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Growing sophistication in RTM.

Sophisticated process monitoring and control with CRT display and SPC/SQC reporting; highly stable hydraulic and electric-servo pumping systems; a low-cost "cassette" mold system; computerized process simulation--all these and more are signs that RTM is outgrowing its "artsy-craftsy" phase and becoming a truly industrialized manufacturing process. Most of the RTM news covered here was gathered at the recent annual SPI Composites Institute show and conference in Washington, D.C. (Last month, Part I of our report on the meeting reviewed new materials and reinforcements.)


One look at the 14-in. color-graphics CRT display on this new control system (photos) and it's obvious that RTM has come a long way from its early days. Liquid Control Corp., North Canton, Ohio, developed this control and monitoring system for automotive and aerospace customers who have a growing interest in RTM's potential. The new system can control or monitor 10 dispensing machines. (It can also be used for other two-component systems, such as robotic adhesive/sealant dispensing and electrical potting/encapsulation.) It contains an industrially hardened IBM-compatible PC in a caster-mounted console together with a standard keyboard, sealed membrane function keypad, 40-MB hard-disk drive, 3 1/2-in. floppy drive, and termination points for up to 500 sensor and control inputs/outputs.

Typical process variables that can be controlled and monitored include ratio, pressure/vacuum, temperature, injection time, flow rate, and material usage. Min./max. process windows can be set for each variable, for which the screen displays set and actual values, as well as alarms. Software is menu-driven for ease of use, and is multitasking to permit real-time interfacing to a variety of controllers and computers. An animated graphics package allows custom-designed displays. Optional software modules include SPC/SQC chartin, trending, report generation, historical graphic analysis, and so on. Price ranges between $25,000 and $50,000. (CIRCLE 1)



Sophistication has also come to RTM pumping hardware. For one thing, in order to take advantage of computer control and monitoring, a more precisely controllable method of pumping than traditional pneumatics is required. Continuous, nonpulsating delivery is desired in order to produce higher quality parts. Current pneumatic pumping systems tend to produce a burst of rapid flow at the start of mold filling, which can cause fiber displacement or "washing." Then, as backpressure builds up in the mold, flow slows considerably, frequently resulting in air entrapment and voids. What's needed is a pumping system that can ensure constant flow rates against varying back-pressure by means of either a hydraulic or constant-speed electric drive.

Liquid Control has taken the latter approach with its new, large-capacity system, the first model of which has been built for the National Institute of Standards and Technology (NIST), Gaithersburg, Md., to study RTM process modeling (see below). This machine has individual electric servomotors driving piston-type lance cylinders for A and B components. The system has a microprocessor controller with a keypad, LED display, and menu-driven software to set ratio, shot size and flow rate. Digital readouts of temperature and pressure are also provided. This machine can handle ratios from 1:1 to 5:1 and is upgradable to 30:1. Shot sizes from a few ounces to over 1 gal can be selected, and multiple shots can be used for parts that cannot be filled in one stroke. Flow rates can be controlled from 0.1 to 5 gal/min. (This machine can also be controlled by the CRT system described above.)

Material feed tanks have electric, variable-speed agitators, high/low level indicators with alarms, vacuum degassing capability, and temperature control up to 250 F. There's also recirculation capability for each component. A remote injection block and motionless-mixer assembly has start/stop buttons and a pressure transducer with high/low alarm setpoints to monitor injection pressure. Price range is $75,000-150,000. (CIRCLE 2)

Venus-Gusmer, Kent, Wash., has chosen a hydraulic-powered lance-cylinder approach to achieve uniform, controlled fill rates and pressure in a new "Ram" system designed for epoxy advanced-composite RTM. Equipped with its own self-contained hydraulic system, the Ram is designed to deliver a 1 1/2- to 3-gal shot of premixed epoxy at rates of 10-100 cc/min (up to 400 cc/min is optionally available with a change in gear ratio). Pressure can be set up to 150 psi with a pressure-relief valve. Premixed epoxy is delivered to the single cylinder from a heated pot (self-tuning PID temperature control up to 300 F is provided, along with heated hose). The operator sets the total shot size and injection rate on the control panel, and then backs off the cylinder with air until it fills to the preset volume. Cylinder displacement is monitored and controlled by means of a linear encoder. Injection rate and temperature are displayed digitally on the front-panel LED. The machine can be controlled and monitored by a remote computer. Price range is around $25,000-35,000. (CIRCLE 3)



