JPH0780226B2 - Injection compression molding method for ultra high molecular weight polyethylene - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is excellent in impact resistance, abrasion resistance, self-lubricating property, chemical resistance and the like, and is capable of performing highly accurate molding. Therefore, it relates to an injection molding method of ultra-high molecular weight polyethylene, which is recently being widely used for various machine parts, household products, etc., and particularly, in the case of multi-cavity molding, between moldings or molding shots. The present invention relates to an injection compression molding method for ultra high molecular weight polyethylene, which aims to eliminate variations in weight or shape.

[Conventional technology]

Regarding the injection molding method of ultra-high molecular weight polyethylene, conventional ultra-high molecular weight polyethylene was injected into a mold cavity with a volume 1.5 to 3.0 times the injection resin amount at a shear rate of 50,000 sec ^-1 (at the injection port). Then, a method of compressing the mold cavity volume so as to reduce the mold cavity volume to a volume smaller than the aforementioned volume by 2.0 times or less of the injection resin amount (Japanese Patent Publication No. 57-30067) or a large number of this method. Method for improvement (Japanese Patent Publication No. 60-58010)
It has been known.

The method of Japanese Patent Publication No. 57-30067 described above works well in single-piece injection molding, but when multi-piece injection molding is performed, variations in weight or shape between molded products may occur. The reason is that the resin remains in the resin flow path, and this resin is introduced into the cavity in a so-called powder form. Therefore, the method of Japanese Patent Publication No. 60-58010 solves it in a very complicated process. That is, the molten resin corresponding to the capacity is injected into the cavity of the volume of the molded product to be obtained through the resin flow path at an injection speed at which it becomes powdery, and the injection is completed immediately before the injection is completed. There is a pressure difference between the resin flow path and the cavity later, and the mold is momentarily opened to 1.2 times the cavity volume or more at any time until the resin does not solidify in the resin flow path. The resin in the resin flow path is introduced into the cavity in powder form, and then the cavity is compressed to its volume while the resin in the cavity is in a molten state.

[Problems to be Solved by the Invention]

In the conventional method, as described in Japanese Patent Publication No. Sho 60-58010, ultrahigh molecular weight polyethylene is filled in the mold cavity in a powder state, but it is necessary to be in the powder state until the filling is completed. In a multi-cavity mold, unless the resin pressure in the mold cavities is high to some extent, the cavities cannot be balanced and the resin weight varies from cavity to cavity.
In order to fill the cavity in the powder state, it is necessary to inject at a high shear rate until the filling is completed, but if the mold cavity volume remains fixed, the filling pressure will gradually increase with the filling, The filling speed decreases, and the powdery ultra high molecular weight polyethylene changes to a nodule,
This nodular resin causes delamination of the molded product. Therefore, in order to maintain the high-speed injection state until the completion of filling and to apply a certain amount of compression to the cavity, it is necessary to have a certain pressure difference before and after the gate until the completion of filling. For this purpose, the cavity volume should be strengthened during filling, and if the cavity volume is expanded while applying a certain amount of pressure, the purpose can be achieved without disturbing the balance of multi-cavity production. Japanese Patent Sho 60-58010
In the method of No. 3, there is a pressure difference between the resin flow path and the cavity after the injection is completed from the time immediately before the injection is completed,
Moreover, it is necessary to determine an arbitrary time until the resin does not solidify in the resin flow path. The time immediately before the end of injection is easily determined from the injection time, but the time when the resin is not solidified in the resin flow path depends on the melting point of the resin, the mold temperature, the injection time, and the conditions are constant. Then, after the injection is completed, it is necessary to repeat the experiment in which the time until the mold is opened and the cavity is expanded is repeated, and it is necessary to determine by observing in what state the resin was introduced into the cavity (Japanese Patent Publication Sho 60). -58010 gazette, column 6, lines 21-43).

Further, in the case of the method of Japanese Patent Publication No. 60-58010, it is necessary to experimentally find the time immediately before the injection is finished and the time when the resin does not solidify in the resin flow path by the extremely laborious method as described above. There is inconvenience.

