JPH0511729B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for press molding thermoplastic resin. [Prior art] The press molding method for thermoplastic resin, in which a molded product is obtained by plasticizing and melting a thermoplastic resin, supplying it between molds, pressing and cooling it, has the following advantages compared to the general injection molding method. There are some characteristics. Molten resin is supplied between unclosed molds, and the pressure from mold clamping causes the mold surface to uniformly flow and shape, so molding can be performed with lower supply pressure and lower mold clamping pressure compared to injection molding. As well as being able to
Since the shear stress and orientation strain during the inter-cavity flow process is low, there is little deformation such as twisting or distortion of the molded product. In addition, since the pressure is applied in the thickness direction of the product, the filling properties of the ribs are good, and the uniform pressure during cooling prevents sink marks without leaving any distortion. However, in order to obtain a good product using this type of molding method, the method and timing of supplying molten resin between the upper and lower molds in the mold clamping method are important, and the present inventors have already reported that Publication No. 31929, JP-A-61-
We have filed applications such as Publication No. 22917. By this method, products with good appearance quality can be molded at low cost, and are already used in the production of automobile parts, home appliance parts, housing parts, office equipment parts, etc. The conventional methods described above include providing a hot nozzle such as a shut-off nozzle that can be opened and closed near the molten resin supply port, providing a switching valve in the middle of the resin passage, or providing a mechanism for relatively moving the supply port.
After the supply of molten resin is completed, the resin passage is closed to prevent backflow of molten resin when pressurized, to prevent resin from flowing out due to forward movement of resin during plasticization, and to prevent cooling delays of molded products near the supply port. This was done to prevent poor appearance due to plasticization and to prevent the resin from oozing out due to residual pressure in the resin passage after plasticization. [Problems to be solved by the invention] However, all of the above methods require means and mechanisms for opening and closing, which not only increases costs, but also takes time to change materials, and furthermore, It was difficult to measure the uniformity of temperature, and this often resulted in poor appearance near the supply port. The present invention provides a method for solving the practical problems of such conventional methods. [Means for Solving the Problems] The present invention is characterized in that upper and lower molding molds are attached to the upper and lower platens of a press machine that moves in the vertical direction, and the upper and lower molds are placed between the upper and lower molds. When the is not closed, the upper and
A thermoplastic resin press molding method in which a molded product is obtained by supplying molten resin from a resin plasticization supply device through a resin passage provided in the lower mold, completing mold closing, applying pressure, cooling, and opening the mold. After supplying the molten resin, without closing the resin passage, after the mold closing is completed and the pressurizing force reaches the set pressure, plasticization of the resin in the resin plasticization supply device is started,
This is a thermoplastic resin press molding method characterized in that the screw or plunger is forcibly retreated by an appropriate amount at the end of plasticization, and then mold opening is started. Furthermore, a more preferable method for carrying out the above method of the present invention is to stop or decelerate the closing operation of the mold midway and start supplying the molten resin when the cavity clearance of the upper and lower molds is 50 mm or less, and , when the thickness of the molded product is tmm, t+0.1mm or more (t
5.0mm) or t+1/2tmm or more (5.0>
When t1.0mm) or 1.5mm or more (t<1.0mm
In this method, the process ends when Hereinafter, the method of the present invention will be explained in detail with reference to the drawings. Figure 1 is a schematic diagram of the molding apparatus used in the present invention, and Figure 2 shows the steps of the molding method of the present invention.
FIG. 3 is a diagram illustrating the positional relationship between the upper mold and the screw in one molding cycle. In the drawings, 1 is a mold clamping device, 2 is an upper mold, 3 is a lower mold, 4 is an upper platen, 5 is a lower platen, 6 is a molten resin supply passage, 7 is a molten resin, 11 is a resin plasticization supply device, 12 is a screw. Upper mold 2. is descended from point a at high speed, and the descending speed is decelerated at point b. After receiving the position signal of point b and the delay timer times up, screw 12.
