JPH0750190Y2 - Parison for injection blow molding of bellows products - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a parison used for injection blow molding. More specifically, the present invention relates to a parison used in injection blow molding of bellows products.

(Prior Art) Conventionally, blow molding is often used to form a bellows product from a synthetic resin. However, according to blow molding, since the peak portion of the bellows has a higher blow-up ratio than the valley portion, the wall thickness of the peak becomes extremely smaller than the wall thickness of the entire molded product. For this reason, when using this blow-molded bellows product as an actuator, the wall thickness of the mountain part becomes thin, and the mountain part buckles due to negative pressure, resulting in a decrease in stroke. It will disappear. Further, when it is used as a duct for an air conditioner or the like, if the wall thickness of the mountain portion becomes thin, the heat insulating effect deteriorates. Therefore, if the wall thickness of the parison is increased to make the mountain portion have an appropriate wall thickness, the wall thickness of other portions becomes unnecessarily large, the material is wasted, and the original flexibility of the bellows is impaired. I will end up.

It is also conceivable to form the bellows by injection blow molding. This injection blow molding is a method in which a molten resin is injected into a mold to mold a tubular parison with a bottom, and this parison is surrounded by a blow mold and expanded to obtain a final shape. Therefore, the material to be used is preferably a fluid material having low viscosity in the injection step, but is preferably a material having high viscosity in the blowing step, which is a trade-off. Therefore, the material is usually selected according to the blow process, but the material is difficult to flow in the injection process. For this reason, conventionally, it has been common knowledge that the parison is formed into a tube having a uniform thickness and a low flow resistance. Then, the parison is entirely expanded along the bellows-shaped cavity formed in the blow molding outer die to form the bellows-shaped body.

However, when a bellows product is made using a tubular parison having a uniform wall thickness, the blow-up ratio is locally different in the blowing process, resulting in a product having a significantly different wall thickness. In particular, in the case of a conical bellows product whose diameter gradually increases, for example, in the case of boots for constant velocity joints used in automobiles and the like, even if an attempt is made to form a parison into a taper shape that spreads toward the end in order to make the thickness of the final product uniform,
Since it becomes close to the open-pressure molding, the gas component in the resin appears as bubbles, or the flow becomes complicated and the internal resistance is likely to remain, which causes problems such as sink marks, bubbles, and voids. Further, when molding a long bellows product, it becomes close to the open pressure molding, and there is a problem that bubbles, voids, sink marks, etc. occur in the parison. For this reason, conventionally, it has been considered that the injection blow molding method cannot produce a bellows product having a uniform wall thickness, in particular, a conical bellows product having a divergently increasing diameter.

Therefore, in order to solve this problem of uneven thickness, first,
No. 34211 proposes a technique for making the wall thickness of the parison itself non-uniform.

(Problems to be solved by the invention) However, blow molding is a method of forming a desired product shape by expanding the parison while reducing the wall thickness, so it may occur due to uneven distribution of blow pressure or variation in resin temperature. The thickness of molded products tends to vary, and it is difficult to form a uniform thickness only by controlling the thickness of the parison. In particular, in the case of a bellows product having a complicated molding shape, in particular, in the case of a boot for a constant velocity joint in which a bellows is formed in a conical shape, the angle of the inclined surface of the bellows part is asymmetric, so that it is not possible to uniformly inflate it. It is difficult and easily accompanied by variations in wall thickness. Generally, when the parison is in the shape of a cone, it expands before the surface with a steeper angle, and when the angle is looser, it expands later, so that it is on the left and right of two slopes that form one peak or valley of the bellows. There was a phenomenon of different wall thickness.

The present invention aims to provide a synthetic resin bellows product having a uniform wall thickness. More specifically, it is an object of the present invention to provide a parison capable of molding a synthetic resin bellows product having a uniform thickness by injection blow molding.

(Means for Solving the Problems) In order to achieve such an object, the parison for injection blow molding of the bellows product of the present invention has a bellows-shaped different diameter composed of a peak portion of a pressure wall and a valley portion thinner than this. The mountain portion is formed and the thin portion is provided on the mountain portion.

Further, the thin portion of the parison of the present invention is characterized by being a circumferential groove formed on the inner peripheral surface of the parison.

Further, the thin portion of the parison of the present invention is arranged on a line connecting the center of the mountain portion of the parison and the center of the mountain portion of the blow molding outer mold.

Here, the groove forming the thin portion is not particularly limited in shape, and may have any shape such as a semicircle, a triangle, or a quadrangle, and preferably connects the core and the corresponding mountain portion of the blow molding outer die. This is the adoption of an axisymmetric groove shape with the line as the axis of symmetry.

