Rotational Molding's Pursuit of Perfection

(Published in Rotation Magazine, Volume X1, Issue 3, June 2002)

Sometime in the early 1950’s, the first rotational molding machine tumbled its way into the history books. With a few spare parts and some loving attention, it is probably still running somewhere today (on a recent visit to a molder – they actually told me it was!). You may not agree with the exact date when our modern era began or that many molders still operate older machines but the fact remains that much of rotational molding has not changed in the last 40-50 years. I believe that it will always be that way; a large proportion of our industry will always be built on the entrepreneurial spirit that thrives on the low barriers to entry to our process. However, for those that have a desire to push the technology forward and for those that must in order to survive, much has improved and, I believe, there is more to come.

Rotomolding has a number of key attributes: low-cost molds, freedom of design, short lead times and low equipment costs. This is relative to other processes of course, and our advantages are sometimes offset by the challenges that we face: high labor content, relatively low output rates, limited material choices, wide tolerances and limited mechanical devices on molds that must endure repeated oven and cooling cycles. Developments in the industry that support our attributes and attack our challenges will most likely take seed – those that don’t will fall by the wayside. In pursuing the perfect rotomolding process, there will need to be many attempted improvements in order for a few to succeed. This paper deals with two areas that can contribute significantly to the advancement of rotomolding as a mainstream process: materials and automation.

Material Development: Rotomolding remains a process built largely upon polyethylene. There is a growing range of materials in use but the fact remains that our palette is limited by comparison with other processes such as injection molding or thermoforming.

We have undertaken research work on ABS over the last few years that has shown promise but this requires new direction and a better focus in order to succeed. Work on using nano-composites for rotomolding materials has begun at several locations; researchers at the Queensland Institute in Australia have shown that this could dramatically improve material properties at very low addition levels in existing materials.

Developing new materials is our most important priority; they help to open new markets and applications. The path to developing new grades has been greatly assisted by the work of the researchers at McMaster University in Ontario, Canada. By defining basic material properties such as low shear behavior, the effect of elasticity and particle sintering behavior, they have defined a set of screening tools that can help identify rotomolding candidate materials without the need to actually mold parts. How can we focus our efforts and take advantage of these tools? Try this three-step process:

1. Review the universe of plastic grades available (in excess of 20,000) and use the screening tools to identify a group of materials that have a good chance of being moldable (say 100 grades).
2. Refine this group further by testing to identify a smaller group with the greatest potential for molding (say 10-15 grades).
3. Use this shortlist to direct work at research facilities around the globe for targeted development in conjunction with suppliers of the materials.

Within this search lies the opportunity for all rotational molding associations to cooperate in work that will benefit the entire industry. A centrally coordinated effort funded by all groups would make best use of our limited resources.

Automation: Reducing the dependency of rotomolders on the skill and attention of an operator will be key to improving productivity and quality levels. A drive towards automating the molding process may help develop a process that still uses low-cost molds but with much reduced labor content, fast production rates and controlled quality.

Fully automating the process for general molding operations may seem like a stretch. However, this development path is akin to the space race developments of the 1960s and 70s; we don’t all fly in space, but we all benefit from the spin-offs that it created. Pushing the boundaries of rotomolding will inexorably lead to benefits for all. Let’s take a look at the steps that would be needed to produce a completely automated process:                                                                                                                                                                           Achievable Today?

Which of These Can be Achieved Today?

We cannot eliminate the operator any time soon, but we can try to simplify his work and reduce his involvement to a pick and place mode. Mold handling can be automated and has been successfully implemented in several locations. Automated material dispensing has become more common with systems now capable of identifying molds and delivering material directly into the cavity. Inserts are routinely inserted using robots in injection molding applications – the justification for rotational molding will come with volume products and flexible systems that can be customized to changes in molds. Applying graphics with an inkjet head directly into the mold is not too great a stretch – one to four color inkjet systems capable of reproducing any image on a complex surface already exist. Automating the venting mechanism on a mold would help eliminate the No.1 reported problem of blowholes and simultaneously remove the need for operator involvement. Reliable mold release systems are critical for an automated system – we have a range of products today that have advanced this goal but still require constant review. Teflon coatings and new paint systems for release may offer the best solution. Self-correcting controls have been available now for some time and work continues to incorporate it directly into machine controls. Part ejection on a mold will violate our objectives for low cost molds and may be too complicated for the variety of shapes and sizes of parts that can be molded. Transporting parts away from the machine can be achieved in a variety of methods from conveyors to robots. Verifying quality measurements on-line could be achieved automatically for weight or wall thickness; others such as cure-level may be possible using ultrasonic gauges.

Full automation has been achieved on dedicated product lines in several locations around the world. However, making it logical and simple enough to make it attractive to a general molding audience will not happen overnight. This will require targeted development work to tackle specific issues one at a time until all the pieces are available. The above list could be remapped into three broad paths as laid out in Figure 1:

Figure 1: Development Paths for Automated Rotational Molding

Path #1 represents the greatest technical challenge in providing a heating system that allows the use of mechanical devices on molds reliably during the molding cycle. There are a number of technologies that are working to provide a simplified path for mold operation including direct electrical mold heating at the mold surface (PPA Teo), infrared mold heating (Paul Meuret), oil jacketed molds (Konan Tokushu Sangyo Co., Ltd. and Norstar Molds) and embedded electrical heating elements (Wytkin Design Associates). There has also been recent work performed using high temperature air through jacketed molds. If the heating system can be made to be cost effective, the possibilities for adding venting and ejection systems to molds appear. (Automatic venting was the subject of a brainstorming session for the Cycle Time Reduction Committee at the recent ARM conference in Minneapolis. A range of ideas and concepts were discussed and will appear in another article in Rotation Magazine for member review and comment.)

