JPH0242431Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to an injection molding machine that prevents backflow of molten resin during injection.

(Prior art) In general, in an injection molding machine, a backflow prevention valve that is installed to prevent the molten resin stored at the tip of the torpedo from flowing back into the screw during injection is connected to a valve seat provided at the tip of the screw and the rear end of the torpedo. The screw is made up of three parts: a retaining ring at the torpedo, and a check valve loosely fitted between the two.During injection, the screw moves forward, and as the pressure in the molten resin reservoir at the tip of the torpedo increases, the pressure increases to the level above. Acting on the front surface of the check valve, the check valve is pushed back and comes into contact with the valve seat, thereby preventing molten resin from flowing back into the screw portion.
However, according to this type of conventional check valve mechanism, the check valve does not operate smoothly, especially when using low-grade resin, and as a result, the weight of the molded product varies greatly. On the other hand, in the molding industry, as molding products become larger and cycle times become higher, the use of high-flow, low-viscosity resins increases, and there is an increasing need to reduce the weight variation of molded products.

To explain this concretely using drawings, the fifth
6 and 6 show a conventional check valve, which, as described above, is composed of a valve seat a, a check valve b, and a retaining ring c. In this conventional check valve, while the screw d is rotating, the check valve b is pushed forward by the molten resin pushed out from the screw d, and as shown in FIG. The check valve b and the retaining ring c come into contact with each other and are held by a certain surface pressure. The extruded molten resin passes through the radial groove e of the retaining ring c and is stored at the tip of the torpedo g.

Next, when the required amount of molten resin is stored at the tip of the torpedo g, the rotation of the screw d is stopped, and the screw d moves forward to inject the molten resin stored at the tip into a mold (not shown). do. During this injection, as shown in Fig. 6, check valve b is pushed back and check valve b and valve seat a come into contact with end face B of the valve seat, the gap with valve seat a is closed, and the tip melts. Designed to prevent backflow of resin. Note that the distance between the inner periphery of the cylinder f and the outer periphery of the check valve b is made as small as possible to prevent leakage of the molten resin.

Here, the operating force F due to the resin pressure of the check valve b at the start of injection is expressed by the following equation.

F=P ₁ × (S ₁ − S) + πDLτ−P ₂ S ₂ …(1) However, P ₁ : Molten resin pressure on the torpedo side of check valve b P ₂ : Molten resin pressure on the screw side of check valve b S ₁ , S ₂ : Surface area before and after check valve b (usually S ₁
= S ₂ ) S: Contact area between check valve b and retaining ring c (1-dot chain line in Figure 7) D: Outer diameter of check valve b L: Length of check valve b τ: Acts on check valve Shear stress (τ=μγ〓
It is expressed as μ: viscosity γ: shear rate) At the start of injection, when the screw begins to move forward, pressure is built up at the tip of the screw, and the above operating force F is applied to the check valve b.
to work. In the case of a high-flow, low-viscosity resin, the pressure drop during pressure propagation is small, and P ₁ ≒ P ₂ , and therefore, F=−P ₁ S+πDLτ (2).

Here, the shear stress expressed by τ=μγ〓 has a low value in the case of a low viscosity resin.

Therefore, in the case of a low-viscosity resin, the operating force F of the check valve b is greatly influenced by the contact area S between the check valve b and the retaining ring c. In conventional check valves, S (the area indicated by the dashed-dotted line in Fig. 7) is large, which means that the operating force F is small, resulting in variations in the operation of the check valve and the inconvenience of variations in the weight of the molded product. It was hot.

(Problems to be solved by the invention) The invention solves the problem that with conventional check valves, the operating characteristics of the check valve are unsatisfactory, especially when molding low-viscosity resin, and the weight variation of molded products becomes large. This is a problem that we are trying to solve.

(Means for solving the problem) Therefore, the present invention provides a retaining ring with a radial groove formed at the rear of the torpedo at the tip of the screw, and a small diameter portion of the screw connected to the rear of the retaining ring.
In an injection molding machine fitted with a check valve that moves back and forth, one or more narrow grooves are formed concentrically with the screw shaft, in addition to the resin passage, in one of the contact points between the retaining ring and the check valve. This is a means to solve problems.

(Function) The molten resin will be present in the thin grooves concentrically provided on the end face of the retaining ring or check valve, and in addition to the resin pressure that acts on conventional check valves, the molten resin will be New resin pressure acts on the check valve.

(Example) Hereinafter, an example of the present invention will be described based on the drawings. FIGS. 1 and 2 correspond to FIGS. 7 and 8 showing a retaining ring of a conventional check valve.
FIG. 3 shows the state of the resin pressure acting on the check valve according to the embodiment of the present invention.

The present invention is characterized by the retaining ring or check valve, and the other parts are the same as the conventional ones shown in FIGS. 5 and 6, so the same parts as the conventional parts will be described here. , the explanation thereof will be omitted, and the explanation will focus on the above-mentioned characteristic parts.

