Fluid Jetting for Next Generation Packages - Nordson ASYMTEK 首页

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making it easier to separate a dot form the fluid. Adjusting three different variables could alter the mechanical performance of the Jet. The force of the needle is generated by the compression of the spring. By partially compressing the spring before the needle is lifted, the force from the spring can be increases. This is done by screwing the top of the jet in along a set of threads, and the compression of the spring is measured in the number of turns it is compressed. During testing this value was varied form two to four turns of preload. The other parameter regulating the motion of the needle is the stroke length. This is adjusted by turning a micrometer at the top of the jet, and varies the position of the hard stop of the needle (the distance it is raised off the seat with each stroke). Figure 1. Diagram of the cross section of Asymtek DJ-2XXX Jet. This was tested at values ranging from 0.12 mm up to 1.5 mm. The final parameter tested was the valve on time. This regulated the time between signals (from when the solenoid was first triggered, raising the needle, to when it was discharged, and the needle began to fall). Valve on times that were used in this Pac Tech, Berlin, April 2002 experiment were 5E-2, 6E-2, and 7E-2 seconds (5-7 milliseconds). In addition to these parameters, there were several basic alterations made to the actual jet used. While a standard jet uses either a 30 or 60-mil seat, in this experiment 15 and 20 mil seats were used. The nozzle size used on the jet was also altered. In addition to a standard five-mil nozzle, a four-mil nozzle was also used during testing. In addition to using nozzles and seats that were not normal size, the screw of the micrometer was shortened. In the standard Jet, the thread of the screw is so long that with two or three turns of preload, the micrometer cannot be turned down far enough to contact the top of the needle. With a shortened thread, the Jet could be set to as low as two turns of preload, expanding the range of parameters which could be tested. Theory Numerical analysis was performed to simulate the jetting process and drop formation. Incompressible fluid and non-slip theory was assumed in the simulations presented in this paper. It was determine that the Reynolds number was very low and therefore turbulence conditions were not present. The needle was accelerated by the spring and the transfer of momentum into the fluid causes the drop to form when surface tensions are overcome. The set of equations needed to define the problem consisted of the mass conservation equation ∂ ∂ρρ ∂ + ∂ t ∂ x k b bρρ ⋅ u = kg= kg 0 Where ρ is the fluid density and uk is the velocity component in the k-coordinate. The momentum conservation equation L NM ∂ui ∂ui + uk ⋅ ∂ t ∂x k O QP ∂Γ = ρ ⋅ fi + ∂x Where fi is the force along the i-coordinate and Γij is the stress tensor. ij j
And the third expression is the constitutive equation relating the shear stresses to the tensions uk u u i τij = λ δ ij µ x x x ∂ ∂ + + ∂ ∂ ∂ ∂ Where λ is an elasticity constant of the fluid and µ is the fluid viscosity. The solution of above set of equations is carried out by a method of weighted residuals from variational principles, for instance the mass conservation integral weighted residual equation can be written as F HG Where ũk is velocity vector approximation to be minimized to satisfy the boundary conditions exactly (weighted residual factor), and Φ T is the transpose of a “floating function.” Figure 2 depicts the diagram of the Jet with the initial boundary conditions Figure 2. Diagram of Dirichlet and Neuman boundary conditions used in the numerical solution. Fluid velocities, Vorticity, stream functions, shear stresses and fluid pressures were computed for various needle velocities and needle location. Figure 3 depicts velocity contour bands resulting from the needle Pac Tech, Berlin, April 2002 k F HG j i j I KJ T dΩ⋅ ⋅ uk x k ∂Φ z ϕ ⋅ ~ = 0 ∂ KJ V=0 P=P P=PF>P F>P a V=V o P=P a I V=0 motion prior to impact of the needle and seat. Calculations show that the fluid velocity at the orifice of the nozzle is more that an order of magnitude higher than that of the needle itself. V 0 = 0.66 m/s V N = 8.10 8.0 m/s Figure 3. Resultant velocities from numerical calculations. A close-up view, see figure 4, at the needleseat area shows the resultant velocity field upon impact. By this time fluid flow momentum overcome the surface tension of the meniscus at the orifice, i.e., the Weber number reaches a threshold and the drop is formed and jetted at velocities much larger than needle velocity. Recall that the pressure at the smaller diameter of the needle compared to that of the seat induces high fluid flow. V N =0.66 m/s V VN=19 N=19 m/s Figure 4. Velocity field upon impact of the needle and the seat. It should be observed that the seat appears to show a large area of slow fluid motion “stagnation zone,” near the hardware walls in this particular design. Pressures at the fluid
Page 1: Pac Tech, Berlin, April 2002 Fluid
Page 5 and 6: fit to a fourth degree polynomial (
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Fluid Jetting for Next Generation Packages - Nordson ASYMTEK 首页

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