JP2019072869A - Hollow molding machine - Google Patents

Abstract

To provide a hollow molding machine capable of preventing excessive resin pressure generation causes when, after the completion of the temperature rising in an extrusion machine, the extrusion machine is started at first.SOLUTION: Provided is a hollow molding machine comprising a controller, upon the start of an extrusion machine after the completion of the temperature rising in the extrusion machine, prior to the extrusion at an ordinary extrusion mode, practicing extrusion at the initial extrusion mode. In the initial extrusion mode, by the function of the controller, the rotational frequency of a screw for feed within a prescribed time after the start of extrusion of a synthetic resin by the extrusion machine is limited to the one at least lower than that at the ordinary extrusion mode (step S2). Simultaneously, the machine comprises a resin pressure gauge detecting the resin pressure within the tip part of the extrusion machine, and, when a resin pressure detection value detected by the resin pressure gauge reaches a pre-set resin pressure upper limit value, the rotation of the screw for feed is stopped (steps S4, S5).SELECTED DRAWING: Figure 4

Description

  The present invention relates to a hollow molding machine which prevents generation of excessive resin pressure when the extruder is operated in a state where the synthetic resin inside the die head for hanging down the parison is unmelted.

  The extruder for thermoplastic resin in the hollow molding machine comprises a heating cylinder and a feeding screw rotatably provided in the heating cylinder. A die head for attaching a shape to the molten resin to be extruded is attached to the tip of the heating cylinder.

  In such a hollow molding machine, at the time of molding start-up, the resin remaining in the die head is warmed to a flowable temperature by a heater attached to the extruder or the outer periphery of the die head, and then the extruder is started to Start extrusion.

  However, in reality, because the temperature of the thermocouple used to control these temperatures is different from the temperature of the resin inside the resin flow path, even if the operator looks at the temperature display on the operation display panel, the resin inside the die head is really I do not know whether or not it is in a flowable state.

  Conventionally, after a sufficient time has elapsed after the start of the temperature rise, a skilled operator determines the state of the resin inside the die head based on experience, and determines the start timing of the extruder. In addition, the molding machine manufacturer also takes into consideration the temperature difference between the thermocouple immediately after the completion of the temperature rise and the resin inside the resin flow path, and prevents the extruder from starting until a certain time elapses after the temperature rise completion. Has been incorporated into the control circuit of

  However, since the machine specifications of the molding machine itself and the raw materials actually used for molding are various, the above-mentioned correspondence is not reliable. In the case of using unfamiliar raw materials or when the operator is not accustomed to machine operation, there is a risk that the extruder may be started in a state in which the resin inside the die head is not sufficiently warmed.

  At this time, since the resin flow path in the die head is blocked by the unmelted resin, the internal pressure rises, and the bolt that fastens the die head constituting member is extended, causing breakage of the die head and resin leakage. I was afraid to do it.

  Also, in the hollow forming machine that has conventionally referred to the control of the extruder and the die head itself, for example, as in Patent Document 1, the extrusion amount of the raw material with respect to the number of rotations of each extruder is measured in a multilayer hollow forming machine. When the extrusion amount with respect to the number is stored as data of y = f (x) and the set extrusion amount is set, these extruders set the number of rotations to the set extrusion amount according to y = f (x) .

  Furthermore, as in Patent Document 2, for example, a resin pressure detector provided on the molten resin inflow side of a die (another name of die head) of a continuous extrusion type hollow molding machine samples changes in resin pressure in one cycle, The resin pressure pattern of the cycle from which a good product was obtained is stored in the memory, the resin pressure is fed back with the stored resin pressure pattern as the main setting, and the extruder rotation is controlled by the output signal to control the resin pressure. There was something.

JP 2001-150519 A JP-A-9-94870

  However, in the hollow molding machines disclosed in Patent Document 1 and Patent Document 2, although reference is made to extruder control after the molding operation, the above-described problems at the start of the extruder have not been solved.

  In the present invention, when the extruder is first started after the temperature rise of the extruder is completed, the first extrusion mode is automatically performed prior to the normal extrusion mode to prevent the cause of excessive resin pressure from occurring. It is an object of the present invention to provide a hollow molding machine equipped with control means to be implemented.

