Twentieth century blow molding.
Modern blow molding can be said to have originated in the 1930s, but plastic materials were blown in ancient Egypt and Babylon, and as early as a thousand years ago by peoples in Africa and South America. In the U.S. in the 1880s, celluloid was blown into items such as baby rattles, brush backs, ping pong balls, doll parts, cutlery handles, and jewelry. Methods involved softening sheets with heat or soaking in liquids followed by confining in molds and blowing with air. Hollow articles made from celluloid and cellulose acetate continued to be made this way into the 1930s.
This article details the technical developments in blow molding since 1930. From the beginning to today, the basic incentives for blow molding plastics have been to make articles that cannot be made by other processes or of other materials, or to make them more economically.
Blow Molding, 1930-1944
Enoch T. Ferngren is credited with being the first to make blown plastic items in the modern era. With William Kopitke, he made a mechanical device, sold to the Hartford Empire Co. in 1937, to be used for blow molding. The Plax Corp. was formed for blow molding and by 1940 had a production machine blowing hollow Christmas tree spheres from cellulose acetate. Owens-Illinois Glass Co., also by 1940, blew all the available injection moldable plastics on a modified ram injection machine into 20-cc bottles and tested them for packaging common products. They supplied 100-cc polystyrene bottles to the U.S. Army Medical Corps during World War II. Very few products could be acceptably packaged in the plastics available in 1940.
In this period, blow molding of plastics was attempted with extrusion equipment designed for rubber and usually steam heated, compression presses, ram injection presses, or chemical processing equipment equipped with electrical and mechanical devices. Hydraulics were used for simple motions, and complicated movements were made mechanically, electrically, or pneumatically. Existing equipment did not lend itself to blow molding, and it was necessary to innovate to achieve volume production. By 1944, five companies were doing enough blow molding to be considered to have commercial operations.
Cellulose acetate and polystyrene were being blown in the U.S. and polyvinyl chloride (PVC) in Europe. Documentation of any technical information was not made before 1944, and all equipment was designed by blow molders. The progress made in this period is outlined in Table 1.
LDPE Period, 1945-1956
Low-density polyethylene (LDPE) was patented in 1937, and the first plant was built in 1939. However, all production was used in the war effort, and small quantities first became commercially available only in 1945. Plax had blow molded some experimental bottles from scrap LDPE in 1942, but squeeze bottles were not commercialized until late 1945. By 1949, 23 million bottles/yr were being made and more than 100 different products packaged.
LDPE opened the door for innumerable applications because it is an inert, stable plastic that is easily blow molded. It has desirable physical properties, excellent chemical resistance, and permeation resistance to many liquids, particularly water. All the larger container manufacturers entered into blow-molding development programs. Many plastics machinery builders and molders modified existing equipment for blow molding, and resin producers began research programs to supply improved or new blow-molding materials. In the U.S., the heat sensitivity of PVC discouraged blow molders, but PVC dry blends were being blow molded in Europe. The expanded activity in this period, summarized in Table 2, shows fundamental systems starting to take shape.
Although the plastics industry grew considerably from 1945 to 1956, growth in blow molding was slow and had low priority. Equipment for making large plastic moldings by any means had not been developed, and the focus in the blow-molding business was on small items only. There were never more than seven blow molders at one time prior to 1957, and the 1952 SPI Directory listed only thirteen molders who would do bottle manufacturing.
James Bailey, in an outstanding article in 1945, outlined fundamental approaches to blow molding (Modern Plastics, Apr. 1945, p. 127). He described the indirect, diaphragm, and direct methods, and he predicted the success of extrusion blow and injection blow.
In addition to LDPE, various inventions and improvements emerged between 1945 and 1960. The wheel machine enabled the use of multiple molds to gain high volume. Systems were devised to make large moldings such as carboys. Injection molding was beginning to use reciprocal screws, and plasticating capacity was increased by the use of preplasticizers and accumulator systems. PVC blow molding was expanding in Europe. Commercial blow molding machines were being offered for sale. Blow molding equipment was ready for improved materials to be developed to fulfill users' demands for plastic products.
Early HDPE Period, 1957-1964
Commercial quantities of a new type of polyethylene became available in 1956. High-density polyethylene (HDPE) was considered a rigid plastic in comparison to LDPE; it was stronger, and it provided the overall properties to make products economically competitive with glass, metal, paper, and other plastic products.
At the same time, processes were developed to utilize other plastics as they became available, and PVC, polypropylene (PP), and polycarbonate (PC) became candidates. These plastics could provide properties lacking in the polyethylenes--clarity, grease resistance, rigidity, resistance to specific chemicals, less permeability to specific liquids or gases, and higher temperature resistance. Other plastics were being investigated and formulations were being developed; in particular, modified acrylics were studied for food containers. Limited blow molding was being done with cellulose acetate, polystyrene, nylon, acetal, and blends.
