During injection, for melting plastics filling mold cavity in time, besides enough injection pressure, injection molding machine
also should have certain injection velocity so that melting plastics can have faster flow velocity. During production, rational injection velocity is an important factor for quality of molding products.
In general, low injection velocity has long filling time. Welding marks, heterogeneous density, high residual stress and other defects easily occur on products, particularly, crystalline plastics and thin-wall products having narrow processing temperature range.
Although high injection velocity can reduce temperature difference of melting plastics inside mold, improving effects of pressure transferring, acquiring precise products which have even density and low residual stress and decreasing period of molding, over high injection velocity can cause large numbers of frictional heat when melting plastics flow through nozzle which may result in plastics resolving and discoloring, inspiriting or poor exhausting, having direct influences on the surface of products. Meanwhile, it has higher requirements for program control.
Nowadays, injection velocity is improved constantly, because improving velocity cannot only reduce injection time, but also can acquire high quality products under lower mold temperature, especially when injecting products having thin wall, high injection velocity is precondition for high quality products. To meet different requirements of plastics and structure, the velocity on injection molding machine should have certain adjustable range. The regulation of injection velocity can be realized by the flow rate of oil injected into cylinder. Moreover, multilevel injection is adopted for different structure of products.