  According to the present invention, in order to achieve the above object, in a hollow molding machine equipped with an extruder for melting a synthetic resin and feeding it to a die head with a feed screw, starting of the extruder after the temperature rise of the extruder is completed. Sometimes, control means is provided to execute extrusion in the first extrusion mode prior to extrusion in the normal extrusion mode, and in the first extrusion mode, the feed within a predetermined time after the start of the extrusion of the synthetic resin by the extruder is The number of rotations of the screw is limited to at least a number lower than that in the normal extrusion mode, and the resin pressure in the tip of the extruder is limited to a predetermined value or less.

  More specifically, as described in claim 2, a detection means for detecting the resin pressure in the tip of the extruder is provided, and the resin pressure upper limit value in the tip of the extruder in the first extrusion mode It is desirable that the rotation of the feed screw be stopped when the resin pressure detection value by the detection means reaches the resin pressure upper limit value.

  As a more preferable aspect, as described in claim 3, the number of rotations of the feeding screw in the first extrusion mode is calculated by the following equation (1).

Nr = Nmin + α .... (1)
However, Nr is the number of rotations of the feeding screw when executing the first extrusion mode, Nmin is the minimum number of rotations of the feeding screw specific to the corresponding hollow molding machine, and α is based on the synthetic resin material used for molding It is the number of revolutions determined by

  Furthermore, as a desirable mode, as described in claim 4, the resin pressure upper limit value is calculated by the following numerical formula (2).

Pu = Pmax-P 'x td .... (2)
However, Pu is the resin pressure upper limit value at the tip of the extruder when executing the first extrusion mode, Pmax is the maximum pressure resistance of the die head, P 'is the rate of increase in resin pressure determined in advance, td is the next It is the deceleration time of the said screw for a screw calculated by Numerical formula (3).

td = Nr / Nd '.... (3)
However, Nr is the number of revolutions of the feeding screw when executing the first extrusion mode, and Nd 'is that the feeding screw is decelerated immediately after the resin pressure at the tip of the extruder reaches the resin pressure upper limit value. The amount of decrease in the number of rotations of the feeding screw per unit time from the start to the stop.

  Similarly, as a desirable mode, as described in claim 5, the predetermined time after the start of the extrusion satisfies the following formula (4).

tl> ts .... (4)
However, tl is a predetermined time when executing the first extrusion mode, and ts is a time until the resin pressure upper limit value in the tip of the extruder is reached when the resin in the die head is not flowable. This time is calculated by the following equation (5).

ts = Pu / P '... (5)
Where Pu is the resin pressure upper limit value of the tip of the extruder when the first extrusion mode is performed, and P 'is the rate of increase of the resin pressure determined in advance.

  Therefore, in the present invention, at the time of the first start of the extruder where the flow state of the synthetic resin in the die head is not sufficient, the number of rotations of the feed screw is limited at least to a number lower than that of the normal extrusion mode. It is possible to prevent a rapid rise in resin pressure in the tip.

  According to the second aspect of the present invention, the deceleration time of the feed screw is shortened when the resin pressure in the tip of the extruder exceeds the resin pressure upper limit value, and a rapid emergency stop of the feed screw is enabled. In addition, by detecting the rapid rise of the resin pressure at the early stage when the flow state of the synthetic resin is not sufficient, the emergency stop operation of the feed screw at the time when there is room for the maximum withstand pressure in the die head. Can be started. Furthermore, after the predetermined time has elapsed, the operator automatically shifts to the extrusion condition manually by automatically shifting to the setting rotational speed and the resin pressure upper limit value in the normal extrusion mode.

  According to the third aspect of the present invention, the number of rotations of the feed screw can be calculated more appropriately in accordance with the specifications unique to each hollow molding machine, the type of synthetic resin material used for molding, and the like.

  In the invention according to claim 4, the resin pressure in the tip portion of the extruder has the resin pressure upper limit value before the feeding screw starts to decelerate until it stops by using the increase speed of the resin pressure fixed in advance. It is possible to predict the value to be increased beyond and to more reliably prevent the die head from being damaged.

  In the invention according to the fifth aspect, by using the increase rate of the resin pressure determined in advance, it is possible to more reliably predict the predetermined time required for the first extrusion mode, and the rapid transition to the normal operation can be realized. To be possible.

  As described above, the present invention prevents excessive increase of the resin pressure by rotating the feed screw excessively at the initial startup of the extruder, and prevents these set values from being set at the initial startup. By automatically limiting the time to a predetermined value, it is possible to reduce the labor of the operator. As a result, it is possible to contribute to preventing the breakage of the die head regardless of the operator's level of skill and without impairing the convenience of the molding machine.

