by Jenny Knodell, IQS Editor
There’s a largely unknown secret lurking in the plastic molding industry. While injection molding, blow molding and thermoforming are the big 3 in terms of fabricating complex, three dimensional plastic products, an untapped resource is hiding. It’s called rotational molding, and it takes an ancient process that the Swiss still use to manufacture hollow chocolate Easter eggs and applies it to plastic molding. Also called rotomolding, you might be surprised to learn that it’s an effective method of fabricating many different products; anything from a doll’s head to a kayak to a plastic tank that holds 20 thousand gallons of abrasive chemicals. It’s cheap, easy and has incredible design freedom. This process generates little waste and creates seamless, single part products, which seriously cuts down on production costs. So, why isn’t it as popular as the other molding processes?
There are two main problems with plastic rotational molding: the large, expensive space-taking machinery and the time it takes to produce small runs of products. Roto molders are huge revolving machines with one or more arms that mount the molds. The larger the mold, the larger the machine. In fact, molders that fabricate items like large industrial tanks often fabricate 1 product at a time. Ample space is also required because the machinery must transport itself into an oven chamber, then to a cooling chamber, all the while revolving on 2 axes. The process can take up to 90 minutes, depending on the size of the product and how long it takes the plastic to cure, which is at a minimum, 20 minutes. Compared to injection molding, rotomolding is painstakingly slow in order to evenly coat the mold walls, prevent the formation of bubbles and properly cure the plastic without using pressure.
Photos courtesy of Quality Industries, LLC.
Recent technological advances, however, have changed the rotational molding game. Microprocessor controls direct all phases, speed and ratio of rotation. Smaller, compact machines that sit in a single chamber that converts from an oven to a cooling area are now commonly used. Before the 90s, it was difficult to obtain an accurate temperature reading and determine whether or not the plastic was fully melted, which wasted time and resulted in imperfect products. Today, rotational molders are equipped with temperature sensors and an internal mold camera, so operators know exactly when the plastic is ready to cure.
Rotational molding definitely caters to a niche market, mostly fabricating large plastic tanks and vats. While it is unquestionably the most cost effective way to produce a quality and long lasting hollow plastic tank, there are many other design options that most people don’t know about. The rotomolding industry is becoming much more technically advanced, but is still trying to fight old conceptions and public unawareness. If you aren’t sure if rotational molding meets the demand of your design needs, take these factors into account:
- Your yearly product fabrication is between 1 and 15,000 items
- The design for your part is hollow, large and weighs more than 2 pounds
- The total cost of fabrication is between $2000 and $40,000
- Lead time falls between 4 and 8 weeks