CA1149566A

CA1149566A - Method of producing resin articles by use of plaster mold and the products thereof

Info

Publication number: CA1149566A
Application number: CA000343102A
Authority: CA
Inventors: Yamato Kogure
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-06-25
Filing date: 1980-01-04
Publication date: 1983-07-12
Also published as: DE3001366A1; JPS5910297B2; IT1128089B; IT8047752A0; GB2051654B; DE8000989U1; DE3001366C2; NL7909117A; FR2459719B1; JPS5549237A; FR2459719A1; GB2051654A

Abstract

METHOD OF PRODUCING RESIN ARTICLES BY USE

OF PLASTER MOLD AND THE PRODUCTS THEREOF

Abstract of the Disclosure Method of producing resin articles by use of a plaster mold comprising the steps of sensing the resin injection pressure in the plaster mold by a pressure sensing means and controlling the injection conditions for a molding machine in accordance with the desired optimum pattern of correlation between the diameters of a sprue runner and an air vent,, which pattern is stored in the memory of a computer in advance. With this construction, the molded products with sufficient strength in the portion thereof as well as stable molded properties and improved dementional accuracy can be obtained according to the present invention.

Description

~ackground of the Invention:
1. Field of the Invention This invention relates to a method of producing, by use of a plaster mold, resin articles such as dental prosthetic goods or industrial trial articles and the product thereof.

2. Prior Art Heretofore, the step of charging of resin in the manufacture process of bedded artificial teeth in the dental prosthetic goods was typically performed by manual operation.
Elowever, in such manual operation,the resulting product has unstable dimensional stability due to ununiform pressure polymerization, thus making the manufacture of an article of accurate size impossible. In place of such manual operation, there has been devised a method of resin molding in the field of plaster molds wherein the molding machine can be applied to the plaster mold. However, the molding machine for the plaster mold and that for the metal mold are different basi-cally in injection ~conditions from each other. Moreover, the following problems are met with this kind of molding machine. In the molding machines in general, the injec-tion molding is a process wherein plastic material is fed by a screw into a heated cylinder and plasticized by external heating and the inner friction heat caused by the rotation of screw. The resulting melted material is introduced and charged by mechanical energy into the metallic mold, pressur-ized, and cooled to a molded article with desired quality.
Thus, the plastic material is changed,by external mechanical energy, from solid to liquid state, charged into the mold by e.~ter3lal kinetic energy and again changed into solid through absorption of the energy by the mold. Thus, the process and 35~

the quality of the completed article are greatly influenced by e~ternal molding conditions. In effect, molded products may have defects due to incorrect molding conditions because of short shots, flashing, flow marks, jetting, weld lines, gas burning, air bubbles, clouding, cold marks, strain, warping, cavities, noncompliance to standards, intershot standard errors and interrod standard errors. On the other hand, there may be such molding conditions as cylinder heating temperatures for plasticizing the material, screw RPM, the amount of the material to be stored in the injection cylinder for injection into the mold,metering strokes,screw back pressure, injection pressure for charging and pressurizing into the mold, injection speed, injection time, cooling temperature for ab~
sorption of thermal energy, and cooling time. These condi-tions are critical and indispensable for producing molded articles. For instance, if the cylinder temperature is too hi~h, the properties of the plastic material are lost through deterioration of the material. If the metering stroke is excessive,the resin will stay in the cylinder too long result-ing in deterioration of the material. If the injection energyis too larc~e, the resin may ooze out from the parting line o~ the mold thus causing strain. If the injection energy is too low, the lustre proper to the resin is lost, sometimes causing cold marks. r~Oreover, if the cooling time is short, the molded product may be unstable in shape and warped. Thus, skilled operators were required, that is, the molded products were checked and adjusted by visual inspection of the operators.
The molding conditions are interrelated to one another and the molded product can not be adjusted unless it is known how the conditions of the plastic material are changed under given conditions throughout the entire process, and what is happen-ing in the mold.