(Operation) Therefore, the blow air that flows between the parison and the core during blow molding flows into the groove of the thin portion of the mountain portion of the parison and concentrates the air blow pressure on this thin portion.
Therefore, this thin portion first begins to expand. After that, since it expands in the left-right direction, the wall thickness on the left and right becomes even from the peak to the valley. Especially when there is a thin-walled part on the line connecting the center of the crest of the parison, that is, the center of the crest of the core and the center of the crest of the blow molding outer mold, direct it toward the corresponding crest of the outer mold for blow molding. This is effective because the thin-walled part begins to expand first.

(Embodiment) Hereinafter, the configuration of the present invention will be described in detail based on an embodiment shown in the drawings. The present embodiment is applied to boots for constant velocity joints used in automobiles.

FIG. 1 shows an example of the cross-sectional shape of the parison of the present invention. The parison 1 is formed of a thermoplastic resin into a conical cylindrical shape with a bottom that is similar to the product shape 6, and a conical surface portion 5 thereof is formed with a bellows-shaped different diameter portion 2. Here, the conical cylindrical body shape that is similar to the product shape 6 is, for example, 0.1 ≦ (D−d) / 2L D: the large end side diameter of the conical surface portion, d: the small end side diameter of the conical surface portion, L: It is preferable to have a taper of at least the entire length of the conical surface portion.

The different diameter portion 2 has a pitch P of irregularities (valley portion 4 and mountain portion 3).
Is approximated to the pitch P ′ of the product shape, and the wall thickness Ti of the crest portion 3 is formed thick and the wall thickness ti of the valley portion 4 is formed thin. It is preferable that the wall thickness Ti of the crest portion 3 is a thickness that gives a variation in wall thickness that can be a flow resistance between the crest portion 3 and the valley portion 4. For example, it is formed so as to maintain a thickness relationship of 1.25ti ≦ Ti ≦ 5ti. There is. In the case of Ti <1.25ti, the difference in wall thickness becomes too small to be the same as the conventional one, resulting in sink marks, and in the case of Ti> 5ti,
This is because the difference in wall thickness becomes too large and bubbles may be generated therein. This difference in wall thickness is not related to the pitch P of the unevenness, and the flow resistance due to bending of the different diameter portion 2 increases as the pitch decreases, but the thickness difference may be small, but the flow resistance due to bending increases as the pitch increases. Since it decreases, it is necessary to increase the wall thickness difference. Also, the thickness ti of the thin portion 4
If is too thin, the flowability of the resin is impaired and defects occur, so it is necessary to secure a minimum wall thickness that does not hinder the flow. Therefore, considering the relationship between the pitch and the minimum wall thickness, the wall thickness Ti, ti
Should be selected.

Further, the difference in diameter between the crests 3 and the valleys 4, that is, the step h (see FIG. 1) is preferably close to the product shape 6 in order to keep the blow-up ratio of the crests 3 as small as possible.
On the other hand, since it must be taken into consideration that the flow of resin during injection is obstructed, a value is selected so that the resin material is in a state of gentle bending compared to the product shape.

Further, the mountain portion 3 is provided with a thin portion 7. More preferably, the thin portion 7 is arranged on a line connecting the center of the mountain portion 3 of the parison 1 and the center of the corresponding mountain portion of the blow mold outer mold (not shown). The thin portion 7 is formed, for example, by forming a groove 8 in the circumferential direction on the inner peripheral surface of the parison 1. The cross-sectional shape of the groove 8 is not particularly limited and may be any shape such as a semi-circle as shown in FIG. 2 or a triangle or a quadrangle as shown in FIG.
Preferably, a line-symmetrical groove shape with a line connecting the peak portion of the core die and the corresponding peak portion of the blow molding outer die as the axis of symmetry is adopted.

That is, a semicircular ridge 9 is circumferentially formed in the mountain portion of the core mold 11 forming the parison 1, preferably at the center of the mountain portion of the outer die for injection molding corresponding to the center of the mountain portion. Is formed on a line O connecting the two.

Further, it is preferable that the difference h between the crest 3 and the trough 4 of the parison is close to the product shape 6, that is, large in order to keep the blow-up ratio of the crest 3 as small as possible. Since it has to be taken into consideration that the flow of the resin is obstructed, a value is selected so that the resin material is in a gently bent state as compared with the product shape.

The parison configured as described above and the conical bellows product molded using the parison are molded as follows.

