Part 1 of this paper discussed the origins of the injection molding process and the development and use of the helical screw for conveying and melting polymers.
The plasticating unit used for melting and mixing on an injection moulding machine performs the same basic functions as the plasticating unit of an extruder. The difference lies in the fact that the screw moves backwards in injection moulding and thus the plasticating unit in an injection moulding machine can be considered to be a reciprocating extruder.
While screw design was considered to be important in extrusion, it was often considered to be less import in injection moulding. The major difference is that in a reciprocating system, the process is cyclic instead of continuous and the screw design plays a key role in maintaining cycle to cycle consistency for the resulting, molded plastic parts.This second part of this paper discusses the use of barrier / mixing screw technology for the plasticating of polymers in the injection molding process from the early 1960’s to today’s sophisticated injection molding equipment.
As Part 1 of this paper explained, simple three-zone screws like the one shown in Figure 1, are commonly known as ‘General Purpose (GP) Screws’, which are designed to process as much thermoplastic material as possible in an injection molding process, while providing a reasonable level of quality.
This screw design was adequate in the early years and in many situations still today, but it did have and frequently still has its drawbacks, for example when the parts that are being manufactured have specific quality requirements especially if high throughput is required.
Another situation in which GP screws provide is when masterbatches are mixed with virgin resins. The use of these color concentrates is more economical than using pre-colored resins, since this reduces waste and reduces a company’s inventory costs. Blending the virgin resins with color concentrates required more mixing than the GP screw geometry could readily provide, because the three zone screws lack shear and mixing elements. As a consequence of the inadequacy of these screws in this application, many believed that GP was an abbreviation for ‘Generally Poor’ and not ‘General Purpose’.
The Conair Auto-color™ blender provided one of the earliest attempted solutions. This piece of equipment automatically pre-blended the materials at the throat of the injection molding machine. Prior to the development of the Auto-Color™ many processors would simply put the neat resin and the color concentrate pellets in a drum and tumble them together often by just rolling the drum across the shop floor. With the introduction of the Auto-color™ blender, better mixing was possible.
This automatic pre-blending gave significant improvements in the color uniformity of the molded part, but it would still be necessary for the machine operator to make processing adjustments to obtain color shade uniformity. Most often this was done by increasing the back pressure on the injection unit.
As time passed, it became more apparent that the need for improved mixing quality was a necessity for the injection molding process. Various workers used ideas developed for plastics extrusion screws, attempting to obtain further improvement by incorporating additional mixing devices into the injection molding screw design.
Carl F. Schnuck and his colleagues, at the Farrel-Birmingham Company developed one of the earliest extrusion screw mixers (figure 2) in 1952 for which they were granted US Patent 2,680,879.
Screw designers at other plastics machinery manufacturers who understood plastics processing, screw design and mixing began to develop other types of mixers. Individuals such as Street, LeRoy, Maddock, Gregory and Hsu developed several different types of distributive and dispersive mixers, some of which are shown in Figure 3. For example, in 1968, R.B. Gregory showed that the introduction of mixing sections lowered melt temperature, allowed faster production rates and yet gave an extrudate with improved properties.
Most of these mixers adopted from the extrusion process into the injection molding process. Most of this work was done in the late 1960’s and 1970’s.
As the injection molding industry started to really grow in the 1970’s, individuals, such as, Bill Willert and Paul N. Colby, started to develop new mixing elements and also started to use barrier screw technology for injection molding screws.
In principle, barrier screws used for injection molding operate in the same way as in extrusion. The screw channel is divided into a solids channel and a melt channel. The barrier flight essentially separates both the channels and helps to homogenize the melt.
Maillefer introduced the barrier screw for extrusion in 1959, in which the screw design included a barrier in the transition section of the screw (Swiss Patent 82,535/59). Bill Willert patented a barrier-type screw in his US Patent 4,330,214 (Figure 4), which was granted on May 5, 1982, and was eventually used for injection molding applications.
After the expiration of the Dray-Lawrence US Patent 3,650,652 on March 22, 1989, Paul N. Colby started to use the Dray-Lawrence barrier screw technology for injection molding applications.
With the development of the two U.S. patents described above, the injection molding industry stepped into the 20th century in the area of plasticating of polymers. The new barrier screw technology, provided a way to increase plasticating rates and thus throughput by15 to 20%.
At this time many new barrier mixing sections were developed. LeRoy at Union Carbide Corporation patented a dispersive mixer in US Patent 3,485,192. This mixer eventually became known as the Maddock Mixer (Figure 5) and was the first device that was used for improving the melt quality of the polymer before it was injected into the mold.
This was a period of innovation (late ‘70s through early ‘90s) for the injection molding industry. Paul N. Colby was responsible for a great amount of the development that was done in the United States. One of the most popular and well-known mixers in the injection molding industry was the Spirex Pulsar® Mixer. The Pulsar® mixer, shown in Figure 6 was granted on June 21, 1988. This new distributive mixing device was a break through in screw design for injection molding which was the market area that Spirex primarily pursued.
Following the development of screw technology by Dulmage, Saxton, Le Roy, Maddock, Kruder, Gregory and others, there was a mad race by many other inventors in the United States to develop new mixing devices. Robert Dray provided the “Dray Mixer” (3,788,612). Jim Frankland invented the StrataBlend® Mixer (4,639,143). Spirex patented the Z-Mixer (5,318.357). Westland Corporation patented the Eagle® Mixer (5,215,764). Womer patented the V-Mixer (5,798,077), Pulsar®II (5,816,698), StrataBlend®II (6,488,399) and Nano™Mixer (6,497,508) while at Spirex and at Xaloy. Also, Womer patented the MeltStar II Mixer (6,547,431) for Milacron.
Over the past 50 years, starting with Bill Willert’s innovation of using barrier technology for the injection molding process, along with the advancements by Paul N. Colby and others, the injection molding of plastics has gone through great improvements to increase the product quality and production rates for the plastics industry.
Although simple three zone screws for injection molding must function with a wide range of requirements and materials, the sophistication of today’s plastics business has put enormous pressure on the homogeneity (melt quality) of the melt such that the needs cannot be achieved with GP screws.
New developments in screw designs will continue to provide molders with exciting opportunities to increase their productivity. As long as plastics continue to be injection molded, innovators of the plastics industry will continue to develop newer screw designs for processing a wide variety of plastic materials.
Timothy W. Womer, CPlasTTim Womer, is the President of TWWomer & Associates, LLC and was the 2006-2007 President of the Society of Plastics Engineers and a member of the SPE Extrusion Board of Directors for the past 20 years. At the Society of Plastics Industries (SPI) NPE 2012, Tim has been inducted into the Plastics Hall of Fame.
Tim is a recognized authority in plastics technology and machinery with a career spanning over 35 years; having worked for other companies like Xaloy, Inc., Spirex Corporation, Conair Group and NRM Corporation.
Tim has designed thousands screws that have been used in all areas of single-screw plasticizing such as extrusion, blow/injection molding. Numerous patents have been issued for his inventions of screws, mixers, processes and other products.