1. Field of the Invention
The present invention relates to a liquid dispenser including a hollow cylinder to be filled by a viscous liquid, whereby a predetermined constant quantity of the viscous liquid can be discharged from the front end of the cylinder.
2. Description of the Prior Art
This type of liquid dispenser, often called as a "syringe", is used to repeatedly discharge with high frequency a predetermined small quantity of viscous liquid by which the cylinder is filled, such as paste- or cream-like electronic materials, adhesive agents, etc. The cylinder is generally composed of a transparent material, and is filled by the viscous liquid and supplied with a pressurized air pulse in use so that the liquid is to be uniformly urged by the pressurized air and discharged through a needle at the front end of the cylinder. In this instance, the liquid level within the cylinder is lowered each time the liquid is discharged through the needle.
Specifically in the case of a liquid with a high viscosity, there arises a problem that the liquid tends to be attached onto the inner wall of the cylinder and the liquid level at the center region of the cylinder is lowered by an increased amount as compared to the peripheral region. In the worst situation, when the depth of the liquid at the center region of the cylinder becomes extremely small, the pressurized air also may be discharged through the needle without properly urging the liquid, making it difficult or practically impossible to discharge a desired constant quantity of the viscous liquid.
This problem is particularly significant when the dispenser is used to discharge an opaque viscous liquid. That is, when the liquid is maintained attached to the inner wall of a transparent cylinder, the lowered liquid level at the center region of the cylinder cannot be visually observed from outside through the cylinder. Thus, the discharge of the viscous liquid may be continued without realizing that the pressurized air also is being discharged.
Heretofore, various proposals have been made in an attempt to avoid the discharge of the pressurized air and ensure a satisfactory discharge of the viscous liquid alone. One approach generally adopted in this respect is to accommodate a plunger within the cylinder, which is applied with a pressurized air pulse and undergoes an axial sliding displacement within the cylinder for uniformly urging the viscous liquid filling the space defined by the cylinder and the plunger.
Various examples of the liquid dispenser incorporating such a plunger will be explained below with particular reference to FIGS. 1A to 1D.
A basic example of the liquid dispenser shown in FIG. 1A includes a cylinder 1 in which a plunger 2 is simply axially slidably accommodated in the cylinder 1, for discharging a viscous liquid 3 within a space defined by the cylinder 1 and the plunger 2. The plunger 2 has an outer diameter which is made slightly smaller than the inner diameter of the cylinder 1. When the viscous liquid 3 contains solid matters, as is the case in cream-like soldering materials, such solid matters tend to be squashed or jammed into an annular clearance between the cylinder 1 and the plunger 2 to impede a smooth operation of the plunger 2. Thus, it is still difficult to satisfactorily achieve the intended discharge of the constant quantity of the viscous liquid 3.
When the plunger 2 has an outer diameter which is substantially smaller than the inner diameter of the cylinder 1, as shown in FIG. 1B, the solid matters in the viscous liquid 3 can be prevented from being jammed into the annular clearance between the cylinder 1 and the plunger 2. However, because an increased amount of the viscous liquid 3 is admitted into the annular clearance in a relatively unrestricted sense, the plunger 2 may be dipped into the liquid 3. Therefore, it is not only difficult for the plunger 2 to properly achieve the intended functions, but also it becomes impossible to perform a visual observation of the liquid level from outside.
According to another example shown in FIG. 1C, the liquid dispenser incorporates a plunger 4 which is provided with one or more seal rings 5a, 5b made of a resilient material. The seal rings 5a, 5b effectively prevent entry of the viscous liquid 3 into the annular clearance between the cylinder 1 and the plunger 4, although the solid matters contained in the viscous liquid still tend to be jammed into the clearance under a resilient deformation of the seal rings, to impede a smooth operation of the plunger 4. Besides, a frictional resistance occurs between the inner wall of the cylinder 1 and the seal rings 5a, 5b, and makes it difficult properly to displace the plunger 4 by applying the pressurized air pulse to the top surface of the plunger 4.
Still another example is shown in FIG. 1D, wherein the liquid dispenser incorporates a hollow plunger 6 which is provided with a pair of axially spaced flanges 7a, 7b. The plunger 6 has a substantially U-shaped longitudinal section, with an inner peripheral surface and a bottom surface which are applied by a pressurized air pulse as shown at 8, in order to axially displace the plunger 6 downwardly in FIG. 1D, and also to urge the flanges 7a, 7b tightly against the inner surface of the cylinder 1. The flanges 7a, 7b are axially slidably guided by the cylinder 1 so as to effectively achieve the intended functions; i.e., to prevent the viscous liquid 3 from being admitted into the annular clearance between the cylinder 1 and the plunger 6, and also to prevent the solid matters in the viscous liquid 3 from being jammed into the clearance.
In operation of the liquid dispenser of FIG. 1D, the viscous liquid 3 is discharged from the needle (not shown) by applying a pressurized air pulse to the pressure receiving surfaces of the plunger 6. When the constant quantity of the viscous liquid 3 has been discharged from the needle and the supply of the air pulse is subsequently stopped, the restoring force of the viscous liquid which had been in a compressed state tends to push back the plunger 6 away from the needle. On this occasion, as the case may be, a small amount of air is sucked from the top end region of the plunger 6 into the annular clearance between the cylinder 1 and the plunger 6, as shown at 9. Once the air 9 has been sucked into the clearance, it is retained therein without being discharged toward the top end region of the plunger 6. Rather, the amount of air 9 retained in the clearance tends to increase during the repeated operation of the plunger 6. It has to be noted that a similar problem takes place in the example of FIG. 1C, as well.
When a substantial amount of air 9 is retained in the annular clearance between the cylinder 1 and the plunger 4, 6 in the examples of FIGS. 1C and 1D, the pressure of the air pulse applied to the plunger 4, 6 is transmitted to the viscous liquid 3 through the air 9 within the annular clearance, which more or less undergoes a compression during a downward displacement of the plunger 4, 6. Thus, the transmission of the air pressure to the viscous liquid 3 is retarded, with a resultant fluctuation in quantity of the viscous liquid 3 which is actually being discharged from the needle. In the worst case, due to the compression of the air 9 within the annular clearance, the viscous liquid 3 cannot be discharged from the needle at all. This problem is particularly serious in the example of FIG. 1D, wherein the air pressure is also used to urge the flanges 7a, 7b against the inner wall of the cylinder 1.