Resin Pumps for Closed Molding

By Hank Yeagley

The title of this article, Resin Pumps for Closed Molding, was intended to catch your attention. If you are reading this it must have worked. However, this title is somewhat misleading. A more accurate, but much longer, title could have been: A Review of the Equipment, Hardware, and Software that is commonly used for Transferring Liquid Thermoset Resins from a Storage Container to the Cavity of a Mold. You can see why we went with the shorter title.

Before exploring pumping equipment for closed molding we should probably describe the molding processes we are referring to. Closed molding has been around for quite a while. Its roots date back more than fifty years to the Marco molding method, which was the first serious closed molding process for composites. Pre-preg (wet) vacuum bagging, liquid composite molding (cold molding), and RTM were, for many years, about the only commonly used closed molding processes. Yes, pultrusion, SMC, BMC, and RIM have also been with us for a while, but they were outside the realm of what most composite molders know as closed molding.

In more recent years a variety of so-called ‘low cost’ closed molding processes have been developed that are viable for a wide range of composites molding applications. Several of those low cost processes, including Vacuum Infusion (VIP), Light RTM, and Closed Cavity Bag Molding (CCBM) have become fairly widely adopted. These low cost processes share many characteristics, primary among which is the use of vacuum. In these processes the resin is often injected into the mold cavity under relatively low pressure. However, in many circumstances, the resin can also be ‘infused’ into the mold cavity without an actual pump. (Atmospheric pressure does the pumping in this case.) The use of a resin pump in conjunction with vacuum is often referred to as a push/pull system.

The growing popularity of closed molding has led to a proliferation of new resin injection machines. New machines have been introduced at both the top and bottom of the price/feature scale. Those machines range from inexpensive manually operated no-frills machines to fully computerized and automated machines with every bell and whistle imaginable. In this article we will review the more common methods of resin delivery, beginning with the least complex.

In what is probably the height of closed molding simplicity, resin is simply poured from a container right into the mold. Liquid Composite Molding, or ‘Cold Molding’, mentioned above, is a common and long established process that uses this method of resin delivery. The reinforcement is loosely placed in the open mold, and a pre-measured quantity of catalyzed resin is poured onto the reinforcement. Exactly how and where the resin is poured is an inexact science that is learned by trial and error. After the resin is poured, the mold is closed, usually by a press. Despite the process name ‘Cold Molding’, the molds used in this process are often heated. Go figure. During the 80’s there was a flurry of activity with a proprietary vacuum clamped molding process, called ‘Prestovac’, that also used this method of resin delivery. In this process a lightweight mold set was slowly clamped using vacuum, rather than a press, thus forcing the poured resin to disperse itself through the reinforcement.

Many of the ‘infusion’ processes use a resin delivery system that is almost as simple. The resin is sucked into the mold through a hose. Again the resin is pre-catalyzed in a container. This method is somewhat more complicated than ‘cold molding’ because hoses and injection ports are used, and because complete mold filling is not quite as much of a given as in the previously mentioned processes. There can be many more variables, and things that can go wrong. Locating injection and vacuum ports correctly and tracking down vacuum leaks are two issues that infusion molders often have to contend with.

Upping the complexity another notch, positive injection pressure can be used. In the previous example, just raising the resin container above the level of the mold would create a slight head pressure. Normally, however, some kind of specialized equipment is used to deliver the resin to the mold under pressure. This specialized equipment can be nothing more than a painter’s pressure pot. One commercially available resin injection machine simply used a section of steel pipe with caps on the end to create a ‘pressure pot’. Although ‘pressure pot’ injection equipment is simple (no moving parts) and relatively easy to maintain, it does have some drawbacks. Like all the previous delivery systems, pressure pot systems are a ‘hot pot’ system requiring a pre-measured and pre-catalyzed quantity of resin. Like all ‘Hot pot’ systems, their virtue is simplicity. Their drawback is that the clock starts ticking when the catalyst is added to the pot. From that moment on it is a race against time, and sometimes the clock wins. Pressure pot systems can be wasteful as well as a source of frayed nerves when the pot ‘kicks’ before cleanup can be accomplished.

Although ‘hot pot’ systems can be, and often are, used for low volume production, medium to large volume molders usually rely on some form of meter / mix / dispense equipment. But, whereas a pressure pot might be purchased for a few hundred dollars, meter / mix equipment can involve an investment ranging from several thousand dollars to more than fifty thousand dollars.