  According to the second aspect of the present invention, the deceleration time of the feed screw is shortened when the resin pressure in the tip of the extruder reaches the resin pressure upper limit value, and the emergency stop of the feed screw is achieved. to enable. Furthermore, by detecting the rapid rise of the resin pressure at the early stage when the flow condition of the synthetic resin is not sufficient, the emergency stop operation of the feed screw is started at a time when there is room for the maximum withstand pressure of the die head. can do.

FIG. 1 is a front view showing an embodiment of a hollow molding machine according to the present invention. FIG. 2 is a partially omitted side view of the hollow molding machine shown in FIG. 1; FIG. 2 is a block diagram showing a control circuit of an extruder of the hollow molding machine shown in FIG. 1; FIG. 5 is a flowchart showing a part of the process flow of the control circuit shown in FIG. 3; The graph which made resin pressure and rotation speed the vertical axis at the time of extruding from an extruder in resin unmelted state, and made the horizontal axis time.

  Hereinafter, an embodiment of a hollow molding machine according to the present invention will be described based on FIGS.

  The hollow molding machine according to the embodiment of the present invention, as shown in FIG. 1 and FIG. 2, has an extruder 1 for melting and extruding a synthetic resin as an operating part for molding, and an extruder 1 at the tip of a parison. It comprises a die head 2 to be dropped, a parison cutting device (not shown) below the die head 2, a molding die 3, a blow device 4, a mold clamping device 5, a die moving device 6 and a molded article takeout device 7.

  The extruder 1 includes, for example, a hopper 8 for charging a pellet-like synthetic resin material, and a feed screw (not shown) for transferring, kneading, melting, and discharging the synthetic resin inside the extruder. . The die head 2 is a cross die head for two heads, a head portion 2a which branches the molten resin flow extruded by a screw, a die portion 2b which makes the molten resin flow branched from the head portion 2a downstream, and a die at the lower end of the die portion 2b And a lip portion 2c consisting of a core and a core.

  The parison cutting device (not shown) is for cutting the upper end of the cylindrical parison hanging from the lip 2c of the die head 2, and includes a cutter holder having an electric heating cutter.

  As shown in FIG. 2, the molding die 3 is composed of a pair of split dies 3a and 3b, and since these split dies insert a tubular parison to form a hollow molded article P, each split die is not shown. The two corresponding cavities are formed side by side.

  The molding die 3 is integrally formed with the mold clamping device 5 by the mold moving device 6 provided with the accompanying electric motor 6a, directly below the two die heads 2 and the left and right blow nozzles 9, 9 of the blowing device 4. It can reciprocate alternately in the horizontal direction between the positions directly below.

  The blowing device 4 includes right and left blow nozzles 9 which are inserted into the parison and blow compressed air above the respective two cavities of the molding die 3 and a blow nozzle drive device 10 for raising and lowering these nozzles. ing. The blow nozzle drive device 10 includes a variable speed type blow device driving motor connected to a control unit (not shown).

  The mold clamping device 5 drives the split molds 3a and 3b at the time of opening and closing and mold clamping operations, and the front platen 11 and the rear platen 12 with the split molds attached thereto and movably spaced by a predetermined distance are formed It is made to move in synchronization with the parting line of the mold 3 in the approaching and separating directions. In FIGS. 1 and 2, reference numeral 13 denotes a front platen support plate to which the front platen 11 is attached.

  The takeout slide unit 7a of the molded article takeout device 7 holds the hollow molded article P during mold opening and conveys it outside the hollow molding machine.

  The molding condition setting device 20 is for inputting various hollow molding conditions, and is attached to the safety door 30. The molding condition setting device 20 includes an operation display screen 21 for inputting molding conditions.

  As shown in FIG. 3, the control device 50 in the hollow molding machine includes, in addition to the extruder 1 shown in FIGS. 1 and 2, a die head 2, a blowing device 4, a mold clamping device 5, a mold moving device 6, and a molded article. A programmable logic controller (hereinafter, this programmable logic controller is abbreviated as "PLC") 40 which controls the drive of the drive unit such as the takeout device 7 is mainly provided. The operation display screen 21 of the molding condition setting device 20 is electrically connected to the PLC 40. The PLC 40 is also electrically connected to the inverter 42 so as to drive the drive motor 43 of the extruder 1 electrically connected to the inverter 42. Further, the PLC 40 is connected to a temperature controller (not shown) so that temperature control of the extruder 1 and the die head 2 can be performed.

