CH632225A5 - Process for making a fired moulding from particulate material - Google Patents

Description

The present invention relates to a method for producing a fired shaped body made of particulate material, wherein binder is removed from the shaped body in such a way that the shaped body does not swell and thus no shear or tensile forces act on the shaped body before firing.

The production of moldings from particulate material is known to the person skilled in the art. The respective particulate material is mixed with a binder and the mixture is then shaped into the desired shaped body, which in this state is referred to as an “unfired shaped body”. The unfired molded body is then fired for the purpose of fusing the particulate material and expelling the binder, the desired molded body having the appropriate surface texturing and strength being obtained.

In the production of moldings described above, it was found that the binder is a necessary evil since it is required to form a mold which can be used in practice, but must be removed before the mold can be sintered.

This problem has been known for a long time and efforts have already been made to remove the binder from the unfired molded article before firing, as described, for example, in US Pat. No. 2,939,199 as well as in GB Pat. No. 808,583 is. While advantages can be achieved by the procedure described there over known earlier processes in which the binder was not removed before firing, moldings produced according to the teachings of the cited patent documents still show the tendency to crack during the removal of the binder, as well as during of burning. This is based on the fact that the binder is removed from the unfired molded body by means of a solvent while the binder is in solid form. It is known that when the binder and solvent are mixed, the mixture obtained tends to expand. If the binder is now in solid form, it cannot move freely within the unfired shaped body, and the expansion within this shaped body can exceed the Van der Waals forces that hold the particulate material together. This creates cracks, which are then supported during the final firing, even if the formed body still retains its shape to some extent.

These disadvantages are practically eliminated by the method according to the invention as defined in claim 1. In short, the unfired shaped body is heated to a temperature above the melting point of the binder and the solvent for the binder is then introduced very slowly into the unfired shaped body in the vapor phase. The combination of binder and solvent expands as in the known processes. However, since the binder has previously been heated to at least its melting point, the pressure within the unfired shaped body due to the expansion of the combination of binder and solvent is reduced by the movement of this combination through the cavities within the unfired shaped body. In this way, a substantial proportion of the binder can be removed from the molded body without breaking or cracking. The effective forces, for example the Van der Waals forces, which hold the particulate material together in the unfired molded body, must be regulated by regulating the formation of the combination of binder and solvent by slowly adding the solvent to the same or greater strength than the swelling

2nd

s

10th

15

20th

25th

30th

35

40

45

50

55

60

65

forces are kept. Since not all of the binder is removed in the vapor phase, the unfired molded body is subsequently introduced in its entirety into a bath of the liquid solvent which has previously been heated to a temperature above the melting point of the binder, with the majority of the remaining binder being removed from the molded body . Here, in turn, no breakage or cracking occurs, since the space created by the gaps in the unfired molded article enables the binder to be virtually completely removed by diffusion from the molded article. Any residues of binder and solvent evaporation can now be removed from the unfired molded article by a preheating treatment. The unfired molded article which is now practically completely free of any binder can then be fired or sintered in a known manner in order to achieve the finally finished molded article.

The aims of the invention are therefore:

The creation of a process for the production of moldings from particulate material, for example ceramic material, metal or cermet;

the creation of a method for removing binder from an unfired molded body by heating the binder to at least its yield point before the solvent extraction;

the creation of a method for the extraction of binder from an unfired molded article, the binder being heated to at least its flow limit and the solvent being used in the vapor phase;

the creation of a method for removing binder from an unfired molded article, wherein the binder is heated to at least its melting point and the solvent is then used in the vapor phase at a sufficiently low speed to dissolve the binder and exit from the unfired molded article through its cavities to an extent to make it possible to avoid breaking and cracking of the shaped body caused by swelling of the combination of binder and solvent and that the cohesive forces between the particles in the unfired shaped body are not exceeded by these swelling forces.

