Mould Dimension

All rubber products exhibit shrinkage after cure, mainly due to the thermal expansion which occurs at vulcanization temperature. Moulded rubber goods are never as big as the moulds in which they are cured. The difference between the dimensions at room temperature of the finished goods and of the mould expressed as a percentage is called the shrinkage from mould dimensions. 

Mould dimension - The size of the mould is limited by the size of the moulding press. The mould size limits the number of cavities that can be cut in it. Plate thickness should never be less than 9.5 mm at the thinnest point. The closing plate should be 18 mm thick so that dowel pins and locating pins remain firm. 

Measured as a percentage, ASTM D1299. (Important for mould dimensions where allowance for shrinkage has to be made. This usually ranges from 0.5% to 5.0% depending on the plastic employed.)... 

The analysis of mould filling requires rheological and thermal data for the plastic, and the mould dimensions. Polymer manufacturers usually provide shear flow curves at a range of temperatures these can be approximated by a power law relationship over a limited range of shear strain rates. In the days before computer analysis, flow lengths of short shots were determined in spiral test cavities, as a function of the injection pressure. However, the geometry of this constant cross section mould differs so much from most other moulds that the flow lengths in the two types of mould do not correlate well. 

Nearly no shrinkage of finished products versus mould dimensions and no dips arise even for thick walled parts. 

All abrasive cleaning processes may significantly change mould dimensions over a period of repeated cleaning cycles. For this reason it is imperative that regular dimensional checks are made and recorded for each mould. 

Mould dimensions must be increased to take account of the average shrinkage of 1.5-2% for HOPE and 2-3% for LDPE, according to the thickness of the item (Figure 6.1). For filled PE, account must be taken of the reduction in shrinkage of 1.7 to 1.1 % when the filler content increases from 15 to 45% independent of the filler type. 

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. 

The allowable dimensional variation (the tolerance) of a polymer part can be larger than one made of metal - and specifying moulds with needlessly high tolerance raises costs greatly. This latitude is possible because of the low modulus the resilience of the components allows elastic deflections to accommodate misfitting parts. And the thermal expansion of polymers is almost ten times greater than metals there is no point in specifying dimensions to a tolerance which exceeds the thermal strains. 

Following the tolerance stack through the end assembly, the bobbin dimension of 22 mm from the outside face to the back face of the magnetic pole is analysed next. This characteristic dimension does not include the tolerance on the impact extruded pole. The pole is to be moulded into the bobbin and the pole face is considered to be part of a mould related dimension. The bobbin is injection moulded using 30% filled polybutylene terephthalate (PBT). The tolerance assigned to the bobbin dimension is 0.035 mm. 

The pole has a eharaeteristie dimension of 8 mm from the rear of the bobbin to the reeess faee and has a toleranee assigned to it of 0.02 mm. From the pole reeess faee, the tube base toleranee is the last eomponent to make up the toleranee staek. The brass tube has been given a dimensional toleranee on its base of 0.2 0.025 mm. Note, the dimensional toleranee on the plunger is 28 0.05 mm, but the analysis will eoneentrate on the silieone rubber seal length of 6 mm beeause this is moulded onto the plunger and again is a mould related dimension. 

Note The injection/compression moulding process capability maps 1, 2 and 3 are used for large parts with a major dimension greater than 50 mm typically and/or for large production volumes. Map 4 is for injection moulded parts that have a major dimension less than 150 mm and which are produced in small volumes. 

BS 7010 1988 A System of Tolerances for the Dimensions of Plastic Mouldings. London BSI. PD 6470 1981 The Management of Design for Economic Manufacture. London BSI. 

Another common problem on moulding is voids. These occur when a moulding or extrudate cools and hardens rapidly on the surface. On further cooling the moulding or extrudate cannot shrink inward because the outside dimensions are fixed and therefore the molecules are pulled towards the surface of the product, generating internal voids. At the same time molecules in the inner... 

The above process is limited to simple shapes whose principal dimension is not more than four times, and preferably less than twice, that of the next largest dimension. More intricate shapes must be made by machining or in some instances by a coining operation which involves stamping a sintered moulding of the same weight and approximate dimensions as the finished part at 320°C. 

