The function of meter-mix machines in the liquid silicone molding process.

Liquid silicone rubber (LSR) is a two component reactive chemical with a viscous paste-like consistency. The A and B components are shipped to the user in separate containers, either five gallon or 55 gallon, and must be mixed together in equal volumes to produce the chemical reaction which changes the liquids into a solid rubber. Heat accelerates this process, with cure taking days at room temperature and seconds at temperatures above 250 [degrees] F. The meter-mix machine is the first mechanical element in the molding process. Its function is to remove the A and B silicones from their containers and deliver them, mixed in the proper ratio, to the injection molding machine. When going about choosing the right machine, all options should be considered. As the industry has grown, so have the choices. First, let's take a look at what a meter-mix machine should be able to do.

Functions of the meter-mix in the liquid silicone injection molding process

The primary function of the meter-mix is to deliver accurately metered 100 x 100 ratio A and B materials to the injection molding machine feed throat. Liquid silicones are designed to produce good cured physical properties, such as durometer, tear strength and tensile strength, when they are metered at slightly off specification ratio. For example, the cured properties will be the same at 100 x 97 or 97 x 100 ratio as at the specified 100 x 100 ratio.

However, the meter-mix machine must be capable of producing the specified 100 x 100 ratio within [+ or -] 1%. The high accuracy is needed both for insurance against mixing off tolerance product and for reasons of economy. Since the A and B components are purchased in equal volumes, any rubber left over because of inaccurate metering cannot be used and is wasted.

The meter-mix machine must be capable of pumping a wide range of material viscosities. Available viscosities range from pourable liquids, less than 100 Kcps, to extremely thick 4 million cps paste. Metering accuracy must be maintained throughout the range.

The liquid silicone molding process is inherently clean because the chemicals are sealed into a closed system. Ambient air with its accompanying dust and moisture does not contact the product from the time drums are installed on the meter-mix machine until finished parts are removed from the mold.

Therefore, it is important that the meter-mix machine not add contaminants. Pump bearing and seals must be made of materials that will not transfer microscopic particles into the silicone. All metal parts must be corrosion resistant to prevent rust from forming. Hoses must be lined with Teflon or other thermoplastics, since conventional rubber lined hydraulic hose contains sulfur, which will contaminate the silicone and sometimes inhibit cure.

Silicone purity is especially important when molding implantable medical parts or parts which contact blood or drugs in their final application. Industrial silicone parts also benefit from pure silicone because contaminants often reduce the physical properties of the cured rubber. The meter-mix machine must also not draw any air into the mix, since small bubbles will reduce part strength and in a clear part will be considered a cosmetic defect.

The meter-mix machine should have the ability to add third and fourth components when needed, at the ratios and accuracy required. These include pigments, medical additives and self bleeding oils.

The mix machine must thoroughly mix A and B components, as well as small amounts of third and fourth streams. Because of the various styles of injection molding machines, the amount of mixing required varies.

It is important for the mix machine to supply a uniform pressure at the injection molding machine barrel inlet. A consistent supply pressure to the barrel will fill the barrel to the same pressure each cycle, resulting in shot size repeatability.

Basic components included in a meter-mix machine

The high viscosity of the material makes a ram and follow plate assembly necessary. The ram, either pneumatic or hydraulic powered, provides the pressure needed to force the follow plate into the container and pressurize the silicone under the plate. The pressure forces the viscous silicone to the center of the plate and aids in filling the pump. The seals on the follow plate prevent leakage and stop contaminates from entering the container.

A drum transfer pump, double or single acting, may be used to do the actual metering or to supply material to a separate metering pump.

The metering pump drive cylinder(s) may be powered by air or hydraulic. The ratio of the cylinder to pump area should provide up to 4,000 psi pressure available to force the viscous silicone through the hoses and manifold/mixer assembly to the injection machine barrel.

Hoses for A, B and pigment carry the fluids from the metering pumps to a fluid manifold assembly. The manifold incorporates check valves for each fluid to prevent back-flow, which could result in cured rubber in the hoses. The check valves also isolate manifold pressure from the individual pump pressures, which is useful in monitoring pump performance.

A mixer, either static or dynamic, should be included. The degree of mixing needed will depend on a number of factors which are detailed later in this article under mixing silicone.

Two general types of meter-mix machines are used to supply liquid silicone to injection molding machines, including direct from the drum, and transfer and meter.

Direct from the drum metering machines

In a direct from the drum meter-mix machine, the A and B metering pumps are mounted in the ram and follow plate assemblies. The metering pump draws fluids from the drum and meters them through hoses to the manifold and mixer assembly. The only wetted components are the follow plates, pumps, hoses, manifold and mixer. This type of meter-mix machine has the smallest number of wetted components which must be cleaned when changing materials. Most dedicated five gallon machines are of this type, since they are often used in short-run applications. Whether using five or 55 gallon machines, the direct from the drum design affords the least time consuming and most economical material changes.

Controls on these units vary from simple pneumatic or hydraulic fixed ratio machines to PLC controlled variable ratio. The PLC controlled machines have the ability to allow both A and B pumps to operate individually for easier pump priming, testing and base purging. Pump monitoring, along with alarms, messages and printouts, are available. Options such as closed loop ratio control and pressure transducer based feed pressure control are also available.

