INJECTION MOLDING OF THERMOPLASTICS IN SANDWICH MOLD EMPLOYING DESYNCHRONIZED OPENING PERIODS

Abstract: A method of injection molding a thermoplastic ma­ terial to produce a product in a sandwich mold (19) comprising at least two cooling cavities and at least two ejection ports (1, 23) and a single runner system (13) for feeding all the cooling cavities, where each ejection port passes through a retracted period and a protracted period which together make up its produc­ tion cycle, where the retracted period comprises an injection period and a cooling period and where the pro­ tracted period comprises an opening period, an extended period or point in time, and a closing period, the method comprising the step of desynchronizing the opening period of a first election port with the opening period of a second ejaction port.

Description: INJECTION MOLDING OF THERMOPLASTICS IN SANDWICH MOLD EMPLOYING DESYNCHRONIZED OPENING PERIODS FIELD OF THE INVENTION This invention generally pertains to a production method employing molds and specifically to injection molding of thermoplastics using a sandwich mold which has more than one ejection port. Heretofore, injection molding of thermoplastics using a sandwich mold has employed synchronized opening periods of the ejection ports, and has normally utilized a gear system to serve this purpose. SUMMARY OF THE INVENTION The present invention is a method of injection molding a therm- plastic material in order to produce a product in a sandwich mold comprising at least two cooling cavities and at least two ejection ports and a single runner system for feeding all the cooling cavities. Each ejection port passes through a retracted period and a protracted period which together make up its production cycle. The retracted period comprises an injection period and a cooling period, and the protracted period comprises an opening period, an extended period or point in time, and a closing period. The method comprises the step of desynchronizing the opening period of a first ejection port with the opening period of a second ejection port. The first ejection port is defined as the ejection port which begins its opening period first, but when the two ejection ports begin their opening periods simultaneously, the first ejection port means the ejection port which ends it opening period first. For description of prior art with synchronous opening periods the first ejection port means any random chosen one of the two ejection ports. The method of the invention provides many unusual and surprising advantages to the prior art of thermoplastic injection molding methods, where synchronized opening periods of the ejection ports have been employed. Some of those advantages are as follows: Advantage number 1. when the injection period of both injec- tion ports are simultaneous and when beginning the opening period of the second ejection port subsequent to beginning the opening period of the first ejection port, the cooling period of the second ejection port will extend into the opening period of the first ejection port thereby providing more cooling time for the product which is ejected from the second ejection port. This is an advantage in the simultaneous injection molding of prodt-ts such as containers and lids, where the two products need a different length of time to solidify during their respective cooling periods, so that the product which needs the longer cooling period to solidify may be ejected from the ejection port with the longest cooling period. This advantage occurs in the embodiment illustrated in Figures 3, 4, 3, 6, 7, and 8. Advantage number 2. when ending the opening period of the second ejection port subsequent to ending the opening period of the first ejection port, the products which are ejected from the first ejection port are ejected spaced in time from the products which are ejected from the second ejection port. This is an advantage usually in the packing procedure where separation of the ejected products may more easily be accomplished, and provides a more continuous supply of work in the packing procedure. This advantage is also accomplished under the conditions leading to advantage number 1 above and under conditions leading to advantages numbers 3, 4, and 5 below. This advantage occurs in all the illustrated preferred embodiments. Advantage number 3. when beginning the closing period of the first ejection port prior to ending the opening period of the second ejection port, the length of the full stroke of the clamping unit is reduced, thereby reducing the size of the required clamping unit. This advantage occurs in the embodiments illustrated in Figures 3, 5, 8 and 9. Advantage number 4. When synchronizing the closing period of the first ejection port with the opening period of the second ejection port only 50% of the length of the prior art stroke of the clamping unit is required. This advantage is also accomplished under the conditions leading to advantage number 5 below. This advantage occurs in the embodiments illustrated in Figures 3, 8 and 9. Advantage number 5. when beginning the injection period of