  Here, the extruder 1 shown in FIGS. 1 and 2 was started first after the temperature rise of the extruder 1 was completed separately from the extrusion mode (normal extrusion mode) in the normal operation. At the same time, it has a function of automatically executing a first extrusion mode for preventing an excessive resin pressure generation cause in advance. The first extrusion mode of the extruder 1 is also performed based on a command from the control device 50 having the PLC 40 as a main element, as in the extrusion mode in the normal operation. Therefore, the control device 50 having the PLC 40 as a main component not only functions as control means for executing the normal extrusion mode but also functions as control means for executing the first extrusion mode. The details of the first extrusion mode will be described later.

  The PLC 40 is electrically connected to a resin pressure gauge 44 provided at the tip of the extruder 1 as a resin pressure detection unit. Thereby, when the resin pressure gauge 44 detects the resin pressure in the tip portion of the extruder 1, the detected value is transmitted to the PLC 40. The PLC 40 monitors the resin pressure in the tip of the extruder 1 detected by the resin pressure gauge 44. Then, in the first extrusion mode, the resin pressure measurement value (resin pressure detection value) detected by the resin pressure gauge 44 is compared with the preset resin pressure upper limit value, and the resin pressure measurement value reaches the resin pressure upper limit value. In this case, the stop signal of the extruder 1 is transmitted from the PLC 40 to the inverter 42. Then, the inverter 42 gradually reduces the rotational speed of the drive motor 43 of the extruder 1 and the feed screw in accordance with the reduction amount of the rotational speed of the feed screw set in advance, and finally Stop it.

  In the hollow molding machine configured as described above, during normal operation, the molten resin having passed through the extruder 1 and the die head 2 is extruded from the die head 2 as a cylindrical parison and hangs down. The suspended cylindrical parison is accommodated in the open mold 3 and transferred to the position immediately below the compressed air blowing device 4 by the mold moving device 6 while being clamped by the clamping device 5. The blowing device 4 lowers the blowing nozzle 9 and drives the molding die 3 into the parison in the molding die 3. Compressed air is blown from the blowing nozzle 9 into the parison in the molding die 3 and an expanded parison is formed on the molding die 3 as shown in FIG. The molded product is formed by pressure contact with the omitted cavity. Then, while the molding die 3 is opened, it moves immediately below the die head 2 which is the original position, while the blow nozzle 9 suspends the molded article, and the molded article take-out device 7 takes out the molded article P from the blow nozzle 9. Take it out. The resin pressure upper limit and the number of rotations of the feed screw in the extrusion mode (normal extrusion mode) in the normal operation are, of course, different from those in the first extrusion mode in the storage unit of the control device 50. It is set in advance.

  Before performing such a normal operation, the above-mentioned first extrusion mode is automatically executed in the extruder 1.

  The processing procedure of the control device 50 for the execution of the above-mentioned first extrusion mode is as shown in the flowchart of FIG. That is, when the operator presses the extruder operation button (not shown) at the start of the process (step S1), the controller 50 shown in FIG. The set rotational speed is set to Nr (step S2). Then, the extruder 1 is started (step S3). The set number of revolutions Nr of the feed screw in the first extrusion mode is preset in the storage unit of the control device 50, and the details thereof will be described later.

  As described above, the resin pressure in the tip portion of the extruder 1 is detected by the resin pressure gauge 44 and taken into the control device 50 shown in FIG. Therefore, the control device 50 shown in FIG. 3 determines whether or not the measured value of the resin pressure at the tip of the extruder 1 is less than the resin pressure upper limit value Pu (step S4). The resin pressure upper limit value Pu is preset in the storage unit of the control device 50, and the details thereof will be described later. If "No" in step S4, that is, if the resin pressure measurement value is not less than the resin pressure upper limit value Pu, the resin pressure measurement value is no less than the resin pressure upper limit value Pu; A "Warning" message is displayed on the operation display screen 21, and screw rotation of the extruder 1 is stopped (step S5).

  Thus, when the measured value of the resin pressure is not less than the resin pressure upper limit value Pu, as described above, there is a high possibility that the resin flow path in the die head 2 is blocked by the unmelted resin. When the operation is continued, the internal pressure rises, and for example, the bolt for fastening the component members of the die head 2 may be extended, and the die head 2 may be damaged and a resin leak may occur. Therefore, when the measured value of the resin pressure is not less than the resin pressure upper limit value Pu, screw rotation of the extruder 1 is stopped.

