Industrial aspects of polymer processing

Transcription

1 Course MP10 Lecture 2 Industrial aspects of polymer processing Ben, I just want to say one word to you, just one word plastics Dr James Elliott 2.1 General model of polymer processing Overview of the various stages: Mixing Addition of additives to alter physical, mechanical or aesthetic properties of the finished product. Melting Liquefy polymer and destroy memory of previous chain configurations. Melt flow Flow and shaping of the liquid by mechanical processes Cross-linking Formation of chemical cross-links to induce dimensional stability Cooling and solidification Transfer of heat away from polymer, crystallisation or glass formation. 1

2 2.2.1 Polymer processing: mixing Can add reinforcing fillers (e.g. carbon black, silica) to toughen the polymer, or non-reinforcing fillers (e.g. calcium carbonate, china clay) to cheapen the mix. Plasticisers are used in cases where it is desirable to increase flexibility, for example PVC is often plasticised with a high boiling point ester. Chemical cross-linking agents need to be added to thermoset polymers, for example in the vulcanisation of rubbers using sulphur. Can add chemical blowing agents (e.g. sodium bicarbonate) to create polymer foams. Use of pigments or dyes to colour the product Polymer processing: mixing Only at very high viscosity can both distribution and dispersion occur simultaneously. Usually aim for distributive mixing when blending dry powders or low viscosity liquids. Aim for dispersive mixing when compounding rubbers or high viscosity liquids. 2

3 2.3 Polymer processing: melting Need to be aware of heat transfer issues, as degradation temperature of most polymers is not far above T m. Not only is there heat supplied to the polymer by an external source, but there is also frictional heating due to the heat dissipated by shearing a highly viscous fluid. Often, the viscous heat is a considerable proportion of the total heat required to work the polymer. So much so, in fact, that it is often possible to make the process selfsufficient after the first few batches have been melted via external heating. Crystalline polymers (HDPE, PP) require far higher inputs of energy to melt than amorphous ones (upvc, PS). 2.4 Polymer processing: melt flow Many polymer processing methods involve a polymer being driven through channels of various dimensions (extrusion dies, injection moulding gates and moulds). The resulting orientation induced in the polymer depends on the Deborah number for that process, and can give rise to several non-ideal effects such as: Melt fracture On die entry, extensional flow gives rise to tensile stress which exceed that of the melt, causing flow instabilities. Die swell and surface texturing On die exit, molecules close to the walls of the channel become shear-oriented and the extrudate swells anisotropically on cooling. At high shear rates, causes surface texturing effects such as sharkskin, orange peel and bambooing. 3

4 2.5 Melt fracture Melt fracture occurs when laminar streamlines are disrupted by too sharp a die entry profile. Can reduce this effect by tapering the die entry profile so that streamlines remain laminar. 2.6 Die swell and bambooing Die swell is a result of elastic recovery of compression. Bambooing is a result of the release of wall friction. 4

5 2.7 Process memory More generally, process memory is the effect by which the melt remembers what it has seen before entering the die. Quantify using Deborah number. Flow from bend to die: T relax = η/e = 10 5 /10 3 = 100 s. T proc = 10 s. N deb = 10 process is elastic Flow from through die: T relax = η/e = 10 3 /10 5 = 0.01 s. T proc = 0.1 s. N deb = 0.1 process is viscous 2.8 Compression moulding Squeeze molten polymer between hydraulic press. 5

6 2.9 Injection moulding Control head pressure using screw angle and variable die/shaft profile Blow moulding Not to be confused with film blowing (which is an extrusion-based process). 6

7 2.11 Extrusion Control head pressure using screw angle and variable die/shaft profile Film blowing Produces less oriented films than extrusion. 7

8 2.13 Polymer processing: cooling, solidification Often the most limiting factor in the process is the transfer of heat away from the polymer. This is particularly a problem for thermoplastics, where dimensional stability is not achieved until the temperature falls well below T m. Due to the relatively low thermal conductivity of polymers, the designer must keep the wall thicknesses as small as possible to reduce the total cooling time to reasonable values. Even so, typical cooling times for thermoplastic processes are between seconds (the largest proportion of the process time). Interlude: industrial video Compression moulding Injection moulding Extrusion Blow moulding (Rotational casting) (Vacuum forming) 8

9 C-MOLD Finite element package for process modelling and optimisation. Takes as input the thermo-physical data on fluid, and uses this to do finite element simulations including the effects of: Temperature Melt flow velocity Shear stress and strain Cooling and solidification Allows the optimisation of process design as a function of parameters such as melt temperature, die shape, ram speed profile and process time C-MOLD : Filling velocity 9

10 C-MOLD: Temperature profile C-MOLD: Filling shear rate 10

11 C-MOLD: Wall shear stress Processing design Exercise in controlling melt flow velocity (MFV) and melt flow area (MFA) using C-MOLD. [Taken from C-MOLD Design Guide.] 11

12 Processing design Change injection gate position. [Taken from C-MOLD Design Guide.] Processing design Change ram-speed profile. [Taken from C-MOLD Design Guide.] 12

13 Lecture 2 summary In this lecture, we reviewed some practical aspects of polymer processing, and described a range of industrial processes used to form polymers into useful objects. We reviewed their general uses, and advantages and disadvantages when applied to the manufacture of a range of types of product. We also saw how process design can be optimised using finite element packages such as C-MOLD. In the next lecture, we will look at a case study of the modelling of processing of liquid crystalline polymers. 13