Sinks and Voids

All injection-moulded thermoplastics are prone to sink marks and voids in areas where sudden changes in section thickness occur, or over ribs and bosses. Semi-crystalline polymers such as PP are more prone to sinks and voids. Voids occur when the external skin of the moulding is rapidly cooled and becomes sufficiently rigid to support the contraction of the underlying melt. Sinking or surface depression occurs in localised thick sections where internal mass contains sufficient heat to keep the polymer in molten stage and erystallises slowly producing sink marks. 

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For Hifax PP/EPDM blends, a general purpose screw is recommended. The clamping pressure increases with part size. The nozzle should be as short as possible with orifice larger than usual. All tools should be vented trapped air may cause localized burning, surface sinks and voids. Keeping the moving half of the mold cooler than the stationary part aids demolding. 

Injection molding machines Resin should be dried < 0.02 %. Do not use resin that has been out of the dryer for > 20 min. Cold molds are difficult to fill and require higher injection pressures. Hot molds, generally, give better finish and less molded-in stress. Melt temperature is very sensitive to very small changes in rpm or back pressure despite sensor or controller set point. Measure with hand held pyrometer or laser sensor. Need slower fill rate for spme-gated parts to prevent blush, splash or jetting. If the walls are > 5 mm, then slow fill helps reduce sinks and voids. 

Extruded parts are normally one-dimensional and of a simple cross-sectional shape (curtain rail, rainwater gutters, pipe, or films which are produced from a very wide die and hauled off at a faster rate to produce orientation). In the extrusion of sections, the part thickness will affect distortion, and may lead to sinking and voiding. The polymer chains are extended in the direction of extrusion and the properties are correspondingly anisotropic. 

High thickness favours sinking, warpage and shrinkage, porosity and voids. Designing ribs or gussets can reduce wall thickness. 

Through a compression and decompression action the density of the mold contents is increased, and the more intimate contact with the mold walls improves the heat reduction. The surface of the molded part is substantially improved and voids as well as sink marks disappear. The improved dissipation of the heat reduces the cycle time up to 15% depending on the polymer. 

The microstructure of a silicone resin emulsion paint is shown in Fig. 3. The overview even at low magnification (Fig. 3a) indicates that one binder forms thick films (black arrows) into which sinks some of the approx. 300-nm-sized white pigment (titanium dioxide). This is the organic polymeric binder. Because of the deficit of organic polymer, pore voids are open and voids between fillers are only partly filled (Fig. 3c), i.e., only small adhesive bridges are present (black arrows in Figs. 3a,c), which determine the binding power. 

In the latter reaction, additional lattice sites are created, so that the vacancies must be annihilated at vacancy sinks, or voids and cavities are created. In the case of prevailing anion diffusion, growth of oxide lattice and consumption of metal lattice occur at the interface... 

In any case, the effect of the thickening of the stud, plus the gussets, will increase the mass of plastic at the base of the stud and, especially with high-shrinkage materials, sinks or voids can be expected if fast cycling of the mold is planned. If voids or sinks are not permissible, the molding cycle must be increased to allow this area to be packed out. 

Changes in wall thickness not only interrupt flow, but also give rise to problems of differential shrinkage, including sink marks, voids, warping, and moulded-in stress. Sink marks are particularly prominent opposite ribs in mouldings made from semi-crystalline polymers, as illustrated in Fig. 8.6. 

Parts should be designed with the minimum wall thickness that will provide the required structure. Also, walls should be kept as uniform as possible. Any wall variation should blend as gradually as possible. Where considerable wall variations are required, it would be better to make several parts and assemble them. Thick and thin sections in the same part can result in voids, sinks, and warpage. 

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