For applications where such high-tech solutions aren't needed, RTM Systems, Inc., Indianapolis, brought out the Megaject II RTM system with catalyst slave pump. Whereas the previous Megaject system used premixed polyester resin and catalyst, this version uses a standard Glas-Craft 11:1 resin pump and slave pump to deliver 2 1/2 gal/min (32 lb/min) with either MEKP or BPO initiator. The cart-mounted system includes resin and catalyst recirculation, air purge, solvent pressure pot, timed solvent-flush cycle to prevent solvent waste, and a mold over-pressure protection system that automatically shuts off the pump when pressure rises above a preset limit, and resumes pumping after the pressure drops. A timer and whistle warn of the need to solvent flush after a shot, but even if material is allowed to gel in the head, cleanup is said to be easy--just replace a silicone sleeve.

The operator can dial in the pump pressure and number of strokes (200 cc each) to fill the mold. Automatic shut-off after the preset stroke count is optional. Also optional is a built-in vacuum system for the mold with a gauge and on/off switch on the pump control panel. System price is $12,995. (CIRCLE 4)

Incidentally, RTM Systems (a joint venture of Glas-Craft, Inc., Indianapolis, and Plastech T.T. of England) also introduced two auxiliary items for RTM processors. One is a roller knife with a replaceable tungsten carbide blade, said to cut through multiple plies of dry reinforcing fabrics and uncured prepregs more safely and accurately than box knives or razor blades. The rolling blade eliminates distortion or pulling of fibers or yarns commonly experienced with straight blades. It's also available in an electric-powered version. (CIRCLE 5)

RTM Systems says it also has an improved heater cloth for embedding in tool faces to provide integral heating. The cloth is available by the foot in 6- and 12-in. widths. It can stretch up to 35% to follow complex curvatures more easily than previous nonstretch heater cloth. Up to 25 volts will generate mold surface temperatures up to 338 F (up to 392 F if the tooling resin permits). Heat output is 50-200 w/sq ft, and heatup time can be as short as 4 min to reach 140 F. (CIRCLE 6)


As the sizes of RTM parts increase, so do the tonnages of clamping presses built to handle the molds. Cannon Tecnos of Italy, which builds sophisticated SMC presses for automotive and other applications, recently delivered two large double-shuttle RTM clamps (550 tons and 300 tons) to an Italian processor. Platen sizes are 86 x 82 in. and 86 x 52 in. Because of the large size, the presses were built with a tilting top platen that "books" 90 [degrees] (like many RIM clamps) into a vertical position for ease of mold servicing. The lower molds shuttle out of the press to permit gel coating or mounting inserts without delaying production. Cannon Tecnos presses are supplied here by Cannon U.S.A., Mars, Pa., which plans to start building such presses here by year's end. (CIRCLE 7)

Another SMC press builder has been building large RTM and SRIM presses lately. Williams-White, Moline, Ill., delivered four 180-ton RTM clamps to Rockwell International Corp., Louisville, Ky., where Class-A hoods are molded for a new Ford truck. Williams-White has also delivered an 800-ton SRIM clamp to Rockwell's Butler Polymet subsidiary in Lenoir, N.C. This press has a lower platen that shuttles into a combination demold station and mold-release spray booth behind the press. (CIRCLE 8)


What's said to be a relatively low-cost means of improving the productivity of RTM is being offered for license in the U.S. by Lindqvist Business Development Corp., St. Paul, Minn. The RPS Process has been used for five or six years by its developer, Racing Plast AB, a Swedish processor. The process maximizes press productivity by using multiple molds with a single press and allowing most of the cure cycle to take place in the mold but outside the press.