moves from the retracted position to the forward limit and transfers the molten resin to the lower mold 3. The resin is supplied between the upper and lower mold cavities through a resin passage 6 provided in the upper and lower mold cavities. Screw 1
2. Upon receiving the position signal, the upper mold 2. The screw 12 is increased in descending speed and lowered from point d to point e to pressurize the supplied molten resin 7. When the applied pressure reaches the set pressure, the screw 12 receives the signal and after the delay timer times up. ．． 12. The rotation of the resin material is started to start plasticizing the resin material. Screw 1
2. The resin material is sent forward while being plasticized, and then retracted until it reaches the bottom. After receiving the position signal of screw 12. Forced to retreat until the end. On the other hand, the upper mold 2. The above screw 12. After receiving the position signal and time-up of the delay timer, it starts to rise from point f and returns to its original position at point h. Here, the supply of molten resin starts when the cavity clearance between the upper and lower molds is 50 mm or less,
And when the thickness of the molded product is tmm, t+0.1mm
or more (when t5.0mm), or more than t+1/2tmm (when 5.0>t1.0mm), or more than 1.5mm (t
< 1.0mm) position of point b so that it ends at
It is preferable to set the lowering speed of the upper mold from point b to point d and the supply speed of molten resin from point to point. The reason for starting the supply of molten resin is when the upper and lower mold cavity clearances are narrow, less than 50 mm.
This is to supply the molten resin so that it comes into contact with the upper and lower mold cavity surfaces with an appropriate pressure. If it is wider than this, it will not come into contact with the upper and lower mold cavity surfaces with an appropriate pressure. If molten resin is supplied in this state, poor appearance such as fine wrinkles (hereinafter referred to as cold marks) will occur on the contact surface of the mold during cooling. The cold mark can be improved by increasing the mold temperature, but increasing the mold temperature requires a long cooling time, which is not preferable. In addition, the reason why the supply of molten resin is ended at the above clearance is to suppress the shear stress when the molten resin flows into the cavity during material supply to a low level.If the material is supplied with a clearance smaller than this value, shear stress As a result, deformations such as twisting and warping are likely to occur, and a larger supply pressure is required, which requires a supply device with a larger capacity. Furthermore, there is no practical effect of applying pressure in the thickness direction of the product.
Advantages such as rib filling properties are lost. The reason why the mold closing operation is stopped or decelerated midway is to ensure that the molten resin is supplied at the above-mentioned position. In the above explanation, an example was shown in which the lowering speed of the upper mold is increased and the upper mold is further lowered in response to a position signal indicating completion of molten resin supply, but it is also possible to lower the upper mold continuously. (hereinafter referred to as continuous method), this re-speed increase is not necessarily required. In addition, when speeding up again or temporarily stopping the upper mold to supply molten resin (hereinafter referred to as the "temporary stop method"), restarting the lowering of the upper mold is as described in this explanation.
This may be done by receiving a molten resin supply completion signal, or may be carried out independently of the molten resin supply by, for example, timing up a timer from the point b signal. However, in the temporary stop method, if there is a time lag between the screw reaching the point and the supply of molten resin ending and the upper mold starting to descend, the molten resin will temporarily stop spreading between the mold cavities, and the This is not preferable because ring-shaped unevenness (hereinafter referred to as ring mark) occurs. In the present invention, the resin is plasticized without closing the resin passage, but plasticization starts after the mold is closed and the pressurizing force reaches the set pressure, and the screw or plunger is forcibly retracted by an appropriate amount at the end of plasticization. , and by opening the mold after plasticization and forced retreat, it prevents backflow when pressurizing molten resin, prevents resin from flowing out due to forward movement of the material during plasticization, and prevents cooling delays near the supply port of the molded product. In addition, it is possible to prevent the resin from oozing out due to the residual pressure in the resin passage after plasticization is completed. vice versa,
Three conditions: plasticization is performed after the mold is closed and the pressurizing force reaches the set pressure, the screw or plunger is forcibly retracted by an appropriate amount at the end of plasticization, and the mold is opened after the plasticization and forced retraction are completed. If any of these conditions is not met, the above problem occurs. Among these three conditions, plasticization is first performed after the mold is closed and the pressurizing force reaches the set pressure, so that the screw or plunger is held in the forward position until the pressure rise is completed (Fig.