the first ejection port subsequent to ending the protracted period of the first ejection port, and ending the injection period of the second ejection port prior to beginning the protracted period of the first ejection port, it is possible to place the injection periods in separate disjunct periods whereby the full injection power of the injection unit is available during each injection period. This is of particular importance in the injection molding of products which have a wall thickness of less than 1 mm. This advantage always occurs in the embodiments illustrated in Figure 9, and may be accomplished in all the other embodiments. Advantage number 6. Together with all the conditions described above is the advantage of reduced cycle time; following the description of the preferred embodiments, a calculation of the total production cycle time of each embodiment will be Set forth. This advantage occurs in all the illustrated preferred embodiments. Some of the advantages described above may sometimes occur also under other conditions than the conditions described, just as some of the conditions described may sometimes result in other advantages. Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description thereof. It should be noted that with very few exceptions, any of tn- above mentioned advantages does not exclude any other, and that a advantages may be accomplished while employing a sandwich mold wn¯:h hÅas only one filling orifice. The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1A is a diagrammatic partialltop sectional view of an injection moulding machine and a sandwich mould assembled in such a manner as to operate in accordance with an embodiment of this invention; Figure 13 is a diagrammatic, side sectional view of the injection molding machine and the sandwich mold of Figure 1A taken as indicated by section line B-B; and Figure 2 through 9 are each a chronological series of diagrammatic partial side sectional views of the sandwich mold of the injection molding machine of Figure 13. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 1A and 13 show a part of a conventional injection unit 8 of an injection molding machine with injection nozzle 9. Also shown is a conventional clamping unit 18 of the injection molding machine comprising a stationary platen 10 and a moveable platen 15; tie rods are not shown. Also shown is a conventional sandwich mold 19 comprising a front mold part 3, an intermediate mold part 4, and a rear mold part 5. The front mold part 3 is fastened to the stationary platen 10 and the rear mold part 5 is fastened to the moveable platen 15. A first ejection part 1 is positioned between front mold part 3 and intermediate mold part 4, and a second ejection port 2 is positioned between intermediate mold part 4 and rear mold part 5. A runner bushing 12 with a filling orifice 11 extends from the intermediate mold part 4 toward the tip of the injection nozzle 9. The runner bushing 12 and the intermediate mold part 4 contain a runner system 13 of a hot runner or insulated runner type. During an injection period the tip of the injection nozzle 9 forms a seal with the filling orifice 11 of the runner bushing 12. The sandwich mold 19 does not have a conventional gear system to synchronize the opening period of the first ejection port with the opening period of the second ejection port. Illustrated is a conventional ejector cylinder 17 with a piston 16 which, via a piston rod 14, is fastened to the intermediate mold part 4. The ejector cyclinder 17 accordingly does not serve its conventional purpose, steering an ejection or stripper unit, but is, in this figure, used to steer the intermediate mold part 4 into any position between the front mold part 3 and the rear mold part 4 in order to desynchronize the opening period of the first ejection port 1 with the opening period of the second ejection port 2. Figures 2 through 9 show the sandwich mold 19 of Figure 1 comprising front mold part 3, intermediate mold part 4 and rear mold part 5. Also illustrated is a first ejection port 1 and a setnd ejection port 2, and a first ejected product 6 and a second ejected product 7. FIGURE 2 illustrates a prior art production method with synchronized opening periods which operates by passing through the following chronological periods. FIGURE 2A shows the first ejection port 1 and the second ejection port 2 both at the end of their retracted period which comprises the injection period and the cooling period, and both at the beginning of their opening periods. Figure 23 shows the first ejection port 1 and the second ejection port 2, both at the end of their opening periods and at the beginning of their extended periods. Two produced products 6 and 7 are being ejected. Since both the beginning and the end of the opening periods of both ejection ports 1 and 2 are simultaneous, the opening periods are synchronous. Figure 2C shows the first ejection port 1 and the second ejection port 2, both at the end of their extended period and at the beginning of their closing period. The two produced products 6 and 7 have been ejected. Figure 2D shows the