Plastic
ROTATIONAL MOLDING
It seems as though the hottest topic going these days is Rotational Molding or “Rotomolding”.  Rotomolding is used to produce hollow parts like ping pong balls, parts with complex geometries and for parts that have an inner and outer wall such as large tanks. This process has burst into prominence recently thanks to refinements such as the use of Transvectors.

What is rotomolding? In short, rotomolding coats the interior walls of the mold with a powder polymer by rotating the entire mold within an oven. As the mold temperature climbs, the polymer bakes against the walls of the mold. But deep cores in some molds are somewhat insulated from the heat of the oven which causes the walls to be thinner in those areas. If the heat could be better distributed into the core areas, wall thickness would be consistent throughout the part.

The stainless steel Model 903XSS Transvector is a perfect fit for this application. Resistant to the high oven temperatures, the 903XSS can be mounted directly onto the mold framework. Coiled copper tubing is often used as compressed air supply line because it absorbs heat from the oven and pre-heats the compressed air that feeds the 903XSS. The amplified volume of heated air is then directed into the core and can be regulated for precise distribution.

Because many rotomolded parts are so large, the costs associated with scrap can be devastating, and the methods previously used to produce more consistent wall thicknesses were elaborate and inefficient. Cycle times within the oven can now be minimized which shortens the curing / cool-down time and in turn means more parts per hour. Rotomolders who are using transvectors are astonished that a problem that had been so difficult could be resolved so easily and inexpensively. This has added even more steam to the growing excitement about the potential for this molding process.

Our just introduced 9O1XSS and 902XSS Stainless Steel Transvectors are a timely addition to our line. With 3 sizes to choose from, the perfect size or combination of sizes will satisfy virtually any rotomolding application.

As you know, Rotomolding is primarily used to form large hollow plastic parts that are difficult or impossible to make with any other molding process. The entire mold is rotated inside a 600F (315C) oven for as long as 17 minutes depending on the part and plastic being used. The mold then swings into a cooling chamber where fans and / or water are used along with an exhaust.

Above 212F (100) cooling is accomplished by vaporization of water on contact with the mold (evaporative cooling). Below this temperature, the movement of air combined with a continuous water quench (conductive cooling) is the most effective means of cooling. The Fogging Sprayvector delivers 20-60 micron size water droplets in a high velocity air stream making it an effective product for both evaporative and conductive cooling. It may even be desirable to discontinue the water flow at some point (easily done with a solenoid) and utilize the Sprayvector as an air amplifier for some portion of the cooling cycle.

Although Rotomolding is over 40 years old, recent refinements to the process and demand for more complex molds are fueling interest in new techniques and processes. Refinements in the cooling phase may assist in controlling “shrink” and in optimizing cycle times.

Their fully automated process produces a headlight bracket at the rate of one per minute, 24 hours a day ! (Translation: 1440 Brackets per day). Each bracket measures 5”h x 16”l x 1/16” thick.

Efforts to reduce the amount of time required to produce one part were being concentrated down in the “cool down “ phase of the process between forming of the part and trimming of the gate. (The gate is a 3” long plastic tail that drifts from 1/4” dia. to 1/2” dia. at the point the tail joins the bracket.)

SOLUTION: Through testing, it was determined that a temperature drop of 50F, concentrated on the 1/2” dia. area of the tail was necessary to produce a cut to their quality standards. The engineer elected to install a 308-35-H Vortex Tube to supply a cold air stream directed onto this critical section. The engineer connected the 308-35-H Vortec Tube to an 80PSIG compressed air supply and discovered a full 20 seconds were shaved off the cycle time instead of the 5 second improvement he had hoped for

SUMMARY: The number of parts produced in a 224 hour period jumped from 1440 units to an amazing 2,160 units.

 In the thermoforming process, a plastic sheet is vacuum formed over heated metal shapes, creating a formed part, (such as a door liner, dashboard, etc.). In order to retain its shape, the part needs to be cooled immediately after forming to a temperature less then 140F

The manufacturer was trying to use a hand-held paint gun type water atomizer to provide enough evaporative cooling to drop the temperature of the part. The hand atomizer gun delivered a highly non-uniform spray so there was a tendency for water drops to fall on the formed plastic sheet. The water drops turned to steam as soon as they fell on the hot formed parts, creating surface imperfections. The size of some parts also prevented the operators from achieving uniform spray coverage and, consequently, uniform part cooling.

SOLUTION: Both problems disappeared when two Atomizing Sprayvectors were installed, replacing the hand held atomizers. The Sprayvectors were connected to filtered, factory compressed air at 60 PSIG and the city water supply, through a Pressure Regulator, at 20 PSIG maximum. The Atomizing Sprayvectors delivered a uniform, high velocity atomized liquid spray that was captured and disbursed by two fans to ensure equal, uniform spray coverage and cooling, of the parts. Water flow is controlled by solenoid and is cycled "on" for only 10 seconds after the fans start circulating the air over the parts.

SUMMARY: Using the two Atomizing Sprayvectors allowed the manufacturer to produce the high quality parts that he needed. A secondary benefit was realized when it was discovered that using the Sprayvectors also shaved 30 seconds off the 90 second cycle time!

This "fix" was so successful that the manufacturer is retrofitting their other 10 machines.

One example of surface cooling using Curtain Transvectors or Round Transvectors would be found in the converting industry. A manufacturer of printed polystyrene (plastic) labels needed to cool down the surface of the plastic after printing. Part of the printing process involves use of an infrared heat lamp to “set” the ink. Since the label line was moving at 300 FPM (feet per minute), the labels did not have time to cool and the ink was smearing as the labels were wound on reels.

SOLUTION: When the manufacturer installed a Model 921-12 Curtain Transvector to blow room temperature air on the printed labels after the heat set process, the temperature on the labels dropped by 200F, eliminating the problem with the smearing ink.

Converter uses a 921-12 (12”) Curtain Transvector to cool the surface of a polystyrene web that travels at a rate of 300 feet per minute.

Blow molder of 2 liter bottles uses a 921-12 (12”) Curtain Transvector to blow off water from bottles after a washing process prior to labeling.

Converter cools the heat seal seam on a bag manufacturing operation using a 106-8-H Vortex Tube that blows cold air through a stainless steel tube attached to the seal bar on the machine.

Manufacturer of plastic injection molded parts uses a 208-25-H Vortex Tube to replace water cooling for a small 4 cavity die.

Manufacturer of water bed mattresses uses a 308-35-H Vortex Tube to cool down a vacuum forming mold.