Injection moulding weld lines

Also with injection moulding weld lines occur frequently. The best known example is the eye for the handle of a pail, which is the spot at which the stresses are highest during use ... 

In summary, therefore, processing, and in particular injection moulding, can introduce limited chemical degradation, local polymer orientation, orientation of short fibre reinforcements, internal stresses, warpage, shrinkage and defects such as weld lines and voids. 

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. 

A 508 mm diameter cycle wheel, having five spokes, is injection moulded using a toughened nylon. It is gated at the hub and is 4 mm thick. Comment on the maximum flow lengths and whether the positions of the weld lines may cause problems. 

A rectangular box measuring 20 X12 cm and 10 cm deep is injection-moulded from a gate at the centre of the base. Assuming that at any given time the speed of the flow front is the same everywhere in the mould, determine the positions of the weld lines. 

Internal lubricants typically reduce bulk viscosity by being partially compatible with the polymer, with the lubricant s soluble component helping to open the polymer chain, and the less soluble portion of the lubricant molecule providing intermolecular lubrication (e.g., fatty alcohols, esters, ethylene-vinyl acetate waxes). They promote flow with a reduction in die swell. In injection moulding, the provide improved weld line strength, clarity, die filling and rate of production. In extrusion it reduces head and back pressure while affecting paintability. However, an increase in internal lubricants lowers heat distortion. 

External lubricants typically provide lubrication between the polymer and the metal surface of the processing equipment, and include PE homo polymers, paraffins, esters, metallic soaps, amides, fatty acids and oxidised PE. In injection moulding they help with mould release and reduce the process temperature. However, an excess of external lubricants can promote plate out, cause delamination, lower weld line strength and cause surging with slow fusion. 

Selden and co-workers [59] measured weld line tensile strength of injection moulded glass fibre reinforced PA 6 and talc filled PP. Fracture surfaces were also examined using SEM. 

For fast injection and consistency of moulding, the melts are momentarily ( 0.1s) pressurized by the screw rams before injection into the mould - this feature is programmed into the moulding cycle. Fast injection also helps avoid weld lines due the less time available for the melt to cool before welding takes place. Ten-stage programmable injection speed controls for both cylinders is incorporated on the machine together with closed-loop facilities. 

How Line. Streaks of different reflectance from the surrounding area, arising in glass and in injection moulded parts, from the flow pattern of material during forming the part see chill mark weld mark. 

Components often include discontinuities which are not present in the testpieces from which performance data are derived. Welded joints are an obvious example apart from the incipient weakness to which all materials are subject, reinforced materials are especially vulnerable, because there will be no fibres crossing the boundary. In injection moulded components, weld lines formed where two streams of melt have met are always likely to display weakness again, reinforced materials are more susceptible. 

Another cause of failure in PP artefacts is the poor quality of the weld lines. Weld lines are inherent weak positions due to the poor frsion of the material at molecular level. However, the strength of welds can be maximised by favourable moulding conditions such as sufficiently high melt temperature, high tool temperature, heated metal inserts and fast injection times. Similarly to other plastics, more attention should be paid to filled or reinforced grades of PP. 

Figure 8.1 Conventional multiple injection moulding always causes weld lines in the plastic, and there is a risk of air being trapped between them [1]...

Figure 8.2 Sequential moulding of a box with flexible hinge provides freedom in locating the weld line. 1,2- injection points...

Weld lines in injection-moulded immiscible blends model predictions and experimental results. Polymer, 36, 2033-2042. 

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