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A. 0. LIERMANN and D. E. MILLER G e n e r a l Electric
Co., Silicone
Products Department, W a t e r f o r d ,
N.Y.
Mixing Equipment
. . . for
Silicone Rubber M a n u f u c t u r e Dough mixers and rubber mills are compared with regard to
4 product quality 4 energy requirements T H I S DISCUSSION compares the suitability of two types of batch mixers commonly used in processing silicone rubber -dough mixers and rubber mills.
Comparison of Mixer Types Standard water-cooled, two-roll mills, such as those used for handling organic rubber, are well suited for the manufacture of most silicone rubber materials. Only the manufacture of very liquid, pastelike materials cannot satisfactorily be handled on mills. Differential roll speeds of 1.1/1 to 1.4/1, with peripheral speeds of 100 feet per minute on the fast roll are quite satisfactory for a 22inch diameter mill. Unlike organic rubbers, silicone rubber compounds usually go to the fast roll. Also, since many silicone rubber compounds have low green strength, it is usually the practice to equip the fast roll with a scraper plate. Standard medium to heavy duty dough mixers, readily available from several manufacturers, are also used, especially by the major producers of silicone gums and rubbers, for processing silicone rubber. These mixers can be equipped with either tangential or overlapping blades. Sigma shaped, dispersion, and other types of blades are used. Either the jacket or the blades, or both, can be equipped for heating and/or cooling Tilting bowl units are usually used to provide versatility. To maintain the excellent electrical properties of silicone rubber. stainless steel material of construction for dough mixers is best suited to minimize contamination. The mixing energy requirement per pound of silicone rubber is summarized in Table I. T o obtain these data, recording power meters were attached to the drives of dough mixers and of rubber mills for a series of silicone rubber materials of soft, medium, and hard consistency. The areas under the power meter chart curves were then
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4 safety 4 economics
integrated, and, after deducting no load power requirements, the horsepowerhours per pound of material output calculated. These results show that as the final Mooney viscosity of the material increases, the energy requirement per pound also increases roughly in proportion. The energy required in mixing silicone rubber in horsepower-hours per pound is approximately the same in a dough mixer as in a rubber mill. However, there is apparently considerable difference in the total mean horsepower applied to a pound being mixed on a rubber mill at any given instant as compared to a dough mixer. The energy input per pound being mixed on a rubber mill is high but the time over which this energy is applied is short. I n a dough mixer the energy applied per pound is low, but it must be applied over a much longer period of time in order to do an equivalent mixing job (Table 11). P r o d u c t Uniformity. While the length of time required for mixing in a dough mixer is many times that required on a rubber mill, the dough-mixer batch size is perhaps 10 to 20 times as large. The typical batch size for a 22- X 60inch mill is 100 pounds. A typical charge to a 300-gallon dough mixer is approximately 2000 pounds. The drive power requirements for these two machines are about the same. Thus, if
T a b l e I. The Total Mixing Energy Requirements for Soft, Medium, and H a r d Consistency Silicone Rubber Compounds W e r e Determined by Equipment Power Measurements
llaterial Compound
Soft Medium Hard
XIooney Viscosity, 4 Mixi., 25O C Slachine 24.5 48.5 69.5
D. mixer D. mixer R. mill
1lp.Hr. ’Lb. 0.035 0.050 0.100
T a b l e 11. Total M i x i n g Energy Required for a Medium Consistency Silicone Compound Is About the Same f o r Dough Mixers and Mills; but the Rate of Energy Is Not
llean
Time Pow PI dpplied,
AIdchine
Hp ’Lb
Hr
Hr/Lh
Rubber mill Dough mixer
0.08 0.006
0.6 7.5
0.050 0.045
Hp
-
silicone rubber is produced on rubber mills, many more batches must be produced than if silicone rubber is produced in dough mixers. This, of course, necessitates more testing to ensure the same quality. I n addition to having larger numbers of increments for the same output, the
The silicone industry has matured from a “ w a r baby” 78 years a g o to an indusfry whose sales are over $60 million a year. According to a U. S. Tariff Commission report, 7959 silicone rubber sales were $19 million or over 30% o f total silicone sales volume. Silicone rubber applications a r e extremely diverse. The silicone rubber products which have been devised t o fill the needs of these applications take a wide variety of forms, f r o m soft liquidlike room temperature vulcanizing silicone rubber materials, t o hard, tough, high green-strength wire and cable compounds. W i f h so wide a variety of silicone rubber materials available, selection o f mixing equipment presents the engineer or the rubber fabricator with a real problem when capital investment must b e kept at a minimum.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
quality variation between increments made on the mill is larger than the quality variation of increments made on a dough mixer. The reasons for this are :
Salient Features of Mixing Equipment Dough Mixer
Rubber
Versatile unit More uniform product
Operation of a rubber mill is greatly dependent on the skill of the operator. The operator usually decides the rate of filler addition, and the rate of cross blending. Whereas, in a dough mixer the ingredients are usually added all at once or a t fixed time intervals and the machine and its instruments control the mixing cycle from that point on. Higher random filler losses can occur on a mill, especially where it is well ventilated to provide operator safety and cleanliness. This is particularly true when using the high surface, very low bulk density silica fillers which are used in the manufacture of general purpose and high strength silicone rubbers. Smaller mill batches require higher weighing accuracy, or conversely, lead toward proportionately greater weighing errors. Also, the greater number of mill batches are subject to a greater number of possible operation errors. The larger batch size of the dough mixer tends to dampen out input raw material quality variations.
