Rotational Molding Industry Overview
Perhaps the earliest reference to a molding process closely related to the principles of rotational molding used today was a technical paper from Dresden dated around 1780-1790, which described a process for producing hollow china figures in a mold that was heated from the outside and moved in a rock and roll motion to provide even coverage.
A further detailed description of rotational molding dates back to 1855 with a patent taken out by R. Peters in Britain. This involved a rotating mechanism producing “two centrifugal motions at right angles to each other” by means of beveled gearing still typical of most modern machinery. This used “molten metal, or other materials” as the molding media to form metal ordnance shells. The theme of producing artillery shells was continued with the issuance of a patent in 1865 in the US to T.J. Lovegrove.
F.A. Voelke described a method including a polymer in a US patent in 1905 for the production of articles using paraffin wax. Development continued in the United States with the production of chocolate Easter eggs and rabbits (1910 Baker & Perks).
In 1920 Powell made mention of the commonly used ratio of 4:1 between major and minor axes of rotation at slow rotation speeds. His patent covered the use of plaster for making hollow objects. In 1932, Landau patented the ‘Rock and Roll’ machine concept for molding wax bottles.
With the introduction of PVC plastisols in the early 1940s, rotational molding in its modern form was born. Vinyl plastisols (liquid PVC with a plasticizer) were used in children’s toys, display items and novelty goods, and were the only materials available in bulk for quite some time. The ease of processing, flexibility and forgiving nature of PVC made it widely accepted and, after patent issues at the Sun Rubber Company were resolved, the industry grew strongly through the 1950s and 1960s. However, plastisol was not sufficiently strong for larger applications and further development would have to wait for other materials to arrive.
The first cast aluminum molds for rotational molding were produced by Allen Kelch of Kelch Aluminum Molds and Bud Lamont of Plasti-Cast between 1949 and 1950. Casting in plaster allowed mold makers to produce thin walled molds with surface properties suitable for rotational molding.
Low-density Polyethylene materials were in common use during the 1940s in other processes. However, it was Bud Lamont who rotationally molded the first polyethylene part (a small Mickey Mouse) in 1953 using powder scraped off a solid sheet using a wire brush.
The rapid expansion of the use of polyethylene was aided by the introduction by Pallman in 1955 of a method for grinding polyethylene to a powder at moderate temperatures, thus eliminating the expense of cryogenic techniques. Availability of cheap versatile material meant increased development so that tanks in the range of 400-1,000 gallons (1500-4000 liters) capacity were being produced prior to 1959.
Also around this time (1956), Thomas Engel of Germany developed the Engel Process for producing open parts from polyethylene powder. Essentially this was a ‘slush molding’ process in which a hot mold was filled with powder and heated from the outside (typically with an open flame) until a suitable thickness of material adhered on the inside surface. Excess material was poured out of the mold for use in the next part. The process was licensed around the world, but was subsequently superseded with the advent of biaxial rotation.
In the early 1960s U.S.I. Chemicals (now Equistar Chemicals) developed their ‘Microthene’ low-density polyethylene powders for rotational molding. One of the early markets that took advantage of this was the toy industry. This was the largest market area for rotationally molded products; one-piece hobbyhorses being a major demonstration of the complex shapes that it could produce. However, information on technical aspects such as cycle times, effects of wall thickness and part shape was scarce or non-existent, and this lead to a series of symposiums held at the National Plastics Exposition (NPE) in 1963, and also by USI during the 1960s.
The first linear low and medium density polyethylenes were produced by DuPont Canada in 1960 under the Sclair trademark using the solution process developed by DuPont US. The worldwide rotomolding community rapidly accepted these materials and by the end of the 1970s DuPont was the largest supplier to the rotomolding industry worldwide. In 1970, the linear materials were made available to the US and other suppliers, such as Union Carbide, began to offer gas-phase materials.
In 1964 McNeil-Akron developed the modern standard for machinery with the introduction of the multi-arm turret. Over the ensuing years, equipment controls may have improved but the same essential elements have remained largely unchanged since then. Indeed many of those same machines continue in full-time production today (albeit with many more controls and sophistications).
As the base for rotational molding grew, interest in other materials developed. Formed Plastics, Inc. molded the first polycarbonate parts in the USA in 1968. These are the light globes commonly seen in many outdoor applications that make use of the transparent and tough characteristics of this material. Phillips Chemical also developed cross-linked polyethylene in the late 1960s. Chemical additives produced rigid bonds between adjacent polyethylene molecules, thereby increasing Environmental Stress Crack Resistance (ESCR) and impact properties.