Now, according to this embodiment, as shown in FIGS. 1 and 2, a thin groove 2 concentric with the retaining ring is formed in the end surface of the retaining ring 1 at the portion that comes into contact with the check valve b. . One or more narrow grooves 2 are formed with a width of 0.5 to 1 mm and a depth of 0.2 to 0.5 mm. In this embodiment, the cross section of the narrow groove 2 is semicircular, but other shapes such as rectangular, trapezoidal, and triangular can be selected, which are sufficient to propagate resin pressure and do not cause problems in retention. If the capacitance does not occur, it can be shaped as appropriate.

Even in an injection molding machine equipped with a check valve configured in this manner, the function of storing molten resin in the storage portion at the tip of the torpedo is no different from the conventional one. That is, it is similar to the one shown in FIG. 5, but it is sufficient to replace retaining ring c in FIG. 5 with retaining ring 1. When screw d rotates, check valve b The extruded molten resin pushes the resin forward and is held by the retaining ring 1. The extruded molten resin passes through the gap between the valve seat a and the check valve b, through the space h formed by the inner surface of the check valve b and the outer surface of the sleeve i, and passes through the radial groove e of the retaining ring 1. It is pushed out to the tip of the torpedo g, and stored in a storage section (not shown) at the tip for the next injection.

Next, during injection, when the screw d starts moving forward, a pressure _P1 will be generated at the tip of the torpedo. Contrary to the _case during plasticization, pressure is propagated from the radial groove e of the retaining ring 1 to the screw side end surface B of the check valve b through the space h, but at this time a pressure drop occurs and the pressure is Torpedo tip pressure P ₁ on end face B
Less pressure is applied.

On the other hand, the molten resin in the narrow groove 2 of the retaining ring 1 also propagates pressure and acts on the torpedo side end surface A of the check valve b with P ₁ ′<P ₁ .

From the above, the operating force F ₁ of the check valve b according to this embodiment
is as follows.

F ₁ =P ₁ ×(S ₁ −S)+P ₁ ′×S″+πDLτ−P ₂ S ₂ (3) where S″ represents the area of the plane of the narrow groove 2.

Here, in the case of a low-viscosity resin, P ₁ ≈P ₂ S ₁ =S ₂ , so equation (3) ultimately becomes F ₁ =P ₁ S+P ₁ ′×S″+πDLτ (4).

Here, by comparing equations (2) and (4), it can be seen that the force acting on check valve b due to the resin pressure increases by the second term of equation (4). In reality _, the pressure acting on the torpedo side end surface A _of check valve b is
As shown in the figure, there is also a boundary pressure P ₁ ″ at the boundary of the contact part,
P ₁ is occurring, contributing to an increase in the operating force of the check valve. Boundary pressure is generated by molten resin that has entered through the corner chamfers and minute gaps.

As described above, the thin grooves 2 are formed in the retaining ring 1, and as shown in FIG. Even if it is formed, it has the same effect.

(Effects of the Invention) Since the present invention is constructed as described in detail above, the operating force of the check valve is improved and variations in the weight of molded products can be suppressed. Furthermore, in addition to the pressure propagation effect, the molten resin in the narrow groove slides in metal contact due to the relative speed difference between the check valve and the contact part of the retaining ring when the screw rotates, so it is in an atmosphere where it is prone to wear. However, the molten resin flowing through the narrow grooves of the present invention can be expected to have both a lubricating effect and a cooling effect on the sliding parts. On the other hand, since high shearing force acts on the narrow grooves when the screw rotates, the retained resin is gradually renewed.

[Brief explanation of the drawing]

FIG. 1 is a plan sectional view of a retaining ring of a check valve for an injection molding machine showing an embodiment of the present invention, and FIG.
-Z sectional view, Figure 3 is a conceptual diagram of the pressure acting on the torpedo side end face of the check valve at the start of injection, Figure 4 is a front view of the check valve when a narrow groove is formed in the check valve, 5 and 6 are side sectional views of conventional check valves for injection molding machines in different operating states, FIG. 7 is a plan sectional view of a retaining ring in the conventional example, and FIG. 8 is a sectional view of a conventional check valve for injection molding machines. It is a YY sectional view of. Explanation of the main parts of the figure, 1... retaining ring, 2, 4...
...Narrow groove, b...Check valve, e...Radial groove.

Claims (1)

[Scope of utility model registration request] In an injection molding machine, a retaining ring with a radial groove formed at the rear of the torpedo at the tip of the screw is provided, and a check valve that moves back and forth is inserted into the narrow diameter part of the screw connected to the rear of the retaining ring. An injection molding machine characterized in that one or more narrow grooves are provided concentrically with the screw shaft, in addition to the resin passage, on either one of the contact points of the retaining ring and the check valve.