  On the other hand, if the determination result in step S4 is "Yes", the determination in step S4 is repeated until the predetermined time t1 seconds have elapsed since the start of the extruder 1 (step S6). The details of the predetermined time tl seconds will be described later.

  When the process in step S6 is completed, the "completion" message and the "cancel" button of the first extrusion mode are displayed on the operation display screen 21 of the molding condition setting device 20 (step S7).

  Then, when the operator depresses the "release" button of the first extrusion mode, the condition at the time of normal operation, that is, the set number of rotations of the feed screw of the extruder 1 and the resin pressure upper limit of the tip of the extruder 1 The setting value in the extrusion mode (normal extrusion mode) in the normal operation is automatically shifted (step S8). It is needless to say that the setting values in the normal operation are also set in advance in the storage unit of the control device 50.

  The set number of revolutions Nr of the feed screw in the first extrusion mode in step S2 is calculated by the following equation (1) and stored in advance in the control device 50.

Nr = Nmin + α .... (1)
However, in the equation (1), Nr represents the set rotation speed of the feed screw when the first extrusion mode is performed, and Nmin represents the minimum rotation speed of the feed screw inherent to the corresponding hollow molding machine. In addition, α represents the number of rotations determined based on the type of synthetic resin material used for molding. The set number of revolutions Nr of the feed screw in the first extrusion mode is a value smaller than the set number of revolutions of the feed screw in the normal mode.

  Similarly, the resin pressure upper limit value Pu at the tip of the extruder 1 in the first extrusion mode is calculated by the following formula (2) and stored in the control device 50 in advance.

Pu = Pmax-P 'x td .... (2)
However, in Formula (2), Pu represents the resin pressure upper limit at the tip of the extruder 1 when executing the first extrusion mode, and Pmax represents the maximum withstand voltage of the die head 2. Further, P 'represents the increase rate of the resin pressure determined in advance, and td represents the deceleration time of the feed screw calculated by the following equation (3).

td = Nr / Nd '.... (3)
However, in Equation (3), Nr represents the set number of revolutions of the feed screw when executing the first extrusion mode as described above, and Nd ′ is the resin pressure at the tip of the extruder 1 as the resin pressure The amount of decrease in the number of rotations of the feed screw per unit time from when the feed screw starts to decelerate to immediately after the upper limit value is reached is represented. The value of this Nd 'is determined in advance by experiment or the like.

  The predetermined time t1 after the start of extrusion in step S6 satisfies the following formula (4), and is stored in advance in the control device 50.

t1> ts .... (4)
However, in the equation (4), tl represents a predetermined time after start-up of the extruder when the first extrusion mode is performed, and ts is until the resin pressure at the tip of the extruder 1 reaches the resin pressure upper limit value Pu. Represents the time calculated by the following equation (5).

ts = Pu / P '... (5)
However, in Formula (5), Pu represents the resin pressure upper limit mentioned above, and P 'represents the increase rate of the resin pressure previously defined similarly as mentioned previously.

  The graph shown in FIG. 5 shows the result of an experiment in which the resin was extruded from the extruder 1 in the resin unmelted state, and the ordinate represents the resin pressure and the number of revolutions, and the abscissa represents the time. . The measurement was performed using a screw diameter of 55 mm, and the die head was the same as the two-die die head 2 of FIG. Extruder 1 and die head 2 are filled beforehand with HDPE (high density polyethylene, MI = 0.35 g / 10 min, melting point 133 ° C) as a synthetic resin, temperature of extruder 1 is raised to 180 ° C and die head 2 is raised to 123 ° C. The rotational speed was 35 rpm, and the resin pressure upper limit was 35 MPa.

  In FIG. 5, the solid line indicates the transition of the measured resin pressure value, the dotted line indicates the transition of the screw rotation speed, and the broken line indicates the resin pressure upper limit value (35 MPa).

  First, as the extruder 1 is started, the screw rotation speed is increased at a predetermined acceleration until it reaches a predetermined set rotation speed (N1 → N2 in the drawing). In FIG. 5, only the numeral parts of the symbols such as N1, N2 .. etc. are indicated by circled symbols, and the same applies to the subsequent symbols such as P1, P2 .. etc.

  When the synthetic resin is fed into the die head 2 by the feed screw, the resin pressure measurement value increases a little later (P1 → P2).

