Impact weld lines

This paper deals with the control of weld depth penetration for cylinders in gold-nickel alloy and tantalum. After introducing the experimental set-up and the samples description, the study and the optimization of the testing are presented for single-sided measurements either in a pulse-echo configuration or when the pump and the probe laser beams are shifted (influence of a thermal phenomenon), and for different kind of laser impact (a line or a circular spot). First, the ultrasonic system is used to detect and to size a flat bottom hole in an aluminium plate. Indeed, when the width of the hole is reduced, its shape is nearly similar to the one of a slot. Then, the optimization is accomplished for... 

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]. 

Blends of PC/PET/TPU with EVAc-GMA and optionally MBS or ABS, have good flexural modulus, strength, weld line strength, solvent resistance, and impact behavior [Laughner, 1994]. PC blends with a polycaprolactone-polyurethane resin, TPU Pellethane , and either MBS or MBA showed similar behavior [Henton et al., 1993]. 

However, a plate that deforms to this degree is not in danger of brittle fracture at low impact energy. Hence, the value of the test is to screen injection mouldings for brittle failure, at forces well below fmax, due to orientation, weld lines or a weak surface structure. 

An important application for polybntylene is plumbing pipe for both coimnercial and residential use. The exceUent creep resistance of polybntylene aUows for the manufacture of thiimer wall pipes compared to pipes made from polyethylene or polypropylene. Polybutylene pipe can also be used for the transport of abrasive fluids. Other applications for polybutylene include hot melt adhesives and additives for other plastics. The addition of polybutylene improves the environmental stress cracking resistance of polyethylene and the impact and weld line strength of polypropylene. Polybntylene is also used in pack aging applications. 

In these blends, ABS s role is to improve flowabihty and reduce cost, while that of PSF is to improve the shape retention at high temperatures. ABS/PSF blends are compatibilized either by phenoxy, EVAc-GMA, or SMA copolymers. They have good processability, high notched Izod impact strength, plateability, hydrolytic stability, and economy. However, they may show poor surface and weld-line strength. Arylon and Mindel A are examples of the commercial ABS/PSF alloys, while Ucardel is an example of PSF blends with SAN. Evolution of ABS/PSF blends technology is summarized in Table 1.19. 

PP/HDPE =1 1 Processability, weld-line strength, low-T impact strength Moorwessel et al. 1974... 

HDPE was blended with 50 wt% PP, having matched melt flow index, to give good flowability, weld-line strength, surface quality, and low-temperature impact strength... 

The multiple functions of mica have been outlined in Chapter 1 of this book, along with an example of its role in the search of multifunctional fillers for polypropylene compounds for automotive applications. Mica-reinforced thermoplastics such as polypropylene, polyethylene, nylon, and polyesters are now established in a variety of automotive applications and consumer products where mica supplements or replaces glass fibers and other mineral fillers. The wider use of mica in many applications has been limited by low impact strength and low weld-line strength in certain plastics. These issues are the focus of continuing R D efforts by materials suppliers and compounders/molders. 

Other applications include hot-melt adhesives and additives for other plastics. The addition of polybutylene improves the environmental stress cracking resistance of polyethylene and the impact and weld line strength of polypropylene. Polybutylene is also used in packaging applications. 

Figure 18.41 The effect of weld lines on the falling weight impact strength of polypropylene copolymer at 23 °C. Source Author s own files)...

Seldon [10] measured the weld line strength and impact, and flexural and tensile properties of injection-molded specimens of glass fiber-reinforced polyamide 6 and talc-filled polypropylene (PP). Further information was obtained by examination of fracture surfaces by scanning electron microscopy. 

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