The present inventor earnestly studied an injection molding method which eliminates the inconvenience of opening the mold at each time point and does not cause delamination even in a large number of molded products, and to complete the present invention. I arrived.

[Means for Solving the Problems]

That is, the present invention is characterized in that, in the injection compression molding method of ultra-high molecular weight polyethylene, the resin is injection molded while applying a pressing force from the back of the movable mold so as to keep the pressure in the mold cavity constant. The present invention is to propose a method for injection compression molding of ultra high molecular weight polyethylene.

Further, in the present invention, as a means for holding the molding pressure after the injection and injection of the resin into the mold cavity, the closing valve provided in the nozzle part of the injection machine is closed immediately after the injection, or the injection is performed. A method of holding by the screw of the cylinder can be adopted.

Furthermore, the pressure in the mold cavity at the time of injection is set so that a constant pressure can be maintained by a counterbalance valve in the hydraulic circuit to the pressure cylinder connected behind the movable mold, and at the time of resin injection. The object can be achieved by applying a constant back pressure to the movable mold so as to allow the mold cavity volume to expand.

The ultrahigh molecular weight polyethylene used in the present invention has a molecular weight much higher than that of ordinary polyethylene and is known as polyethylene which is difficult to melt-mold, and is obtained by, for example, Ziegler polymerization. When the intrinsic viscosity [η] measured in decalin at 135 ° C is 3 dl / g or more, especially 10 dl / g or more,
And the melt index (ASTM D 1238F M120) is 0.01g
Good results are obtained when applied to polyethylene of 10 min or less.

In the present invention, the temperature at the time of injection of ultra-high molecular weight polyethylene is not particularly limited as long as it is higher than the melting point of the polyethylene and lower than the decomposition temperature, but is generally 150 to 300 ° C., more preferably 170. Injection temperatures of ~ 240 ° C can be used.

[Operation] In the method of the present invention, in order to keep the pressure inside the mold cavity constant, a pressure preset by a compression cylinder is applied from the back of the movable mold, and a predetermined plasticized and melted state is injected from the injection cylinder side. By injecting a quantity of ultra high molecular weight polyethylene, the pressure inside the mold cavity increases as the resin is filled into the mold cavity.

The pressure of the injected resin pushes back the piston of the compression cylinder added together with the movable mold, and the pressure of the resin injected into the cavity is maintained at a substantially constant state and the resin is filled into the mold cavity. Will be.

Moreover, the pressure of the compression cylinder that acts on the movable mold during injection is balanced with the pressure of the resin in the cavity, and at the same time, the buffer function of the compression cylinder occurs in the movable mold, and as a result, each cavity of the multi-cavity The injected resin is evenly filled and maintains a uniform pressure.
It is molded without causing sink marks and deterioration of molding accuracy.

Further, the pressure of the resin injected and injected into the mold cavity is given by the screw in the injection cylinder, and when the resin is filled in the cavity, the advance of the screw is stopped as it is, or the nozzle is installed. By closing the shutoff valve and maintaining it until the resin solidifies,
It is possible to obtain a molded product in which delamination does not occur.

The pressure in the compression cylinder that is attached to the movable mold to secure the initial volume of the cavity can be set by a counter balance valve provided in a hydraulic circuit that operates the compression cylinder.

〔Example〕

 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic view of an apparatus for carrying out the injection compression molding method for ultrahigh molecular weight polyethylene according to the present invention, in which (1) is inserted into a cylinder (2) of an injection molding machine. A screw, (3) an injection cylinder for injecting ultra high molecular weight polyethylene by pressing the screw (1), (4) a closing valve (rotary valve), (5) a gate of a fixed mold (6) Nozzle part connected to the part (6 '),
(7) is a movable mold which forms a cavity (9) by the fixed mold (6) and the intermediate mold (8).

