Shrinkage voids

These bubbles may originate from the adhesive packaging but can also be due to shrinkage of the adhesive as it cures. Most of the acrylic-based adhesives will shrink on cure slightly whilst epoxies tend to show lower shrinkage. Applying smaller volumes of adhesive will minimise the shrinkage voids and so in some applications it may be necessary to apply the adhesive in two lots . 

Flfnme 111 Optical micrograt of shrinkage voids In an alunlna moulded part. 

Pressure testing of the finned oil cooler in Fig. 15.29 revealed leaks. Examination of the interior of the cooler after sectioning in the vicinity of the leaks revealed a small cavity in the weld zone in the corner of some fins (Fig. 15.14) and porous areas inside the channel in the welded zone in other fins. Microstructural examinations of specimens cut through the sites revealed interconnected voids resulting from either shrinkage during solidification of the weld or lack of fusion of the base metal and weld metal. 

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. 

Normal reheat treatment can partially restore blade properties however, it does not appear to be capable of full property recovery, although the miscrostructures are comparable to new blades. This shortcoming implies that cavitation may be present and was not removed by conventional reheat treatment. Hot isostatic press (HIP) processing is an alternative that ensures void removal. It has demonstrated its ability to remove even gross internal shrinkage porosity in investment castings. The results of HIP treatment... 

Uniform wall thickness Wall requirements are usually governed by the load, the support needs for other components, attachment bosses, and other protruding sections. Designing a product to meet all these requirements while still producing a reasonably uniform wall will greatly benefit its durability. A uniform wall thickness will minimize stresses, differences in shrinkage, possible void formation, and sinks on the surface it also usually contributes to material saving and economy in production. 

There are certain basic rib-design guidelines that should be followed (Fig. 3-9). The most general is to make the rib thickness at its base equal to one-half the adjacent wall s thickness. With ribs opposite appearance areas, the width should be kept as thin as possible. In areas where structure is more important than appearance, or with very low shrinkage materials, ribs are often 75 or even 100 percent of the outside wall s thickness. As can be seen in Fig. 3-10, a goal in rib design is to prevent the formation of a heavy mass of material that can result in a sink, void, distortion, long cycle time, or any combination of these problems. 

Shrinkage The transition from room temperature to a high processing temperature may decrease a plastic s density by up to 25 %. Cooling causes possible shrinkage (up to 3 % ) and may cause surface distortions or voiding with internal frozen strains. As discussed in other chapters, this situation can be reduced or eliminated by special techniques, such as controlled cooling under pressure. 

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. 

In reality, around an inclusion embedded in a matrix a rather complex situation develops, consisting of areas of imperfect bonding, permanent stresses due to shrinkage, high stress-gradients or even stress-singularities, due to the geometry of the inclusions, voids, microcracks etc. 

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. 

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