CENTERING MOLDING ASSEMBLIES

E. I. Suzdal’tsev,1 D. V. Kharitonov,1 and A. N. Khramov1

Translated from Novye Ogneupory, No. 12, pp. 13 – 18, December, 2008.

Original article submitted October 22, 2008.

Methods are surveyed for centering molding assemblies as used to mold large ceramic blanks from aqueous

slip. Existing and promising future methods of centering are examined, with emphasis on their advantages and

shortcomings. The analysis shows that video centering is the most promising method.

The molding assembly (Fig. 1) is a basic component in

producing ceramic blanks by slip casting in porous molds,

which consists of two basic parts: the core 1 and the porous

mold 2, which provide respectively the internal and exterior

profiles of the shaped blank 3. When this assembly is

brought together, the main problem is good centering of the

core relative to the outer mold, since its accuracy largely

governs the quality of the blanks, and also affects the toler-

ance allowed for mechanical working and consequently the

consumption of material.

This is particularly important for making large ceramic

blanks (height up to 1500 mm and diameter at the base up to

450 mm) such as nose cones for flying vehicles, where the

thickness differences in the nose part sometimes attain two or

more millimeters.

As regards improved core centering, there are many de-

vices and methods designed to do this in making large ce-

ramic components from aqueous slip. In a patent [1], a de-

vice is proposed (Fig. 2) that employs self-orientation of the

core. That effect in the opinion of the authors of the patent

[1] is attained because the core 1 joined to the supporting

flange 3 by a spherical hinge 5 and elastic insert 4 takes a

strictly vertical position under gravity. However, although

that idea is original, it is difficult to apply under actual indus-

trial conditions for the following reasons. Firstly, in self-ori-

entation (under gravity) of the hinged core, one needs very

accurate adjustment of it on the coordinate axes of the basic

surfaces relative to the central axis of the inner surface of the

porous mold. Secondly, with such an assembly, the porous

mold must be set strictly vertically to eliminate any deviation

between the axes of the core and the inner side of the mold.

Failure to meet these conditions may lead to the axes of the

core and the inner face of the porous mold not coinciding,

which produces thickness variation in the formed blank.

One can avoid lack of coincidence between those axes in

the opinion of the authors of a patent [2] by using the device

shown in Fig. 3. When the core 1 has been set up in the cav-

ity of the porous mold 2, a centering device 4 is placed in the

slip pouring unit 3, in which the centering rod 5 is placed co-

axially with a central hole, with scope for axial displacement.

The centering rod is introduced into the hole at the nose of

Refractories and Industrial Ceramics Vol. 49, No. 6, 2008

441

1083-4877�08�4906-0441 © 2008 Springer Science+Business Media, Inc.

1ONPP Technology Group, Obninsk, Kaluga Region, Russia.

Fig. 1. Molding assembly for making

ceramic blanks by slip casting: 1 ) core;

2 ) porous mold; 3 ) blank.

Fig. 2. Molding assembly for producing

ceramic blanks from aqueous slip [1]:

1 ) core; 2 ) porous mold; 3 ) supporting

flange for core; 4 ) elastic insert; 5) sphe-

rical hinge.

the core, which produces coincidence between the axes of

the core and the mold. Tension bolts fix the position of the

core in the matrix. The slip is supplied from the bottom

through the filling hole, which fills the body of the centering

device and the internal cavity formed by the core and the

mold. Before the solidification starts, the centering rod is

withdrawn from the internal cavity to provide a component

with a closed tip.

This design at first sight is attractive for its simplicity,

but attempts to implement it with large items have been un-

successful for the following reasons. The operation of insert-

ing the centering rod in the tip of the core is a fairly laborious

one, particularly for blanks of height more than 1000 mm.

Even if that operation is performed successfully, the tension

bolts used to fix the core position (whose mass may exceed

100 kg) results in deformation of the centering rod, which is

usually a metal rod of diameter not more than 5 mm. Also,

during testing of equipment with such a centering device

placed directly in the slip injection unit, a further adverse

factor was encountered. When the slip is injected, it enters

into the body of the centering device, and thus produces the

deficiency of impregnation for the tip part of the blank,

which in turn leads to cracks (sometimes even through ones)

in the tip.

Another way of making components with equal wall

thickness in any section (deviation of the wall thickness re-

duced to 0.2 mm) is by providing exact coaxial setting of the

matrix and the core, which in the opinion of the author of the

patent [3] involves using a single removable supporting

flange to attach the model and the core (Fig. 4). The basic

idea of the centering method in that patent [3] is as follows.

One first generates the inner surface of the porous mold

(Fig. 4a ), for which one uses an assembly consisting of the

model 1 and supporting flange 3, which are rigidly attached

by guide bolts to the metal body of the mold 3. Then the re-

sulting space is filled with gypsum. Then on forming the

blanks, the model 1 is replaced by the core 4; one uses the

same supporting flange 3 (Fig. 4b ). In the opinion of the pat-

ent’s author [3], a single supporting flange rigidly attached

by guide bolts at the same points (as in the model) favors a

coaxial position for the core relative to the inner surface of

the porous mold.