(10) is located behind the movable mold (7), and when the molten resin is injected and injected into the cavity (9), the movable mold (7) is maintained so that the cavity (9) can be kept at the initial volume. It is a compression cylinder for applying a compression force. (11) is a counterbalance valve for setting the compression force of the compression cylinder (10), that is, the pressing force that opposes the pressure of the resin in the cavity (9). It is provided in a hydraulic circuit connected from 13) to the piston side (10a) of the compression cylinder (10) via an electromagnetic switching valve (12). In order to secure the initial volume of the cavity (9) associated with the forward / backward movement of the movable mold (7),
It is preferable to restrict advance of the rod (10b) of the compression cylinder (10) or the movable die (7) by a mechanical stopper or limit switch (neither is shown). (14) is a relief valve.

(21) is a screw position detector for measuring the axial movement of the screw (1) by the injection cylinder (3), (22) is a stroke setting device of the screw (1), and (23) is a screw ( A comparator for comparing the stroke set value of 1) with the actual measurement value detected by the screw position detector (21), (24) a control device controlled by a signal from the comparator (23), ( 25) is a flow control valve for controlling the hydraulic oil supply amount from the hydraulic pressure source (26) to the injection cylinder (3) by the command of the control device (24), and (27) is the command of the control device (24) A shut-off valve drive device controlled by (28) is a relief valve.

Next, an injection molding method using the above apparatus will be described with reference to the sequence showing the movement of FIG.

Ultra high molecular weight polyethylene
340M "(Mitsui Petrochemical, melt index 0.01g /
An intrinsic viscosity η16.7 dl / g) of 10 min or less was used.

(1) Immediately before injection, the closing valve (4) is closed, and a predetermined amount of plasticized and melted ultra high molecular weight polyethylene is stored in the tip of the screw (1) in the cylinder (2).

(2) Mold closing is performed by a known means with the closing valve (4) closed.

(3) With the mold closed, the solenoid a of the switching valve (12) is excited, and hydraulic oil is supplied from the hydraulic oil supply source (13) to the piston side (10a) of the compression cylinder (10). ,
Set the cavity (9) to the initial volume.

(4) When the cavity (9) is set to the initial volume, the solenoid (a) of the switching valve (12) is deenergized.

(5) Then, in response to a command from the control device (24), the closing valve driving device (27) is driven to open the closing valve (4), and at the same time, the injection cylinder (3) is operated to move the useful resin into the cavity of the mold. (9) High speed injection with high pressure. Regarding the movement of the screw (1) by the injection cylinder (3), the flow rate control valve (25) and the relief valve (28) in the hydraulic oil supply passage are controlled.

The screw position detector (21) detects that the screw (1) has moved forward by the preset value of the stroke setting device (22) by the operation of the injection cylinder (3), and the comparator (23) detects it. As soon as the signal is sent to the control device (24), a command signal is sent from the control device (24) to the closing valve drive device (27), the flow control valve (25) and the relief valve (28), and the injection operation is started. Stop. At this time, the cavity (9) is filled with molten resin.

In this molten resin injection step, as the molten resin is filled in the cavity (9) through the nozzle (5), the internal pressure of the cavity (9) increases and the movable mold (7) is pushed back to move the cavity (9). Shows a tendency for the volume to expand.

However, the pressing force of the compression cylinder (10) acts on the back of the movable mold (7), and the compression cylinder (1
The pressing force of 0) tries to equilibrate with the injection pressure of the molten resin. The pressing force of the compression cylinder (10) on the movable mold (7) is set by the counter balance valve (11) provided in the hydraulic circuit, and the resin filled in the cavity (9) is used for molding. It is designed to maintain the required pressure.

Even if the mold has a large number of cavities, an external force is applied and balanced so that the internal pressure of the cavity (9) is maintained at a constant pressure. A uniform pressure is maintained inside the cavity (9).

Immediately after the required amount of molten resin is filled in all the cavities (9) of the mold, the closing valve (4) is closed, so that the resin flow in the mold portion (6 ') is also instantaneously interrupted. After that, the nodular molten resin does not enter the cavity (9).

(6) In the above state, the solenoid (a) of the switching valve (12) is excited, and working oil is added to the piston side (10a) of the compression cylinder (10) at the pressure set by the relief valve (14), The piston rod (10b) of the compression cylinder (10) is positively advanced to apply pressure to the molten resin filled in the mold cavity (9). By this operation, the amount of shrinkage of the molded product due to the temperature decrease when the resin is cooled and solidified is compensated by the operation of pressing the moving mold (7).

