Cleveland — Rotational molders learned about molding polypropylene, a dry color blend with good impact resistance, and old-school methods to make better parts at the Society of Plastics Engineers Rotational Molding Conference.
Sandy Scaccia said the rotomolding sector knows all about polyethylene, by far the most common rotomolded material.
"The current reality is we are a polyethylene-based business. Rotational molding has made their living on polyethylene for 50 years," he said. "But let's consider some other options."
Scaccia is president of 4Roto in Cedarburg, Wis., a sister company to Norstar Aluminum Molds Inc. 4Roto handles distribution of Rotoworx polypropylene compounds in North and South America.
PP is used in a lot of injection molded parts, such as bumper fascia and resin furniture. But rotational molding has been slow to adopt the material.
He said PP is not intended to be a replacement for PE. And polypropylene is more expensive. But Scaccia said PP can open up a wider range of demanding rotomolding applications because it has superior mechanical properties, lower shrinkage and good abrasion resistance, he said.
And it's autoclavable, making PP a good choice for medical and food sectors. Scaccia said PP is easy to mold.
"It's just an easy transition" for rotomolders, he said.
Other advantages of PP include good long-term creep resistance. Rotomolded PP items also hold up well on vacuum testing, he said.
Scaccia listed other potential markets, including automotive tanks, underground applications like tanks, pipe, drainage lines and manifolds for buried pipe, manhole covers and solar water heaters.
"It is intended to give you opportunities to get to places you aren't at now, without a lot of difficulty," Scaccia told the rotomolders.
"It's the next logical step," he said.
The SPE Rotational Molding Conference drew 100 attendees to Cleveland June 3-6.
Edwin Tam described Teknor Apex Co.'s TekTuff dry color blends. The company originally developed it for injection molding then moved on to blow molding. Last year, Teknor Apex introduced TekTuff for rotomolding. The company incorporates an impact modifier into the dry color.
Tam, manager of new strategic initiatives and business development, said pre-colored compounds for rotomolding gives very good properties. That solves a problem with standard dry colors, where higher loadings can reduce impact resistance in the part, he said.
At the SPE conference, Tam outlined testing Teknor Apex did for dispersion and impact strength.
Rotomolding veteran Bruce Muller gave conference attendees a dash of simple tips. For powder quality, "The better the grind, the shorter the cycle." He said when you run material with hairs and fines, it takes longer to get the air out, extending cycle times.
High-intensity mixing can reduce the fines.
"It's not like a bowl of spaghetti. Therefore, it'll flow in the mold and densify more quickly," said Muller, who runs Plastics Consulting Inc. in Palm City, Fla.
Swirling in a part surface is another problem, and Muller said it can be caused by static in dry colorant, caused by the pulverizing and conveying processes. "The way to identify static is, it's always in the same place in the part," he said.
To reduce static swirl, Muller said to try reducing the rotation speed of the arm and adding grounding to all equipment. Rotomolders can also add in-process antistatic material when mixing dry color.
Muller also talked about venting and gave a list of laboratory equipment every rotomolder should have.