first ejection port 1 and the second ejection port 2, both at the end of their closing period and at the beginning of their retracted period, which ends the production cycle of both ejection ports. FIGURE 3 illustrates a preferred embodiment of the invention which operates by pasing through the following chronological periods. Figure 3A shows the first ejection port 1 at the end of its retracted period and at the beginning of its opening period and shows the second ejection port 2 some time during its retracted period. Figure 33 shows the first ejection port 1 at the end of its opening period and at the beginning of its extended period. A first produced product 6 is being ejected. Figure 33 also shows the second ejection port 2 some time during its retracted period. Figure 3C shows the first ejection port 1 at the end of its extended period and at the beginning of its closing period. The first produced product 6 has been ejected. Figure 3C also shows the second ejection port at the end of is retracted period and the beginning of its opening period. Figure 3D shows the first ejection port 1 at the end of its closing period and the beginning of its retracted period, and shows the second ejection port 2 at the end of its opening period and beginning of its extended period. A produced second product 7 is being ejected. Figure 3E shows the first ejection port 1 some time during its retracted period and shows the second ejection port 2 at the end of its extended period and the beginning of its closing period. The second produced product 7 has been ejected. Figure 3F shows the first ejection port 1 some time during its retracted period, and shows the second ejection port 2 at the end of its closing period and the beginning of its retracted period. Advantages numbers 1, 2, 3, 4 and 6 occur in this embodiment. FIGURE 4 illustrates a preferred embodiment of the invention which operates by passing through the following chronological periods. Figure 4A shows the first ejection port 1 at the end of its retracted period and at the beginning of its opening period, and shows the second ejection port 2 some time during its retracted period. Figure 43 shows the first ejection port 1 some time during its opening period and shows the second ejection port at the end of its retracted period and the beginning of its opening period. Figure C shows the first ejection port 1 at the end of its opening period and the beginning of its extended period. A first produced product 6 is being ejected. Figure 4C also shows the second ejection port 2 some time during its opening period. Figure 4D shows the first ejection port 1 at the end of its extended period and the beginning of its closing period. The fi produced product 6 has been ejected. Figure 4D also shows the sue end ejection port 2 at the end of its opening period and the begi ming of its extended period. A second produced product 7 is being ejected. Figure 4E shows the first ejection port 1 some time during its closing period and shows the second ejection port 2 at the end of its extended period and at the beginning of its closing period. The second produced product 7 has been ejected. Figure 4F shows the first ejection port 1 at the end of its closing period and the beginning of its retracted period, and shows the second ejection port 2 some time during its closing period. Figure 4G shows the first ejection port 1 some time during its retracted period and shows the second ejection port at the end of its closing period and the beginning of its retracted period. Advantages numbers 1, 2, and 6 occur in this embodiment. FIGURE S illustrates a preferred embodiment of the invention which operates by passing through the following chronological periods. Figure SA shows the first ejection port 1 at the end of its retracted period and at the beginning of its opening period, and shows the second ejection port 2 some time during its retracted period. Figure 53 shows the first ejection port 1 at the end of its opening period and the beginning of its extended period. A first produced product 6 is being ejected. Figure 53 also shows the second ejection port 2 at the end of its retracted period and the beginning of its opening period. Figure SC shows the first ejection port 1 at the end of its extended period and at the beginning of its closing period. The first produced product 6 has been ejected. Figure SC also shows the second ejection port 2 some time during its opening period. Figure SD shows the first ejection port 1 some time during its closing period and shows the second ejection port 2 at the end of its opening period and the beginning of its extended period. A second produced product 7 is being ejected. Figure SE shows the first ejection port 1 at the end of its closing period and the beginning of its retracted period, and shows the second ejection port 2 at the end of its extended period and the beginning of its closing period. The second produced product 7 has been ejected. Figure SF shows the first ejection port 1 some time during its retracted period, and shows the second ejection port 2 at the end of its closing period