The blow molding by many different methods of all these materials expanded the areas for development. Stress cracking needed to be overcome, and standard tests were developed. Labeling difficulties led to flame treating and other surface treatments. Many empirical tests were developed for materials and products, but standardization was not accomplished by 1964.
The technology to really understand polymer properties was extensively enlarged during this period. Rheological, morphological (electron microscopy), molecular-weight, and other studies of important factors contributed to improvements in blow molding.
By 1959, there were 97 blow molders and 24 foreign and domestic blow-molding machinery manufacturers, increasing in 1960 to 100 and 55, respectively. In 1964, some 350 to 400 machines were reported to be in use, 75% of which were owned by six companies. By 1963, 195 million bottles/yr were being made. Asian-built machines began to enter the U.S. in this period.
In Europe in 1960, extruded tubes were being cut to parison lengths and then blown, and a machine was reportedly producing 10,000 bottles/hr by this process. In 1963, three companies were supplying small injection-blown containers, and Continental Can was making 75,000 HDPE quart bottles/day on a huge wheel machine.
At the end of the early HDPE period, which is outlined in Table 3, blow molding was well established with fairly good equipment, but many refinements were still necessary. Materials were being held to tighter specifications, and improved formulations regularly entered the market. Extruders were designed specifically for blow molding with better screw designs, ram-type accumulators were performing well, reciprocating screws had found their place, free extrusion was widely used, calibrated necks had almost the quality of molded necks, and many novel designs were perfected. Molds were well designed with more durable metals, better cooling and venting. Variable orifices and controlled extrusion rates produced better parisons. Downstream operations of handling moldings, deflashing, trimming, reaming necks, flame treating, and decorating were automated.
HDPE Period Ends, 1965-1970
HDPE continued to dominate the blow molding industry throughout the remainder of the 1960s. By 1966, 280 dairies were using HDPE bottles, and the first generation of lubricating-oil containers was being made. Despite considerable investment, an effort to win public acceptance of returnable plastic milk containers was unsuccessful. In-house blow molding grew tremendously--in the dairy industry, which had 500 machines in operation, in the pharmaceutical industry, for cleaning products manufacturers, and in other market areas--for many reasons, but primarily because reliable molding equipment and systems could now be purchased. As equipment became more sophisticated, many manufacturers were unable to meet the competition and withdrew.
The shortcomings of polyolefins led to many efforts to improve properties via treatment. Fluorination and other chemical treatments had some success. While radiation improved many properties, the process was impractical. Coating with materials such as polyvinylidene chloride (PVDC) and vinylidene-acrylonitrile copolymer was considered too difficult. Materials other than polyolefins were clearly required, and they would need innovative processing.
PVC continued to be used extensively in Europe, particularly for blow molding bottles, and it was an active candidate in the U.S. for its clarity, chemical resistance, and barrier properties. However, it did not progress much beyond the experimental stage in the U.S. until the late 1960s. Early on, it was shown that free extrusion processes could be successful, provided that corrosion-resistant metals were used, temperatures closely controlled, and equipment operated to prevent parison sag. PVC suppliers developed more heat-tolerant resins and formulations, and changed additives to gain FDA approval. At this time, PVC created many problems with reciprocating screws, ram accumulators, and injection molding.
Experimentation resulted in a new process, stretch blow molding, in which the polymer molecules of a molding are oriented by stretching the parison vertically at a temperature further below the melt temperature than normally used and then immediately blowing. The Orbet process was a patented process for this, and commercial machines were engineered to stretch blow PP, PVC, and other plastics. Oriented PVDC was produced in Japan, but it did not become important in the U.S. Essentially clear PP bottles were produced by stretch blow molding and marketed for liquid detergents and other products where the desired clarity could not be obtained with polyethylene.
From the beginning, blow-molding machines needed a means to automatically sequence the steps in the operation. Various systems were used, and the technology generally evolved from one operation triggering the next to a central timer signaling all the operations. Signals were usually transmitted electrically to the functioning location, and mechanical systems were eliminated. Temperature controllers had been on-off, limit switches were all over the machine, and timers would start-stop only. One of the first improvements was in gaining control of extrusion by controlling the extrusion rate, the size of the orifice at a given time, and the length of a parison. Anticipatory temperature controllers, advanced timers, better hydraulic valves, and other devices were added so that the programming of every function could be considered. For example, positive feedback and linear motion transducers could sense and regulate a hydraulically operated mandrel, making it continuously variable. Such controls enabled bottle weight to be reduced by 20% to 30%.