By operating in this way, it is possible to prevent sink marks from being formed in the molded product and to improve the dimensional accuracy.

Further, while this operation is being performed, since the flow passage of the nozzle (5) is closed by the closing valve (4), the screw (1) is retracted and rotated to move the next molten resin for injection. Is plasticized and weighed.

(7) After the resin in the mold cavity (9) is cooled and solidified, the mold is opened by a known means.

(8) After that, an eject operation for taking out the molded product is performed. At this time, the compression cylinder (10) can also function as an eject cylinder.

When the compression cylinder (10) is used as an eject cylinder, the solenoid (a) of the switching valve (12) is de-energized after the above operation, and the solenoid (b) is energized instead, so that the piston of the compression cylinder (10) is The rod (10b) is retracted to prepare for the next molding operation.

The ultra-high-molecular-weight polyethylene molded articles thus obtained all had a certain amount of weight, and a product which did not cause layered peeling even when cut off in appearance was obtained.

In the above embodiment, the pressure of the resin filled in the cavity is held by the "closing" operation of the closing valve (4) provided in the nozzle part of the injection molding machine. The same effect can be obtained by keeping the screw of (1) stopped at least at the position where the injection is completed.

〔The invention's effect〕

According to the method of the present invention, molding conditions can be set irrespective of the time immediately before the end of injection as in the prior art or the time when the resin is not solidified in the resin flow path. This is possible, and the time required for obtaining molding conditions is shortened. In addition, the quality of the molded products is good, and there are no variations in the molded products when a large number of them are taken, and it is possible to mold efficiently.

Further, according to the method of the present invention, when the eject cylinder equipped in the molding machine is diverted to the compression cylinder, it can be carried out by incorporating the counter balance valve in the conventional hydraulic circuit, so that no special device is required. Reasonably, injection molding of ultra high molecular weight polyethylene can be performed.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an apparatus for carrying out the injection compression molding method of the present invention, and FIG. 2 is a sequence diagram showing a molding operation. (1) ...... Screw (2) …… Injection cylinder (3) …… Injection cylinder (4) …… Occlusion valve, (5) …… Noble part (6) …… Fixed mold, (6 ′) ...... Gate part (7) …… Movable mold, (9) …… Cavity (10) …… Compression cylinder (11) …… Counter balance valve (12) …… Electromagnetic switching valve (13) …… Operating oil supply Source (14), (28) …… Relief valve (21) …… Screw position detector (22) …… Stroke setter (23) …… Comparator, (24) …… Control device (25) …… Flow rate Control valve, (26) …… hydraulic power source (27) …… blocking valve drive

Claims (5)

[Claims] 1. A method for injection compression molding of ultra high molecular weight polyethylene, characterized in that the resin is injection molded while applying a pressing force from the back of the movable mold so as to keep the pressure constant in the mold cavity. Injection compression molding method for ultra high molecular weight polyethylene. 2. The method for injection compression molding of ultra-high molecular weight polyethylene according to claim 1, wherein after the injection of the resin into the mold cavity, the molding pressure is maintained on the injection side. 3. After injection injection of resin into the mold cavity,
The injection compression molding method for ultrahigh molecular weight polyethylene according to claim 1 or 2, wherein a closing valve provided in the injection nozzle portion is closed. 4. After injection and injection of resin into the mold cavity,
The method of injection compression molding of ultrahigh molecular weight polyethylene according to claim 1 or 2, wherein the molding pressure is maintained by a screw of an injection cylinder. 5. The pressure in the mold cavity at the time of injection is set so that a constant pressure can be maintained by a counterbalance valve in a hydraulic circuit to a compression cylinder arranged behind the movable mold, and The method for injection compression molding of ultra-high molecular weight polyethylene according to claim 1, wherein a constant back pressure is applied to the movable mold so as to allow the mold cavity volume to expand when the resin is injected.