Injection moulding shrinkage

Dried granules may be injection moulded, blow moulded and extruded without undue difficulty with melt temperatures of 220-260°C. The moulding shrinkage (0.003-0.004 cm/cm) is very low and a somewhat greater than normal draft may be required in deep draw moulds. 

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. 

Another example is the injection moulding of a pail, where the injection takes place at the bottom. The most removed part, the brim of the pail, will solidify first, thus impeding the shrinkage of the remaining part, in particular of the bottom. The bottom is, therefore, heavily stressed and may spontaneously break in shock loading. A better construction of the mould can prevent this drawback, namely by making the bottom not flat but somewhat hollow, so that the stresses easily disappear by a minor deformation of the bottom. 

CONTRIBUTION OF FLOW DEFORMATION TO THE SHRINKAGE OF INJECTION MOULDED PVC... 

The dimensional reproducibility, between a number of injection mouldings from a mould, is usually extremely good. However, if two parts are to be assembled without the application of excessive force, the separation of the assembly points must fall between close limits. The linear shrinkage S of a moulding is defined by... 

A frequent problem in repeated processing is chain scission. It result in a reduction of melt viscosity thus making extrusion difficult, an increase in the brittleness of the material making demoulding after injection moulding difficult, and an increase in melt shrinkage (Table 3.3). Furthermore, the final properties of the material in terms of dimensional stability, impact resistance, chemical resistance, and electrical properties are adversely affected. Several additives... 

Both figs. 12a and b show also somewhat unexpected results, in so far as a density decrease with pressure occurs at all depths in the sample. The reduction of density due to pressure is minimum at the sample surface and grows with depth. Furthermore, the majority of the density change is observed by varying the pressure from 0.1 to 8 MPa, which is quite low especially if comp)ared with the typical pressure values used in injection moulding. This experimental result may be relevant for modeling the shrinkage and the internal stress distribution in injection molded products. Particularly important is the fact that this effect is more pronounced in the bulk of the sample. 

Linear thermal expansion testing helps to determine if failure by thermal stress may occur in products and materials. Precise knowledge of the CTE can be utilized to estimate the thermal stresses. This aspect makes CTE to an important property of the used fiber for composite materials. A rule of mixtures is sufficient for calculating the CTE of polymers filled with powder or short fibers. In case of long libers, the rule of mixtures is valid perpendicular to the reinforcing fibers. Molecular orientation affects the thermal expansion of polymers. Processing also affects CTE, for semicrystalline polymers this fact is very important. For that reason, CTE measurements are often used to predict shrinkage in injection moulded parts. 

Plastics - Injection moulding of test specimens of thermoplastic materials - Part 4 Determination of moulding shrinkage... 

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