5~:;6 . As the molding conditions are not constant, but are changing at all times to a more or less degree, the product quality per each shot can not be maintained easily. In effect, the plastic material may be changed in its conditions through changes in hydraulic pressure, oil temperature, mold temperature and heating temperature. The molding machine must remain stable under the effect of these changes. Fluctuations or discrepancies in the product quality in the absence of these changes are a-ttributable, among plasticizing factors, to air discharge from the resin, material purity, mixing of regener-ated articles, mixing of compound material and viscosity of regenerated articles. These factors have been confir.ned experimentally to account for a majority of fluctua-tions occurring in the products and these factors are extremely difficult to be placed under effective quality control.
The optimum injection molding should be so conducted that an optimum amount of evenly plasticized molten resin is charged into th~ mold at an injection energy suited to the mold, the injection energy is maintained accurately until sealing of the sprue and/or a gate and the product is cooled for a proper cooling time interval at a proper cooling tempe-rature. The problem is then how to control these factors.
The status functions for the plastic ma-terial comprize three parameters such as resin pressure, resin capacity and resin temperature. It has been proposed to provide a sensor in the mold for sensing the resin tempera-ture and feeding the sensed temperature back to heating means mounted at the entrance to the gate. However, such control is practically a matter of great difficulty in view of a response in high injection speed of the resin which may be of the order of less than one second.
The resin capacity can be made constant by maintaining the mold clamping state constant. An increase in the charged quantity until the cavity is filled with resin can be sensed indirectly through pressure increase in the resin. Thus, -the basis for control is resin pressure. In other words, as shown in Fig. 8, showing the relation between the mold inside pressure and time, such defects as flow marks, jetting, weld lines, gas burning, bubbles, cloud, cold marks, strain and warping occur during charging (time point A in the drawing); such defects as flashing, or strain occur immediately before completion of charging (time point B); such defects as short shot, flashing, warping, cavity nonconformance to standards and intershot standard errors occur during pressure holding (time point C) and such defect as warping or shot occurs,during cooling (time point D). Thus, a majority of defects are caused during charging. Accordingly, the basic condition for molding of resin is the resin pressure. Hence, it has been desired for long to provide a method for molding of resin under optimum conditions and by `use of a plaster mold, wherein the correla-tion among the injection speed, inside pressure of the mold ~0 and the holding pressure on one hand and the sprue,diameters and lengths with air vent diameters and lengths on the other hand, are clarified and the corresponding technical measures are used for producing resin articles having desired physical properties and dimensional stability.
Summary of the Invention:
As the resin is pressurized and charged abruptly or at high speed from an injection opening into a mold, the air in the mold is compressed and the internal pressure of the mold becomes momentarily high, thus a resistance within the mold ~. .
~' ~ ,.

becomes high. This momentary abrupt increase in the pressur~
in the mold lowers the speed of the resin about to be charged into the mold at a high speed of less than one second, thus inducing the cooling of the resin. However, in the injection mold as the time in which the resin is cooled is inversely proportional to the distance traversed by the resin and the thickness of the resin product, the charging operation must be completed before the resin is cooled and hardened. ~ppli-cation of excessive pressure to ~he mold in disregard of the above leads to an increased pressure remaining in the mold and lowers physical properties and stability of the resin.
Moreover, molding may become impossible i~ the product has both thick and thin portions. It is therefore a primary object of the invention to provide a method and an apparatus in which the optimum correlation between the injection speed and pressure and the aL~r vent diameter can be obtained.
According to the invention there is provided a method of injecting molding of resin by,use of a plaster mold comprising the steps of providing a sprue runner passage-way having a diameter of 0.5 to 20 mm and an air vent having a corresponding diameter of 0.1 to 5.0 mm in the plaster mold, sensing the injected resin pressure in the plaster mold by pressure sensing means, and controlling the injection speed of the resin to be injected within the range of 0.01 to 10 seconds according to the sensed pressure so as to change the inner pressure in the plaster mold within the range of 30 to 1200 kg/cm2 in a predeter-mined optimum pattern stored in a memory of a computer below the destruction point of the product to be molded.
It is an advantage o~ the invention, at least in preferred forms, that it can provide a method for coping ~.i .