The above objects are achieved by the invention, and preferred embodiments, which are immediately understandable for a person skilled in the art, are explained in the following with reference to the drawings, for example. The drawings show:

1 shows a diagram of the treatment time in liquid solvent as a function of the wall thickness of the unfired molded article;

Fig. 2 is a schematic representation of a device suitable for solvent extraction, partially in section.

In the introduction, it is pointed out that the following explanations of preferred embodiments of the invention are made primarily with reference to particulate ceramic material, but the invention can also be carried out with particulate material made of plastics, metals, alloys, cermets and any other materials, as long as these materials held together by means of a binder and, after removal of the binder, can be melted under the action of heat. In the manufacture of precision parts, in particular made of metal or ceramic, it was previously necessary to achieve these parts

632225

to subject the finished product to the required close tolerances for mechanical precision finishing. According to the known methods, such precision parts could usually not be produced directly with economical yield or reliability using known injection molding methods, in particular if their cross section exceeded several millimeters. This is because

that very strong shrinkage as well as a high rejection rate due to breakage occurred if an extremely long treatment time was not used for the solvent extraction, but this made industrial utilization impossible. The present invention now enables the direct production of moldings from such particulate materials with the formation of end products with the necessary precision without the need for mechanical post-processing or other expensive process steps. As a result, the manufacturing costs of such precision parts are significantly reduced.

The individual process steps for the production of fired moldings according to the invention include:

1) Select material;

2) mixing;

3) deformation;

4) removal of the binder;

5) heat treatment at a relatively low temperature;

6) Burning or sintering.

The primary starting materials are primarily the particulate material from which the end product is to be made and a binder. If desired, the starting material may contain other additives such as antioxidants, deflocculants or the like. It is desirable that the end product be made of a particulate material of spherical shape, and that the diameter of the particles be as small as possible and within a relatively small particle size range. This enables a maximum packing density, the gaps between adjacent larger particles being filled with smaller particles made of the same material. The particle size is preferably at most 4 µm with an average particle size of 1 (in or below. The melting point of the binder used is expediently below the sintering temperature of the particulate material forming the final shaped body. For the reasons explained below, a mixture of several binders, of which The minimum amount of binder required should suitably be sufficient to cover the entire surface of all particles of the particulate material present, and the maximum amount of binder is the amount which covers all the spaces between the particles of the particulate material filled in and remains inert to these particles during the process steps used, preferably the mixture used as the starting material contains 65-70% by weight of particulate s material, remainder binder, with higher proportions of particulate material being preferred as long as the entire surface of all particles of the particulate material is covered with binder. Mixtures of only 40% by weight of particulate material have already been obtained,

3rd

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

632225

as well as those with 90% by weight of particulate material, successfully used.

The mixing of particulate material and binder into a practically homogeneous mixture and for coating the entire surface of each of the particles with the binder can be carried out in a known manner. This also breaks up agglomerates of particulate material, so that practically the entire surface of each particle is covered with the binder. Agglomerates usually arise from particles of small particle size w due to their tendency to stick together, due to the relatively higher surface forces of small compared to larger particles.

The mixture of particulate material and binder can then be deformed in a mold to form an unfired molded body. The most common is injection molding, although extrusion, vacuum or blow molding or any other known type of deformation can be used. The deformation taking place in the method according to the invention can take place in a known manner 20 and is not a feature essential to the invention.