Such lenses may be made by machining from rod. More recently processes have been developed where the monomers are cast polymerised in tiny plastics moulds whose cavity corresponds to the dimensions of the lens and using procedures very reminiscent of those described for the manufacture of acrylic sheet (see Section 15.2.2). 

This expression therefore enables the thickness of the moulded article to be calculated from a knowledge of the die dimensions, the swelling ratio and the mould diameter. The following example illustrates the use of this analysis. A further example on blow moulding may be found towards the end of Chapter 5 where there is also an example to illustrate how the amount of sagging of the parison may be estimated. 

In practice the clamping pressure will also depend on the geometry of the cavity. In particular the flow ratio (flow length/channel lateral dimension) is important. Fig. 4.42 illustrates typical variations in the Mean Effective Pressure in the cavity for different thicknesses and flow ratios. The data used here is typical for easy flow materials such as polyethylene, polypropylene and polystyrene. To calculate the clamp force, simply multiply the appropriate Mean Effective Pressure by the projected area of the moulding. In practice it is... 

These calculations can give a useful first approximation of the dimensions of a thermoformed part. However, they will not be strictly accurate because in a real situation, when the plastic sheet is being stretched down into the cold mould it will freeze off at whatever thickness it has reached when it touches the mould. 

Calendering can achieve surprising accuracy on the thickness of a sheet. Typically the tolerance is 0.005 mm but to achieve this it is essential to have very close control over roll temperatures, speeds and proximity. In addition, the dimensions of the rolls must be very precise. The production of the rolls is akin to the manufacture of an injection moulding tool in the sense that very high machining skills are required. The particular features of a calender roll are a uniform specified surface finish, minimal eccentricity and a special barrel profile ( crown ) to compensate for roll deflection under the very high presurres developed between the rolls. 

For a steel mould of the same dimensions and thickness, a quick calculation (A = 11 W/m K, Cp = 480 J/kg K and p = 7850 kg/m ) shows that the steel mould would take three times longer to heat up. However, in practice, steel moulds are less than a third of the thickness of aluminium. Therefore, although aluminium has a better thermal conductivity, steel moulds tend to heat up more quickly because they are thinner. 

Change in dimensions of an unvulcanised rubber (calendered sheet or extruded section) on cooling from the processing temperature. Also the volume contraction of a moulded rubber product on cooling from vulcanising temperature. See Coefficient of Thermal Expansion (Volumej. Shrinking... 

With compression moulding, to ensure dimensional consistency, it is necessary to allow the excess material to move away from the edge of the cavity so that the lands between the cavities can contact with minimum thickness of rubber (flash) between them. Spew grooves and channels are provided of sufficient dimensions to accommodate this excess, and also to allow the escape of air from the mould cavity. In some cases, where the shape is complex, it may be necessary to provide extra venting to allow air to escape from a blind area, where it is likely to be trapped. 

Framework of load compensator on plane wings injection moulding of carbon fibre reinforced PEEK replaces the aluminium alloy previously used. This part plays a critical role in plane safety and must resist the static and dynamic stresses and hydraulic fluids. The grade selected after many tests has a high fluidity allowing the manufacture of parts with dimensions of 200 mm by 400 mm. With 30% carbon fibre reinforcement, this PEEK grade ... 

ISO 7823-3 2003 Plastics - Poly(methyl methacrylate) sheets - Types, dimensions and characteristics - Part 3 Continuous cast sheets ISO 8257-1 1998 Plastics - Poly(methyl methacrylate) (PMMA) moulding and extrusion materials - Part 1 Designation system and basis for specifications ISO 8257-2 2001 Plastics - Poly(methyl methacrylate) (PMMA) moulding and extrusion materials - Part 2 Preparation of test specimens and determination of properties ISO 12017 1995 Plastics - Poly(methyl methacrylate) double- and triple-skin sheets - Test methods... 

DNPT)) were studied using a gas evolution apparatus. The decomposition temperature of ADC decreased with both DNPT and 4,4-oxybis(benzenesulphonyl hydrazide) (OBSH) blending and this affected the structure and properties of the resulting foams. Using a tube mould for an extrudate to vulcanise the NR/EPDM extradate in a hot air oven was found to control the expansion and foam dimensions. The NR compositions affected the foam structure and properties. 16 refs. 

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