Since the polymer can be compressed, high pressure results in a high density product. Minimizing the pressure and heat losses is possible by shortening flow length due to absence of the sprue, and the central gate location in the moulds using HRS. Therefore, the same pressure level is more influential in the cavity of the mould using HRS than the mould using CRS. When investigating both run- ner systems comparatively, it can be seen that higher weights of injected parts are achievable at the same process temperature in the case of HRS. The increase of density varies between 2.02%  and  3.58%  for ABS polymer,   and

0.43 and 3.39% for PP  polymer.

7. Conclusion

In this experimental study, optimal process conditions, variations in length-wise and width-wise shrinkage rates, warpage rates and densities of samples produced of ABS and PP polymers were determined with respect to the changes in process temperature and injection pressure when using HRS, in comparison with CRS. It was observed that the required injection pressure in HRS was considerably lower. When using HRS, injection moulding process can be performed at lower process temperature and injection pressure than the case of using CRS. It was noted that the pressure gain can reach up to 33.33%  for  ABS  and 42.85% for PP. Such a saving in required power results in accordingly less energy consumption by the injection moulding machine and hence smaller machines with less power can be utilized for producing relatively large compo- nents. This gain reduces the requirement for mould clamp- ing force, increases the lifetime of the mould and injection machine, and allows the significant cuts in production costs. It was observed that the shrinkage rate decreased with increasing injection pressure for both runner systems. Results showed that the usage of HRS decreases the shrinkage rates for both of the polymers in comparison with CRS. This shrinkage-decreasing effect of HRS results from more influential packing stage due to late solidifica- tion of the gates, lower heat and pressure losses and   better 论文网

fluidity of molten plastic.

When using HRS on multi-cavity moulds, nozzle-gate can be located symmetrically for each cavity. Thus, weak- ening tendency of packing pressure on outer regions from the gate can be reduced by decreasing the ratio of flow length to thickness. This provides decrease in pressure losses, contributes to achieving more influential packing stage and reduction in shrinkage and   warpage.

It was observed that the shrinkage and warpage rates generally increase with the increase of process temperature. It was noted that the shrinkage rates in length and width were not the same. This persity caused by cooling condi- tions and flow direction of melt in the cavity is decreased with using HRS.

It is found that higher sample densities are achievable when using HRS than in the case of using CRS. It is noted that the increase in density can reach up to 3.58% and 3.39% for ABS and PP polymers, respectively. Increase in weight and density of injected parts implies that their mechanical properties become better due to denser structure. Another interesting point is that low rates of shrinkage and warpage occur when the weight is high. In order to produce the sam- ple having dimensional stability and low rate of shrinkage and warpage, the injection pressure providing the highest sample weight can be adopted by experimenting different values of pressure at defined  temperature.

References

[1] Nikbay N. Application of hot runner systems on plastic injection moulding. Mechanical Engineering Chamber Seminar Notes, Istan- bul, 14–16 October 1998 [in Turkish].

[2] Bushko WC, Stokes VK. Solidifications of thermoviscoelastic melts. Part 3: effects of mold surface temperature differences on warpage and residual stresses. Polym Eng Sci 1996;36(3):322–35.