and Tool Company
molders, there are three clearly defined and well-designed types of injection molding machines: COMPLFITDLY HYDRAULIC.Hydraulic power is used for the closing and locking of dies, as well as for the forcing of the plastia through the material cylinder (Figure 2). MDCHANICAL. All operations are mechanically carried out (Figure 3). Power is supplied from a crank for locking mol&
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Comparative Costs of Injection and Compression Molding Some idea as to the economy and speed of i n j e c t i o n molding in comparison with conventional compression molding commonly used for thermoplastics and rubber is given in the case of a certain a u t o m o b i l e radiator ornament which was first produced by the compression method. Under the best operating conditions, twentycavity molds were required to produce one hundred and seventy pieces per hour because of the time required for heating the mold, for heating through the cold powder, and finally for chilling the piece prior to its ejection. When the manufacture of this ornament was transferred to the injection process, & twocavity die produced the same number of pieces per hour, and less finishing was required since there was no flash t o be removed. While the saving in mold costs is n o t directly proportionate to the number of cavities required, it is a considFigure 4 (Above). Injection Press of Reed Prentice Corporation erable item. InjectionFigure 5 (Below). Injection Press of The Watson-Stillman Company molded pieces have no flash or fin. I n many i n s t a n c e s o n l y very small channels are used, the resulting stalks are and forcing the plunger forward. Follow-up pressure is mainmerely broken off a t the edge of the article, &d no furtained by springs. ther finishing is required. Slightly heavier stalks may be COUBINATION HYDRAULIC AND MECHANICAL. Hydraulic clipped while the pieces are still warm. This contrastsfavorapower is used t o force the material through the stock cylinder, bly with the compression technique which must have flash and the dies are locked by a mechanical toggle or wedge, which in general is actuated by a second hydraulic cylinder (Figures 4, 5, removed by filing, grinding, or scraping, with a consequent and 6 ) . loss of material and increase in labor. However, the manufacturers of compression molding It is estimated that there are approximately a thousand equipment have made numerous improvements, such as of these three types of injection machines in commercial semiautomatic and automatic presses, and i t is recognized use in the United States. While each specific type has its that for some purposes compression molding has advantages proponents, they all give eminently satisfactory service. over injection molding. This is particularly true in the While there seems to be no limit to the capacity of a macase of very large articles. chine which might be developed, practical charges in the field To illustrate the cost advantages of injection over comtoday are generally less than one pound. Nevertheless, pression molding for a typical article approximately 0.25 inch there is at least one manufacturer of such equipment who thick and weighing 56 grams, the comparative estimates feels that the ultimate size of injection machines will depend shown in Table I were made. These figures do not represent only on what the purchaser is willing to pay. This process is a complete over-all cost, in that items other than cost of ideally suited for application where large numbers of small material which are equal in both cases have not been inarticles are to be produced a t high speeds. cluded.
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Uses Articles which are in commercial production by this process vary from a tiny doll’s eye, weighing only a fraction of a gram, to the windows in the housing of a roadside gasoline pump, weighing several pounds; others are housings for accounting machines, speedometers, clocks and instruments, dashboard panels, sunglass frames, telephone housings, plates, novelty jewelry, etc.
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The possibilities of a technique of this sort seem to open new vistas to rubber technology, provided rubber could be compounded with vulcanization accelerators which would not cause vulcanization a t temperatures required for flow but would cause the rubber to vulcanize completely in a short time after injection into a die.
Aclrnowledgment
The authors are indebted to A. F. Randolph for his assistance in reviewingand revising this paper, to J. E. Teagarden for a compilation of the bibliography, and to The Hydraulic Press COSTSOF INJECTION AND COMPRESSION Manufacturing Company, The Watson-Stillman Company, TABLE I. COMPARATIVE MOLDING Reed-Prentice Corporation, De Mattia Machine and Tool Company, and Lester Engineering Company for valuable Compression Injeotion suggestions, technical data, and photographs. Material $0.1050 $0.1050 Molding and finishing labor Addi,tional finishing and polishing, packing, shipping Die cost Amortization and interest on equipment cost Electricity, steam, water Total
0.0094 0.0026 0.0080 0.0024 0.0070
0.1343
0.0047
0.0005 0.0020
0.0027
0.0010
0.1169
The use of the injection molding technique for thermosetting compounds such as phenol-aldehyde resins has been studied experimentally by a number of equipment manufacturers (2, 3) as well as by producers of such thermosetting compounds. So far as can be ascertained, there has been little or no commercial use of this technique; however, satisfactory articles have apparently been produced by modifying the technique outlined above for thermoplastic materials to the extent of using comparatively low cylinder temperatures in conjunction with elevated die temperatures to cause a final setting up in the die rather than in the cylinder.
