MOULD COOLING

The temperature of the plastic melt decreases with outward flow from the feed point, and the temperature of the water increases as it flows through the mould. This effect could be utilised to counter the temperature drop in the material so that all the portions of the moulding are cooled at the same rate, i.e., using a co-current system of cooling where the cold water enters to a point close to the feed systems and travels outward in the direction of melt flow. This would ensure, for all practical purposes, the 

Narrow cores may prevent the incorporation of adequate cooling channels. In such instances, the cores tend to overheat and, apart from increasing the cooling time, this can cause the walls on each side of the core to shrink towards each other after the moulding is ejected. Advantage may be taken of the superior heat transmission characteristics of some alloys, such as beryllium-copper, to improve this situation. The core is then provided with generous cooling at the base. 

Turbulent flow in the cooling channels is more efficient than laminar flow. This can be achieved by decreasing the size of the channels or by increasing the flow rate. This runs coimter to popular belief that large cooling channels are always better. It is difficult to achieve turbulent flow with oils or antifreeze solutions due to their higher viscosities. 

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It is immediately apparent that in many processes involving rubber heat flow across the interface between two surfaces has to be considered. This is true in mixing, moulding, cooling after processing and conditioning of test pieces but, nevertheless, very little attention has been paid to the measurement of the coefficient. The effect of the heat transfer coefficient on net heat flow is greatest with thin articles and where one of the materials is a gas. It is probably reasonable to assume a value of infinity for the transfer coefficient when rubber is pressed into intimate contact with a metal but in other cases it will be finite. 

In the cooling stage of processing, the product must completely solidify before the mould can be opened. Figure A.4 in Appendix A shows the dimensionless temperature profiles at different times since the start of cooling. Products cooled from both sides, such as injection mouldings, cool faster than products of the same thickness (extruded pipe, blow moulded and thermoformed) where one side is effectively thermally insulated. The temperature profiles are given in terms of the Fourier number, a dimensionless time defined by Eq. (A.26) as... 

The injection moulding machine is an 80-ton clamp Negri-Bossi. Temperatures in plasticizing and injection sections are 185 C. The mould cooling water is at 70°C. 

Injection moulding Processing cost high in comparison to amorphous materials. Mould cooling critical. Warpage and dimensional tolerances difficult. 

Thick ribs Increase mould cooling in rib zone. Alter product design. Move injection site. 

Fig. 19 Processing route to directly form simple geometries from woven polymer fibre plies. Fabric plies (or a piecmisolidated fabric plate) (a), may be placed in a constraining Irame (b) to prevent shrinkage if preheating is required and also to prevent the fabric plies being pulled excessively into the mould during forming. The clamped fabric plies may then be preheated in an overt, and then stamped in a cold mould (c), or placed cold in a mould and heated in situ. The part may he consolidated in the mould, cooled and removed (d). Adapted from [22]...

Good Planning needed for effective Mould Cooling , C E Waters. Plastics Technology, 1970,16, (12), 43. 

With thermoplastics the main purpose of the mould system is to minimise both the cycle time and thermal differences in mould part cooling. Mould cooling is therefore essential for both cost saving and quality control. Uniform cooling improves product quality by preventing differential shrinkage, high residual stress and mould release problems. 

Since the process is basically independent on the ceramic powder composition while driven by the ice growth, samples with homogeneous isotropic rounded porosity could be obtained pouring the slurry into moulds cooled at the liquid nitrogen temperature of -196 °C, as found for other materials (Landi et al 2008). Similarly, samples with graded porosity, from globular to elongated shapes, could be obtained within the same sample (Landi et al., 2008). 

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