More accurate monitoring of the rotomolding process and new tools that improve mold sealing and product quality were presented at the 28th annual fall meeting of the Association of Rotational Molders, held in Kansas City, Mo., in November.
Control in the mold
A new process-monitoring system provides real-time data on conditions inside the mold to help speed cycles, save energy, reduce rejects, and troubleshoot problems. K-Kontrol is a new mold-temperature measurement system from 493K Ltd. in Northern Ireland, a firm created to commercialize the results of R&D at Queen’s University of Belfast. “Most rotomolders rely on a repetitive, timing-based processing approach, but ambient process temperatures change from cycle to cycle, as does the material, and so do internal mold-wall temperatures. These variations can result in under- or over-cured parts,” says company director Gerath McDowell. K-Kontrol captures the temperature of the inner mold wall, the resin, and the air in the mold throughout the process. The system is priced around $16,000.
Queen’s University’s previous mold-monitoring system, called Rotolog, consisted of an insulated temperature transmitter bolted onto a mold. It received data from sensors in the mold and relayed them to an external data-acquisition system. The system had to be detached before the device became overheated, so it could only operate as a non-permanent set-up tool.
The K-Control system overcomes this limitation by means of an electrical rotary joint or slip ring that can withstand high oven temperatures. The rotary joint is permanently mounted on the machine arm. A signal wire extends from the rotary joint through the center of the arm’s mounting plate into the mold, where it is connected to the sensors. The transmission wires can be placed in the machine’s air lines.
Temperature data—said to be accurate to ±4.8° F—travel through a high-temperature signal cable in the main body of the molding machine to a radio transmitter. The signal is transmitted to a PC data-acquisition system. A screen displays the data in real time and stores them in a database for quality-control purposes. The software conducts statistical analysis of the incoming data to highlight quality-control problems in real-time. Control signals can be sent back to the molding machine for automatic closed-loop control. Internal mold-temperature setpoints can be used to actuate signals for mold movement through the successive steps in the process.
“Live mold-temperature feedback allows a molder to control the heating and cooling rates without using a preset cycle,” says McDowell. Instead, the actual mold temperatures determine timing of the process. The system can be used to balance cycles if there are a number of different parts being molded.
493K also developed K-Paq, a portable device that measures internal mold pressures and temperatures in real time, allowing users who pressurize their molds to optimize the pressurization phase. Molders may pressurize the mold during resin heating in order to expunge bubbles from the part and obtain a smooth part surface. Or they can pressurize the mold during cooling to hold the shrinking part against the cold mold and thereby cool it faster.
K-Paq consists of an electronic transmitter mounted on the mold that measures mold pressure using a sensing device installed through the mold’s vent pipe. Temperature is measured with k-type thermocouples in the mold. The transmitter is designed to withstand oven temperatures up to 662 F.
Meanwhile, existing process control equipment is being upgraded. Ferry Industries updated its Rotocure 2.0 “system manager” software to monitor more process parameters and recognize more alarm conditions (over 150 at present). It is now Windows-based. The system can display oven-temperature profiles, store and retrieve mold set-up recipes, and record scrap codes. It keeps a track of process performance and problems, and can generate trend reports.
Parting-line gaps that can adversely affect strength or appearance of rotomolded parts can be remedied with the new Parting Line Sealer from Mold In Graphics (MIG) Systems. The Sealer is a polymeric material that resists flow at high molding temperatures. Made from fractional-MI polyethylene and other proprietary materials, it is a pliable compound that cures in the mold during the molding process and fuses to the outer skin of the part. Excess material can be easily trimmed away using standard flash-removal tools.
The PL Sealer is applied directly to problem areas in the mold, where it forms a barrier. It can help overcome problems with damaged, worn or mismatched mold parting lines, completely sealing gaps or voids. The barrier prevents the entry or escape of gases that could create voids or blowholes in the part.
MIG also rolled out a new lower-temperature version of its Millennium series permanent graphic transfer, which is printed on a see-through polymer film. The new version allows for the transfer of a graphic into a mold at 90 to 130 F. The first product in the line, the Ultra High-Temperature version introduced a year ago, is intended for hotter molds—110 to 165 F or more. The Millennium series can be used with all polyolefins, says Scott Saxman, senior v.p. of sales.
More new equipment
STP Equipment offers a new rotational molding machine with independent operation of each arm. The line is geared to optimize output when molding different parts simultaneously and some parts have long cooling cycles. The new Tornado series can have three or four arms in a straight or offset design. Encoders on the motors for each arm control the position and monitor rotation. Arms can deliver a swing of up to 160 in. Arm rotation (major axis) ranges up to 10 rpm on smaller models with swing below 120 in. and up to 5 rpm on larger machines.
The company is developing a new version of its HVS rock-and-roll machine. The formerly raised cooling stations have been lowered 3 ft to floor level for easier access to the arms. The rock-and-roll series has three stations—a central oven and a cooling station on each side.
A new supplier of pulverizing systems that downsize pellets into 35-mesh powder introduced itself at the show. Powder King LLC, formed last April, offers single- and dual-mill systems capable of pulverizing typical resins such as rigid PVC, LLDPE, LDPE, HDPE, or crosslinked PE at rates from 600 to 2000 lb/hr or more, says Jim Hummel, president. Hummel previously worked with Reduction Engineering, a major supplier of pulverizing equipment.
Powder King’s system features patented disposable pulverizing disks said to be 30% larger than comparable models and to last up to five times longer than conventional disks. Disposable disks save the time and expense for resharpening. A patented airflow management system keeps the mill cool without traditional water jackets. New sifter technology is said to perform well with a six-deck screening area, while other systems use eight or more decks. Operator work platforms provides safety and save time during cleanouts. The system comes with Allen-Bradley controls.