The exudation treatment is intended to remove a large proportion of the binder from the unfired molded article obtained from the deformation, the aim being to remove a maximum proportion of the binder by this treatment, but to leave sufficient binder in the molded article to disintegrate or crack therein to prevent. In the case of a one-component binder, the unfired molded body can be inserted into a container and heated in this to at least the melting point 30 of the binder, so that the binder is in a flowable state. A solvent for the binder in gaseous state is then introduced into the container, the boiling point of the solvent preferably being above the melting point of the binder 35. The solvent is introduced into the container at a sufficiently slow rate that the swelling produced by the combination of solvent and binder is not sufficient to exceed the forces 40 which act to hold the particles of the unfired shaped body together. This means that the sweating out of the combination of solvent and binder from the unfired shaped body must take place at practically the same or higher speed than the formation of a new, swollen combination of solvent and binder 4s within the unfired shaped body. Solvent in the gas phase is introduced into the initially practically evacuated treatment chamber until some solvent appears in the liquid phase. The time required for this process step depends on various factors, including the size and shape of the unfired molded article and the solvent and binder used in each case. A preferred period of addition of solvent in the gas phase corresponds to that which was determined by initially evacuating the ss treatment chamber and introducing the solvent over a period of 1 h in an approximately linear manner, so that the pressure in the treatment chamber after the introduction had ended for 1 h is about 1 bar. After completion of the treatment of the removal of binder by gaseous solvent, a substantial proportion of the binder has been removed, as a result of which points of increased permeability have been formed in the cavities between the particles of the unfired molded article. The unfired molded body is then introduced into a liquid bath of the solvent at a temperature above the melting point of the binder, virtually all of the residual binder being removed. It should be noted that something

Binder and solvent may and may remain, and what will be done as described below. The time required to remove binder in the bath from liquid solvent depends on the wall thickness and shape of the unfired molded article. The treatment time as a function of the wall thickness is shown in Fig. 1 of the drawing.

Although the combination of solvent and binder within the cavities in the unfired molded article expands in the same way as explained above in relation to known processes, neither crack formation nor breakage occurs in the unfired molded article, since the amount of binder remaining is sufficiently low that the Passages in the cavities are sufficient to cope with the expansion and to allow the remaining binder, with or without solvent, to be exudated into the liquid solvent.

The unfired shaped body is then removed from the liquid solvent and, although it is practically free of binder, can still contain some binder which may be left behind or combined with the solvent and which is not completely leached out of the unfired shaped body. This residual binder and any solvent present is now removed by firing at a low temperature. Such low temperature firing treatments are well known to those skilled in the art and include treating the molded article in an oven in which the temperature is raised slightly above the boiling point of the solvent and maintained for a longer period of time, preferably at least 1 hour, to remove all solvent residues from the unfired molded article and then raised to a temperature above 100 ° C. and maintained for 1-2 hours in order to remove any water that may have got into the unfired molded article. Thereafter, the temperature is raised to the temperature required to remove the binder by oxidation, reduction or evaporation and then slowly in the range of 320-430 ° C. within approximately 3 days. The duration and temperature of the heat treatment vary depending on the particular particulate material and binder used. The duration of the heat treatment of 3 days also varies depending on the wall thickness of the unfired molded body in practically the same ratio as shown in FIG. 1 with regard to the removal of the binder. If desired, the low temperature firing treatment can be carried out for sensitive parts at a lower temperature, with the final burnout then taking place during the sintering treatment.

The treatment temperature is always below the sintering temperature of the particles of the unfired molded article.

The sintering treatment is carried out according to the standard method in a known manner. In the case of metallic moldings, the sintering is usually carried out in a reducing or neutral atmosphere and in the case of ceramic oxide materials in an oxidizing atmosphere. After appropriate sintering in the sintering furnace in a known manner, the end product is removed from the furnace.

Moldings produced as described above showed shrinkage to a calculable extent and were relatively free from cracks and the like. It has been found that precision products made of metals, ceramics and cer-mets can be produced reproducibly and with high yield.

It is also possible for the binder, which is expensive in relation to many of the particulate materials used in most cases, to be in the form of the remaining combination

nation of solvent and binder is recirculated and these can be separated into binder and solvent for reuse.

While the above explanations were made with respect to a one-component binder, it has proven expedient to use a two-component binder under certain conditions for the following reasons. As mentioned above, the particulate material is mixed with a binder to improve the shaping of the unfired molded article, the binder providing adhesion of the particles, flowability, ductility, lubrication and the like. If, as described above, this binder is then removed by leaching, the unfired molded article obtained still shows its original physical state and has generally retained its size and shape. However, the leached green body has lost the weight of the leached binder and is more fragile because at least some of the bonding properties created by the binder have been removed.