  Since the synthetic resin inside the die head 2 is in the unmelted state, blocking by the unmelted resin occurs in the resin flow path inside the die head 2, and the measured resin pressure value continues to rise and reaches the resin pressure upper limit (P2 ).

  At this time, as shown in step S5 of FIG. 4, the stopping process of the feeding screw is disclosed, and the screw rotation speed is decelerated according to a predetermined deceleration (N2 → N3), but the feeding screw is also during this Because it is rotating, the resin pressure measurement value continues to increase exceeding the resin pressure upper limit (P2 → P3).

  Finally, after the screw rotation is completely stopped (N3), the resin pressure gradually decreases due to the backflow of the resin to the extruder 1 side (P3 → P4).

  As apparent from the graph of FIG. 5, it is possible to read the transition of the resin pressure measurement value under measurement conditions when the resin inside the die head 2 is not melted, and the setting rotation of the feed screw in the first extrusion mode It can be used to determine the number Nr, the resin pressure upper limit value Pu, and the time t1.

  In particular, each value can be determined more appropriately by the above-mentioned mathematical expressions (2) to (5) by obtaining the increasing speed P ′ of the measured value of the resin pressure.

  As described above, according to the hollow molding machine of the present embodiment, when the extruder 1 is first started after the temperature raising of the extruder 1 is completed, the extrusion mode in the normal operation (normal extrusion mode) is set. By automatically executing the first extrusion mode, it is possible to eliminate the cause of excessive resin pressure generation.

1 ... extrusion machine 2 ... die head 20 ... molding condition setting device 21 ... display screen for operation 40 ... programmable logic controller (PLC)
43: Extruder motor 44: Resin pressure gauge (resin pressure detection means)
50: Control device (control means)

Claims (5)

In a hollow molding machine equipped with an extruder which melts a synthetic resin and feeds it to a die head by means of a feed screw,
Control means for performing extrusion in the first extrusion mode prior to extrusion in the normal extrusion mode when starting up the extruder after the temperature rise of the extruder is completed,
In the first extrusion mode,
The number of revolutions of the feeding screw within a predetermined time after the start of extrusion of the synthetic resin by the extruder is limited to at least a number of revolutions lower than that of the normal extrusion mode.
A hollow molding machine characterized in that the resin pressure in the tip of the extruder is limited to a predetermined value or less. And a detection means for detecting the resin pressure in the tip of the extruder.
The resin pressure upper limit value in the tip of the extruder in the first extrusion mode is set in advance,
The hollow molding machine according to claim 1, wherein when the resin pressure detection value by the detection means reaches the resin pressure upper limit value, the rotation of the feed screw is stopped. The hollow molding machine according to claim 2, wherein the number of rotations of the feeding screw in the first extrusion mode is calculated by the following equation (1).
Nr = Nmin + α .... (1)
However, Nr is the number of rotations of the feeding screw when executing the first extrusion mode, Nmin is the minimum number of rotations of the feeding screw specific to the corresponding hollow molding machine, and α is based on the synthetic resin material used for molding It is the number of revolutions determined by The hollow molding machine according to claim 2 or 3, wherein the resin pressure upper limit value is calculated by the following equation (2).
Pu = Pmax-P 'x td .... (2)
However, Pu is the resin pressure upper limit value at the tip of the extruder when executing the first extrusion mode, Pmax is the maximum pressure resistance of the die head, P 'is the rate of increase in resin pressure determined in advance, td is the next It is the deceleration time of the said screw for a screw calculated by Numerical formula (3).
td = Nr / Nd '.... (3)
However, Nr is the number of revolutions of the feeding screw when executing the first extrusion mode, and Nd 'is that the feeding screw is decelerated immediately after the resin pressure at the tip of the extruder reaches the resin pressure upper limit value. The amount of decrease in the number of rotations of the feeding screw per unit time from the start to the stop. The hollow molding machine according to any one of claims 2 to 4, wherein the predetermined time after the start of the extrusion satisfies the following formula (4).
tl> ts .... (4)
However, tl is a predetermined time when executing the first extrusion mode, and ts is a time until the resin pressure upper limit value in the tip of the extruder is reached when the resin in the die head is not flowable. This time is calculated by the following equation (5).
ts = Pu / P '... (5)
Where Pu is the resin pressure upper limit value of the tip of the extruder when the first extrusion mode is performed, and P 'is the rate of increase of the resin pressure determined in advance.

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