Tooling Advances Let Rotomolders Make New Kinds of Parts

Sixteen-foot-long dump-truck bodies weighing over 1100 lb and the new ability to make hollow parts with internal ribs and other solid sections are just two of many striking new developments in rotomolding technology. Last month, we reported on several machinery innovations that were presented at the annual fall conference of the Association of Rotational Molders (ARM). This story examines recent advances in tooling, materials, and additives.

One really hard body

An impressive example of a heavy-duty structural application for rotomolding is the all-plastic dump-truck body from Reiter Industries Inc., Dickinson, N.D. This maker of steel roll-off waste containers developed the crosslinked HDPE product and the molds and machine to produce it. Molded in sizes from 10 to 16 ft long x 92 to 96 in. wide x 24 to 46 in. high, the part is up to an inch thick and weighs up to 1150 lb, says Jack Butsch, sales and marketing manager. It's built to withstand 500 ft-lb of impact at –40 F and to handle boulders and chunks of concrete in payload weights similar to an all-steel dump body. Reiter has sold more than 300 units since 1999 at prices of $6500 to $8500 apiece. The product was selected as one of the "Top 100 Products of 1999" by Construction Equipment Magazine, Des Plaines, Ill.

These huge parts are molded in a two-cavity mold on a proprietary machine run by Poly Processing Co. Inc. in Monroe, La. Cycle times are 4-5 hr. The plastic part slides into a twin-rail steel subframe that was designed by Reiter. The steel frame is mounted on the truck chassis like a conventional steel body. The plastic body attaches to the frame using only five bolts.

Butsch says the plastic dump body has several advantages over steel. It is pigmented, eliminating the need for paint, and it is not susceptible to denting, which can accelerate wear. It won't rust and is resistant to chemicals, salt, sand, industrial sludge, and ash. It has a slicker surface, so the payload doesn't adhere to it in hot or cold weather, and the material will slide out of the dump body easier and at a lower dump angle than from a steel body. This can cut unloading time and yield more payload dumps per day.


Mold actions do wonders

New mold technology in which sections of the mold move during the forming cycle can add solid features to a hollow product such as internal or external ribs, internal reservoirs, solid flanges or handles, living hinges, mounting tabs, undercuts, or overlapping sections. Some of these features are difficult or impossible to make with conventional molds, says rotomolder SJS Industries Inc. in Medina, Ohio, which developed the new TRIP (Transfer Rotational Injection Process) Mold. This novel tooling may help rotomolding compete with injection molding on some low-volume jobs. SJS has its TRIP molds built to order by Wheeler Boyce in Stow, Ohio. SJS will make the technology available for licensing.

The word "injection" in the name of the patent-pending process doesn't actually involve injection molding at all, says SJS president Don Schraegle, but relates to the crispness of the added features, which can be produced to tight tolerances. "In traditional rotomolding, if you wanted a flange, it had to be an inch thick. With our process, you can make that flange as thin as a quarter inch, position it anywhere on the product, and even add mounting holes." Schraegle says developments under way may make it possible to rotomold parts with threaded bung openings.

The TRIP technique uses a machined or cast aluminum mold with one or more articulating sections that compress or push the material after it has been melted and distributed over the inside of the tool. The movable sections are spring loaded and are actuated manually with a trip rod. A TRIP mold with five moving mechanisms is being engineered to create a footed bin.

Incorporating structural supports can permit molding parts with 20% greater usable volume, Schraegle says. Other benefits of the TRIP mold include the ability to eliminate metal inserts or foam used for stiffening or support. TRIP molds reportedly produce parts with a smooth interior so there is no material hang up or bridging. TRIP molds can be used with conventional rotomolding equipment and will not incur a cycle-time penalty, according to SJS.


Material introductions

A new two-part thermoset polyurethane elastomer for rotomolding has been released by the Formulators Group of PolyOne Corp. (formerly Geon Co. and M.A. Hanna) in Cleveland. Rotothane material is a two-part liquid system that is processed under conditions similar to vinyl plastisols and polyolefins. Rotothane can be molded into complex rigid or flexible parts with good chemical and abrasion resistance. Once parts A and B are mixed, the system has a 20-45 min pot life at room temperature. Cycle times are typically about 8 min. Hard versions can have 65 Shore A to 85 Shore D hardness and flexural modulus up to 140,000 psi or more. Softer formulations can have high dart impact at –40 F.

At least three grades of a new high-stiffness, high-impact HDPE are offered by Jerico Plastic Industries Inc., Akron, Ohio. The resin is offered in densities from 0.940 to 0.950 g/cc and MIs from 3.5 to 5 g/10 min. Flexural modulus ranges from 110,000 to 140,000 psi. Jerico will precompound colors for minimum orders of 3000 lb.

Parts reportedly can be made up to 14% lighter while retaining stiffness, thanks to a pair of new HDPEs from McCann Plastics Inc., North Canton, Ohio. One, called Future 8, has 0.950 g/cc density, 3.0 MI, and flex modulus of 165,000 psi. The other, Future 2, has 0.957 density, 2.8 MI, and 185,000 psi flex modulus. Both have tensile strength of 3700-3800 psi and HDT of 230 F at 66 psi.


Colors & mold releases

A standard line of 25 dry colors for rotomolding is new from Chroma Rotational Molding Division, Inc., McHenry Ill. Six formulations have FDA approval, and five have anti-swirling agents. The line includes a number of shades of yellow, green, blue, and red, as well as white, tan, black, orange, and pink. The products can be formulated to minimize molding problems such as streaking and "star bursts."

Chem-Trend Inc., Howell, Mich, has come out with Perma-Mold 2562, a new solvent-free, high-slip parting-line release for rotomolded parts. The release helps maintain clean parting lines and eliminate flash sticking and pinholes near the parting line. Clear liquid Perma-Mold can be applied with a disposable foam brush and does not require curing or drying time.


Apply post-mold graphics

Post-mold decals that retain their color after prolonged outdoor uv exposure are new from Mark-it Co., Batavia, Ill. Its new Post-Mold Graphic XTL line has been tested by rotomolders for more than two years. An ongoing outdoor ASTM test shows only slight fading after 12-months, the company reports.

The XTL graphic system consists of an ink-transfer decal and activator. The surface is wiped clean, then heated slightly with a flame to oxidize the surface. The decal is dipped in the activator, then applied to the substrate with a squeegee. The paper backing is pulled away and the substrate is wiped down with a sponge. The activator softens the top layer of the transfer so it can bond to the substrate. The graphic adheres to polyolefins. It comes in a range of colors and effects, including pearlescent, metallic, and a new color-shifting decal called Chameleon Mark.