Computer process control comes to rotomolding.

Rotational molders will soon have the opportunity to step up to computerized process control and to remote real-time monitoring of machine and production status in the plant. Two new software developments in on-line control and monitoring - one from a processor and the other from a machine supplier - are both approaching readiness for commercial marketing.

One new development improves machine productivity through sophisticated temperature control, while the other provides a means of gathering and analyzing mold and machine data during the process. The system can collect data from a single machine cell or a whole plant.

These and other developments were presented to the 409 attendees at the annual spring meeting of the Association of Rotational Molders (ARM) in San Antonio, Texas. Also reported there were new material introductions from three suppliers. Also, ARM announced that it will host Rotoplast '97, a one-day trade show and exhibition for rotational molders, to be held October 6 in Westmont, Ill., in conjunction with ARM's fall meeting October 5-8 in Oak Brook, Ill. Seventy to 80 exhibitors are expected.

TABLE 1 - DOW'S NEW DOWLEX LLDPE FOR ROTOMOLDING

                        NG 2429 N     NG 2431 N     NG 2432 N

MI, g/10 min                4.0           7.0           3.8

Density, g/cc               0.935         0.935         0.939

Vicat Softening
Point, F                    244           244           253.4

Heat Deflection
Temp., F                    167           137               -

Flex Modulus
(1% Secant), psi          76,850         76,850        89,900

Tensile Modulus,
psi                       65,250         72,500        79,750

Falling Dart
Impact, J/mm                17            15             15

MONITORING THE MOLD

A brand-new development in process monitoring for rotomolding comes from Remcon Plastics, Inc. of Reading, Pa., a rotomolder whose new IRT (Infra-Red Thermometry) technology provides continuous monitoring of mold temperature as a means to optimize the process. The system uses a non-contacting infrared camera mounted in the oven and another one in the cooling station. The sensors remotely monitor the temperature of the outer mold surface.

This monitoring system can collect data continuously 24 hours a day. Temperature data can be collected up to 20 times a second and sent to a personal computer. The latter creates mold heating and cooling curves, from which an optimized curve can be mapped out.

TABLE 2 - ICC INDUSTRIES' NEW FARALLOY FR-LLDPE
(Preliminary Data)

Property                         PE-400           PE-401

MI, g/10 min                       7.5              4.5

Density, g/cc                      1.02             1.15

Tens. Strength [at]
Yield, psi                         1900             1600

Flex. Modulus
(1% Secant), psi                  78,600            87,00

ARM Impact [at] -40 F,
1/8 in., ft-lb                      15              10-15

UL 94 Listing [at] 1/8 in.
(all colors)                         V-2 V-0

The outer mold temperature is used as a reference point to indicate inner mold temperature, and is used as a way to determine optimum times and temperatures for heating, cooling, and demolding, says Paul Nugent, Remcon's director of technology.

Real-time monitoring can help improve repeatability from cycle to cycle, even if ambient conditions in the plant change or, for example, an operator varies the speed at which the mold moves into and out of the oven.

The IRT system has been in development for about 18 months and used in production at Remcon's plant for six months on a three-arm carousel machine from Ferry Industries, Stow, Ohio. Remcon uses the IRT system to determine an optimal demold time for a product, then it examines how different process variables lengthen or shorten the time needed to get to that ideal demolding temperature. In the near future, Remcon plans to offer IRT for sale to other processors.

CIM FOR ROTOMOLDING

Another new development comes from rotomolding machine builder Ferry Industries, Inc. It is developing Rotobase, a process and production monitoring system that allows you to see graphic displays of how every machine in a plant is performing from a remote PC terminal, says Ferry president Harry Covington.

Rotobase receives information from each machine through a PLC linked by an Ethernet or similar communication network. Rotobase can pulldata from each machine and drop it into standard or custom-designed screen charts, such as a cycle-time and fault-status summary screen or a production report by shift. Percentage of good and scrap parts made by each machine can be represented by a bar chart, while causes of rejects (e.g., overcuring, warping, or bad trimming) can be shown on a pie chart for each shift or other period. Rotobase also keeps detailed records of all process parameters and individual operator, shift, or plant performance over time.

Covington says commercial release of Rotobase is a few months off. For the future, Ferry plans to develop Rotobase into a machine control as well as monitoring system. To that end Ferry and Remcon are working on adaptive control software that will regulate the heating and cooling rates based on the optimum heating and cooling profile stored in the database.

NEWS IN POLYETHYLENE

On the materials side, more information is now available from Dow Plastics, Midland, Mich., on its new Dowlex NG resins that are said to be the first octene LLDPEs for rotomolding (see Table 1 and PT, May '96, p. 12). Nu-Plast Inc. of Brampton, Ont., and Buffalo, N.Y., North American distributor for Dow's rotomolding products, says another grade, aimed at large parts, is in the works. A developmental version has 15 MI and 0.935 g/cc density.

Nu-Plast also distributes a new octene-based MDPE containing a semi-conductive additive package. Black, uv-stable Tecrothene C501 is a 500-micron powder with 4 MI and 0.935 density. It comes from Rotec Chemicals Ltd. of the U.K. The material's static-dissipative properties are said to be permanent and independent of ambient humidity and reportedly do not significantly affect resin properties. Surface resistivity at 50% R.H. is 3-5 x [10.sup.5] ohm/sq; flexural modulus is 80,000 psi; and tensile yield strength is 2500 psi.

Nu-Plast also offers a 7 MI 0.935 density Tecrothene rotomolding grade that produces a "stone effect." The product is based on Dowlex NG2431N resin with uv stabilizer and comes in five colors that mimic the surface textures of sandstone, marble, and granite. It reportedly imparts good definition due to the high melt flow of the base resin.

A new flame-retardant version of Superlink 110 crosslinkable PE is available from Wedtech, Inc., Brantford, Ont. Besides a UL 94V-0 rating, it offers high stiffness and low-temperature impact strength. It has a density of 1.010 g/cc, tensile yield strength of 3200 psi, tensile strength at break of 2700 psi, and flexural modulus of 130,000 psi. Impact strength at -40 F of a 0.250-in.thick rotomolded part is 50 ft-lb. The resin comes in standard or custom colors.

Two new flame-retardant polyethylenes for rotomolding were developed by ICC Industries Inc.'s New Product Development Center, Dover, Ohio. Both materials are listed with Underwriters Laboratories. Faralloy PE-400 is classified UL 94V-2 at 3 mm or 1/8 in. Faralloy PE-401 has both 94V-0 and 95-5VA ratings at 1/8 in. ICC claims that other PE resins require 1/4-in. thickness to achieve V-0 and that they do not meet 5VA at all. See Table 2 for properties.