For medium to large volume production, dual component meter / mix / dispense systems are most always used. Their virtue is speed and convenience, and their drawback is cost, complexity, and maintenance.

The least cost and most straightforward dual component delivery system is the double pressure pot system. This is the same type of pressure pot that was used with the ‘hot pot’ method only now there are two of them. With this type of system a 1 to 1 mix ratio is the standard. The mix head can be nothing more than two ball valves and a removable static mixer. This is not a positive displacement system so component ratios are not going to be exact. However, if part A and part B are properly formulated, an exact 1 to 1 ratio is usually not essential. When using a double pot system, off ratios can be minimized by observing two rules of thumb. First, when plumbing the system, both pots should be manifold to the same air regulator, and all resin hoses and fittings should be exactly the same on both sides. Secondly, both the A and B sides of the resin should be the same viscosity and temperature.

Next, in terms of increased cost and complexity, are peristaltic pumps. Peristaltic pumps squeeze resin through a pliable plastic tube just like toothpaste is squeezed from a tube. Peristaltic pumps have a number of desirable attributes. They are relatively inexpensive, quite simple to maintain, and very easy to clean. (Only the tubing comes in contact with the resin) This type of pump also is fairly easy to set up to pump two or more components as long as the mix ratio is not extreme (20 to 1 ratios are a practical limit.). Peristaltic pumps are positive displacement in principle, but in practice some slippage can occur, such as in the automatic transmission of a car. For that reason precise accuracy of mix ratios may be difficult to achieve and maintain. If the resin component mix ratio is critical, then peristaltic pumps would probably not be a good choice. On the plus side, peristaltic pumps can be an excellent choice for use in a hot pot system because of the ease with which the tubing can be cleaned, or replaced if necessary. The primary drawback of peristaltic pumps is that the plastic tubing, depending on type, can only withstand up to about 30 psi. That works for VIP, and vacuum assisted RTM, but it doesn’t work for many other applications.

The next step up in price is the gear pump. They work in fashion similar to peristaltic pumps but are truly positive displacement. Also like peristaltic pumps, gear pumps can be sized and driven to provide almost any multiple component mix ratio. Gear pumps can be used in many meter / mix applications. Their strong suit is in metering. They can be set up to deliver a constant non-pulsing volume of material per minute regardless of back pressure (within reason). Their drawbacks are that they don’t tolerate abrasive filler, are not easy to service, and don’t lend themselves to quick and easy ratio adjustments. Gear pumps are well suited for pumping urethane resins and are often found in RIM pumping equipment.

The most common type of resin pumping equipment found in typical FRP shops employs the use of positive displacement piston pumps. Piston pumps are available in a variety of designs and configurations, and they are popular for good reason. First like gear pumps they are true positive displacement pumps, which allows them to meter material very precisely. That is of prime importance when a 1000 to 1 mix accuracy may be required (for instance 100 parts resin to 1.25 parts MEKP). But, whereas gear pumps excel at delivering constant flow rates at the expense of regulated pressure, piston pumps excel at delivering resin at a controlled injection pressure, but at the expense of regulated flow rates. (This generalization applies to resin pumping equipment, not all hydraulic systems) In most composite closed molding situations, controlling injection pressure is more critical than controlling fill rates. Taking a little extra time to fill a mold is much preferable to rupturing a hose or blowing a mold apart and spraying resin everywhere.

Two types of piston pumps are commonly found on resin pumping equipment... single action and double action. They both do the same job. Single action pumps pump only on the ‘down’ stroke but are simpler in construction, easier to maintain, and perhaps more reliable. Double action pumps pump on both the ‘down’ stroke as well as on the ‘up’ stroke, providing a smoother resin flow. (a V8 runs more smoothly than a 4 cylinder) Both types work very well for resin injection or dispensing. In spray applications the smoother flow of the double action pump can be an advantage. In line accumulators are often used on spray equipment to further dampen the pulsing effect that is typical of all multi-stroke reciprocating pumps. (It should be noted that a piston pump can be constructed large enough to completely fill a mold cavity in a single stroke. Such a pump is called a ‘lance’ type pump and does not create the pulse of a multi stroke pump. For practical reasons lance pumps are not common.)