Higher power input per pound Higher machine output per unit of investment cost Conversion to continuous mixer possible Product shaped or formed One level operation; less explosive hazard ; easier clean out
Atmosphere control Higher product yields Potential labor savings
It is readily apparent that the doughmixer output does not exhibit as wide a variation in physical properties as does identical material produced on a rubber mill. A 1 to 2% yield increase is obtained with dough-mixer production compared to mill production. This again is attributed to the lower loss of the very fine silica filler because the dough mixer is an enclosed machine. Higher batch-to-batch variations on a mill may be attributed in part to variations in the quantity of energy received, and the manner in which energy is received by the material being mixed, even though the same operator is doing the mixing and attempting to do it uniformly. Table IV shows that the energy received by each batch in a series of mill batches of the same material, on the same mill, made by the same operator, in sequence, can be quite variable, even though the time required
I n Table 111, the product uniformity is compared for a series of batches of the same formulation made in a dough mixer, and on a mill. The average values of tensile, elongation, and Shore hardness are shown in each case along with the calculated standard deviation.
per batch is fairly uniform. While Table IV does not purport to explain the difference in product variations between mill mixing and dough mixing in its entirety, it is offered as food for thought for mill run product variations. The floor space required for equjpment, work area, and aisles is approximately the same for equivalent output dough mixers and rubber mills (Figure 1). Mill sizes and mixer sizes are defined by the numerals adjacent to the points on the curve. Table IV. Lower Product Uniformity of Mill Manufacture M a y Be Due to Variations in Energy Input Variation of ampere-minutes per mill run with batches of 50-durometer rubber
Run No. 1
Table 111.
Dough Mixers Produce a More Uniform Product than Rubber Mills Cure: 24 hr. a t 250'
Machine Type Rubber mill Dough mixer
C. and 0.83 pt. Cadox TS-50 per 100 pts. Compound
yo Elongation
Tensile P.S.I.
No. of Batches
2
U
38 33
1009 1040
142 94
-2
U
428 448
63 29
Shore Hardness A
-2
U
50.7 53.0
Mill
3.8 2.3
2 3 4 5 6 7
8 2 U
u/x,
%
Time Required, Min.
Ampere-
22.5 19.5 22.0 21.0 24.0 21.0 21.0 18.5 21.2 1.71 8
190 169 276 218 226 204 173 174 204 36.1 18
;I
10000
t'
9
s
Minutes per Run
f
loo0
800
Y
CI
600
g loo400
MILLS-
I
9
LOO
I
I
I
I
I
800 HMO 1200 1400 1600 MACHINE OUTPUT-THOUSAND PWNDS PER MACHINE YEAR
400
600
I
BOO
2000
0
20
400 800 800 1000 1200 1400 1600 MACHINE OUTF'UT-THOUSAND POUNDS PER MACHINE YEAR
IS00
2 00
Figure 1. Mills and dough mixers of equivalent output require about the same floor space
Figure 2. For equivalent machine outputs, dough-mixer equipment costs are higher than rubber mills
Floor space includes space occupied b y equipment, work space, and associated aisle space. Machine output, is based on 5000 machine hours per year
Equipment costs include mixer or mill, drive and motor only. Mixer cost assumes vacuum, jacketed, tilting, stainless steel unit. Machine output is based on 5000 machine hours per year
VOL. 58, NO. 9
SEPTEMBER 1961
703
With respect to material handling, the rubber mill offers three very important advantages:
DOUGH MIXER EQUIPPED FOR COLD MIXING
DOUGH MIXER EQUIPPED FOR HOT MIXING
- 2d YI 2
1 0 x 2 0 IN. 1
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Equipment Costs. Figure 2 shows equipment cost of mixer or mill, and drive, as a function of its output. Output, expressed as pounds of a typical medium silicone rubber produced in a machine year, is arbritarily taken at 5000 machine hours. Dough-mixer costs and outputs used here assume jacketed, tilting, stainless mixers suitable for vacuum operation. This figure also points out two important aspects of equipment cost: For equivalent outputs, doughmixer equipment costs are considerably higher than rubber mill costs. The slope of the dough-mixcr equipment cost curve is steeper than the mill cost curve and, therefore, as equipment size increases, the cost differential increases. Labor cost is the largest element of mixing operating cost. Labor costs of machines giving equivalent output differ primarily in the batching labor requirements, and the extent to which more than one machine can be operated by one man. One man cannot readily operate two batch rubber mills simultaneously. On the other hand, two (but usually no more) dough mixers in parallel can sometimes be operated by a single man, assuming proper instrumentation, materials handling facilities, and the right product mix and scheduling sequence. For example, if the outputs of 26- X Winch mills are roughly equivalent to those of 260-gallon dough mixers, as implied in Figure 1 or Figure 2, the labor cost for operating two parallel 260-gallon dough mixers could be approximately half the labor cost of operating two 26- X 84-inch mills. However, since labor cost differences vary widely from one situation to another, no attempt has been made here to itemize them. Only the generalization that dough mixers offer the potential for labor cost reduction is made.