This system of positioning the core relative to the mold-

ing surface with a single removable flange has been used in a

series of molding assemblies for various sizes and profiles.

Experience over many years with these assemblies has

shown that the design does not justify itself. This is con-

firmed by Fig. 5, which shows the differences in thickness in

the nose parts of blanks of two different types. The first type

is of height 650 mm and diameter at the base 250 mm, while

the second type is correspondingly 1200 and 400 mm. Sets

of 100 components of each type were produced. Figure 5

shows that only 15 – 18% of all these blanks had thickness

differences less than 0.5 mm, while the main 70 – 75% had

thickness differences in the range 0.5 – 1.5 mm. That spread

is characteristic of all types of item no matter what their size

and profile.

There appear to be two basic reasons for the high thick-

ness differences in these molded blanks. Firstly, most experts

on molding equipment suggest that the main reason is the

cleaning of the mold surfaces before each use, which leads to

gradual reduction (not always uniform) in the thickness of

the layer of gypsum on the inner surface, and consequently

increase in the thickness of the blank and the thickness dif-

442 E. I. Suzdal’tsev et al.

Fig. 4. Essential scheme for making the working surface of a po-

rous mold (a) and forming a blank (b): 1 ) model; 2 ) mold frame-

work; 3 ) supporting flange; 4 ) core.

Fig. 3. Scheme for centering device

proposed in a patent [2]: 1 ) core; 2 ) po-

rous mold; 3 ) slip injection unit; 4 ) cen-

tering device; 5 ) adjustable rod.

Fig. 5. Thickness differences of molded blanks of the first type ( )

and second type ( ).

ferences. However, that suggestion is discounted by Fig. 6,

which shows the changes in thickness difference in relation

to the number of uses of a given mold (this applies to the

complete cycle of operations with a mold before relining

with gypsum, namely 11 cycles). Figure 6 implies that there

is no regularity in the possible increase in thickness differ-

ences with the number of cycles; the process is purely cha-

otic and random.

A second cause, which in our opinion is the sole one, is

the fairly large tolerances in the units for adjusting the flange

bearing the core and adaptation to the porous mold. As a

rule, the clamping is provided by three or four guide bolts,

and even a slight difference in the tensioning of one of the

bolts leads to a substantial deviation of the core tip from the

central axis. Then one can regulate the bolt tension to adjust

the position of the core tip relative to the inner surface.

Effective regulation of the tip position is impossible

without good instrumental measurements. Up to the present,

this has been done only with an internal gauge, which is in-

serted through the injection hole into the cavity formed by

the core and mold. One measures the gap between the tip of

the core and the inner surface in two or three mutually per-

pendicular sections at a distance of 10 – 15 mm from the end

of the tip, with adjustment by means of the bolts, which al-

lows one to reduce the thickness differences to 1 mm. It is

difficult to obtain more accurate centering by that method be-

cause of the impossibility of making measurements at the

same points. Also, this instrument can be used only on

blanks whose diameter for the injection hole is not less than

20 mm.

These shortcomings can be avoided by a centering

method proposed in a patent [4]; it is based on the tip of the

assembly containing a centering device (Fig. 7) made of four

micrometer elements fitted with adjustable rods, which are

located in mutually perpendicular directions. Before the

working surface of the gypsum mold is produced, the center-

ing device is attached to the framework of a mold and the

readings of the micrometers are set to zero; after injection

and solidification of the gypsum the model is extracted and

the remaining gypsum mold contains the installed microme-

ter devices (Fig. 7a ). Then the core is inserted. The installed

micrometers in the centering devices are used in measuring

the distances to the tip of the core at four points in one plane,

with regulation of the core position relative to the gypsum

mold (Fig. 7b ). Before the slip is injected, the moving rod of

the micrometer device is taken to the zero position (Fig. 7c ).

Experiments have shown that this method enables one to

center the core quite accurately relative to the working sur-

face of the mold. One should note that this operation is very

laborious.

All the above centering methods are mechanical, and

they have common deficiencies such as insufficient accuracy

and considerable effort involved.

Rapid advances recently have led to large numbers of

methods of centering (accuracy attaining 0.001 mm) on the

basis of lasers and various sensors. A literature search has

shown that in the main all existing systems are directed to re-

solving well-known and persistent tasks such as centering

the pulleys of belt transmissions, maintaining coaxiality in

horizontal and vertical pump shafts, reduction gears, and

shaft drives, and so on. Unfortunately, we found no ready-

made systems for centering a core relative to the internal sur-

face of a porous mold. It was thus necessary to develop cen-

tering in nonstandard ways, e.g., by the use of inclinometers,

Centering Molding Assemblies 443

Fig. 6. Dependence of thickness differences in shaped blanks on

number of operations employed with gypsum mold.

Fig. 7. Centering scheme [4]: 1 ) model; 2 ) mold framework;

3 ) micrometer component with mobile rod; 5 ) core; 6 ) slip.

Fig. 8. Centering scheme using inclinometers.

laser centering, and video centering. We consider each of

these methods separately.

