Sink mark

Because of low injection pressure, some cost savings are possible in mold and press constmction. Mol ding cycles are somewhat longer than for injection molding. The part must be cooled in the mold long enough to be able to resist swelling from internal gas pressure. In stmctural foam parts there is almost a total absence of sink marks, even in the case of unequal section thickness. Stmctural foam has replaced wood, concrete, sohd plastics, and metals in a variety of appHcations. 

Abrupt changes of section cause poor flow and differential shrinkage, giving sink marks (Fig. 28.11 - you can find them on the surface of many small polymer parts), distortion, and internal stress which can lead to cracks or voids. The way out is to design in the way illustrated in Fig. 28.12. Ribs, which are often needed to stiffen polymer parts, should have a thickness of no more than two-thirds of the wall thickness, and a height no more than three times the wall thickness. Corners are profiled to give a uniform section round the corner. 

Fig. 28.11. Changes of section cause distortion, sink-marks and internal stresses in moulded plastic ports.

The internal pressures can prevent the formation of sink marks, particularly on faces opposite to reinforcing ribs. 

Thick mouldings may be produced, again without distortion such as sink marks. 

Variations in thickness without sink marks due to presence of an internal pressure caused by the entrapped gases. 

However, as will be shown in Chapters 4 and 5, it would not be good design practice to use a plastic beam Section 7.6 mm thick. Such a section would take a long time to mould and would probably have sink marks on its surface. It would be preferable to use thinner wall sections and introduce ribs to provide the desired stiffness. This is illustrated below. 

The recommended radius not only reduces the brittleness effect but also provides a streamlined flow path for the plastic melt in the mold cavity. The radiused corner of the metal in the mold reduces the possibility of its breakdown and thus eliminates a potential repair need. Too large a radius is also undesirable because it wastes material, may cause sink marks, and may even contribute to stresses from having excessive variations in thickness. 

As discussed in the previous section on injection molding, a sink mark almost always occurs in extrusion on a flat surface that is... 

Fig. 3-34 Sink marks can be eliminated by creating a design, rib, or serration.

These are usually self-adhesive, precut, printed patterns on a substrate that are simply adhered to the surface of a product. Decals generally use a transparent plastic film while labels are usually on an opaque plastic, metallic and multilayer sandwich base. Labels of sufficient thickness are useful for hiding unavoidable appearance problems such as gate and sprue removal areas, sink marks, blushes, splays, and weld lines. 

As reviewed there is much to consider. Examples include cooling as the product sets up results in different shrinkage rates for thicker versus thinner sections in the different processes. This results in either external waviness or sink marks, or warpage and internal voids, as the product contracts. Flat surfaces are difficult to maintain but not impossible to attain using certain processes. High speed of flow to fill the cavity of the mold is impeded going around square corners, so provision for radii and fillets are important. 

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. 

Parts are less sensitive to sink marks, thickness variations and residual stresses. Thicknesses, 5-15 mm, can be higher than with dense compounds bosses and ribs can be oversized. 

Avoid thick sections. Thick sections add to the material cost and increase sink marks caused by shrinkage. 

Attention at the design stage to differences in section thickness also can be beneficial. The position of features such as strengthening ribs can be important—they may give rise to sink marks on the opposite surface, the unsightliness of which may be exaggerated by painting. Marks of this kind are especially noticeable when otherwise a surface is flat and featureless. 

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