US 2715256 A
Description (OCR text may contain errors)
g- 1955 P. F. SIEGRIST 2,715,256
APPARATUS FOR MAKING THIN CERAMIC PLATES Filed March 19, 1952 2 Sheets-Sheet 2 it; 3nventor Bu 2 W W WW (Ittorneg Aug. 16, 1955 P. F. SIEGRIST APPARATUS FOR MAKING THIN CERAMIC PLATES 2 Sheets-Sheet 1 Filed March 19, 1952 8 PM 202 g W 5 United States Patent APPARATUS FOR MAKING THIN CERAMIC PLATES Paul F. Siegrist, Erie, Pa., assignor to Erie Resistor Corporation, Erie, Pa., a corporation of Pennsylvania Appiication March 19, 1952, Serial No. 277,350
2 Claims. (CI. 25-17) In barium titanate and the like ceramics, there is need for flat plates of the order of 10 mils thick for electrical condensers and piezo-electric devices. Thicker plates have been made by pressing in pill machines, but when the pill machines are applied to the 10 mil plates, the variations in density cause warpage during firing.
This invention is intended to make thin flat plates which will not warp during firing by extruding a ribbon which is cut while plastic into the desired lengths.
In the drawing, Fig. 1 is an end view of the extruding die; Figs. 2, 3, 4, and are sections on the correspondingly numbered lines in Fig. l; and Fig. 6 is a side view of the apparatus.
The extrusion of flat plates of ceramic of the order of one inch wide and mils thick (the thickness of from 23 sheets of standard bond paper) would not appear to be possible to persons familiar with plastic extrusion of ceramic materials. It would be expected that the ceramic would be so unevenly distributed that the fired plates would be hopelessly warped. However, by the use of the extruding die illustrated in Figs. l-5, inclusive, a continuous ribbon can be extruded which can be cut into a variety of rectangular, trapezoidal, and other shapes which will be flat when fired.
As is customary in extruding, the plastic ceramic mix is prepared under conditions insuring the absence of occluded air and the extruding head is likewise provided with vacuum for removing air. No attempt has been made to illustrate the provisions for removing air, as these are well understood.
The die illustrated in Figs. l-S, inclusive, is based upon the premise thatif all of-the ceramic particles in the plastic ceramic mix flowing through the extruding orifice of the extruding die are traveling at the same velocity, then the extruded ribbon will remain flat when fired, because no strains will exist in the ceramic which would tend to appear during the firing operation and to set the fired ceramic in a warped condition. With this result, there is for example no tendency for the sides of the ribbon to shrink concave inward, nor for the cut edges of the ribbon to bow outward as would be the case if the ceramic at the center of the extruded ribbon were moving faster than the ceramic at the outer edges. Another advantage is that the extruded ribbon may be cut along various angles crosswise of the ribbon and these angular adges will remain straight when the cut pieces are fired.
The discharge orifice 1 of the die is of much smaller cross-sectional area than the entrance mouth 2. Ordinarily, the mouth will be several hundred times the crosssectional area of the orifice. The entrance 2 to the die starts with a generally circular shape and tapers down toward the extruding or discharge orifice 1 having top and bottom sides 1a and 1b and ends 10 and 1d. The spacing of the sides and ends of the orifice corresponds to the width and thickness of the fired plate plus the firing shrinkage. The rate of taper is generally uniform throughout the first part of the travel of the mixture through the die. At the region indicated by the reference numeral 3,
Fatented Aug. 16, 1955 the die has tapered down to a generally elliptical shape or, in other words, a shape which is gradually approaching the shape of the discharge orifice. The taper from the entrance 2 to the region 3 is not critical. An eifort is made to have the material gradually increase in velocity and to have the material moving at substantially the same velocity throughout the cross-sectional area. However, the cross-sectional area is still large and the modifying efiect of friction against the inner surfaces of the die is not too great. However, from the region 3 toward the discharge orifice 1, the internal shape of the die is quite critical. At the central region represented by the section on line 22 of Fig. 1, the taper becomes more abrupt than would be necessary to bring the upper and lower sides of the die to the thickness of the orifice 1. As shown in Fig. 2, there is a rather long throat 4 in which the upper and lower sides are quite close together so as to exert a substantial friction drag on the material flowing therebetween. The upper and lower sides of the throat 4 are still more widely separated than the upper and lower sides 1a and 1b of the discharge orifice 1, but the throat is sutficiently restricted so as to slow down the travel of the material therebetween. Toward the outer edges of the discharge orifice 1, as indicated by Fig. 3, the section on line 3-3 of Fig. 1, the taper of the upper andlower sides of the die is just sufficient to bring the die to the dimensions of the orifice 1 at point 5 just behind the orifice. Point 5 is very close to the discharge side of the orifice 1 and it might be said that there is only sutficient distance between the point 5 and the discharge side of the orifice 1 to provide a surface of sufficient length so that it will maintain its shape and will not erode away. It would not be improper to term the point 5 as part of the discharge orifice 1, since it is so close to the discharge side of the orifice. In the region indicated by section line 33, the taper of the die is such as to cause a uniform increase in velocity of the material. At this point, there is no need to slow down the material, nor to speed it up. In this region, the friction effect of the upper and lower sides of the orifice 1 is at an average value. Toward the center of the orifice, as indicated by Fig. 2, the friction is at less than average value and it accordingly wasnecessary to taper the die more abruptly so as to produce the throat 4 which offered additional resistance so as to bring the resistance of the throat 4 plus the resistance of the orifice 1 up to the average value present on section line 33. At the portion of the die in back of the outer corners of the orifice 1, as indicated by Figs. 4 and 5, the taper from the region 3 toward the orifice is very greatly slowed down until just back of the orifice when the tapered walls are smoothly merged into the back edge 6 of the orifice by fillets 7 and 7a, the fillets 7 merging into the ends 1c and 1d and the fillets 7a merging into the upper and lower sides 1a and 1b. In the portion of the die in back of the corners, the die accordingly offers less resistance than in the other portions and tends to speed up the material flowing toward the corners of the discharge orifice 1. The friction at the corners of the discharge orifice is at a maximum and it is accordingly important that the friction in back of the corners be decreased so that the sum of the two frictions will equal the average value present on the section line 3 3. In short, at the central part of the discharge orifice 1 where the friction offered to material flowing through the orifice is at a minimum, the friction in the section of the die upstream of the orifice is materially increased so as to bring the over-all friction to the value present on section line 3-3. Similarly, at the corners of the discharge orifice 1 where the friction oifered to material flowing through the orifice is a maximum, the friction on the die section upstream is decreased so that the sum of the die and orifice friction will equal the average value appearing on section line 3-3. As a result of this die construction, all of the particles flowing through the orifice 1 are flowing at the same speed with no tendency for the particles at the center or edges to move faster or slower than the intermediate portions of the extruded ribbon. This results in an extruded ribbon of uniform density and furthermore a ribbon in which the particles of the plastic ceramic mix are uniformly stressed so that there are no non-uniform strains in the extruded ribbon which would tend to be relieved during the subsequent firing operation and cause warpage in the fired piece.
