Innovative plastics specialist Tecni-Form Ltd. (Stone, UK) re-imagined the scope of rotational molding to produce an iPad-enabled foot-measurement device. The product has been rolled out to Clarks’ UK shoe stores.
The award-winning Clarks’ iPad foot gauge was designed and developed for the precise measurement of children’s feet by Designworks Windsor Ltd. The company initially challenged Tecni-Form to produce the device’s main body in white, medium-density polyethylene, specifying an exceptional surface finish to complement the appearance and feel of the iPad case.
Although rotational molding, using relatively low-cost tooling, makes good sense for large products (such as trash bins and kayaks) produced in sub-five-figure quantities, the technique is not conventionally prescribed to achieve a flawless, gloss finish and dimensional accuracy. The Tecni-Form team however, was eager to demonstrate its advances in the process, such that surface finish limitations are now a thing of the past. Additionally, the cost of rotational molding tools for the main body of the device was between one tenth and one twentieth of that estimated for a pair of imported injection molds for the same product.
Craig Allen, Senior Design and Project Development Engineer, told PlasticsToday that, given the advances in this technology, “the rotational molding process is often wrongly portrayed.” He noted that there are many rotational molders in the UK, “vastly more than in the U.S., but out of those huge numbers there are relatively few of us who want to advance the process forward.”
Tooling cost for rotational molding is a clear advantage when compared with injection molding, for example, but, Allen explained, “for small products this advantage is actually much less and not necessarily the main reason one would opt for the rotational molding process.”
One advantage in particular of roto-molding over injection molding is that Tecni-Form was able to produce a one-piece component for the iPad foot-measuring device. “To make this product in an injection molding process would require fastening together two molded parts,” Allen said. “We are also able to put kiss-points into the product and this makes for a very strong and rigid component.”
One of the requirements for the foot-measurement device was that it had to support a 25-stone (weight) child. “We were able to make a hollow, one-piece unit that easily had the structural integrity to support this weight without the need for any reinforcement,” said Allen. “The molding itself is quite intricate and it is probably fair to say that when I began my career in rotational molding some 18 years ago, we would have balked at making this part. However, materials have improved over the years and we can mold very difficult features and small radii, basically all the things designers love but we rotational molders dislike.”
Tecni-Form’s expertise in rotational molding and reaction injection molding (RIM) has given the company a reputation for excellence among its clients, which include Bentley, Aston Martin, Volvo and Caterpillar. The company also provides mold-tool design, precision process control and machining techniques to meet the specifications of these customers.
Material suppliers worked with Tecni-Form to formulate a specific, high brightness, white, medium-density polyethylene roto-molding powder. The tailor-made formula assisted with the complexity of the molding shape and final gloss of the product.
With the surface finish achieved, Tecni-Form also had to be able to manufacture to a +/-0.5-mm machining tolerance to ensure a clearance fit between the main body and a steel footplate insert. This eliminated a potential dirt trap on the upper face of the assembled device. In production, the component is molded fractionally oversized and a CNC 3D-machining station is used to create the correct recess profile to accept the steel plate.
A combination of uniform wall thickness and very low residual stress promote high strength in rotational molding. When Designworks tested production samples, the strength of the molded part reached the 350-kg load limit of the test rig, more than twice the specified minimum failure load for the component. Tecni-Form has completed the delivery of 2,500 units of the iPad enabled foot measuring device for Clark’s UK stores and has already begun fulfilling orders to equip Clark’s branded outlets across Europe and beyond, said the company.
Allen noted that Tecni-Form recently built a new facility specifically for the company’s RIM capabilities, which he said is a “recent technology” for the company. “RIM is very synergistic with the company’s rotation molding, very much suited to lower volume products," said Allen, adding that it "serves the company’s existing markets.”