Mold filling weld lines

Macroscopic product problems that can result from poor control in injection molding include, but are not limited to voids and sink holes on the surface generally due to poor mold filling or low pressure, incomplete mold filling, weld lines and flow marks, warping or distortion of parts, high shrinkage, and so forth. 

Weld line With moldings that include openings (holes), problems can develop. In the process of filling a cavity the flowing melt is obstructed by the core, splits its stream, and surrounds the core. The split stream then... 

Weld line Weld or knit fines where two parts of a melt join while flowing into the mold cavity can result in problems. The quality of the weld depends on the temperature of the material at the weld point and the pressure present in the melt after flowing from the gate. The higher the temperature and pressure, the more complete the weld and the better the product performance and appearance. Bringing the material to the weld point at a higher temperature and pressure requires rapid filling of the mold cavity (Chapter 3, BASIC FEATURE, Weld Line). 

Figure 4.48. Diagram illustrating propagation of the front line and appearance of welding lines (dotted lines) at filling a plane mold with inserts.

We discuss some of these regions in detail below. In addition, we concern ourselves with the overall flow pattern during filling. Recall that the manner in which a mold is filled— that is, the location of the advancing melt front—affects the weld-line location and the orientation distribution and may be responsible for poor mold filling conditions. 

The most characteristic feature of injection molds is geometrical complexity. In such molds there is a need to predict overall flow pattern, which provides information on the sequence in which different portions of the mold fill, as well as on short shots, weld line location, and orientation distribution. The more complex a mold, the greater this need is. The irregular complexity of the geometry, which forms the boundary conditions of the flow problem, leads naturally to FEMs, which are inherently appropriate for handling complex boundary conditions. 

Overall Flow-Pattern Simulation (a) Develop a computer model to simulate, with the FAN3 method, the filling of a shallow mold, assuming constant gate pressure, isothermal flow, and incompressible Newtonian fluid, (b) Simulate the filling of the mold in Fig. 13.8, Case 1, identify the shape of the advancing front at various times, and the location and shape of the weld lines. 

The SCORIM Process. It is possible to attach the SCORIM unit to the injection molder to manipulate the filling process of the mold so that in the area of the weld lines the resin is mixed [16]. This will improve the structural strength substantially and at the same time the unwanted orientation of metallic pigments is prevented. This process typically requires a 10-15°C higher temperature of the resin and the mold. The process is based on a prolonged flow ability of the polymer. 

At the end of this injection process (80-90% filled), the two pistons will hydraulically separate the mold from the nozzle. The missing amount of polymer is now added from the small cylinders, countering the phases in the mold and the different molten Section 5 of the plasticized plastic is moved back and forth two to three times over the theoretical weld lines. After completion of this mixing process the cylinders are synchronized (phase B) (Fig. 14.18). 

The fihngate (Figure 4-4) functions to the same principle as the diaphragm gate. It is used with flat moldings and assists cavity filling. (Leaves neither weld-lines nor other markings on the article s surface). 

The mold must be vented to allow for gas escape such vents must be placed near weld lines and also near the last areas to be filled. Typical vents are slots 6 to 13mm (0.236 to 0.512in) wide and 0.01 to 0.03mm (0.0004 to O.OOlin) deep such slots are located on the mating surface of one of the mold halves. If a negative pressure device is available it may be possible to vent the mold into the water channels. This can speed up mold filling, reduce component burning and, reduce the cycle times. 

Knit lines, also known as weld lines, occur in parts made of the processes in which the plastic fills out a mold (cold press, compression. 

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