Weld Line Strength Research Articles

Weld line studies on transfer‐molded thermosetting composite materials

AbstractMultiple gating and the presence of inserts make weld lines (also known as knit lines) almost unavoidable in commercially produced moldings. These weld regions may be significantly weaker in strength than the bulk material away from the weld line. Hence, a molded section containing a weld line will only be as strong as the weld line, no matter how strong the surrounding bulk material. The influence of processing history on weld line performance is essential to optimally designed transfer‐molded composite products with discontinuous‐fiber‐reinforced molding materials. This study has therefore been directed toward experimentally characterizing the weld line strength of a bulk molding compund (BMC) and a thick molding compound (TMC) as a function of the mold temperature, fill time/fill rate, and the postbake operation.

The relationship between processing conditions, morphological structure and mechanical properties of knit lines of an injection molded semi‐crystalline thermoplastic

AbstractThe structure‐property relationship of injection molded polypropylene samples with knit lines was investigated. Double gated tensile bars were injection molded under different molding conditions in order to study the influence of molding parameters on crystallinity and spherulite diameter. It was observed that the degree of crystallinity and spherulite size were highly dependent on cooling rate. To improve weld line strength and elastic modulus, an increase in crystalline fraction and the diameter of the crystalline aggregates seems to be necessary.

Improvement of Weld Line Strength in Injection Molded FRTP Articles

Abstract Weld lines generated in fiber reinforced thermoplastics (FRTP) injection molded parts are considered to be of poor quality and lead to reduction of the mechanical strength. A multi-cavity ...

Injection welding of polyamides

AbstractIn the following article, we will discuss welding of polyamides inside the injection mold. This process is called injection welding. The process of injection welding makes it possible to bond polyamides with quite different characteristics together without adhesive. In this way, one can achieve economic as well as constructional advantages. We will discuss two different methods in this article. In the first process, two melts are injected onto each other simultaneously; with the second technique, the melt is injected onto a thermoplastic insert. The weld line strengths of injection‐welded parts depend on material and process conditions. With an amorphous polyamide, the weld line strength can be attained up to the full tensile strength of the material, while a glass‐fiber‐reinforced polyamide falls back onto the tensile strength of the basic polymer. Injection welding achieves the greatest weld line strength when two melt streams are colliding together with the highest possible injection pressure. If a melt is injected onto an insert, the temperature of the melt should be as high as possible.

Weldline integrity of reinforced plastics: Effect of filler shape

AbstractThe paper is a preliminary report on glass fiber and glass flake orientation in the weldline zone of injection molded reinforced polypropylene. Two types of weldlines were studied using simple shape molds: one where two melt streams meet head‐on and become immobilized, the other where the weldline formation is followed by additional flow. In the weldlines of the first type, which are characteristic of current standard molds used to test weldline strength, the anisometric particles are almost perfectly oriented perpendicular to flow. As a result, the weldlines of this type tend to be weak and brittle. In the second case when the weldline is formed by merging of melt streams separated by an insert, particle orientation in the weldline area remains different from other areas of the sample for long distances from the insert. It is shown that in molds where the weldline formation is followed by laterally expanding flow the change of particle orientation is faster than for unidirectional flow.

Performance prediction of weldline structure in amorphous polymers

AbstractThe presence of a weldline generally reduces the mechanical strength of injection molded parts. A typical remedy to eliminate the problem of weak weldline structure has been to increase the melt temperature. This, however, is not an acceptable solution in some situations. A general solution to the weak weldline problem requires an in‐depth understanding of the thermomechanical history of the injection molding process. A theoretical model for the strength of weldlines is presented that provides a comprehensive physical insight of the bonding process at the weldline interface. The model is based on the self‐diffusion of molecular chains across the polymer‐polymer interface and the frozen‐in orientation that remains parallel to the interface. Both factors are analyzed separately and then superimposed to predict the strength of weldlines from known processing conditions and geometry. Experimental results show good correlation with predictions.

Applications of polymer blends: Emphasis on recent advances

AbstractIn the past decade, polymer blend technology has achieved an important position in the field of polymer science. With increased academic and industrial research interest, the application of polymer blend technology to commercial utility has grown significantly. This review on the applications of polymer blends will cover the major commercial blends in the categories of styrene‐based polymer blends, poly(vinyl chloride) blends, polyacrylate blends, polyester and polycarbonate blends, polyolefin blends, elastomer blends, polyelectrolyte complexes, and interpenetrating polymer networks. New developments in polymer blend applications will be discussed in more detail. These systems include linear low‐density polyethylene blends with either low‐ or high‐density polyethylene, styrenemaleic anhydride terpolymer/ABS (acrylonitrile‐butadiene‐styrene) blends, polycarbonate/poly(butylene tetephthalate) blends, new PPO/polystyrene blends, and tetramethyl bisphenol A polycarbonate/impact polystyrene blends. Areas for future research to enhance the potential for polymer blend applications will be presented. The need for improved methods for predicting miscibility in polymer blends is discussed. Weldline strength is a major property deficiency of two‐phase systems (even those with mechanical compatibility), and future research effort appears warranted to resolve this deficiency. The use of polymeric compatibilization additives to polymer blends has shown promise as a method to improve mechanical compatibility in phase‐separated blends, and will be expected to be the subject of future research programs. Finally, the reuse of polymer scrap is discussed as a future application area for polymer blends. Unique applications recently proposed for polymer blends include immobilization of enzymes, permselective membranes, reverse osmosis membranes, selective ion‐exchange systems, and medical applications using polyelectrolyte complexes.