WO1988000116A1 - Process and system for injection molding of objects from thermoplastic material - Google Patents

Abstract

In the process described, in order to avoid surface grooves running along the joint lines (15), the temperature of the otherwise cooled injection mould (11) is increased by means of a heating device (19) in a spatially limited strip-like region following the joint line (15), except in the surface region of said joint line and for a period of time which starts at the opening of the mould (11) and ends when it is completely full. This way the formation of grooves at the joint lines can be avoided without increasing the duration of the injection cycle.

Description

 Method and arrangement for the injection molding of objects made of thermoplastic material

The invention relates to a method and an arrangement for the injection molding of objects made of thermoplastic material in an injection mold effecting weld lines on the object. Injection molds for objects with openings or differences in wall thickness, as well as injection molds with several gate openings intended for filling, create several flow streams of the melt during mold filling, which collide with their flow flow fronts. Since the flow stream fronts of the flow streams, which are normally heated far above the melting temperature, can cool prematurely on the injection mold which is cooled to shorten the production cycle times, notches are formed along the weld lines on the finished item, which notches have the appearance and the mechanical properties of the item affect.

In the case of objects that are subject to high quality requirements, the weld line notches have so far been removed by subsequent processing, such as polishing or painting.

From US Pat. Nos. 2,182,289 and 2,360,023, it is known to improve the appearance and strength of weld lines of injection-molded articles by the fact that the injection mold, cooled to a temperature considerably below the melting temperature of the thermoplastic material, in the Area of the weld lines is heated, for example, by electrical resistance heating cartridges. However, the temperature to which these areas are heated is also considerably below the melting temperature in order not to prolong the cooling phase and not to impair the appearance and the molded part properties too much. The heating devices are constantly switched on in the known method. In a clock-controlled injection molding machine, the known method undesirably leads to a considerable increase in the cycle time.

DE-OS 1 938 496 discloses orientations and to prevent residual stresses in injection molded parts made of thermoplastic material by the injection mold being alternately heated and cooled again in the cycle cycle of the injection molding machine. The injection mold contains channels in its mold wall, through which hot oil or water vapor is pumped to heat the mold, while a coolant flows through the channels for cooling. During the heating phase, the injection mold is at least partially heated to a temperature which is considerably above the melting temperature. The considerable heating of the injection mold leads to a substantial increase in the cycle time of cyclically operating injection molding machines.

From DE-OS 2 151 697 it is also known that

To at least partially coat the mold surface of injection molds with a thermally insulating plastic material. The insulation layer leads to a brief increase in the surface temperature, as a result of which the surface gloss of the object is increased. It has been shown that this method is only of limited use for avoiding weld line notches, since the temperature of the injection mold must also be increased, which undesirably increases the cycle time.

Finally, it is known from FR-A-2 386 957, AT-C-183955 and JP-A-58-134 722 that the melt injected into the injection mold is additionally produced by microwave heating, high-frequency heating or ultrasonic heating to warm up.

It is an object of the invention to provide an injection molding process according to which articles made of thermoplastic material can be produced with a comparatively short cycle time, without visible seam seam notches appearing on the surface. It is also the task of the inventors fertilizing to provide an arrangement that makes it possible to produce articles made of thermoplastic material free of weld line notches with a comparatively short cycle time.

The invention presented in the claims takes advantage of the fact that, in the case of cyclically operating injection molding machines, there is sufficient time between opening and ejecting a currently manufactured object, on the one hand, and completely filling the closed injection mold in the next injection cycle, on the other To heat the mold surface to a contact temperature that is only slightly less than the melting temperature of the thermoplastic material. The contact temperature is to be understood here and in the following as the temperature that the mold surface has when it comes into initial contact with the injected thermoplastic material. In the case of amorphous thermoplastic material, the melting temperature is to be understood as the softening temperature and in the case of partially crystalline thermoplastic material the crystallite melting temperature. Since the area to be heated is limited to a comparatively narrow strip of the mold surface along the weld line of a few millimeters on both sides of the weld line, the additional amount of heat to be removed from the surface area of the weld line during the holding and cooling phase is comparatively low, so that there is no noticeable increase in the cycle time overall.

The method according to the invention is best carried out on a clock-controlled or clock-controlled injection molding machine which plasticizes the thermoplastic material, injects it under pressure into the injection mold and opens the injection mold for the removal of the object and then closes it again. Which supports the strip The area of the mold surface of the injection mold in the area of the weld line which heats up can then be controlled by a heating time control which is synchronized with the machine control or regulation of the injection molding machine. The heating device controlled by the heating time control must be designed in such a way that it has heated the strip-shaped area of the mold surface adjacent to the weld line to the desired contact temperature in the area of the melting temperature at the latest at a time when the two have

The flow streams of the melt forming the weld line meet and the compression phase has not yet been completed, so that thermoplastic material can flow into the injection mold to compensate for the weld line notch. It has proven to be expedient if the heating device is switched off as soon as the volumetric filling of the injection mold is reached.

