Mold cooling rate

A higher mold temperature favors a uniform melt cooling rate, minimizing residual stresses, and improves the surface finish, mold release, and product quality. The mold cooling rate affects finished product quality. Polyether type TPU can set up better and release better. 

Injection molding is the most commonly used processing technique for engineering thermoplastics. Typically, the polymer pellets are melted and the melt pulled forward by means of a screw as in extrusion, so filling a mold under appropriate pressure. The shape of the mold, the number and relative location of the injection devices, and the mold cooling rate determine, together with the intrinsic properties of the material, the final quality of the molded articles. Very complex... 

Table 9.7. Effect of Mold Cooling Rate on the Part Properties...

Number of cavities, layout and size of cavities/runners/gates/cooling lines/side actions/knockout pins/etc. Relate layout to maximize proper performance of melt and cooling flow patterns to meet part performance requirements preengineer design to minimize wear and deformation of mold (use proper steels) lay out cooling lines to meet temperature to time cooling rate of plastics (particularly crystalline types). 

In continuous extrusion blow molding, the preform is continuously produced at the same rate as the article is molded, cooled, and released. To avoid interference with the preform formation, the mold-clamping step must be rapid to capture the preform and move it to the blow mold station. There are various modifications of this that allow essentially continuous operation. 

Crystallization temperature and the heat of crystallization at various cooling rates (or the ejection temperature of the molded part) ... 

The polymers used and some of their physical properties are listed in Table I. Polymers were mixed and blended on a two-roll mill at 450 K. Samples were compression molded at 450 K for 7 min and cooled in the press with tap water for 5 min. ASTM D412 6.35-mm (Va in.) dumbbells were cut parallel to the mill grain from sheets having 1.9-mm (75 mils) thickness. Instron tensile tests were carried out at least 48 hr after molding. Pull rate was 50.8 cm/min (20 in./min). 

Crystallization and shrinkage were influenced by cooling rate (part thickness and mold temperature). While the oil-heated mold maximized crystallization, cooler (water-heatable) molds produced crystallinity levels of 25% or better (Chapter 1). Parts molded with the high mold temperature did exhibit better surface finishes. Shrinkage was relatively low for all processing conditions and design variables. 

Fig. 13 Influence of the cooling rate on the impact performance of /S-modified PP (MFR 0.3 dg min-1). The specimens were compression molded and tested at 1.5 ms-1. The arrows indicate the temperature at which the ductile-brittle transition occurred...

Fig. 14 Spherulitic structure of compression molded samples (core) obtained with different cooling rates a rapid, b conventional and c slow...

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