), to prevent backflow of molten resin when pressurized. Also,
Plasticization is started after the mold is closed and pressurized and completed before the mold is opened, thereby preventing resin from seeping out from the supply port of the molded product during plasticization or appearance defects due to delayed cooling in the vicinity of the supply port. Furthermore, by forcibly retracting the screw or plunger by an appropriate amount at the end of plasticization, and opening the mold after the plasticization and forced retraction, it is possible to prevent poor appearance near the gate due to the resin gushing out when the mold is opened, and to prevent the resin after the mold is opened. Prevents bulges. In the above description, an example has been shown in which the screw or plunger is forcibly retracted by an appropriate amount even before the start of plasticization, but this may be omitted depending on the case. Next, more specific examples and comparative examples will be described. [Examples 1 to 7] Using the apparatus illustrated in the drawings and the method of the present invention, polypropylene (PP), ABS resin (ABS), and polymethyl methacrylate resin (PMMA) were used.
Molded products with a diameter of 300 mm and thicknesses of 1.0, 2.0, and 5.0 mm were molded. In both cases, there is no backflow of molten resin during pressurization, no noticeable cooling delay near the molded product supply port during plasticization or after plasticization, and no resin leakage.
The obtained molded product was free from deformation and had a good surface appearance. [Comparative Examples 1 to 3] Except for starting plasticization before increasing the pressure (Comparative Example 1), or excluding forced retraction of the screw (Comparative Example 2), or starting mold opening during plasticizing. (Comparative Example 3) Molding was carried out in the same manner as in Example 3, except starting at (Comparative Example 3). In Comparative Example 1, there was a backflow of molten resin,
Sink marks occurred near the gate, and in Comparative Examples 2 and 3, resin oozed out when the mold was opened, resulting in poor appearance around the gate, and furthermore, resin oozed out even after the mold was opened. [Comparative Examples 4 and 5] Except that the cavity clearance at the start of supply of molten resin was as large as 55 mm (Comparative Example 4), or the cavity clearance at the end of supply was 2.
Example 3 except that it is small (comparative example 5)
Molding was carried out in the same manner as. Although there was no backflow of molten resin or seepage of resin in either case, the surface appearance of Comparative Example 4 was poor, and the molded product of Comparative Example 5 was highly twisted (deformed). The molding conditions and evaluation results of the molded products of the above Examples and Comparative Examples are summarized in Table 1.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a molding device used in the present invention;
FIG. 2 is a diagram illustrating the relationship between changes over time in the position of the upper mold and the position of the screw in the process of the molding method of the present invention. 1: Mold clamping device, 2: Upper mold, 3: Lower mold, 4:
Upper platen, 5: Lower platen, 6: Molten resin passage, 7: Molten resin, 11: Material plasticization, supply device, 12: Screw.

Claims (1)

[Claims] 1. Between the upper and lower molding molds that move in the vertical direction,
While the upper and lower molds are not closed, molten resin is supplied from the resin plasticization supply device through the resin passages provided in the upper and lower molds, and the molds are closed and pressurized.
In a thermoplastic resin press molding method in which a molded product is obtained by cooling and opening the mold, after supplying the molten resin, the mold is closed without closing the resin passage and the pressurizing force reaches the set pressure. , a method for press molding a thermoplastic resin, characterized in that plasticization of the resin in the resin plasticization supply device is started, and upon completion of plasticization, a screw or a plunger is forcibly retracted by an appropriate amount, and then opening of the mold is started. . 2 When the supply of the molten resin is started when the mold closing operation of the mold is stopped or decelerated midway and the cavity clearance of the upper and lower molds is 50 mm or less, and the thickness of the molded product is t mm. t+0.1mm or more (t
5.0mm) or t+1/2tmm or more (5.0>
When t1.0mm) or 1.5mm or more (t<1.0mm
The method according to claim 1, characterized in that the method is carried out in such a way that the process ends at the time of .