For example, a core mold 11 is placed in a cavity mold 10 that is a split mold.
After installing, close the cavity mold 10 and the neck mold 12. Then, the parison 1 is molded by injecting molten resin around the core mold 11 from the small end side [FIG. 4 (A)]. At this time. Cavity mold 10 and core mold 11
In order to form the bellows-shaped different diameter portion 2 which is close to the product shape and the peak portion 3 and the valley portion 4 which give the predetermined thickness difference, the irregularities 13A, 13B and the groove 8 having a predetermined depth and shape are formed. For example, a semicircular ridge 9 is formed. After the injection molding, the cavity mold 10 is opened, and the parison 1 is taken out with the core mold 11 being supported [Fig. 4 (B)]. Then, the mold is transferred to the blow mold 14 [Fig. 4 (C)], the blow mold 14 is closed, the air blow-in port 15 provided in the core mold 11 is opened, and air is blown in for molding [Fig. 4]. (D)]. At this time, since the peak portion 3 is thicker than the valley portion 4 and is hard to cool and retains heat, the fluidity is higher than that of the valley portion 4 and is easily extended, so that the blow ratio is increased. Moreover, the blow air that flows between the parison 1 and the core mold 11 during blow molding flows into the groove 8 of the thin portion 7 of the mountain portion 3 of the parison 1 to concentrate the air blow pressure on the thin portion 7. Let Therefore, this thin portion 7
First begins to expand. After that, since it expands in the left-right direction, the wall thickness on the left and right becomes even from the peak portion 3 to the valley portion 4.
In particular, when the thin-walled portion 7 exists on the line connecting the center of the mountain portion 3 of the parison 1, that is, the center of the mountain portion of the core mold 11 and the center of the corresponding mountain portion of the blow molding outer mold 14, the blow molding outer metal Mold 14
This is effective because the thin-walled portion 7 first starts to expand toward the corresponding mountain portion of. Therefore, the shape of the product as the final shape after the blow molding has a thickness difference smaller than that of the parison shape, and is substantially uniform.

After cooling, the neck mold 12 and the blow molding outer mold 14 are opened, and the molded product 16 is taken out [Fig. 4 (E)].

The above-described embodiment is an example of a preferred embodiment of the present invention, but the present invention is not limited to this, and various modifications can be made without departing from the scope of the present invention. For example, it can be applied to a parison for molding a bellows product having a tubular shape as a whole, such as a steering boot. Further, in the present embodiment, the groove forming the thin portion is formed on the inner peripheral surface side of the parison, but it is not particularly limited to this, and a groove, a depression, or the like is formed on the outer peripheral surface side as necessary. You may do it.

(Effect of Device) As is clear from the above explanation, the parison of the present invention is
Since the bellows-shaped different-diameter portion including the thick peak portion and the valley portion thinner than this is formed in accordance with the product shape and the thin portion is provided in the peak portion, the blow ratio of the peak portion is set. Can be made smaller than the conventional tubular parison, and it is possible to prevent the mountain portion from being extremely thin and to concentrate the air blow pressure on this thin portion to start expansion from the thin portion.

Therefore, since the parison expands straight in the radial direction and then expands in the left-right direction, the wall thickness on the left and right is even from the peak to the valley, and a bellows product with a substantially uniform wall thickness can be obtained. You can Especially when there is a thin-walled part on the line connecting the center of the crest of the parison, that is, the center of the crest of the core and the center of the crest of the blow molding outer mold, direct it toward the corresponding crest of the outer mold for blow molding. Since the thin-walled part first begins to expand, it is effective for making the wall thickness uniform.

Further, according to the present invention, since the bellows-shaped different diameter portion is provided, the turbulence of the resin flow and the generation of bubbles caused by the near-opening pressure molding near the divergent conical surface portion are continuously bellows-shaped. It is possible to form a conical bellows-shaped parison without sink marks, air bubbles, and voids, by applying appropriate flow resistance by means of flexible bending to eliminate it.

[Brief description of drawings]

FIG. 1 is a cross-sectional view schematically showing an embodiment of a parison for injection blow molding of a bellows product of the present invention, for a conical bellows parison. FIG. 2 is an enlarged sectional view showing a main part of the parison of FIG. FIG. 3 is an enlarged cross-sectional view of a main part showing another embodiment of the thin portion. 4 (A) to 4 (E) are manufacturing process principle diagrams for explaining the molding of the parison of the present invention and the blow molding of the bellows product using the parison. 1 ... Parison, 2 ... different diameter part, 3 ... mountain part, 4 ... valley part. 7 ... Thin portion, 8 ... Groove.

Claims (3)

[Scope of utility model registration request] 1. A parison for injection blow molding of a bellows product, wherein a bellows-shaped different-diameter portion including a thick peak portion and a valley portion thinner than the thick peak portion is formed, and the thin portion is provided on the peak portion. A parison for injection blow molding of bellows products. 2. The parison for injection blow molding of a bellows product according to claim 1, wherein the thin portion is a circumferential groove formed on an inner peripheral surface of the parison. 3. The bellows product injection according to claim 1, wherein the thin-walled portion is arranged on a line connecting the center of the ridge of the parison and the center of the ridge of the blow molding outer mold. Parison for blow molding.