5~
with such a situation that, while abrupt pressure increase in the mold as encountered during injecting molding can be avoided by air venting, the vent is stopped by hardened resin at the next moment and the resin so far charged into the mold starts to be hardened and be subjected to shrink-age. Thus, the correlation between the air vent and sprue and the mold holding pressure must be clarified.
Moreover, if the molded product has considerable thickness,the dimensional stability can not be maintained unless the holding pressure is applied immediately, because the resin once charged into the mold is subjected to cooling and shrinkage. The problem is then how to control this and how to cope with such situation that while decreased pressure due to shrinkage may be compensated by addition of resin, there is no way for charging of resin if the sprue is stopped completely through hardening before the molded article has assumed a completely stable condition. Thus, it is another advantage of the invention, at least in preferred forms, that it can provide a method solving this problem through clarification of the correlation betwe~en the thickness and the sprue diameter and length.
It is another advantage of the invention, at least in preferred forms, that it can provide a me-thod for imparting physical properties and stability in case the molded product has both thick and thin portions. It is of no avail to consider onl~ the shrinkage caused during molding, and charging into the mold must be com-pleted before hardening of the resin. To this end, injection and charging must occur in a very short time by using an eleveated injection speed. However, if the air at the thick portion is ~ 6 --~1 compressed to dwell in the thin portion, the air resistance given by the thin portion will increase, for delaying the resin flow during charging and resulting in cooling and reduces the injection charging effect. Thus, a shorter article may be produ-ced, or the physical properties and accuracy may not be obtained.
On the other hand, increasing the injection speed and pressure recklessly leads to unstable quality and destruction of the plaster mold. In this case, the technical problem to be solved is that the correlation between the air vent and the diameter and length of sprue must be taken into consideration, which is capable of hardening the resin which enables the air vent to seal with respect to demention and accuracy thereof to supplement and to stabilize the resin for contraction or shrinking so as to secure the discharge of air within the mold and properties thereof.
It is another advantage of the present invention, at least in preferred forms, that it can provide a plaster mold cavity such that the shape of the wax pattern to be embedded in the plaster mold may be maintained faithfully.
In preferred embodimentsr wax pattern, sprue, gate and air vent are mounted in a desired manner in a plaster mold, then plaster is injected into the plaster mold, then the air bubbles in the plaster are removed by vacuum defoaming, then the mold is heated for re-moving the wax pattern, and the resin is injected from a molding machine after washing. At this time of injection, the air within the mold is removed by an air vent having a diameter decided by calculation in consideration of the correlation between the sprue, and gate diameters and lengths and the pressure inside the mold while charging ~he resin in the plaster mold. At the same time, the resin pressure is sensed, by a ~ . .

resin pressure sensor provided to the sprue, until the resin is filled to the terminal part of the cavity, cooled, and solidified. In addition, a setting pressure to be included within the maximum allowable range of the pressure in tne mold short of insufficient pressure or overpressure is found out. A controller is used to control the hydraulic system of the molding machine so that this setting pressure may be main-tained until sealing of the mold gate.
Brief Description of the Drawings:

Fig. 1 is a longitudinal section of a mold according to the present inve~tion showing the molding of a molded product having a uniform thickness;

Fig. 2 is a longitudinal section of a mold according to the present invention showing the process of integral molding of a molded product having a thick portion A and a thin portion B;
Figs. 3 to 5 are perspective views of molded products;

