Meter/mix machine

Urethanes are processed as mbber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The Hquid prepolymers are cast using automatic metered casting machines, and the thermoplastic peUets are processed like aU thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensUe, and resistance to many oUs is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

FIGURE 17.6 Operating schematic of an automatic metering, mixing, and dispensing machine for adhesive systems.6... 

Figure 2. Custom RIM metering and mixing machine with fixture, for the fill of aluminum baseball bats with a small amount of flexible foam.

On account of their low viscosity, liquid silicone rubbers are processed chiefly by liquid injection molding (LM), a technique which resembles the injection molding of plastics and is combined with a meter/mix unit developed specifically for the process [7]. Additional homogenization is effected by a static mixer located between the mixing station and the feed screw of the injection molding machine (Fig. 5). 

Prepolymer is prepared as already described and cooled to a temperature between 70 and lOO C. This is then used as one stream in the continuous mixing machine. The other stream consists of water, emulsifying agent (e.g. sulphonated castor oil), and a foam stabilizer (silicone oil). Different hardnesses can be achieved by varying the NDI content together with the water content. Density control is obtained by the metering of precise quantities of reaction mixture into the mould which is then closed to enable the material to expand to fill the mould without overspill. After removal from the mould the product requires an oven cure of 24h at 110 C. 

Reprinted from ref. 42 by courtesy of Machine Design, Penton/IPC Inc. Meter-mix systems required for room temperature cure. 

In order to give our readers a good idea of what can he done in large installations, we describe hereunder a metering and mixing machine which is used in large plants for furniture and woodworking. 

This is the first parameter to consider. In some cases, the pot life is long enough so that the two components may be kept from one to several horns before application, but in other cases the pot life may be only a few minutes and this requires a metering and mixing machine that prepares the mix immediately before application. When the pot life is very short (a few minutes), the flow rate of the mix must be large enough so that the mixed products may be evacuated and used before the mix could harden into the equipment. 

In addition to the above described, stationary production of refrigerators, the circulating transport systems as well, as more developed concepts are used [9]. The molds will pass a metering and mixing machine. Filling is done either from top or bottom. 

The most popular meter, mix and dispensing machines have been designed around gear pumps and pistons, or a combination of both. This chapter will explain how both systems function, their strengths and weaknesses. 

In conclusion, the methods of dispensing reactive materials are varied. We are seeing the meter, mix, and dispense machine being integrated into computer controlled production lines. Dispensers are working successfully with... 

No premixing or preforming is necessary. The meter-mix unit supplies direct to the injection moulding machine in a fully enclosed air-free system. 

A number of injection moulding machines currently produced in Europe and the USA can be supplied already modified for LSR and provide, together with the meter-mix units, virtually a turn-key operation. Mould design is generally straightforward and duplicates many existing techniques used for rubber and plastics. Runner systems and gates can be much smaller than usual and this reduces material waste. 

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