In order to keep the amount of heat supplied as low as possible, the aim is for the heating device to provide one

Surface area of the weld line, if possible, adjacent area of the mold surface or of the thermoplastic material is heated. The heating device expediently comprises elongated, electrical resistance heating cartridges which are installed as close as possible under the surface of the mold in the wall of the injection mold, in such a way that they essentially extend along the weld line. On the one hand, the distance between the heating cartridge and the mold surface should be as small as possible, on the other hand, however, the remaining material cross section of the wall cannot be made arbitrarily small as a result of the high pressures which occur during the filling of the mold. A distance of approximately 2 mm between the mold surface and the heating cartridge has proven to be suitable.

The shape geometry and in particular the position of the cooling necessary cooling channels and the like make it impossible in some applications to install the heating cartridge just below the mold surface in the mold wall. Since an increase in the distance between the heating cartridge and the mold surface increases the heat capacity and thus the cycle time, it is provided in a preferred embodiment that the heating device, which is preferably in the form of an electrical resistance heating cartridge, does not directly heat the wall of the injection mold, but instead a bridge part made of a material that is more thermally conductive than the mold wall. The heating device can thus be installed at a comparatively large distance from the area of the mold surface following the strip seam, and the bridge part transfers the heat to this area. The bridge part expediently consists of Berylliu copper. The use of a bridge part made of a material which is a good conductor of heat also has the advantage that its end adjacent the weld line can easily be adapted to a more complicated shape of the weld line course, so that the heating power can be brought close to the mold surface even with curved mold surfaces . In addition, the bridge part easily absorbs the high pressure forces.

In order to keep the influence of the heating device on the cycle time as low as possible, and last but not least to prevent overloading of the heating devices, the heating time control expediently comprises a temperature control circuit which uses a temperature sensor to control the temperature in the vicinity of the mold surface to be heated detected. The temperature sensor is preferably part of the heating cartridge.

The area of the mold tending to weld lines not only can be heated by the fact that the area adjacent to it region of the mold wall is heated to a contact temperature slightly below the melting temperature, but heating devices can also be used which directly heat the melt in the bonding area. High-frequency or microwave sources radiating in strips are suitable. However, strip-shaped ultrasound sources are particularly suitable, since the heat capacity of the material to be heated means that the mold wall can be kept very small after the mold wall does not have to be heated as well. Due to the particularly short heating time, the latter type of heating the area of the weld line can also be used in methods and arrangements in which the contact temperature of the melt in the area of the weld line is raised to values which are considerably higher than the melting temperature, ie the Softening or crystallite melting temperature. This also considerably reduces the inhomogeneity of the plastic in the area of the weld line and thus improves the mechanical properties (eg strength).

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. It shows:

1 shows a schematic illustration of an injection molding machine on which the method according to the invention can be carried out; 2 shows a plan view of a mold half of an injection mold that can be used in the context of the invention;

Fig. 3 is a sectional view through the injection mold, seen along a line III-III in Fig. 2; Fig. 4 is a sectional view of a variant of an

Injection mold with bridge part and usable within the scope of the invention

5 is a sectional view through the injection mold, seen along a line VV in FIG. 4.

1 schematically shows a conventional injection molding machine 3 equipped with a screw piston 1, which, controlled by a machine controller 5 in a cycle, plasticizes thermoplastic material by heating and compression and injects it via a sprue opening 7 into a mold interior of an injection mold, generally designated 11 . The injection molding machine 3 heats the thermoplastic material to a temperature which is considerably above the melting temperature, i.e. in the case of amorphous materials via the softening temperature or in the case of partially crystalline materials via the crystallite melting temperature. The walls of the injection mold 11 contain a system of cooling channels 13 through which coolant circulates, which constantly keeps the walls of the injection mold at a temperature below the melting temperature in order to allow the injection-molded object to solidify and demold. The machine controller 5 not only controls the filling phase, but also the compression phase that follows until the injection mold 11 is completely filled, and the subsequent holding pressure phase of the injection molding machine 3. The machine controller 5 also controls a mold-closing unit (not shown) which controls the Injection mold 11 opens and closes again and, if necessary, ejects the molded object.