In order to improve the handling properties of the leached, unfired molded article, the binder originally used can contain a second binder component which is not leached out and thus gives the leached, unfired molded article improved handling properties.

It has been found that in many cases the leachable portion of the binder can be cottonseed or soybean oil, in which case "Freon" TF is used as the leaching agent and the non-leachable component of the binder can be polyethylene. The use of these materials has the advantage that the leachable component of the binder is already in a liquid state at room temperature. The leaching agent for the oil is initially used in the gaseous state and then, like in the case of a one-component binder, in the liquid state, so that no or at most minimal heating is necessary for the treatment for removing the binder. If such a binding and leaching agent is used, the leaching treatment can thus take place even at room temperature. After completion of the leaching treatment explained above, the leached, unfired molded article still contains the second, non-leached binder component and is now easier to handle, i. H. it can be stored, stacked and subjected to further deformation treatments. The second binder component is usually an organic type that is heat decomposable so that it can be removed during firing. The weight ratio of the two binder components is preferably such that only sufficient non-leachable binder remains to prevent deformation of the unfired molded article.

For other uses, it may be appropriate to use a three component binder, two components of which can be leached out by different solvents. In this case, leaching the first binder component using a first predetermined combination of temperature and solvent and leaching the second binder component using a second predetermined combination of temperature and solvent, as discussed above, the third binder component could be removed in a subsequent firing treatment . A binder composed of several different components can thus be used.

In such a three-component binder, the first component can advantageously be polyethylene or polypropylene.

632225

panglycol or polyvinyl alcohol, all with water as solvent, the second component advantageously polystyrene, with methylene chloride as solvent, or di-octyl phthalate, with «Freon» TF as solvent, and the third component one of the agents usually used for reinforcement, such as polyethylene, that decomposes when it burns.

Another useful three component binder can include an oil, a wax, and polystyrene.

It is easy to see that, taking into account the principles set out above, a binder can be built up from any desired component.

In some cases, it may be appropriate to use an additive to fill up part or all of the space originally occupied by a binder component, for example to achieve certain electrical properties or as an adhesion promoter for a metallic deposit as a primer for later metallization. This can easily be achieved by using the appropriate filler material for the space created during the leaching of the unfired shaped body in a known manner, for example by vacuum deposition or the like.

example 1

A particulate mixture of 90% by weight of aluminum oxide produced by the Bayer process and 5% by weight of talc and kaolin as flux was ground in a ball mill to achieve the desired particle size and particle size distribution of the aluminum oxide. The alumina had an average particle size of 0.6 µm and a particle size distribution in the range of 3-0.2 µm, although there were undoubtedly some particles above and below this range. The mixture was filled into a sack made of a soluble film and pressed isostatically in a hydraulic press in a sack under a uniform pressure of 552 N / cm 2 to a rod, which was then vacuum-impregnated with Carnauba wax to form an unfired molded article.

In an industrial steam degreaser according to FIG. 2, the unfired molded body 1 obtained was placed on a suction box 3 on supports 5 and 7 above the bottom of the container 9. A heating band 11 extends around the container 9 under insulation 13, and cooling coils 15 are arranged in the upper part of the container 9 in order to condense volatile vapors back into the container. In the vicinity of the unfired molded body 1, a thermometer 17 for reading the temperature inside the container 9 is attached, and a dropping funnel 19 with a tap 21 is used to introduce solvent. The dropping funnel 19 contained tri-chloroethylene. The container is closed airtight to maintain a vacuum in the container with a lid 23 and provided with a manometer 25 and a connection valve 27 for evacuating the container in a known manner.

