Shrinkage semi-crystalline

Shrinkage and coefficient of thermal expansion are those of semi-crystalline polymers, that is to say, rather high. The absorption and swelling by moisture exposure are high (see Figure 4.54). Creep depends on reinforcement, moisture content and temperature. 

When designing plastic parts it is often recommended that the part have uniform thickness. This is especially true for semi-crystalline polymers where thickness variations lead to variable cooling times, and those in turn to variations in the degree of crystallinity in the final part. Variations in crystallinity result in shrinkage variations, which lead to warpage. However, it is often necessary to design parts in which a thickness variation is inevitable, i.e., extrusion profiles with thickness variations as shown in Fig. 6.14. 

RWM type b is semi-crystalline material and needs up to 5 min to build up the required handling strength. After this open time, the wettability ends and simultaneously high forces are built up in the adhesive material. Solidification results in shrinkage and leads to stress cracks at the interface of the bonding and in the adhesive. It behaves like candle wax. The cut-off string is minimal, since a short chain semi-crystalline binder is used. 

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. 

Plastics shrink in molds during the cooling process. Shrinkage for amorphous plastics is less than for semi-crystalline plastics. Shrinkage is larger in the thickness direction than other directions. Orientation of plastic occurs in the direction of flow. 

Amorphous thermoplastics display very low shrinkage when they solidify, typically between 0.5% and 1%. Semi-crystalline materials shrink very much more, usually between 1.5% and 5% depending upon the particular material. 

The higher shrinkage with the semi-crystalline materials is due to the repeat units along the molecular chains being of such a form that they can pack very closely together in an ordered marmer. By use of appropriate moulding conditions it is possible to vary the extent of the crystalline areas. For example. 

Amorphous plastics are basically transparent (exceptions styrene copolymers containing butadiene (B), such as SB and ABS). Amorphous plastics have lower shrinkage values than semi-crystalline compounds, so with amorphous compounds higher levels of accuracy can be obtained at lower cost than with semi-crystalline materials. 

Shrinkage Processing shrinkage 1-2%. Postshrinkage with semi-crystalline PET only becomes negligibly small at mould temperature of 140 °C. 

Amorphous materials can be thermoformed with less residual stress than semi-crystalline resins. Retained volume is nearly 100 percent for monolayer COC-LLDPE films with COC content between 15 and 30 weight percent. This result reflects the amorphous nature of COC which is imparted into the films, even at relatively small amount. Less stress means less post-form shrinkage or snap-back, giving the flexible package the appearance of a rigid one. Film structures with high LDPE content tend to have lower retained volume. Formed cavities from other semicrystalline materials, such as nylon and PP, usually suffer some loss in volume. 

---