The injection mold 11 can form a breakthrough or the like in the object to be molded

Contain mold core 14, which divides the melt flowing into the injection mold 11 via the sprue opening 7 into two partial streams, which reunite after the mold core 14 to form a weld line indicated at 15. Such weld lines also occur in others

Shape configurations, for example when filling over several sprue openings or in the case of step-like changes in the wall thickness of the object to be cast. In FIG. 1, equidistant flow fronts of the melt flows are drawn in at 17 for better explanation. The field of flow fronts clearly shows that the through the

Mold core 14 divided partial flows in the region of the weld line 15 to each other and stretched in the tangential direction and sheared in the radial direction. In the areas of the weld line adjoining the inner surface of the mold, this can lead to an elongated surface notch on the object following the weld line 15. Such weld line notches are undesirable because they affect the appearance and quality of the injection molded article.

In order to avoid weld line notches of this type, the injection mold is provided with heating devices 19, which make it possible to heat the weld line for a limited time only in the area of a narrow strip of the mold surface projecting only a few millimeters above the weld line. The heating device 19 and the strip-shaped region which it heats in a substantially congruent manner in FIG. 1 extends essentially over the entire length of the weld line 15.

A heating time control synchronized with the machine control 5 switches on the heating device 19 at least for part of a time interval in each cycle of the injection molding machine 3, which begins with the opening of the injection mold 11 for the demoulding of an object and with complete filling of the injection mold 11, i.e. the end of the compression phase or the beginning of the holding and cooling phase. During this period, the heating time control 21 keeps the temperature in the surface area of the weld line at one

Temperature setting member 23 adjustable value. The fact- Real temperature in the surface area of the weld line is detected by means of a temperature sensor 25. The heating time control 21 can only respond to the opening and closing of the injection mold 11. Expediently, however, adjustable timing elements are provided via adjusting elements 27, which are triggered when the injection mold 11 is opened and which define a predetermined switch-off delay for the heating device 19.

The heating time control 21 heats the surface area of the weld line in the time interval between the opening of the injection mold 11 and the end of the compression phase of the next injection molding cycle to a contact temperature which, in the case of amorphous thermoplastic material, is only a little, preferably about 5 C below the softening temperature in the case of partially crystalline thermoplastic material, it is only slightly, preferably about 10 C, below the crystallite melting temperature. Surface notches along the weld line are completely avoided in this way. Since the area to be heated is spatially limited to the surface area of the weld line, the heat capacity is comparatively low, so that the cooling phase is not extended accordingly. Since the heating already begins with the opening of the injection mold 11, a period of time already provided in the cycle is used for the heating. Overall, the cycle time is therefore not increased or is increased only insignificantly.

The heating device 19 can be a strip-type radiating high-frequency source or a strip-type radiating microwave source. In particular, however, it can be a strip-shaped ultra-sound source, which enables it to be particularly narrow

To heat areas in a particularly short time so that if necessary, cycle temperatures above the melting temperature can also be achieved in the areas of the weld line near the surface.

Injection molds which can be used in conjunction with the machine according to FIG. 1 and which have elongated, electrical resistance heating cartridges as the heating device are explained below. Parts having the same effect are provided with the reference numerals of FIG. 1 and to distinguish them with a letter. For further explanation, reference is therefore made to the description of FIG. 1.

2 and 3 show an injection mold 11a which contains a bore 35 in its wall 31 at a close distance 33 of, for example, 2 mm below the inner mold surface 9a delimiting the weld line 15a, in which an electrical, elongated, cylindrical. stand heating cartridge 19a sits. The heating cartridge 19a extends essentially over the entire length of the weld line 15a and is exchangeably screwed into the bore 35. It carries the temperature sensor 25a of the heating time control from FIG. 1 at its tip. The diameter of the heating cartridge 19a is comparatively small in order to keep the material volume of the wall 31 to be heated as low as possible. Cartridges with a diameter of a few millimeters are suitable.

The position of the weld line relative to the injection mold can in some cases make it impossible to install the resistance heating cartridge directly adjacent to the surface of the weld line. This can be the case, for example, if the weld line area is not accessible in its longitudinal direction from an outer surface of the mold, or if the location most suitable for the installation already contains other structural components of the mold, for example cooling channels. In the injection mold 11b of FIG. 4 and 5 heats an electrical resistance heating cartridge