and at the beginning of its retracted period. Advantages number 1, 2, 3, and 6 occur in this embodiment. -FIGURE 6 illustrates a preferred embodiment of the invention which operates by passing through the following chronological periods. Figure 6A shows the first ejection port 1 at the end of its retracted period and at the beginning of its opening period, and shows the second ejection port 2 some time during its retracted period. Figure 63 shows the first ejection port 1 at the end of its opening period and at the beginning of its extended period. A first produced product 6 is being ejected. Figure 6B also shows the second ejection port 2 at the end of its retracted period and at the beginning of its opening period. Figure 6C shows the first ejection port 1 some time during its extended period. The first produced product 6 is still being ejected. Figure 6C also shows the second ejection port 2 at the end of its opening period and at the beginning of its extended period. A second produced product 7 is being ejected. Figure 6D shows the first ejection port 1 at the end of its extended period and at the beginning of its closing period. The first produced product 6 has been ejected. Figure 6D also shows the second ejection port 2 at some time during its extended period. The second produced product 7 is still being ejected. Figure 6E shows the first ejection port 1 at the end of its closing period and at the beginning of its retracted period, and also shows the second ejection port 2 at the end of its extended period and the beginning of its closing period. The second produced product 7 has been ejected. Figure 6F shows the first ejection port 1 some time during its retracted period and also shows the second ejection port 2 at the end of its closing period and the beginning of its retracted period. Advantages numbers 1, 2, and 6 occur in this embodiment. FIGURE 7 illustrates a preferred embodiment of the invention which operates by passing through the following chronological periods. Figure 7A shows the first ejection port 1 at the end of its retracted period and at the beginning of its opening period, and shows the second ejection port 2 some time during its retracted period. Figure 73 shows the first ejection port 1 at the end of its opening period and at the beginning of its extended period. A first produced product 6 is being ejected. Figure 73 also shows the second ejection port 2 at the end of its retracted period and at the beginning of its opening period. Figure 7C shows the first ejection port 1 at the end of its extended period and at the beginning of its closing period. The first produced product 6 has been ejected. Figure 79 also shows the second ejection port 2 at the end of its closing period and the beginning of its extended period. A seconc produced product 7 is being ejected. Figure 7D shows the first ejection port 1 at the end of its closing period and at the beginning of its retracted period, and shows the second ejection port 2 at the end of its extended period and at the beginning of its closing period. The second produced product 7 has been ejected. Figure 7E shows the first ejection port 1 some time during its retracted period and shows thé second ejection port 2 at the end of its closing period and the beginning of its retracted period. Advantages numbers 1, 2, and 6 occur in this embodiment. FIGURE 8 illustrates a preferred embodiment of the invention which operates by passing through the following chronological periods. Figure 8A shows the first ejection port 1 at the end of its retracted period and beginning of its opening period, and shows the second ejection port 2 some time during its retracted period. Figure 83 shows the first ejection port 1 at the end of its opening period at its extended point, and at the beginning of its closing period. The first produced product 6 has almost been eject- ed, and there is no need for any extended period for a very fast ejection. Figure 8C shows the first ejection port 1 at the end of its closing period and the beginning of its retracted period, and shows the second ejection port 2 at the end of its opening period, at its extended point and the beginning of its closing period. A second produced product 7 has almost been ejected, and there is no need for any extended period for a very fast ejection. Figure 8D shows the first ejection port 1 some time during its retracted period and shows the second ejection port at the end of its closing period and at the beginning of its retracted period. Advantages numbers 1, 2, 3, 4, and 6 occur in this embodiment. Neither ejection port 1 nor 2 passes through any extended period but only- through an extended point in time because the produced products 1 and 2 were able to be ejected between the opening period and the closing period. FIGURE 9 illustrates a preferred embodiment of the invention which operates by passing through the following chronological periods. Figure 9A shows the first ejection port 1 at the end of its cooling period and at the beginning of its opening period, and shows the second ejection port 2 during its injection period and during