The temperature in the container 9 was heated to 100 ° C. and then to the melting temperature of the carnauba wax of 88 ° C., so that the wax was liquefied. The container was then evacuated to 20 mbar and then trichlorethylene was dropped and evaporated into the container over a period of 1 hour so that there was no liquid on the bottom of the container. After approximately 15 minutes, a liquid began to sweat out of the molded body 1 and to accumulate on the suction box 3. After 1 h, the pressure in the container was 1 bar, and the solvent dripped in from the dropping funnel 19 began to collect and rose to above the upper surface of the shaped body 1, the temperature of the solvent being raised to 88 ° C. over 2 h

5

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

632 225 6

was held. Then, the unfired molded body 1 was fired and sintered, as in the example, removed from the container 9 and dried. 1, being a finished product with excellent

The molded body was found to be crack-free and practically the results were obtained.

All of the carnauba wax binder was from the mold. Examples 1 and 2 were repeated with the exceptions.

body drained. Then the shaped body was removed in order to remove the soft iron removal of the solvent as the particulate material for 1 h in an oven and the sintering in a reducing atmosphere-88 ° C and then to remove any water on a sphere Temperature of 1180 ° C took place. The received

Heated to 100 ° C. Then the temperature in the oven was raised within 1 hour. Results were the same as in Examples 1 and 2. Slowly increased to 120 ° C to remove any residue. The binders can be more expensive than the particulate

Burn solvent. The leached molded material, and in the known processes, were then fired in a furnace in an air atmosphere while these relatively expensive binders were being burned at 1540 ° C. for 1 hour due to their decomposition and then cooled, the loss being lost during the firing. In the described procedure

Heating and cooling cycle lasted a total of 24 hours, the relatively expensive binders from the unfired, of which 18 hours were leached and required to achieve the sintering temperature of 1540 ° C. molded articles by means of a leaching agent. The end product obtained showed that it remained in this leaching agent, from which it had an economically viable density and other properties, and no crack formation. can.

Example 2 Although the invention has been

A particulate mixture as in Example 1 was discussed in specific preferred embodiments - using a 50:50 binder mixture of Car-20 tert, it will be apparent to those skilled in the art that verauba wax and low density polyethylene are made to various variations and modifications to the use of an unfired molded article processed, leached, can reach.

B

1 sheet of drawings

Claims (8)

632 225 PATENT CLAIMS 1) Particulate material and binder are mixed in predetermined proportions at a temperature above the melting point of the binder, so that the binder can harden on cooling and can then be melted by heating, 1. A method for producing a fired shaped body made of particulate material, wherein binder is removed from the shaped body before firing in such a way that the shaped body does not swell and thus no shear or tensile forces act on the shaped body before firing, characterized in that one 2. The method according to claim 1, characterized in that a ceramic material, a metal or a cermet is used as the particulate material. 2) the mixture obtained is deformed under heating and pressure and then the binder is hardened by cooling, 3. The method according to claim 1 or 2, characterized in that one uses particulate material with an average particle size in the range of at most 4 [im, preferably of about 1 [im. 3) the shaped body obtained is heated to a temperature above the melting point of the binder in order to liquefy the binder in the absence of a solvent, 4. The method according to any one of the preceding claims, characterized in that one uses particulate material of practically spherical shape. 4) the heated molded body is introduced into the vapor of a solvent for the binder at a temperature above the melting point of the binder and is left in this vapor for a sufficient period of time to dissolve the binder and the combination of solvent and binder obtained without breaking the molded body to make it sweat out of the molded body, 5. The method according to any one of the preceding claims, characterized in that the weight ratio of particulate material to binder is 7: 3 or more. 5) introducing the obtained molded body into a bath of a liquid solvent for the binder at a temperature above the melting point of the binder, so that the binder is largely removed from the molded body, and 6. The method according to any one of the preceding claims, characterized in that the particulate material and the binder are mixed in such a way that the binder covers practically the entire surface of each particle of the particulate material. 6) the molded article obtained is removed from the solvent bath and then sintered. 7. The method according to any one of the preceding claims, characterized in that the binder consists of at least two components, both of which are not soluble in the same solvent. 8. Molded articles produced by the process according to claim 1.