19b does not directly adjoin the area of a mold wall 41 of the injection mold 11b that is adjacent to the weld line, but via a bridge part 43 made of a material that is better conductive than the mold wall material, for example beryllium copper. The bridge part 43 is seated in a recess 45 of the mold wall 41, which extends to close to the inner mold surface 9b in the region of the weld line 15b heraner- ^'. In order not to endanger the mechanical stability of the injection mold, the cutout 45 also ends here with a small distance of, for example, 2 mm from the interior of the mold. The heating cartridge 19b sits at a large distance in one. suitably arranged bore 47 of the ^'bridge portion 43 and the mold wall 41. In order to weld line 15b toward the bridge portion 43 is tapered in a wedge shape so that only a narrow along the weld line 15b er¬ stretching the strips in the region of the molding surface 9b er¬ is heated. The end of the bridge part 43 which is adjacent to the weld line 15b can also be adapted to curved weld lines without problems and in particular this end can follow the inner mold surface at a constant distance, even if it should itself be curved. For better temperature monitoring, the temperature sensor 25b is located in the bridge part 43 in the vicinity of the conical outlet. However, similar to the temperature sensor 25a from FIG. 3, it can also be part of the heating cartridge 19b.

Claims

P a t e n t a n s r u c h e Process for the injection molding of objects made of thermoplastic material in an injection mold effecting weld lines on the object, in which the injection mold is cooled to a temperature lower than the melting temperature and the thermoplastic material is heated to a temperature higher than its melting temperature , in which the warmed thermoplastic material is repeatedly injected into the cooled injection mold until the injection mold is completely filled, is left in the injection mold until it cools down to a temperature lower than the melting temperature and the object is removed from the injection mold and in which the otherwise cooled injection mold is heated in the area of the weld line, characterized in that the injection mold is located in a strip-shaped area of the area adjacent to the weld line which essentially follows the course of the weld line n mold surface at least during part of the time interval between the removal of the object and the complete -A - the filling of the injection mold for the production of the next article is heated for a limited time to a temperature which is initially only slightly less than the melting temperature of the thermoplastic material upon contact with the injected thermoplastic material. 2. The method according to claim 1, characterized in that the region of the mold surface which is essentially limited to the area of the weld line is heated to a temperature with initial contact of the mold surface with the thermoplastic material for amorphous thermoplastic materials at about 5 ° C is below the softening temperature and, for partially crystalline thermoplastic materials, is 1100 ° C. below the crystallite melting temperature. 3. Arrangement for the injection molding of objects made of thermoplastic material, with an injection mold (11) effecting weld lines on the object to be produced and having cooling devices, with an injection molding machine (3) which is cyclically controlled by a machine control (5) and to which the injection mold is attached (11) is connected and which plasticizes the thermoplastic material, injects it under pressure into the injection mold (11) and opens and closes the injection mold (11) for the removal of the object and with heating devices (19) on the injection mold for heating the mold surface in the area of the weld line, characterized in that the heating device (19) is designed in such a way that it locally delimits a strip-shaped area which essentially follows the course of the weld line (15) and which adjoins the weld line (15) adjacent mold surface is heated and that the heating device (19) is connected to the mas machine control (5) synchronized heating time control (21) can be controlled, which the heating device (19) at least during part of each beginning with the opening of the injection mold (11) and ending with the complete filling of the injection mold (11) by the machine control (5) set time interval. 4. Arrangement according to claim 3, characterized in that the heating time control (21) has a temperature control circuit (23, 25) which detects the temperature in the vicinity of the surface to be heated with a temperature sensor (25) and maintains a predetermined setpoint . 5. Arrangement according to claim 4, characterized in that the heating device has at least one elongated electrical resistance heating cartridge (19a), which lies along the weld line (15a) closely below the strip-shaped area of the mold surface substantially over the entire length of the weld line (15a ) stretches and that the temperature sensor (25a) is installed in the resistance heating cartridge (19a). 6. Arrangement according to claim 3 or 4, characterized in that in a the strip-shaped area A mold part of the injection mold (11b) forming a mold surface, a bridge part (43) made of a material that is better than the mold wall (41), which connects the strip-shaped area of the mold surface with the heating device (19b) located away from the mold surface. 7. Arrangement according to claim 6, characterized in that the end of the bridge part (43) adjacent to the mold surface is one of the weld lines (15b) following and protrudes only a few millimeters transversely to the weld line.

has surface shape. 8. Arrangement according to claim 6 or 7, characterized gekennzeich¬ net that the strip-shaped region of the mold surface adjacent end of the bridge part (43) follows with a substantially constant distance from the shape of the object determining the mold surface. 9. Arrangement according to claim 3 or 4, characterized in that the heating device (19) is designed as a strip-shaped radiating ultrasound source or strip-shaped radiating high-frequency source or strip-shaped radiating microwave source, the radiation for heating the injected thermoplastic material from the strip-shaped area of the mold surface forth into the injection mold (11).