- 7a ~

. Fig. 6 is a plan section showing the inside of the flasks provided with cooling and heating means; and Fig. 7 is a longi,udinal section of another mold according to the present invention showing the molding of a dental prosthetic goods having a thin palatal surface:
Fig. 8 is a characteristic curve of the inner pres-sure of a mold and the time elapsed according to the prior art;
Fig. 9 is a characteristic curve of a setting pres-sure within a mold to be controlledi Fig. 10 is an ideal characteristic curve of theinner pressure of a mold to be controlled according to the present inventioni Fig. 11 is another characteristic curve of the inner pressure of a mold according to ano-her embodiment of the present invention.
Detailed Description of the Preferred Embodiment:
Fig. 1 s~ows a structure of a molding machine according to the present invention wherein a resin product (a) to be molded in a mold cavity 1, has approximately a constant thickness.
A wax plaster 6 is housed as known manner within a lower flask 2 and an upper flask 3 and there is provided a sprue runner 4 and an air vent 5 which extend in opposing directions respectively, of the joint surfaces of -the upper and lower flasks 2, 3.
Plaster 6 is injected in-to the hermetically sealed flasks and air bubbles are removed from the plaster by vacuum ;~
defoaming. After the plaster has been hardened, wax is removed t!7.~t-efrom by heating for providing a plaster pattern 7. In the vicinity of an injection opening 8 whicll leads to a sprue S~

4, there is provided a sensor pin 9 of an internal pressure sensing and controlling device (not shown) for sensing the internal pressure. An injection nozzle of an injection molding machine (briefly shown), having a very high speed injection capability (maximum injection speed, 0.1 second) is applied to the opening 8 for pressure injection of ultra hard resin into the plaster mold 7 at the injection speed of 0.01 to 10 seconds and the injection pressure of 30 to 1200 kg/cm2. At this time, the air held in the mold 7 is compressed, resulting in momentary increase in internal pressure and elevation in the resistance prevailing in the mold. This pressure is relieved smoothly by air vent 5 for preventing an abrupt increase in the internal pressure. After complete removal of the mold pressure, the air vent 5 is sealed hermetically. The numerical values showing an optimum correlation should be stored in a memory within a computer, with the thickness of the molded product and other conditions taken into account within the dimen-sional range of the diameter of 0.1 to 5 mm of t~e air vent ~ for the diameter or perimetric length of 0.5 to 20 mm of the sprue 4. Granted that the abrupt increase in the pressure prevailing within the mold could be avoided by air removal througn the air vent 5, the latter may be sealed at the next moment by hardened resin, and hardening shrinkage of injected resin is started after completion of injection. Therefore, a holding pressure must be applied immediately for the holding time of 0.05 to 60 seconds for compensating such shrinkage.
At this time,the air vent 5, used for relieving the pressure within the mold, must be sealed completely.
In the embodiment of Fig. 1 wherein the molded product has a uniform thickness, the molding operation is not encountered with considerable difficulties. However, in the g _ 5~

case of the molded product having a larger thickness, the nigher the temperature at which the resin is heated, the more the shrinkage which the product undergoes when the latter is held and cooled in the mold. As a countermeasure to such shrinkage, the sensor pin 9 of the inner pressure detection and control device is provided to the sprue 4 and the destruction pressure of the plaster Mold is memorized in advance in the computer so that a suitable amount of resin can be supplied into the mold for compensating resin shrinkage in accordance witll the decrease in the internal pressure. It is to be noted that, if the sprue 4 has been completely solidified before tile injected resin has been molded completely and stably, the control function of the sensor pin 9 may be lost. In this consideration, the optimum correlation between the diameter ancl length of the sprue 4 and the thickness of the molded product is stored in the memory of a computer so that the optimum conditions may be instructed by the computer for the purpose of preventillg the solidifica-tion of the sprue 4.
Fig. 2 shows an embodiment of the invention wherein the molded product has a non-uniform thickness and thus has a tllick portion A and a thin portion B which must be molded simultaneously.
In this case, it is of no avail to take solely the shrinkage into considera-tion during molding, and hardening of the resin should not occur until the resin has been comple-tely injected into the mold. Thus, an increased injection speed is required in comparison with that used in the embodi-ment of Fig. 1. First of all, if the air in the thick portion A is compressed and held in the thin portion B, -the air pressure thereof becomes large and delays the resin flow during the.