the beginning of its cooling period. Figure 93 shows the first ejection port 1 at the end of its opening period and at the beginning of its extended period. A first produced product 6 is being ejected. Figure 93 also shcws the second ejection port 2 some time during its cooling period. Figure 9C shows the first ejection port at the end of its extended period and -at the beginning of its closing period. The first produced product 6 has been ejected. Figure 9C also shows the second ejection port some time during its cooling period. Figure 9D shows the first ejection port 1 at the end of it: cooling period and during its injection period and during the be, nning of its cooling period and shows the second ejection port at the end of its cooling period and at the beginning of its opening period. Figure 9E shows the first ejection port 1 some time during its cooling period and shows the second ejection port at the end of its opening period and at the beginning of its extended period. A second produced product 7 is being ejected. Figure 9E shows the first ejection port 1 some time during its cooling period and shows the second ejection port 2 at the end of its extended period and the beginning of its closing period. The produced product 7 has been ejected. Figure 9G shows the first ejection port 1 some time during its cooling period and shows the second ejection port 2 at the end of its closing period and the beginning of its injection period. Advantages numbers 2, 3, 4, 5 and 6 occur in this embodiment. In the embodiment of Figure 9 it is possible to leave out Figures 9C and 9F in case the produced products 1 and 2 are able to be ejected with no extended period but only an extended point in time between the opening period and the closing period. In all the preferred embodiments 3 through 9 the ejection port which is nearest to the stationary mold platen has been. the ejection port which opens first during the total production cycle. Embodiments where it is the other ejection port which opens first are equally possible. Many other embodiments are possible, for example combinations of parts of the illustrated embodiments. One interesting embodiment is one where the extended period or point in time of the first ejection port takes place within the opening period of the second ejection port and the extended period or point in time of the second ejection port takes place within the closing period of the first ejection port. Without narrowing the scope of the invention, and only for the purpose of calculating an approximate length of the total production cycle of each method as illustrated in Figures 3 to 9, we may make the following assumptions. (a) that the rear mold part 5 moves back and forth with a certain speed V in relation to the front mold part 3. (b) that the intermediate mold part 4 moves back and forth with a maximum speed equal to V in relation to the rear mold part 5. (c) that the opening distance D- is the same for all ejection ports. (d) that the time T is the time taken to move distance D at speed V. (e) that the extended period is E. This period is usually very short and is typically in the order of O to 1 second. (f) that the injection period is I. (g) that the cooling period is C. The duration of the total production period of the prior art method illustrated in Figure 2 is then: I + C + 4T + E The duration of the total production period of the embodiment illustrated in Figure 3 is: I + C + 3T + 2E So this embodiment has a shorter total production period than the prior art when E is shorter than T which is most commonly the case. The embodiment illustrated in Figure 4 always has T being longer than E. The duration of the total production period is: I + C + 4T This is a shorter total production period than the prior art method of Figure 2. It is also a faster method than that of Figure 4 when E is shorter than T and T is shorter than 2E, but slower when 2E is shorter than T. The embodiment illustrated in Figure 5 also always has T longer than E. The duration of the total production period is: I + C + 3T-+ F Since T is longer than E, this is a faster method than any of the previously described methods of Figures 2, 3, and 4. The embodiment illustrated in Figure 6 always has T shorter than E. The duration of the total production period is: I + C + 3T + F This is a faster method than the prior art method of Figure 2 and faster than the method of Figure 3. This method cannot be compared to the methods of Figures 4 and S since T is shorter than E. The embodiment illustrated in Figure 7 has T = E. The duration of the total production period is: I + C + 4T This is a faster method than the prior art of Figure 2 and faster than the method of Figure 3. The embodiment illustrated in Figure 8 has E = O. The duration of the total production period is: I + C + 3T This is faster than the prior art method of Figure 2, and is same method as illustrated in Figure 3 when the extended periods are reduced to extended points in time. The embodiment illustrated in Figure 9 has a total production period of: I + C + 2T + E This method is the fastest of all the