MOULD FLOW

This rubber has a very high melt viscosity and this was reduced by using a polar flow promoter such as zinc stearate at levels of 9.5 and 19%. This not only reduced the viscosity at low shear rates but also increased the level of pseudoplasticity so that at the high shear rates used in injection moulding flow was even more enhanced. 

NMR microscopy is appropriate to study the flow behavior of complex materials, the flow in complex geometric structures and processes such as extrusion, injection moulding, flow in nozzles, pipes, etc., because the velocity vectors can be directly... 

Flow cracks. Incorrect shape and size of blank, incomplete mould flow high viscosity stock and excessive mould lubricant. Modify the shape, reduce temperature, modify curing system, peptize and pre warm the stock. 

Rough surface distortion Use of scorched blank, less mould flow time, slow closing of the mould. Reject seemingly scorched stock, correct ram speed. 

Scaling up, however, a single spiral mould test is not possible. Therefore, Fritch recommends that test should be carried out at three melt temperatures and three injection rates for a total of nine tests, in such a way that they cover melt temperature and mould flow rate in commercial processing. 

Flows can be classified into streamline, when particles in the fluid follow paths (streamlines) that remain constant with time, and turbulent, when vortices cause unpredictable changes in the flow pattern with time. The changeover occurs at a critical value of the Reynolds number, which is defined as the melt velocity, divided by the viscosity times the channel diameter. The high viscosity of thermoplastic melts causes velocities to be low. Hence, the Reynolds number is very low and the flows are streamline. We will consider steady flows, and ignore the start and end of injection and blow-moulding flows, when the melt accelerates and decelerates, respectively. However, in the RIM process (Section 5.6.5), turbulent flow of the low viscosity constituents in the mixing head achieves intimate mixing. 

A number of manufacturers have introduced ATH grades that are claimed to offer better dispersibility and mould flow. The Dutch company Ankerpoort has marketed a synthetic ATH called Portaflame SG40 for use in cable jackets, resilient flooring, SMC and BMC thermoset compositions, epoxies and carpet backing. 

DuPont s Fusabond AEB-560D is a modified ethylene-acrylate copolymer for use in polyamides. It is claimed to be a cost-effective toughener, more effective than maleic anhydride terpolymers and usable at low temperatures, while improving mould flow, with less of an adverse effect on the flexural modulus. 

Injection moulding Flow behaviour Extrusion strength... 

CR grades are available both as homopolymers and copolymers. Copolymer-based CR grades for injection moulding flow well and are highly resistant to warpage and internal stresses. These grades find application in thin-walled packaging for food and... 

Venting is necessary to avoid a short shot or burning of material and also to allow easy and quick mould filling at a lower injection pressure. In some cases the location of vents is quite obvious, while in complex cases mould flow simulation packages can predict the location of vents before machining the mould. Thin-walled mouldings require high injection speeds. In such cases the location of the vents is important. 

Mould Shrinkage A measurement of the contraction of the moulded material in comparison with the original mould dimensions. Two values may be quoted where material flow induces anisotropy in material microstructure, e.g. preferential alignment of reinforcing phases. Example 2.0/3.0 indicates 2.0 % shrinkage in direction of material flow, with 3.0% shrinkage transverse to mould flow direction. 

Key Words Injection moulding, Flow visualization, Flow simulations. 

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