injection molding to cool the injected resin, thus resul-ting 95~i6 in a lowered injection effect and worsening physical properties of the moldefi produc-t. An elevation in injection pressure and hence in injection speed may overcome the resistance in the mold. However, the plaster may be destroyed. The air vent 5 is essential for avoiding such increase in pressure in the mold.
In addition, the air vent 5 should be of such a diameter and length as to be sealed by the injected resin itself after realization of the above effec-t.
The diameters of sprue 4 and the air vent 5 are differ-entiated in accordance with the size of the products to beIllolded, but in order to compensate the discharge pressure ancl retain the suitable properties of the material by supple-menting the resin for the shrinkage, a proper correlation between the sprue 4 and the air vent 5 is required, which enables the resin capable of sealing the air vent -to harden so as to prevent the secondary pressure from discharging.
The physical properties and dimensional accuracy may be realized with a product having both thick and thin portions by giving it numerical values indicative of such ~0 correlation stored in advance in a control device and having the optimum injection molding conditions instructed for the particular molding operation.
In injection molding not only of industrial products, but of dental prosthetic goods, where not many products are required and the injec-tion molding is to be made in the plaster mold secured by the flasks, the internal pressure in the mold is sensed and adjusted by the sensor pin 9 for securing -the physical properties` and dimensional accuracy of the rnolded product. The sensor pin 9 is connected direct to the sprue 4.
By such means,a product having a thin portion B with the thickness of 0.5 mm and the thick portion haviny a thickness of 20 mm, which otherwise ha~ to be molded separately and connected to one another after molding, can be molded inte-grally and simultaneously, as shown in Fig. 4, through injec-tion molding at an ultra high speed and without the risk of destruction of the plaster mold.
The pressure within the mold of the molding machine is controlled in the following manner. A pressure transducer or sensing means for sensing the resin pressure is mounted near a runner of an injection mold or a cavity gate near the sprue 4 as described before. The resin pressure is measured, with the aid of an oscillograph or other recorder (not shown), Erom the time that molten resin is injected from -the nozzle of the molding machine into the mold until the resin has been completely filled into the cavity 1 and solidified through cooling. Thus, a desired setting pressure c is found out, said setting pressure being included within a tolerable range included within the upper and lower limits a and b short of insufficient charg~ng and overcharging, as indicated in Fig.9.
Then the hydraulic system of -the molding machine is controlled ~0 until sealing of the runner 4 is obtained so that the pressure in the mold may be included in the range of -this setting pressure by a controller.
It may be remarked that, in the above construction, a variable hydraulic pressure supply device, such as a servo valve or a proportional magnetic valve, may be mounted to the injection means for providing a closed loop adap-ted for feed-back of resin pressure witnin the mold so that the inner pres-sure in the mold may be controlled for each shot by the con-troller until sealing of tne sprue 4 is performed in such a manner that the inner pressure in the mold is within the setting pressure as indicated in Fig. 10.