Path #2 has already been achieved. Powder dispensing systems have been directly linked to mold identification tags and machine movement so that the dispensing system can recognize the mold and also pour powder or liquid into the mold evenly over the mold surface. Process controls have been implemented on rock-and-roll machines via slip rings and non-contact temperature measurements are in increasing use for controlling and monitoring the process. These systems close the loop on the process by providing data on the mold directly to the machine control program.

Path #3 has also been successfully implemented in specific applications. Rototech S.r.l in Italy has installed a new automated Mold Exchange System developed by Persico SpA on an Alan Yorke Engineering machine. This system removes grids of molds from the arm of a machine and exchanges them with grids at an operator station at ground level. Figure 2 shows how the system operates with two operator workstations adjacent to the machine.

 Figure 2: Mold Exchange System Conceptual Layout

The first real world implementation is shown in Figure 3. The manipulator arm interchanges two sets of molds creating a ‘virtual’ arm where the operators can work on both sets of molds simultaneously.

Figure 4 shows the manipulator removing a grid from the arm of the machine after disengaging it via a quick-connect mechanism at the head.

Figure 3: Mold Exchange System Manipulator Arms 

  Figure 4: Manipulator Engaging and Removing a Mold Grid from the Machine

Once the mold has been removed, it is delivered to the operator work area for servicing as shown in Figure 5.

 Figure 5: Manipulator Delivers The Grid to the Operator Work Area For Servicing

The system has required considerable development effort between the automation specialists at Persico and the machine programmers at Alan Yorke Engineering. It is currently undergoing in-service trials at Rototech.

Providing a flexible manufacturing base is the objective of this system, one that allows the machine to maintain a steady pace by simplifying access to the molds for the operator and providing a path for molds that are having problems to be removed from the system without interrupting production.

Another style of machine that allows molds to bypass each other has been developed by Reinhardt GmbH. Their ‘linear-machine’ concept (Figure 6) allows molds to move from oven to cooler to service station independently of the other arms in the system. This production flexibility allows parts with significantly different thickness, materials or servicing times to operate simultaneously.

Figure 6: Linear Machine Concept Developed by Reinhardt GmbH

Another concept that combines several of these ideas is shown in Figure 7. The mold exchange system is simplified from handling grids of molds to handling individual molds. A manipulator or robot (2) is used to transfer molds from the arm (1) to a conveyor system (3) that delivers them to operator stations (4a,b,c). One scary part of the concept is that the operator stations could actually be in an air-conditioned area; however, there are limits to the development program and we may be going too far! Some key concepts to consider in such as system include:

·        Molds would be mounted in standardized frames that could be pre-balanced

·        The manipulator could quickly remove all molds from each side of the arm and reload the arm quickly to ensure that the machine is ready to move within a short space of time

·        The mechanism used to lift the molds from the machine arm could also be used to assist in opening the molds in the operator station (pneumatic assist)

·        The operator area could be air-conditioned

·        Molds with long servicing cycles or problems can be bypassed or taken off-line without interrupting production

·        Molds waiting to be remounted on the arm of the machine could be pre-heated to shorten the molding cycle

·        Molds can be added and removed from the system at the conveyor belt without interrupting production

Some key challenges are also evident:

·        Using standardized frames reduces the flexibility of a machine to mix and match complex shapes of molds

·        Ideally this system would work with parts of all the same wall thickness (molding cycle), although some adjustments within a given tolerance can be achieved by varying the oven temperature vs. wall thickness. Large cycle time variations cannot be accommodated on a carousel style machine.

If the mold handling system were combined with a linear style of machine, the issue of mixing different cycles could be overcome by producing a flexible mold servicing cell in conjunction with a machine that can combine flexible cycles.

Figure 7: Individual Mold Handling Work Cell

Molders operating mixed molds and short production runs (i.e. custom molders) could use this concept to smooth production and develop safer and more comfortable work environments for their operators.

Some key points to consider are:

·        The robotic manipulator may be reduced to a simple slide mechanism, reducing cost and complexity.

·        Fully automating the process is not yet practical, but the workload of the operator can be significantly reduced. Much of the physical aspect of rotomolding can be eliminated so that the operator now concentrates on removing parts and inspecting them for accuracy.

·        Interaction between the machine and the manipulator must be kept as simple as possible to allow older machines to be retrofitted and to allow for wear and tear at the head of arms.

·        Retaining the advantage of low-cost molds is important. Adding ejection systems to molds is a long way off. Automation systems must incorporate standard molds.

·        The overall benefits of such a system must be considered in conjunction with the obvious upfront costs. Reduced operator turnover, steadier production rates, improved part quality and longer mold life are all potential benefits that may be achieved.

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