Knit line

During some mol ding and extmsion operations, knit line failures, incomplete mold fill, die drag, and excessive heat buildup, ie, scorch, are problems. Many of these problems are reduced or eliminated by the addition of internal lubricants such as low mol wt polyethylene or Vanfre AP-2 Special, a product of R. T. Vanderbilt. 

Design Considerations for Injection Molded Parts (Parting lines, draft angles, wall thickness, fillets and radii, bosses, ribs, opening formations, shrinkage, gating, vents, potential knit lines)... 

Common problems like insufficient filling-packing and poor dimensional control are often related to the gate size and design. Similarly, gate location is another important factor. They should be located in areas having heaviest cross-section of the part to assure fill-out and elimination of sink marks. Also their position should not facilitate the residual molded stress formation in the part, knit line formation. 

Figure 8. SEM-rutbenium telroxide stained knit line between two DVA s... 

If more than one mix is required to complete the pour, the second mix must be commenced while the first is still very tacky and not surface-cured otherwise, it will not fully blend together and will form a crack or knit line. 

Knit line A visible line seen in a molded polyurethane part that indicates where two flows of reacting polyurethanes have come together and joined. Often caused by flow turbulence, incorrect pour point placement, or poor polyurethane flow. Depending on the severity, the knit line can be a weak point in the molding that may cause future delamination, structural failure, or poor dimensional stability. 

Fig. 13.20 Schematic representation of the impingement and the subsequent flows in the weldline region gray areas indicate cold regions of the melt dashed lines denote regions undergoing extensional flow. [Reprinted by permission from S. Y. Hobbs, Some Observations on the Morphology and Fracture Characteristics of Knit Lines, Polym. Eng. Sci., 14, 621 (1974).]...

R. C. Thamm, Phase Morphology of High-Impact-Strength Blends of EPDM and Polypropylene. Knit-line Behavior, Rubber Chem. Technol., 50, 24-34 (1977). 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

Mg. 27. Thin section profile of the knit-line notch in PC. The highly birefringent region, extending about 25 microns beyond the tip of the notch, reflects a weaketted zone. Reprinted courtesy of Matsumoto ... 

In the following sections the fatigue lifetime results for fom polymers will be described. In aU cases, the samples were pre-notched at knit lines by using doublegated tensile bar injection molded samples. 

The fatigue lifetime data for prenotched (knit-line) samples of poly butylene terephthalate (PBT), a semicrystalline polymer, were unrevealing in that no fatigue crack growth could be induced under any of the conditions teted (Fig. 34). At higher... 

Typically, thermoplastics, because of their high melt viscosity, require higher temperatures and injection molding pressures, or expensive mold fabrication costs to design multiple gate systems and therefore are not cost effective materials for large parts. In addition to cost, the use of multiple gates may also lead to a less structurally sound part as a result of knit lines. 

Prominent knit line How front very prominent Tool or material temperature too low... 

PC/ABS blends have better flow properties than PC, which means that the parts have fewer knit lines, sink marks as well as less molded-in stress. Molded parts offer excellent surface appearance and good paint adhesion. Up to 20% regrind may be used for all grades. 

Experts have noted the benefits of lower melt temperatures and pressures made possible by these additives in molding. Along with cycle-time reductions, lower pressures may allow a smaller injection-molding machine to be used for a job. Part quality may also be improved (with fewer flow lines and stronger knit lines), with less molded-in stress. (And by no means are most of these additives limited to molding processes Chapter 12 addresses their use in extrusion, where they have analogous benefits.) [11-3,11-4,11-9, 11-14]... 

Thin film is inflated from extruded tube consequently to the die exit. The blown film dies could be in general divided to spider and spiral types, as presented in Fig. 4.3. Since spiral dies provide more uniform melt distribution and eliminate weld or knit lines caused by the spider holding the mandrel, they almost replaced... 

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