In Fig. 6, there is diagrammatically shown an apparatus for using the die illustrated in Figs. 1 to 5, inclusive, for making flat ceramic plates. The die is shown mounted in the head 8. The details of the extruder are not important. The extruder is preferably of the type provided with a vacuum connection for removing air. Better results will be obtained, if the mix to be extruded has previously been subjected to de-airing. The ceramic is discharged from the extruding orifice 1 in the form of a ribbon 9 directly onto a belt 10 which travels at the same speed as ribbon. This means that the extruded ribbon is neither stretched nor compressed, as it is carried away from the extruding orifice by the belt. Along opposite sides of the belt are a series of guides 11 having diverging sides 12 defining slots 13 for cutter wires 14 carried by a cutter reel 15. The cutter reel is driven at the same peripheral speed as the conveyor belt 10 and the slots 13, which receive the cutter wires 14, guide the wires through the ceramic ribbon 9 andcut the ribbon without disturbing the position thereof on the conveyor belt. The ribbon should be cut while still plastic so as to avoid rough edges. This is accomplished by locating the cutter adjacent the die so the ribbon 9 has little or no chance to dry.
What is claimed as new is:
1. Apparatus for making thin flat ceramic plates having a thickness when fired of the order of 10 mils, comprising an extruder for the plastic ceramic mix having a die with an extruding orifice having a Width and thickness equal to the corresponding dimensions of the fired plate plus the firing shrinkage, said orifice having upper and lower sides defining the thickness and ends defining the width, said die having a throat upstream wider and thicker than the orifice and tapering down in cross-sectional area toward the, orifice, the rate of taper of the throat opposite the center of the orifice being initially faster than that required to meet the upper and lower sides of the orifice and thereafter smoothly merging into such sides and providing a friction increasing restriction in the throat opposite the center, the rate to taper of the throat opposite the ends of the orifice between the upper and lower sides most remote from the center of the orifice being initially slower than that required to meet the upper and lower sides and ends of the orifice whereby the throat remains wider and thicker than the orifice until just behind the orifice and thereafter smoothly merging into such ends by fillets connecting the throat to the upper and lower sides and ends ofthe orifice at the corners where the sides and ends meet and providing a friction relieving section in the throat opposite the ends of the orifice, and the rate of taper of the throat opposite portions of the orifice being intermediate that at the center and ends, the taper of the throat on each side of the center but less remote than the ends being adjusted so the friction in the throat upstream of the orifice plus the friction in the orifice is constant across the width of the orifice and all particles extruded through the orifice travel at substantially the same speed.
2. Apparatus for making thin flat ceramic plates.
having a thickness when fired of the order of 10 mils, comprising an extruder for the plastic ceramic mix having .a die with an extruding orifice having a width and thickness equal to the corresponding dimensions of the fired plate plus the firing shrinkage, said orifice having upper and lower sides defining the, thickness and ends defining the width,'said die having a throat upstream wider and thicker than the orifice and tapering down in cross-sectional area toward and merging into the back edge of the orifice, the upper and lower sides of the throat being built up at the center to provide a friction increasing restriction in the throat opposite the center of the orifice which decreases in thickness fromthe center toward the ends of the orifice, and the throat, being enlarged both in width and thickness at the edges of the orifice to provide friction relieving sections in the throat opposite the ends of the orifice, said enlarged sections of the throat having fillets smoothly merging into the upper and lower sides and ends of the orifice at the corners where the sides and ends meet, the throat being adjusted so the friction in the throat upstream of the orifice plus the friction in the orifice is constant across the width of the orifice and all particles extruded through the orifice travel at substantially the same speed.
References Cited in the file of this patent UNITED STATES PATENTS 532,085 Murray et al. Jan. 8, .1895 616,170 Wheeler Dec. 20, 1898 688,352 Smalley Dec. 10, 1901 1,114,870 Ehrick Oct. 27, 1914 1,350,722 Goodenberger Aug. 24, 1920 2,245,608 Rogers June 17, 1941