Highlights of the period are given in Table 4. In 1970, progress in blow molding the available polyolefins was approaching a standstill, PVC blow molding was well established, and major efforts were being made to improve polyolefins, take advantage of other plastics, make copolymers, coextrude existing polymers, blend polymers, or use additives to penetrate more markets. Equipment manufacturers were endeavoring to provide the machinery to process all the various materials and combinations of materials being offered.
Thermoplastic Polyester Period, 1971-1978
Large expenditures were made in the late 1960s to develop blow-molded plastic containers for soft drinks. Test marketing was done, and, pending FDA approval, it appeared in the early 1970s that blow-molded bottles would replace the containers then being used. Polyacrylonitriles had been vying for sizable usage in blow molding for a number of years, and three such materials were test marketed. However, final FDA approval was not obtained, and the advent of thermoplastic polyesters prevented the acrylics from gaining new markets.
Although thermoplastic polyester films and fibers had been made for more than twenty years, the plastics industry had never been able to mold them on conventional equipment. In the early 1970s, polybutylene terephthalate was compounded to be injection moldable, and polyethylene terephthalate (PET) was processed by injection molding followed by stretch blow molding. With PET's combination of chemical and physical properties and clarity, and with biaxial orientation, lightweight, almost unbreakable products could be made. This led to a revolution in the blow molding industry.
Stretch-blow-molding machines for other plastics did not perform with PET, so equipment makers were faced with a major challenge. Very large numbers of containers were needed, so high volume machines with automatic handling, assembly operations, labeling, and packaging needed to be developed. In the two-step process, parisons are injection molded, cooled to room temperature, and then reheated to below the melting point and stretch blown in a second machine. The one-step process carries out injection, cooling only to below the melting point, and stretch blowing in the same machine. Some two-step processes extrude a tubular parison in place of injection molding a closed-end parison.
PET took over the beverage bottle market and was considered for some HDPE, PVC, PC, and other plastic applications. Injection blow molding continued to grow. Machines were designed specifically for blow molding; most operated with three- or four-station rotary tables. Both horizontal and vertical injection cylinders were available. Extrusion blow dominated the industry, and injection blow was used for small moldings and non-handled ware. Aseptic molding had some success, and several blow-mold/fill/seal/trim lines were in operation.
Blowing of multilayer coextruded plastics to obtain barrier properties was successfully achieved but not commercialized to any extent by 1978. Accumulator-manifold multihead machines, large multihead wheel blow molders, and large accumulator machines that could produce 55-gal drums or larger containers gave the industry the versatility to make almost any product. All-hydraulic power could be used for movements, and high-torque extruders, high-tonnage clamp, and complicated designs were built. Auxiliary equipment was sized to handle large products or high volumes of small items. Engineering resins and heat-sensitive plastics that had previously been avoided could now be blown, and improvements in all plastic formulations permitted better stabilization, better clarity, FDA approval, and easier processing. High-molecular-weight polyethylene in powder form was invented, providing improved strength and other properties in large parts.
A displacement-blow process, which offered processing at minimum temperatures by inserting a core into a fixed amount of molten plastic to form a parison, proved interesting, but it did not replace conventional operations.
Lower-cost, improved electronic controls, together with servohydraulics and microprocessors, sped the conversion from electro-mechanical to solid-state control. Parison length, finished weight, servo-velocity control, pressures, temperatures, and die configurations could now be closely controlled. Digital data analysis linked to management information systems could follow the processing.
Cooling of blown ware to shorten cycles now had a number of options--exchanging plant air, use of carbon dioxide or liquid nitrogen, use of sub-zero dry air or high-pressure moist air, or use of sub-zero air with water injection.
The highlights of the 1971-1978 period are summarized in Table 5. By 1978, the models and types of blow-molding equipment had almost become too numerous to catalog. A very large number of old machines were idle and for sale. Some really obsolete ones were abandoned, but others were purchased by newcomers for blow molding throughout the world. The number of vendors of new equipment continued to diminish, but a few were entering the field, particularly from Asia.
Current Period, 1979-1991
Since 1978, no single development has dominated blow molding. However, outstanding growth has resulted from advancements such as continuous improvement of stretch blow molding; success with multilayers; systems to thermocrystalize PET and other plastics; expanded use of in-mold labeling; handled PET products; new methods for aseptic blowing and filling; use of robots; and improved and expanded use of solid-state controls, including microprocessor feedback warnings.