Inclinometer centering. An inclinometer measures the

angular deviation of the axis of a rigidly coupled object from

the inherent geophysical axis, which is always directed to the

same point in the Earth, which enables one to relate the di-

rection of two parts of the molding assembly to a vertical di-

rection of the gravitational force.

Figure 8 shows the sensor positioning principle. The

flange attached to the core bears two inclinometers for the X

and Y axes (Cx and Cy sensors)2, and simultaneously the

flange for the mold also has two sensors (the Ôx and Ôy sen-

sors). These sensors are linked in two systems Cx – Ôx and

Cy – Ôy, which measure the deviations of the core axis from

the axis of the mold in the XY plane. The deviation can be ob-

served visually on the monitor. The core position is adjusted

with bolts to bring about coincidence between the centers of

the mold and the core.

This method is the most accurate for the purpose, but

various difficulties arise in applying it. Firstly, the setting

planes for the sensors must be strictly perpendicular to the

axes of the core and mold; the geometrical accuracy in the

manufacture of the molding assembly (tolerance) should be

not less than 0.005 mm. Secondly, on centering in this way,

one must be sure that there are no vibrations and that the

temperature in the building is constant, which is not always

possible. Finally, a substantial difficulty is that the inclinom-

eters have to be nonstandard design to carry out the produc-

tion of the entire range of components. In particular,

the error in determining the degree measure should

not be more than �0.1 angular seconds with an angle

measurement range of �5�, which substantially raises

the cost of the equipment.

Laser centering. An alternative to the above

method of bringing about coincidence between the

axes of the mold and the core is a method of bringing

the center of the filling hole in the bottom flange into

coincidence with the center of the core tip. At first

sight, the simplest way of doing this is to use a simple

system consisting of a laser located at the tip of the

core and a center seeker in the filling hole. However,

that method requires a laser for each core, which is

economically undesirable.

Another way of using lasers is to employ a laser

rangefinder placed exactly at the center of the filling

hole, which enables one to determine very accurately

the distance L to the tip of the core. The centering is

performed by manipulating the position of the core to

obtain L = min. The least value of L indicates that the

tip is exactly located directly above the center of the

filling hole.

It is also possible to use a two-coordinate laser

profile meter (two-dimensional laser scanner) to bring

the center of the filling hole into coincidence with the

center of the core tip. Laser scanners are meant for

contactless measurement and monitoring of surfaces, dis-

placements, dimensions, and so on and for recognizing tech-

nological objects. The method allows one to obtain the pro-

file of the core in relation to the drainage hole in a plane of

section.

The general shortcomings of laser centering include the

need for exact setting and adjustment of the measuring in-

strument in each gypsum mold in relation to the hole in each

assembly of the molding set. This shows that lasers are

largely unsuitable for industrial production on account of the

considerable effort involved in using them and the high de-

grees of skill needed in staff.

Video centering. A digital video camera is used to bring

about coincidence between the calibrated filling hole and the

center of the tip of the core in three-dimensional space. The

essence is as follows: the camera is installed below the as-

sembly opposite the filling hole (lens directed to the hole,

Fig. 9a ); one determines the center of the ellipse; one then

incorporates the correction for the displacement of the center

(from data obtained on previous moldings); an image is

formed of the target; and one automatically determines the

pattern represented by the mold and core and by the point

representing the center; the operator performs the final ad-

justment of the assembly on the basis of the graphical repre-

sentation of the target and the center of the core brought into

coincidence. Clearer representation of the core tip is pro-

vided either by making it reflective (for example, polished),

or by fitting it with a light source (light-emitting diode).

444 E. I. Suzdal’tsev et al.

Fig. 9. Scheme for centering the core relative to the working surface of the

mold by the use of a video camera (a) and example of practical realization (b).

2Instead of two sensors one can use a single biaxial inclinometer.

That method is the best because one can use almost any

digital video camera and the error in the measurement is de-

pendent only on the lens and on the image processing algo-

rithm. Experiments have for example been performed with a

digital camera having a Web interface (1152 � 864 pixels,

Fig. 9b ), with the error in centering the core represented by

0.26 mm on average simply by the use of standard software

for visual monitoring.

This analysis has shown that the most promising center-

ing method for the core relative to the working surface of a

porous mold is video centering. Subsequent research will be

directed to developing this method for introduction into in-

dustry.

REFERENCES

1. USSR Inventor’s Certificate No. 1664560, A Device for Forming

Ceramic Components From Water Slip (V. P. Sumtsov and

Yu. P. Polyanskii) [in Russian], published 23.07.91, Bull. No. 27.

2. Patent 2123928 Russian Federation, A Device for Forming Ce-

ramic Components From Water Slip (V. V. Platonov and

M. Yu. Rusin) [in Russian], published 27.12.98.

3. Patent 2137599 Russian Federation, A Device for Forming Ce-

ramic Components From Water Slip (V. V. Platonov) [in Rus-

sian], published 20.09.99.

4. Patent 2242359 Russian Federation, A Device for Molding

Equal-Thickness Large Ceramic Components From Water Slip

(E. I. Suzdal’tsev, D. V. Kharitonov, et al.) [in Russian], pub-

lished 20.12.04.

Centering Molding Assemblies 445