User talk:AlexKuprich/ Macro squeeze flow during plastic welding

Macro squeeze flow during plastic welding
Macro squeeze flow during plastic welding allows to achieve intimate contact at the faying interface between the two parts, which is necessary for creating a sound weld. Gases entrapped between the asperities of mating surfaces may compromise the quality of the weld joint and need to be removed.

Background and theory
During contact hot plate welding, two plastic parts are brought in contact with a hot plate one on each side of the plate. While surface of the hot plate is not perfectly flat, plastic parts retracted away from that surface resemble its shape and therefore contain small asperities.

Two melted parts then come in contact with each other under pressure. Molten plastic layer flows outwards with the flow lines perpendicular to the line of applied pressure. A stagnation point at which no plastic flow occurs, is located at the center of contact area between the two mating parts. Appropriate macro squeeze flow forces the entrapped gases out of the weld region. A flash is then created as a result of the squeeze flow.

With respect to macro squeeze flow, molten layer at the contact interface does not support the force that is applied to the mating parts. The final melt layer thickness for a particular squeeze time can be defined from the following relationship for rectangular parts:


 * $$\frac{h_0}{h_f} = \left(1+\frac{F\cdot t\cdot h_0^2}{2\cdot L^3 \cdot \mu\cdot w}\right)^\frac{1}{2} $$

where
 * h0 – the initial thickness of molten layer of one part
 * hf – the final thickness of molten layer
 * F – applied force
 * t – squeeze time
 * L – thickness of a part
 * μ – viscosity
 * w – width of a part

Similar equation can be written for a circular bar:
 * $$\frac{h_0}{h_f} = \left(1+\frac{16\cdot F\cdot t\cdot h_0^2}{3\cdot \pi\cdot \mu\cdot R^4}\right)^\frac{1}{2} $$

where
 * R – radius of a bar