The most significant change in equipment has been the use of closed loop systems incorporating microprocessor-based, solid-state electronic controls. Sensors monitor all machine functions and send the information to a computer, and feedback from the computer controls operations. Injection rate, pressures, stroke, clamp force, screw rpm, shot size, bottle weight, orifice size, servohydraulic valves, temperatures, parison programs, and other functions can all be controlled. Diagnostics and process monitoring for production control are done by computer analysis.
Auxiliary equipment continues to be improved and enhanced with solid-state controls and computerized operation. Factories are being air-conditioned, and dehumidifying hoods are being placed on machines. Major advances have been made in cooling systems.
Machine output has increased tremendously in recent years, and the size of blow moldings is becoming unbelievable. One type of stretch-blow machine produces 16-oz soda bottles at a rate of 36,000/hr, and another makes 2-liter beverage bottles at 20,000/hr. Preforms for stretch-blow molding are being made at 12,000/hr. In 1986, there were 30 machines capable of blowing more than 725 lbs of plastic at one time. Melt accumulators holding 250 liters are in use, and 400-lb containers are being made by operating four extruders to produce 3300 lbs/hr. Fourteen-ft-long parisons for windsurfer keels are being blown.
Many old machines are being operated at low outputs throughout the world, but retrofitting with the latest engineering technology has become important. In 1988, the total number of established machine builders was estimated to be 30, of which only six remained from the 55 listed in the early 1960s. Most of the machines are made by a few manufacturers; the rest build only a limited number per year. In recent years, the consolidation of blow molders and blowing equipment manufacturers has accelerated, and ownerships are changing often. The sale of used machines continues to grow. Machines made from 1968 to 1987 by more than 30 companies are currently being advertised.
Highlights of the 1979-1991 period are given in Table 6. Virtually every basic type of thermoplastic is currently being blow molded commercially, and this means that hundreds of formulations, alloys, and blends are being used. Before 1979, resins were usually supplied by the companies that did the polymerization. Today, a sizable portion is supplied by companies that do not manufacture resins. Currently, the fastest growing developments are coextrusion; multilayer; barrier materials; compounds with nucleating agents; hot-fill compounds; and oriented plastics.
Recycling of post-consumer plastics is predicted to disrupt the growth of blow molding. Issues under discussion include: lower quality blow-molded products resulting from the forced use of recycled materials; a decrease in the amount of plastic packaging as a result of public pressure; and a decrease in the number of plastics used (see PE, January 1992, p. 23ff). At this time, it appears that blow molding will undergo changes not created by new plastic materials, engineering developments, or applications needing to be filled.
The great growth of blow molding since 1930 began with a demand for applications and with the inventors who developed the equipment to make the products to fill these applications. The growth accelerated as inventors of equipment and developers of molding compounds worked together to produce the desired blow-molded parts. As those needs were satisfied, other applications were found, new inventions and new materials became available to fill the needs, and the cycle was repeated over and over again. These cycles will continue in the future, but it is doubtful that more giant steps will be taken.
The thousands of individuals and the companies with which they were associated, who contributed so much to the dynamic growth of blow molding, deserve more recognition than has been given to date. The tremendous accomplishments noted in this review suggest the need for an engineering summary documenting the achievements for future generations.
Blow molders in the 1950s recognized the need for a better understanding of materials and the weaknesses of the equipment, but they were unable to rapidly overcome the inadequacies. From the fundamentals established then, however, the industry applied progress from all branches of science and engineering to overcome breakdowns, lack of control, and material problems, and thereby create operations that could function without any major difficulties. The opportunities for blow molding of plastics now appear to be greater than ever.
TABLE 1. Blow Molding, 1930-1944.
Highlights 1935 Ferngren patent: Extruding a molten tube into a closed mold and injecting air. 1935 First modern injection machine imported into the U.S. 1938 BASF extrusion blow molds in Europe. 1939 Plax operates blow-molding machine to make 25,000 items per day. 1939 Electrically heated extruder developed. 1942 Owens-Illinois patent: Injection-blow process with core-pin blow.
Cellulose acetate (1927), PVC (1927), ethyl cellulose (1935), nylon (1935), acrylics (1936), polystyrene (1938), cellulose acetate butyrate (1938), polyvinylidene chloride (1939), experimental LDPE (1942), polyesters (1942).
Basic principles in practice by 1944.
TABLE 2. Low-Density Polyethylene Period, 1945-1956.
1945 LDPE available in commercial quantities. 1949 Horizontal-wheel blow molding-continuous-tube extrusion operating (making 8- to 40-oz bottles). 1950 Kautex offers first blow-molding machine for sale. 1951 Pirelli blows carboys in Italy. 1952 Plax manufacturing 13-gal, Hedwin 15-gal, and Delaware Drum 160-gal carboys. 1955 Kautex shipping blow-molding machines to U.S. 1956 Reciprocating-screw injection machine in use.
