Definition: Weld Line: flow line 1) a visible mark (flaw) which results from meeting of two flow fronts, the weld may be imperfect and weak, very serious for filled materials. 2) In extrusion created by flow front split created by support or spiders inside the die resulting in line parallel to axis.

In many plastic parts especially injection molded there are weld lines, and as the definition implies an area where the flow of plastic welds back together. There is also the same issue in extrusion, though this is occurring within the die itself, especially in tubing, or profiles which have a tube like features or create a hollow feature. In extrusion one can extend the land area of the die to build up back pressure within the die and create a better weld area, and or even increase/decrease the die temperature.

In molding when we have an opening in the part / divide the melt stream (pins, cores etc.)and the material flows back around to form a knitted or welded surface we must rely on the pressure of the plastic at that point to knit together our flow fronts. Since we do in fact have a hot interface due to fountain flow of the material to the front the temperature of the material is not usually the issue in creating a good weld. The real issue is material composition, pressure and elimination of the trapped air.

With material composition the issue tends to be filled materials. The best example is the use of a highly glass loaded material in which has very high physical properties, yet breaks at the weld line. When we consider the actual flow of the material and what it is trying to do at the knit line it becomes obvious that we are only knitting together the base material and in most cases not getting an entanglement of the glass fibers. This is especially true when processing a long glass fiber material or very highly filled material.

How to overcome this issue? One would be obvious in the elimination of the weld line. Another is to locate the weld line in a nonstructural point within the part, and or to try and design the flow so that we get re-entanglement at some point along the weld interface. An example that has been used to some success is to place an overflow, in the path of the weld line thus creating more flow/ movement along the path hoping for a bit more entanglement.

Another issue is the pressure lost in the injection molding process and what is needed at the weld line point. While this can be measured by performing an in mold pressure lose assessment, or possibly using a software simulation program, it is generally agreed that we need a good 3,000 psi plastic pressure at end of fill to pack out the part. If in fact we have not the pressure necessary than the weld line shall be weak whether or not it is a filled or unfilled resin. Thus as the flow paths become longer and pressure lose greater within the mold the issue appears more and more in the performance of parts. This is one reason that dividing the flow at end of fill is avoided in many cases.

A possible solution that is used is to increase the mold temperature, which easily allows the material to flow in a thicker path, maintaining a bit more pressure (less pressure loses). Again this goes back to fountain flow, since there is no flow at the sidewalls of the mold the question is what is the boundary layer thickness, and what has it done to the flow path of the material in regards to volumetric area. Thus a hotter mold means minimum wall on boundary and thicker higher pressure conveyance in center.

Trapped air can be a real cosmetic issue but also a weak point. As material flows around the obstruction, core pin and opening and then tries to knit back together it is always traveling in a curve at the front. Thus the area around the pin closes to the knit area is a void, which includes air that must be compressed out. If we do not vent the back side of the pin, core etc. the air has nowhere to go and can create its own defect, or a notch which can lead to stress riser and possible breakage. On the free flowing side of knit there should be venting in the tool to allow the air to escape. If this is not allowed than the weld line can become very pronounced as the air impedes the plastic from duplicating the mold surface, it also heats up greatly possibly causing the plastic to discolor or to have different gloss level.

Weld lines and knit lines are part of molding. It has to be understood by the designer, tool marker, and processor that they occur, and by quality that they well be in the part. If for cosmetic reason they must not show, than use of texture, various colors, design features such as ribs, bumps etc., they might be hidden. From a structural point they need to be away from stress areas, and basic rules such as 1 to 2 diameters away from the edge of parts for hole must be followed. Understanding the rules one breaks to produce to the design desired and explaining to OEM, designers, customers, prior to building the tool may eliminate many issue in the actual production of the part and also in the finished product.

SL Silvey

silveysplastics@hotmail.com

061211