The molds are thin epoxy shells, or "cassettes," without any built-in clamping or support structure, which keeps the cost down. The lower-mold cassette is placed in a supporting cart or fixture for gel coating, followed by layup of glass reinforcement. A premeasured amount of resin is then poured over the glass, and the top cassette is then placed over the glass and resin.

This sandwich is then placed into a press containing tools made of a special concrete with embedded electric heaters. The hinged press lid is closed by hydraulic power, and then 90-100 psi pressure is gradually applied by inflatable bladders until the resin is distributed throughout the part. After the peak exotherm time has been reached (30-60 min), the cassette is removed from the press and allowed to cure completely. Demolding from the cassette does not take place until the part has cooled to room temperature.

Unlike conventional RTM, glass layup and resin dispensing take place outside the press; one cassette can be prepared while another is curing in the press. Most of the curing and the cooling also occur outside the press. Because the cassette itself is cold (only the concrete press tool is warm), relatively fast-curing, heat-activated resin systems can be used, despite some delay between resin dispensing and press curing. Most of the exotherm heat remains in the cassette, so the concrete press tool maintains a temperature around 100 F without active cooling.

Another advantage over standard RTM is said to be lower styrene emissions, since the mold cassette is left closed until curing and cooling are complete. Normally, RTM parts are demolded much hotter to avoid tying up the press. Leaving the part in the mold cassette longer reportedly also improves part quality and reduces shrinkage.

An automated concept of the RPS Process is shown in the schematic on p. 21, although Racing Plast itself conducts the process manually. It has molded an automotive fan cover, measuring approx. 6 x 6 in. and weighing 1.1 lb with a 7-min press cycle. A 57-lb snowmobile trailer and a 116-lb, 18-ft canoe were each molded in 40-min cycles. An 88-lb aerodynamic front section of a camper took 60 min. The cost of a complete installation could range from around $35,000 for a small part to $300,000 for a large one. (CIRCLE 9)


Numerous papers have been presented at technical conferences in the last few years on experimental studies and computer models for predicting injection times, pressures, and flow patterns through different types and arrangements of reinforcements in RTM and SRIM molds (see PT,Jan. '90, p. 17). Three more presentations of recent work on computer simulation of these processes were presented at this year's SPI composites meeting. The Dept. of Mechanical Engineering, Center for Composite Materials at the Univ. of Delaware, Newark, showed evidence of progress in modeling flow through quite complex, 3-D parts, such as an automotive undercarriage crossmember (see illustration, p. 25). (CIRCLE 10)

The Engineering Research Center at Ohio State University, Columbus, presented flow-simulation software that was able to account for the compressive deformation of fabric reinforcements in the mold caused by the pressure of resin injection. Another OSU paper showed one of the first applications of RTM mold-filling analysis to large-area, complex parts, such as an automotive inner-hood panel and hatchback door panel, each with areas over 20 sq ft. The software included special provisions to account for the bending and stretching of glass fabrics during the preforming process, which locally alters permeability and results in uneven flow penetration. OSU researchers are supplementing this work by applying software approaches developed for sheet-metal stamping to simulate stretching effects of preforming on glass fabric. (CIRCLE 11)

Additional help in understanding the RTM and SRIM processes is coming from the federal government. As noted above, NIST is working on RTM/SRIM computer process modeling in a new cooperative program with the Automotive Composites Consortium (ACC), Troy, Mich. The latter is a partnership of Detroit's Big Three automakers aimed at furthering production technology for structural composites. ACC has already enlisted six other companies in RTM/SRIM development (see PT, Oct. '90, p. 94).
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Title Annotation:part 2; resin transfer molding
Author:Naitove, Matthew H.
Publication:Plastics Technology
Date:May 1, 1991
Previous Article:Unmistakable signs of a slump.
Next Article:Automated molding & testing facilities aid resin quality control.

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