methods described in Figures 2 through 9, but the method is limited to conditions where C is longer than or equal to I + 2T + E. Since the method is faster than all the other methods at least by a period of T, it will still be as fast as the fastest method if the cooling period can be artificially increased up to C + T. When the cooling period can be increased by T, then the method is the fastest method even when C is longer than or equal to I i T f E. So far advantage number 1 has not been taken into account. when the product which is ejected from the one ejection port needs a shorter cooling period than the product ejected from the other ejection port, there may be a saving in the total production period in most of the embodiments. The maximum theoretical saving in each embodiment is as follows: Figure 3 - T + E Figure 4 - less than T Figure 5 - T Figure 6 - T Figure 7 - T Figure 8 - T where C now is the cooling period for the longest cooling product. while the above descriptions contain many specific features, these should not be construed as limitations of the scope of the invention but rather as an exemplification of the preferred embodiments thereof. Many other variations are possible. For example, methods utilizing a sandwich mold with more than two ejection ports and methods wherein the steering of the intermediate mold part is steered in relation to the front mold part and/or the rear mold part by employing other means that the hydraulic ejector cylinder as show-n in Figure 1A. The individual injection periods, opening periods, extended periods or points in time, and closing periods of the different election ports may vary from those of the other ejection ports and a total production cycle may be constructed to optimize the various obtainable advantages. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.

Claims: CLAIMS 1. A method of injection molding a thermoplastic material to product a product in -a sandwich mold comprising at least two cooling cavities and at least Iwo ejection ports and a runner system for feeding the cooling cavities, where each ejection port passes through a retracted period and a protracted period which together make up its production cycle, where the retracted period comprises an injection period and a cooling period, and where the protracted period comprises an opening period, and a closing period, the method comprising the step of: (a) desynchronizing the opening period of a first ejection port with the opening period of a second ejection port. 2. A method according to claim 1 wherein step (a) comprises the step of: (b) beginningtheopening period of the second ejection port subsequent to beginning the opening period of the first ejection port. 3. A method according to claim 1 or claim 2 wherein step (a) comprises the step of (c) ending the opening period of the second ejection port subsequent to ending the opening period of the first ejection port. 4. A method according to maims 1, 2 or 3 comprising the additional step of: (d) ending the injection period of both the first ejection port and the second ejection port prior to beginning the protracted period of the first ejection port. 5. A method according to claim 1, 2, 3 or 4 comprising the additional step of: (e) beginning the opening period of the second ejection port not later than beginning the closing period of the first ejection port. 6. A method according to claim 1, 2 or 3 comprising the additional steps of: (d) beginning the injection period of the first ejection port subsequent to ending the protracted period of the first ejection port and, (e) ending the injection period of the second ejection port prior to beginning the protracted period of the first ejection port. 7. A method according to claim 1, 2, 3, 4 or 5 wherein step (a) comprises the step of: (f) beginning the opening period of the second ejection port not later than at the end of the opening period of the first ejection port and the method comprising the additional step of: (g) beginning the closing period of the second ejection port not later than at the end of the closing period of the first ejection port. 8. A method according to claim 7 wherein step (f) comprises the step of: (h) beginning the opening period of the second ejection port before the end of the opening period of the first ejection port and wherein step (g) comprises the step of (i) beginning the closing period of the second ejection port before the end of the closing period of the first ejection port. 9. A method according to claim 1, 2, 3, 4 or 5 comprising the additional step of: (f) beginning the closing period of the first ejection port prior to ending the opening period of the second ejection port. 10. A method according to claim 1, 2, 3, 4, 5 or 9 wherein step (a)- comprises the step of: (g) synchronizing the closing period of the first ejection port with the opening period of the second ejection port. 11. A product produced by the method of ally one of the preceding claims 1 to 10.

Inventors: SORENSEN, JENS OLE
Applicants: SORENSEN, JENS OLE
Application year: 1982
Main classification: B29C45
IPC codes: B29C45, B29C35