By Design: Part design 301—Weldlines

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

During the injection molding process, melt flows from the gate through the cavity. This is a melt flow process, and that imposes limitations on what can and cannot be molded. The ideal shape for an injection molded part is a poker chip with a centrally located gate. If the poker chip has a uniform wall that is thick enough, the melt will flow uniformly from the gate and reach all of the periphery of the cavity at the same time. This would be an ideal situation. 

If that poker chip contained a deep recess or a through hole, the melt would have to separate into two flow paths that would pass around those obstructions and reunite on the other side. There would then be an interruption in the flow, and the melt would not reach all of the periphery of the cavity at the same time. 

Rotational molding is a sintering process. Thermoforming and blowmolding are stretching processes. These are not melt flow processes. They can mold parts with thinner walls and no weldlines. Yet, in spite of its melt flow limitation, injection molding is the product designer's most frequently specified molding process. 

Weldlines 
Weldlines are an integral part of the injection molding process. Weldlines are the result of nonuniform wall thickness, unusual flow paths, multiple gates, or the restrictions to flow created by core pins that form holes. The partially filled demo part shown in Figure 1 is a classic example of weldline formation. In this instance, the gate is located at the center of the part, near the top. The melt flows uniformly outward from the gate until it encounters the core pins that form the two holes on the right and left sides of the part. As the two flow fronts reunite beyond the core pins, they create weldlines. 

The fundamental problem with weldlines is that they create appearance problems. Also, a molded part is normally weaker at the weldline than in the homogenous surrounding areas. In addition, weldlines make it more difficult to mold a high-quality part. 

The strength and appearance of a weldline are determined by the amount of intermolecular mixing, or diffusion, of the two flow fronts. The amount of intermolecular mixing at a weldline is determined by the shape of the part and the location of the gate, which are both controllable. 

For example, the luggage-tag-shaped parts in Figure 2 are identical, except for the location of the gate. In the left-hand view, the melt flows around the rectangular hole and forms a weldline near the gate. At this location, the melt temperature and pressure are high, and a strong weldline can be produced. As the melt continues to flow through the cavity, the edges of the two flow fronts rub against each other, encouraging intermolecular mixing. In many cases, the weldline will not be visible after a short distance of flow. When the flow fronts meet and continue to flow together, the resulting weldline is referred to as being "dynamic." This type of weldline is also called a meldline. 

In the part on the right in Figure 2, the melt flows the whole length of the cavity before forming a weldline beyond the rectangular hole. After flowing this distance, the melt can have reduced temperature and pressure. Once this weldline is formed, the melt stops flowing. There is no opportunity for the two flow fronts to be mixed, or melded, together as in the part on the left. Weldlines that are formed with no additional melt flow are referred to as static or butt weldlines. Static weldlines located at the extremities of a cavity are normally weaker and more obvious than dynamic weldlines formed close to a gate. 

The progressive short shots of the louver shown in Figure 3 are a classic example of static weldlines. Relocating the gate to the top or bottom of that part would have produced dynamic weldlines on the bezel. These weldlines would have been stronger. The unsightly weldlines also would have been eliminated from the highly visible center area on the louvers. 

Things to Consider 
• The right material. Some plastic materials retain more strength across weldlines than others do. In one experiment, tensile bars were molded with gates at both ends of the part. This gating produced static weldlines in the center of the part. These parts were tensile tested and compared to parts molded with one gate and no weldlines. Polysulfone, nylon 6/6, and polycarbonate retained most of their tensile strength. The loss in tensile strength at the weldline with polypropylene was 14 percent, polyphenylene sulfide 17 percent, and SAN 20 percent. 

The molecular weight of a plastic material and its degree of crystallinity are other considerations. The amount and type of filler and reinforcement also have an effect. Additives such as pigment, fire retardant, and especially lubricant normally reduce the strength of a weldline. 

There is very little good information published on weldline strength. The most reliable source of this data is the manufacturer of the material being considered. 

• Molding considerations. The strength of weldlines in a given material is greatly affected by molding conditions. Increased injection speed and melt and mold temperature are the primary cycling conditions that produce better quality weldlines. Fast injection speeds reduce the time the melt has to cool and, if fast enough, may generate additional frictional heating. Increased melt temperature encourages intermolecular mixing at the weldline. A hotter mold reduces cooling of the melt. The use of adequate venting and high packing pressure also contribute to the quality of weldlines. 

• Design guidelines. In the design of a new part, engineers should consider the appearance and strength limitations associated with weldlines. If the part has holes through its wall, there normally will be weldlines. Nonuniform wall thickness and abrupt changes in thickness can result in unanticipated weldlines. Whether or not there will be weldlines and where they will be located can be determined with simple off-the-shelf moldfilling simulation and analysis programs. 

For example, an analysis of the grillwork shown in Figure 4 would indicate that this part would have static weldlines on the left side and dynamic weldlines on the right side. The static weldlines are in a highly visible area. They will weaken the slats of the grille. They are away from the cavity's parting line and are difficult to vent. The dynamic weldlines on the right side do not distract from the appearance or strength of the slats. They are near the parting line, where they are easy to vent. A preproduction moldfilling analysis of this grille would indicate that it has weldline problems and that redesign should be considered. 

The location and quality of weldlines are determined by the size and shape of the part, the location of the gate, and the way the part is molded. Since a drawing note indicating "no weldlines" is not a satisfactory solution to the problems created by weldlines, there are a few things a designer can do to minimize their effects: 

• Consider weldlines during the part design process. 

• Discuss design with a competent injection molder. 

• Get involved in the mold design and pay attention to how the cavity is gated and vented. Every effort must be made to avoid weldlines in heavily loaded areas on an injection molded part.