New thermoplastics available
LDPE (1945), ABS (1948), SAN (1948), polyethylene-isobutylene blends (1948), polyurethane (1954), HDPE announced (1955), acetal (1956).
Major hydraulic improvements; hot-runner molding being practiced; parison programming being investigated; major improvements in controllers for timing, temperature, sequencing, pressure, etc.; machine size being increased; testing for stress cracking; two-step injection-preplasticizing.
TABLE 3. Early High-Density Polyethylene Period, 1957-1964.
1958 First electronic parison programmer. 1958 Blow-molding machines at NPE for the first time. 1959 HDPE bottles for detergents, bleaches, and cleaners. 1959 Calibrated-neck patent. 1962 Carbon-dioxide cooling patent. 1963 Two-step blow in Europe--extrude parisons, blow. 1963 Double-wall containers marketed. 1963 First patent for coextrusion and blow (Italy). 1964 Very large wheel machine operating for bottles. 1964 First in-house milk container manufacturing. 1964 Marrick process for PVC in England.
New thermoplastics available
HDPE (1956), PC (1957), PP (1958), phenoxy (1962), polyallomer (1962), ionomer (1964), polypropylene oxide (PPO) (1964), ethylene-vinylacetate (1964).
Good injection-blow machines using reciprocating screws; extrusion pressblow machines successful; wheel machines successful; free extrusion widely used; accumulators, manifolds, and shuttles; surface treatments; programming; auxiliaries; blow air controls; rheology studies; neck finishing; handled ware; quality control.
TABLE 4. End of High-Density Polyethylene Period, 1965-1970.
1965 Acetal bottles used for aerosols. 1968 Orbet process for stretch-blow molding. 1969 First testing of polyacrylonitrile carbonated-beverage bottles.
New thermoplastics available
Polysulfone (1965), modified polyacrylonitriles, thermoplastic polyesters (PET and PBT) (1970).
Molding machines specifically for PVC; stretch-blow molding; use of new materials and formulations; greatly improved injection-blow machines; growth of wide-mouth-container manufacturing; major progress in control of the plastic melt; increased use of solid-state controls and programming; standard testing procedures accepted; coextrusion being extensively investigated.
TABLE 5. Thermoplastic Polyester Period, 1971-1978.
1972 Toya Seikan develops multilayer blow process. 1973 PET bottle patented. 1974 Process for biaxial orientation process studied. 1975 Two-step commercial biaxially orienting machines. 1976 First Pepsi PET bottle production. 1977 Japanese offer one-step orienting machines. 1977 55-gal drum in volume production. 1977 Two-step injection-blow molding with rotation.
New thermoplastics available
Lopac (1975), polyacrylate (1975), nitrile barrier resins (1975), polybutylene (1975), Cryolite (1977), styrenic terpolymers (1977), high clarity PVC (1978), HMW-PE, acrylic-modified nylon 6, ethylene-vinyl chloride copolymer.
Superior barrier properties obtained; great increase in use of heat-sensitive resins and engineering plastics; aseptic containers; blow mold/fill/seal/trim; improved injection-blow rotaries (four station); industrial blow machines greatly enlarged ; specifically designed auxilliary equipment for blow-molding plants; electronic controls and microprocessors in use; commercial equipment for stretch and multilayer; greatly improved PVC injection-blow; in-mold labeling successful; major cooling improvements.
TABLE 6. Current Period, 1979-1991.
1983 Multilayer Heinz catsup bottle. 1984 Hot-fill PET. 1984 Extensive in-mold labeling. 1984 Second-generation oil containers. 1984 Wide-mouth PET containers. 1984 5-gal PC water bottles. 1986 Multilayer plastic cans.
New thermoplastics available
HLMI PE (1984); HPT PVC in Europe; nucleated polyolefins; LLDPE; VLLDPE; PVDC-coated PP; ethylene-propylene copolymers; PC blends; PPO blends; PE/nylon 6 alloys.
Improvements in stretch-blow machines; commercial multilayer machines; thermocrystalizing; increased use of in-mold labeling; handled PET bottles; use of robots; expanded use of solid-state controls and computerization; microprocessor-feedback warnings; unique cooling systems; 18 different basic polymers and hundreds of formulations in use; post-consumer recycling; hot-fill container growth; very-high-volume production machines; in-mold trimming; new machines for wide-mouth molding.
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|Title Annotation:||history of blow molding materials and technology|
|Author:||Dunham, Robert E.|
|Date:||Aug 1, 1992|
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