5~

Alternatively, the variable hydraulic pressure supply device is applied in the above construction to the injection means of the molding machine and, as shown in Fig. 11, a desired program control is performed for the injection speed according to the actual mold configuration until the time of pressure increase of each section of setting pressure components, and the setting pressure is maintained until sealing of the air vent, with the program prevailing so far being nullified as the setting pressure Ps is attained.
According to the present invention, in resin molding with the plaster mold 7, the resin is injected at high speed from the sprue of 0.5 to 20 mm in diameter or perimetric length for the pressure holding time of 0.05 to 60 seconds and by using injection molding conditions of 0.01 to 10 seconds of injection speed and 30 to 1200 kg/cm of injection pressure.
The pressure in the mold having a tendency to increase abruptly is relieved by the air vent 4 with the diameter or perimetric length o~ 0.1 to 5 mm. After the mold 7 has been filled with resin, the air vent 5 is sealed by cooled resin, and an amount of resin is introduced so as to compensate the s;lrinkage of the molded product. After filling the sprue 4 is coolecl and harclened. The sensor pin 9 of the control device connected clirect to the sprue 4 senses the internal pressure for controlling this sequence of operations. Thus, the optimum correlation between the sprue 4 and the air vent 5 for securing the dimensional accuracy and desired properties of the molded product can be established at the optimum condi-tion while con-trolling the internal pressure of the plaster mold, tilus yielding the foregoing effec-t.
However, when the time has come for the sprue 4 to harden,the thick portion of the filled mold still undergoes 15~6 cooling and shrinkage beeause of the fact that the thick portion is too thick, the desired properties and accuracy can not be obtained. Thus, as shown in Pig. 6, a cooling pipe 10 is em-bedded in advance at the periphery of the thick portion, while a lleating pipe 11 is embedded wihere the heating is required, so as to improve the overall physical properties and accuracy of the Molded product.
Moreover, wnen manufacturing the plaster mold, mixing of plaster with water and stirring of the resulting mixture are earried out as known manner and when embedding, injection of the plaster is earried out in vacuo prior to the reaction of plasti-eization thereof and at the same time vibration is applied tnere-to, whicA leads to an elaborate or smootll inner surface of tAe plaster pattern, thus improving the accuracy of -the molded pro-duct.
Fig. 7 shows a cross-sectional view of the mold app-lied to a dental prostlletic goods, (such as artificial teeth)to which the above moldinc3 method aeeording to the present inven-tion is applied. In the figure, unless the thiekness of the portion of the rnolded product corresponding to the upper palatal surfaee 12 of the mouth is less than 0.5 mrn, it is normal in view of the human physiology that the physiologieal action of rejeeting an alien or foreign substance enterincJ in the mouth is always performed so as to defend the body by repelling the foreign substance out of the human body. Accordingly, the por-tion must be very thin. In this respect, the artificial teetn neretofore obtained by the conventional method, however, were often broken a, t~e thin portion because the latter can not be effectively opposecl to the chewing ~re5Sure,thus the artificial teeth with thin portion having sufficient strength can not be realized. Recently, the resin with satisfactory properties in this eonneetion has been developed. Ilo~ever, such resin has to 9S~;

be heated from 280 to 400C or higher and exhibits bad fluidityduring injeetion charging. Accordingly, in such resin, in order to eompensate the defects, the desired physical prop-erties and dimensional accuracy can not be attained unless the injection eharging is completed at the injeetion speed of about 0.1 seeond. Moreover, the produet according to the con-ventional technique has a property of absorbing water and in turn it has flexion so that the accuracy is not good. In the conventional injeetion molding proeess, it was not possible to manufacture artifieial teeth with the physiologieally ideal thin portions of less than 0.5 mm as shown in Fig.7, in which the existing resins such as polycarbonate, polyallylate or polysulfonie resin were injeeted into the plaster mold and were molded, in view of the improvement of quality of produet as well as human physiologieal point of view. The injeetion molding of the ideal dental prosthe-tie goods aecording to the present invention has been made possible by the injeetion molding apparatus with very~high injeetion speed, which eomprises the sensor p.ill 9 o~ means for sensing and eontrolling inner pressure of the rnold and the eooling and heating means for plaster mold operable in aeeordanee with tne correlation between the para-meters of the sprue runner 4 and tne air vent 5. ~loreover, stable properties of the material within the plaster and the aeeuraey of the molded produets ean be realized by the present invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of injecting molding of resin by use of a plaster mold comprising the steps of providing a sprue runner passageway having a diameter of 0.5 to 20 mm and an air vent having a corresponding diameter of 0.1 to 5.0 mm in the plaster mold, sensing the injected resin pressure in the plaster mold by pressure sensing means, and controlling the injection speed of the resin to be injected within the range of 0.01 to 10 seconds according to the sensed pressure so as to change the inner pressure in the plaster mold within the range of 30 to 1200 kg/cm2 in a predetermined optimum pattern stored in a memory of a computer below the destruction point of the product to be molded.

2. A method as claimed in claim 1 wherein said injection speed is determined by the correlation between the dia-meter of the sprue and the diameter and the length of the air vent.

3. A method as claimed in claim 1 or claim 2 wherein the mold has a thick portion and the method further comprises the step of providing cooling pipes around the thick portion of the mold.

4. A method as claimed in claim 1 or claim 2 wherein the method further comprises the step of providing heating pipes to portion or portions to be heated so as to produce a heating effect to the mold.

5. A method as claimed in claim 1 or claim 2 wherein, after completion of the injection, the internal pressure is maintained for a holding time of 0.05 to 60 seconds.