New developments were presented at a recent Rotomolding conference. Technologies introduced at the meeting included rotomolding of nylon 6 parts directly from caprolactam monomer and pulsed-air pressurization of molds to make tougher parts, a new source for automated material batching, an anti-microbial mold spray and several new machines-both maxi and mini-were
Making nylon in the mold
A process that polymerizes liquid caprolactam in the mold has been brought to the U.S. Called Rotomolded Anionic Polymerization, or RAP, the process is claimed to offer cost savings relative to using standard nylon 6 resin. RAP has been used in Europe for several years to make fuel tanks, but Centro worked with several U.S. and European companies to improve the safety and productivity of the process. In the RAP process, a robot dispenses liquid caprolactam plus a catalyst and activator into a heated aluminum mold, which is then rotated while the mixture reacts to produce a virgin nylon 6 part. The process is said to be faster than molding with nylon 6 and reportedly makes superior parts free of surface porosity. In addition, RAP makes it easy to customize formulations with pigments, impact modifiers, and uv stabilizers, plasticizers, etc.
Pulsed pressurization and venting method to cut molding cycle times
A new pulsed pressurization and venting method is being developed to cut molding cycle times while also reducing shrinkage and warpage and increasing part quality by eliminating trapped air bubbles. This "DynamiKventing" method comes from a relatively new spinoff from Queen's University Belfast. The firm supplies mold-temperature and pressure measuring equipment based on R&D conducted at the school's Rotational Molding Research Centre.
DynamiKventing delivers pulses of 1- to 3-psi air into the mold in timed cycles. The pulses can be as long as 10 sec and spaced as much as 10 sec apart. The system measures the air pressure and flow into and out of the mold to detect problems such as a blockage in the vent pipe and ensure that the mold is depressurized prior to being opened. Research has shown that part impact strength can improve up to 10%. The company expects to launch the process sometime in 2005.
Material-handling and dispensing systems to automate charge batching
Material-handling and dispensing systems for individual machines or plant-wide use are now available. A of material preheaters, mixers, and automated dispensing equipment reportedly can help trim overall cycle times by up to 25% while reducing scrap and allowing for just-in-time material delivery. The firm offers powder preheating, weighing, blending, and dispensing systems. reSource can supply systems with a central resin preheat station that delivers material to as many as eight gravimetric blending hoppers where different colors and/or additives are blended in. Systems use PC-based controls with easy-to-use touchscreens, recipe storage and retrieval, and real-time reports.
The firm also offers systems that use high-intensity blenders to heat the material so no preheating is necessary. And instead of central blending, individual dispensing systems can automatically blend and dispense mold charges on demand at the press. Charges can be dispensed at up to 10 lb/sec for shot sizes up to 30 lb with accuracy within 0.02 lb (10 g).
No more part discoloration due to the growth of bacteria, yeast, molds and fungus
Part discoloration due to the growth of bacteria, yeast, molds, and fungus can be suppressed with a new antimicrobial aerosol coating for polyolefin parts -a permanent, silver-based spray. The colorless, odorless spray impregnates the part surface to combat microorganisms, which are killed by silver ions. The spray combines silver with a polyolefin and a ceramic that permits controlled release of the silver over time. This biocide reportedly does not affect the part's color or physical properties and is tolerant of rotomolding process temperatures. The coating can be applied in the mold or to the part after molding.
Ferry Industries is developing a huge new shuttle machine with a 26-ft (312-in.) swing diameter, capable of supporting 15,000-lb molds. Model RS7300 is designed to mold large parts such as chemical or agricultural tanks. Also in the pipeline is a rock-and-roll unit for large parts that delivers better wall-thickness uniformity, allowing for possible part-weight reductions. The new RO-6100/3300 machine lifts the oven from both ends and controls the tilting angle of the oven precisely and repeatably. This allows for much more uniform wall thicknesses than conventional rock-and-roll machines, which have a center pivot. The new model can produce tanks up to 20 ft long and 11 ft in diam. It also holds up to 15,000 lb.
Machine builder STP Equipment has re-engineered its High Volume Shuttle (HVS) machines for large parts. These "rocking shuttle" units have a rocking oven. What's new is the operator stands at floor level instead of up on a platform, which has been eliminated. In addition, the touchscreen control interface has been revised with more user-friendly graphic icons. The HVS series takes up half as much space as a conventional rock-and-roll oven. STP is also building its two largest machines ever. These Hurricane in-line shuttle units will have a 220-in. swing radius and will produce septic tanks and a marine product for molders in the U.S. and France.
Prototype or lab-size rotomolding machines are becoming more widely available. One example is Rotoline's new Lab 0.5 model, its smallest yet. The unit has a single offset arm and a cylindrical horizontal oven 18 in. long x 18 in. diam. It is designed for short runs of parts that measure no more than 18 in. in diameter. The whole machine (electrical panel, cart, and oven) is mounted on a single base. Major and minor axes have infinitely adjustable speeds from 0 to 24 rpm. Cooling is done with fans and water-spray nozzles. The PLC controls permit manual or fully automatic operation.
A new lab-sized pulverizing system from Powder King allows for testing small samples.
Powder King, a two-year-old supplier of pulverizing equipment, rolled out its first milling machine for lab-scale work. The new PK18 pulverizer is a scaled-down model of the firm's bigger units. It uses the company's disposable dual-disc and air-flow management technologies and can process samples from 1 to 50 lb at up to 50 lb/hr. The equipment is designed for cleaning in less than 10 min. Other features include a variable-speed drive, vibratory feeder, and gap adjustment for the milling discs from outside the mill with single-stud, push/pull adjusters.
Powder King also rolled out the smallest production pulverizing system in the PK line. The PK120 can grind from 600 to 900 lb/hr of rigid PVC to 20-mesh fineness or 500 to 800 lb/hr of LLDPE to the standard 35 mesh.
(based on inputs from the 29th annual Fall meeting of the Association of Rotational Molders (ARM) in Cleveland)