Virtually all piston type resin pumping equipment is powered be either a hydraulically or pneumatically actuated master cylinder. Pneumatic master cylinders are by far the most common and are typically referred to as ‘air motors’. The beauty of this arrangement is that maximum pump pressure can be easily controlled by controlling the input air pressure to the master cylinder or air motor. Another advantage of this system is that the pump maintains continuous pressure even though the air motor only runs when resin is actually flowing.

In addition to being able to provide constant delivery pressure, piston pumps lend themselves to being ganged (slaved) together to pump multiple components in fixed but easily adjustable ratios. Those of us who can remember the days of the catalyst injector (a pressure pot) can attest to the advantages of the slaved catalyst pump. Another major advantage of the piston pump in FRP applications is that they can be engineered to handle highly filled resin systems that would quickly destroy a gear pump. In that regard contemporary piston pumps have been designed for relatively easy and inexpensive servicing. Replacing wear items such as seals and packing is usually not a major project and can be done as part of routine maintenance.

Piston pumps are available in a variety of ‘ratios’. The ratio that is referred to is the ratio of the master cylinder (usually an air motor) pressure to the resin pump pressure. In practical terms, an 11 to 1 pump would create 11 pounds of resin pressure for every 1 pound of air pressure that is applied to the air motor. Stated another way, the same pump would create 1100 psi of resin pressure when 100 psi air pressure is applied to the air motor. Since closed molding resin injection is accomplished at relatively low pressures (typically 1 to 30 psi), low ratio piston pumps work fine for this purpose. (Much higher pressures are typically needed for spraying resin.)

Most current piston pumping equipment can be had with a lengthy list of convenience options. Stroke counters or flow meters are indispensable for processes such as Lite RTM and CCBM where a measured shot is typically required. Catalyst alarms reduce the risk of dispensing un-catalyzed resin, in-line heaters can maintain a constant resin temperature at the mix head, recirculation loops can be of value when filled or heated resin is being pumped, auto purge and flush circuits simplify the cleaning of mix heads, and auto sprues eliminate the need to manually handle a mix head.

An increasingly important advantage of piston pumping equipment is that it can be easily engineered to incorporate a variety of process control functions. For instance, a feature that progressively varies the catalyst ratio during the injection of large parts can enable faster and more consistent mold cycle times. (The last resin into the mold cures at the same time as the first resin into the mold.) In addition, piston pumping equipment can be easily coupled to other process control equipment. A recent enhancement to closed molding pumping equipment is the addition of an electronic feedback loop that actively controls internal mold pressures during injection. Molds can be equipped with a sensor that monitors the internal pressure in the mold cavity during injection. The signal from this sensor controls the injection speed and pressure so that the internal pressure in the mold never gets above a pre-set level. Remember that LRTM mold halves are clamped only with vacuum, and like all infusion processes, a pressure of less than one Bar should be maintained inside the mold cavity during injection or infusion. A positive internal pressure can cause the mold halves to blow apart. The pressure monitoring feature allows the mold to be filled at the maximum possible rate, and in the least amount of time.

Most all of the pumping equipment suppliers currently offer equipment with a very high level of process control capability. State-of-the-art ‘real time’ process controls not only have the capability of making process adjustments on the fly, but they also enable the monitoring and recording the processes vital statistics. Micro processor controlled systems can store molding parameters for multiple parts, can sequence multiple process steps based on sensor input, and can be tied into plant-wide computer-based SPC and process monitoring software. All of this can lead to improved part quality as well as speed up the molding cycle, thereby improving productivity. This level of process control has long been available to thermoplastic injection molders, but is relatively new technology to the commercial segment of the composites industry.

It goes with out saying that highly sophisticated, computer controlled resin pumping equipment is most suited for high volume automated, or semi-automated closed molding production situations. And it would probably be fair to say, the initial excitement over computer controlled ‘touch screen’ equipped pumping equipment has slackened a bit as the overall cost of owning and maintaining such equipment has been established.

Currently, in our industry, the majority of closed molding applications don’t require a top-of-the-line pumping system with all of the automated controls. For those applications, most of the equipment suppliers offer simple, inexpensive, easy to maintain, manually controlled pumping equipment. In the end, each molder will have to determine what pumping equipment is most cost-effective for their application.

Hank Yeagley has more than 40 years experience in the composites industry. He is a contributing editor to CM: 814.442.8446; h.yeagley@verizon.net.