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INDUSTRIAL AND ENGINEERING
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Machine Output. Total installed cost has been selected as a basis on which to compare the outputs of mills and dough mixers. Output-cost ratios for various sizes of equipment have been calculated and are plotted in Figure 3. The total installed cost used in the calculation of the output-cost ratios of Figure 3 include mixer or mill costs, drive and motor costs, cost of associated equipment, installation cost, and the cost of total operating floor space required valued at $15 per square foot. For equivalent outputs on a total installed cost basis, the mill is considerably more economical than the dough mixer. Note that the output-cost ratio actually declines for larger size mixers whereas the mill output-cost ratio rises, at least for the size range of the mills covered. Also, even if a mixer is equipped only for cold mixing, the output-cost ratio still favors the mill. Other Aspects, Generally, the dough mixer has greater versatility than a rubber mill. The dough mixer can handle the wide variety of silicone rubber products previously mentioned I t can be used to manufacture free flowing pastes for dispersion, as well as very tough, highly loaded compounds. It is readily outfitted for heating and cooling, and for producing material under low pressures. In addition, being a closed device. it permits the use of conditioned and /or inert atmospheresfor example. oxygen-free or moisturefree mixing environments. This versatility is probably of more value to the basic manufacturer than to the rubber fabricator. On the other hand, rubber mills are much easier to clean in going from one material to another due to their readily accessible unbroken roll surface. Dough mixers are difficult to clean because of the complcx shape of their blades, the difficulty of getting at the spaces between the blades and the mixer, and because of packing and gasket contamination.
CHEMISTRY
Unlike the dough mixer, material discharge from the rubber mill is generally readily shaped. o Virtually all rubber mills are onelevel operations, whereas, with the larger size dough mixers, two levels of operation and the installation of steelwork around the machine are required for charging. l i o t to be overlooked is the inherent ability of the rubber mill to be converted to a continuous operation either by itself or in tandem with another mill ( 7 ) . Obviously, the conversion to continuous operation can also minimize the batch mill run variations referred to above . By virtue of the larger batch sizes used, dough mixers do not generally require prebatching as is common in mill production. Also, because the dough mixer is a closed machine, the environment around the machine can be kept cleaner and the product is also free of stray dirt during its mixing. With respect to safety, the closed nature of a dough mixer also results in a lower amount of atmospheric contamination a n d hence a cleaner, healthier operation. However, the confined space in a dough mixer actually can place the atmosphere in the explosive range when flammable materials are present in the mix. With the static charge readily generated in mixing the highly insulating silicones this can result in a violent explosion. Control of the atmosphere with inert gas blanketing avoids these explosive conditions. While similar explosion conditions can occur in mill mixing, the ventilation usually used in rubber mill mixing results in atmospheres which are below the lower explosive limit.
Salient Features of Mixing Equipment The salient features of the dough mixer and the rubber mill are summarized below. Obviously, the selection of a piece of mixing equipment for processing silicone rubber is dictated by the specific application. Output, safety. costs, flexibility, product uniformity, and cleanliness must be considered carefully in the selection of the equipment. However, the overriding differences between mills and dough mixers aremills offer higher output per investment dollar; dough mixers give greater product uniformity; and dough mixers provide greater versatility. Literature Cited (1) Hale, A., Rubber World 141, No. 6,
815-19 (1960).
RECEIVED for review September 22, 1960 ACCEPTED March 29, 1961