Transfer and meter systems

In a transfer and meter system, transfer pumps instead of metering pumps are located in the ram/follow plate assemblies. They supply silicone to a separate metering pump set which is usually located beside the injection molding machine. One set of transfer pumps can be used to supply several meter-mix machines, thus several injection presses. This type of system is appropriate when the same silicone is used at multiple presses and silicone changes are infrequent. Both the direct metering and transfer and meter systems are available in five gallon and 55 gallon sizes. Most 55 gallon systems can also operate from five gallon pails by removing the dram follow plate and replacing it with a pail follow plate.

Options in molding room design

As silicone injection has become more popular, for reasons of space and money, how to best utilize the meter-mix equipment becomes an important question. It takes up less space to use one meter-mix for more than one injection machine. There are a few ways to couple multiple injection machines to one meter-mix.

The simplest way is to put a tee after the mixer and split the flow of mixed material to two or more machines. This is an economical way and is popular during expansion phases of a molding operation. There are a few drawbacks to this method. If all the molds are not being run steady enough, hoses full of mixed material will cure. Mixed silicone normally has a working time of one week. This can vary greatly with make, model and temperature of the silicone. Another drawback is the possibility of having a pressure sensitive process. If both injection machines call for material at the same time, they will be filled at a lower pressure than if they were sequenced. In turn, the barrels will be packed less and short shots may result.

By sequencing the injection machines, the shot volume problem is solved. The machines must be interfaced so when one screw is rotating, the other will wait until the first is done before it starts. The shot volume is okay, but there is still the hose full of mixed material. By teeing the silicone at the meter-mix, adding a hose set, manifold and mixer, and sequencing the machines, both machines will receive the same pressure at the same flow rate.

With any of these scenarios, it is recommended to have an interface valve between the manifold or mixer, and the barrel. Usually air operated, this valve opens only during screw rotation. It does not allow pump pressure to force the screw position from its set point, and it relieves pressure from the elastomeric seal at the rear of the barrel.

Mixing silicone

There are many mixers used to process silicone and nearly as many ideas of which is the best. There are motionless mixers from $1,000 to less than $100 available. Normally suggested for medical part production are stainless elements in stainless steel tubes with stainless fittings. Plastic elements may be used with the same tube and fittings to lower costs. Either of these mixers lend themselves well to water-jacket cooling, used to prolong the interval between maintenance. The length, or elements of mixing required, will vary depending on major factors. First, is a reciprocating screw or a plunger style injection machine being used? Second, is it a two component mix or is a third lower viscosity material being added in a very small volume?

The majority of injection molding machines being used for liquid silicone rubber use reciprocating screw injectors. The rotation of the screw during recharge provides aggressive mixing action. Back pressure is used to force screw slippage and enhance the mixing efficiency. Therefore, if the shot volume is large enough to allow sufficient screw rotation, a static mixer is not needed. A rule of thumb is if the shot volume is greater than 10 grams, a static mixer is not needed. This will vary with screw diameter and low pigment percentages. However, many molders continue to use a static mixer to provide redundancy and make screw speed and back-pressure setting less critical.

Many older transfer and compression presses can be converted to liquid silicone rubber use by adding an after-market injection unit. Most of these units, as well as other new injection machines, utilize a plunger injector. When feeding a plunger type injection molding machine, the meter-mix unit must provide all the required mixing, since no screw is present. A 20 element static mixer will completely mix A and B rubbers. A 32 element static mixer is needed if pigment or additives are being mixed. The quality of mix through a static mixer or reciprocating screw is generally regarded to be equal, since physical properties of the molded rubber parts from plunger and screw type presses are comparable.

Adding pigment and/or additives

Third (and more) stream metering may be accomplished in several ways. Traditionally, a small pump is slaved off the main cylinder that drives the A and B metering pumps, mounted on fulcrum type drive bars allowing the pump stroke to be varied by adjusting its position along the drive bars. The stroke adjustment allows selection of pigment loading, usually from 1% to 5% of A and B volume. Larger pigment or oil pumps are available for higher percentages, but most applications are in the 1% to 5% range. Accuracy of the mechanically slave driven pigment metering pumps is within [+ or -] 0.2%, allowing molders to meet precise color specifications.

If one meter-mix needs to feed two or more injection machines, and they in turn need different or no pigment, a different approach is used. Mechanically independent pigment injectors may be added to the A and B silicone stream at each injection molding machine. This system may be used with either a direct from the drum machine feeding multiple presses, or in a transfer and meter system. The injector can get its signal from either the screw rotate signal from the molding machine, or a flow meter in the silicone line. If the screw rotate signal is used, a flip-flop timer will control the frequency of the injections. With a flow meter, a counter/timer combination will control the frequency.

Feeding pigment to either a pigment pump or pigment injector is accomplished in the same ways. Small pressure pots with a follow plate or a disposable pressurized plastic tube may be used for short runs. For large, long running jobs, a five or 55 gallon ram pump may be used to lessen the interval of pigment refills and re-priming.

Conclusion

When choosing a meter-mix unit, a number of factors needs to be considered:

* If starting out with five gallon pails, is there the possibility of needing to go to 55 gallon drums?

* Is the pump going to feed more than one injection molding machine?

* Are pigments involved, how many, how often and will more than one be used at the same time?

* Are the injection molding machines screw type or plunger?

* Is there a possibility of needing a medical grade machine?

* Will the meter-mix need to be mobile?

* Will the meter-mix be used constantly or sporadically?

* Is there a need for ISO 9000 calibration and certification data?

Having answers ready for these questions will allow the meter-mix supplier to give informed choices to the prospective molder.

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