Figure 6. InJection Press of the Lester
Bibliography (1) Adams, W. H., Jr., Trans. Am. Inst. Chem. Engrs., 30, 317 (1933). (2) Amigo, Alfonso, Brit. Plastics, 10,579-81 (1939). (3) Zbid., 10,628-34 (1939). (4) Amigo, Alfonso, Plasticheskie Massy, 5 (1935). (5) Anonymous, Moltthly Rev. Am. Electroplaters’ SOC.,20, No. 11, 24-8 (1934); Brit. Plastics, 2, 296-300, 574-6 (1930-31),9, 307 (1937); C h m . Age (London) 1933,607-8; India Rubber J . , 81, 11 (1931): Industrial Bull. of Arthur D . Little, Znc., 166,3-4 (1941); Iron Age, 140,No 12,40-5 (1937); J. SOC. Dyers Colourists, 53, 82-6 (1937); Mem. Proc. Manchester Lit. & Phil.XOC., 83, 161-74 (1938-39); Rubber Age (N. Y,), 20, 149-51 (1939); Trans. Inst. Plastics Ind. (London), 5, No. 10,92-107 (1936); Tram. Znst. Rubber Ind., 12, 124-39 (1937). (6) Barnette, L.T.,Modern Plastics, 17,232-8 (1939). (7) Brandenburger, Kurt, Kumtstoffe, 24, 194 (1934); Plastische Massen Wiss. Tech., 6 , 80, 130 (1936), 7, 172-4 (1937); “Processes and Machinery in the Plastics Industry”, 1st ed., pp. 27-47, New York, Pitman Pub. Corp., 1938. (8) Breskin, C.A., IND. ENO.CHEM., 27,1140-2 (1935). (9) Duli, H.,Plastische Massen Wiss. Tech., 5 , 254 (1935). Ellis, Carleton, “Synthetic Resins and Their Plastics”, PP. 358 et sea., New York. Chemioal Catalog Ci,-1923. Hyatt, I. S., and Hyatt, J. W. (to Celluloid Mfg. Co.), U. S. Patent 133,229 (Nov. 19,1872). Zbid., Figs. 4 and 5. Hyatt, J. W., U. S. Patent 114,945 (May 16, 1871). Hyatt,- J. W. (to Celluloid Mfg. Co.),Ibid., 202,441, Fig. 1 (April 16, 1874). Kirkpatrick, S. D.,Chem. & Met. Eng., 42, 539 (1935). Laeis, E. M., Kunststoffe,27, 70-5 (1937);28,62-4 (1938). Linder, H., Plastische Massen Wiss. Tech., 6 , 272 (1936). Merrill, J. K.,Modern Plastics, 18,65-7 (1940). Morrell, R. S.. “Synthetic Resins and Their Allied Plastics”, p. 115, London, Oxford Univ. Press, 1937. Novotny, E. E. (to J. S. Stokes), U. 8. Patents 1,993,942,(March 12, 1936),and 1,997,074(April 9,1935). Pelikan, K. A., Modern Plastics, 7, 19 (1931). Rahm, L. F., “Plastic Molding”, pp. 4, 14,New York, McGraw-Hill Pub. Co., 1933. Randolph, A. F., C h m . & Met. Eng., 44, 25-8 (1937). Rivise, C. W.,Modern Plastics, 7, 13,53 (1931). Shaw, F. H., Plastic Products, 9,310-13 (1933). Shaw, L. E. (to J. S. Stokes), U. S. Patents 1,916,495(July 4, 1933), and 1,919,534(July 25, 1933). Spencer, H. S.,Modern Plastics, 7,35 (1931). Sproxton, F., Chemistry & Industry, 54, 435 (1935). Stadlinger, Hermann, Chem.-Zfg., 56, 409, 431 (1932).
