MOULD ABLE

Cellular Polymers II. Conference proceedings. Edinburgh, 23-25th March 1993, paper 29. 6124 SHAPE MOULD ABLE POLYPROPYLENE PARTICLE FOAMS PROGRESS REVIEW Cousin J R BASE pic... 

Cellular Polymers.Conference Proceedings. London,20th-22nd March 1991,Paper 34. 6124 RECENT DEVELOPMENTS IN SHAPE MOULD ABLE PARTICLE POLYOLEFIN FOAMS... 

If the compound in the mixture includes a clay, there also needs to be present water since its presence renders the material more readily mould-able. The presence of clay and water in appropriate amounts has profound and helpful consequences for the formation of ceramic materials into objects of desired shape. In the absence of clay and water, particles usually require very high pressures to be formed into objects since the friction forces within the assembly of particles have to be overcome. The applica-... 

A number of injection mouldings have been prepared from CAB with about 19% combined acetic acid and 44% combined butyric acid. Their principal end products have been for tabulator keys, automobile parts, toys and tool handles. In the United States CAB has been used for telephone housings. Extruded CAB piping has been extensively used in America for conveying water, oil and natural gas, while CAB sheet has been able to offer some competition to acrylic sheet for outdoor display signs. 

Whilst the injection moulding process has now been widely accepted for phenolics the transition from compression moulding has been less extensive with U-F materials. The basic reason for this is that the U-F materials are more difficult to mould. This has been associated with filler orientation during moulding, which can lead to stress peaks in the finished product which the somewhat brittle resin in less able to withstand than can a phenolic resin. 

Nowadays the position is changing because, as ever increasing demands are being put on materials and moulding machines it is becoming essential to be able to make reliable quantitative predictions about performance. In Chapter 4 it was shown that a simple Newtonian approach gives a useful first approximation to many of the processes but unfortunately the assumption of constant viscosity can lead to serious errors in some cases. For this reason a more detailed analysis using a Non-Newtonian model is often necessary and this will now be illustrated. 

Thermoplastic polymer macromolecules usually tend to become oriented (molecular chain axis aligns along the extrusion direction) upon extrusion or injection moulding. This can have implications on the mechanical and physical properties of the polymer. By orienting the sample with respect to the coordinate system of the instrument and analysing the sample with polarised Raman (or infrared) light, we are able to get information on the preferred orientation of the polymer chains (see, for example, Chapter 8). Many polymers may also exist in either an amorphous or crystalline form (degree of crystallinity usually below 50%, which is a consequence of their thermal and stress history), see, for example, Chapter 7. 

The sheet of paper was made by a mould, composed of a frame covered with fine wiro-cloth, having a movr able edge called a deckel. This mould the workman dipped into tfie. vat, lifting a portion of the pulp on its surface. The water flowed through the wire-cloth, and. left the fibre on the top. The workman then removed the deckel, and passed the mould to another,... 

The year after, the same group developed a system based on the same anti-idiotypic approach and a similar template possessing a chiral centre, in order to investigate the possibility of controlling the enantioselectivity of the reaction uniquely by the direct moulding of the cavity. In this case the polymer was able to give 67% ee as opposed to the control polymer, which produced a virtually racemic mixture [32]. 

Among other things Sphinx sanitary ware manufactures washbasins and lavatory pans by means of slip casting. The moulds must be able to absorb a lot of water, for example 3 to 4 kg for a single basin. 

Microtransfer Moulding. A PDMS stamp is filled with a prepolymer or ceramic precursor and placed on a substrate. The material is cured and the stamp is removed. The technique generates features as small as 250 nm and is able to generate multilayer systems. 

In some cases the raw material is already in a fluid state with thermosets as a resin for casting or impregnating, with thermoplasts, as an exception, sometimes as a monomer which is able to polymerise in the mould. In these cases the forming processes are relatively simple, since they can be carried out on low-viscous fluids which do not require high pressures to be transported. 

Casting is the simplest processing technique the mould is filled with a liquid, which, at ambient temperature, has such a low viscosity that it is able to fill the mould under its own weight. After filling, a chemical reaction should take place to form the material. With thermosets this is a reaction between the components which, prior to filling, have been mixed together, and which react into a three-dimensional network. Sometimes a catalyst is added to initiate or accelerate this reaction. Dependent on the combination chosen, curing takes place at room temperature or at an elevated temperature. 

The user first must select the type of plastic to be used, then the grade appropriate for the purpose—bearing in mind the specification and any special requirements in the mouldings, like transparency, translucency, gloss or matt finish. The suppliers usually are able to supply information to assist the selection, either verbally or in writing, and may be able to help with demonstrations and tests before production is started. On occasion, when large quantities of material are likely to be required, a grade may be developed specially for a particular purpose. 

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