The formation of the Association of Rotational Molders (ARM) in Chicago in 1976 brought together a small group of molders in a trade association with the goal of sharing knowledge, advancing the process and promoting the industry for their common good. Today, ARM continues to represent the industry worldwide, but is also joined by associations in many other regions of the world; the Australasian Association (ARMA) was formed shortly after the North American group and continues to be strong and pro-active; in Europe, there are groups in Italy (AISR), the UK (BPF), France (AFR), Germany (ARM-CE), Poland, Scandinavia (Nordic ARM); elsewhere in the world South Africa (ARMSA) has an active group and China has multiple groups supporting local molders. Almost all of them have come together as part of the Affiliation of Rotational Molding organizations (ARMO) with the goal of promoting rotational molding around the world and, in addition to their own regional meetings, now have regular global meetings scheduled every three years.
In 1978, DuPont Canada, as part of their considerable support for the rotomolding industry, sponsored the first ‘International Meeting of Rotational Molders’ in St. Helier on the Isle of Jersey. More than 200 delegates from twenty-seven countries took part in the two-day event. This was a precursor to the many successful events held by ARM and other growing associations promoting rotomolding worldwide that would be held throughout the rapid growth era of the 1980s and 1990s.
Allied Signal introduced nylon 6 for rotational molding in 1978. It is an extremely tough and chemically resistant material and one of a group of nylon combinations, which have been evaluated for rotational molding. Elf-Atochem (now Arkema) began supplying nylon 11 and 12 versions, which also worked very effectively for rotomolding. However, they are expensive relative to polyethylene and continue to be considered specialty materials.
Research and development efforts were marked by two events in the 1980s; the establishment of research centers at Akron, Ohio and Belfast, Northern Ireland. ARM founded the Rotational Molding Development Center (RMDC) in an effort to concentrate research work. Despite some good technical work on ABS, warpage and cooling, the Center was closed down as the focus of ARM shifted to a decentralized approach involving more than one university. This has led to the involvement of a number of universities across the US. Work began at The Queen’s University of Belfast under the direction of Prof. Roy Crawford. The formation of the Rotational Molding Research Centre followed in 1990 after a series of industry and government sponsored projects. These projects lead to the introduction of the first process control system known as the Rotolog™ as well as work on computer simulation and materials development.
Material developments have continued but more slowly than in other processes; new grades of polypropylene have been introduced and these materials mold reasonably well, but they still suffer from low impact performance and continue to be the subject of development work today. Metallocene (single site catalyst) polyethylene materials appeared during the mid-1990s and continue to be developed through 3rd and 4th generation versions – they offer improved processability and physical properties but are more expensive than standard grades and tend to be used for more technical applications. The latest developments in materials have shifted to biomaterials where PLA has moved forward allowing scratch resistant, paintable parts to be produced.
Today the industry spans the globe with molding operations in almost every country. There are an estimated 2400 companies with some form of rotational molding capability ranging from very basic open-flame rock-and-roll equipment through to the latest fully automatic machines from Persico SpA in Italy. The predominant material used worldwide (over 95%) is polyethylene and consumption stands around 1.4 million tonnes (around 2% of world consumption of polyethylene). Some estimates on the number of molders and consumption can be seen below:
The highest levels of technical expertise are to be found in Europe followed closely by the US; both regions benefit from pressure from strong manufacturing sectors that push molders to develop more and more complex parts. In most regions of the world, rotomolding dominates the production of water storage, water treatment and chemical storage tanks – in fact more than 70% of rotomolding globally is involved in the production of tanks of some form. The average molder is small and entrepreneurial in nature with several machines and is focused on a number of standard products; in the US, where data is most readily available on market size and company details, the vast majority of companies have annual revenues below $10 million with only a handful generating sales above $40 million. Flexibility in terms of design and product size allow rotomolding to be used in an amazing array of industries including products from medical applications to automotive fuel tanks, from children’s toys (think Little Tikes and commercial playground equipment) to boats (almost every kayak maker in the world has some level of rotomolding capability), from industrial applications (almost every commercial floor-cleaner is rotomolded) to furniture (some of the world’s most sophisticated designers have adopted rotomolding, particularly in Italy) and from road barriers to pallets.
Information provided by Paul Nugent, MNOP and RotoWorld® Manufacturing Editor.