Variable mold temperature to improve surface quality of microcellular injection molded parts using induction heating technology

Abstract

Microcellular foam injection molding provides many advantages over conventional foams and their unfoamed counterparts, but its applications are limited by visible surface quality problems such as silver streaks and swirl MICROCELLULAR INJECTION MOLDED PARTSmarks. In this study, we propose a variable mold temperature method to improve the surface quality of molded parts. Electromagnetic induction heating is used in combination with water cooling to achieve rapid mold surface temperature control during the microcellular foam injection molding process. The effect of processing parameters, such as mold temperature, melt temperature, and injection velocity on the part surface quality, was investigated using surface roughness measurements and visual inspection of the molded parts. The results show that using induction heating to increase the mold surface temperature from 100• C to 160• C can decrease surface roughness of polycarbonate moldings from 25 μm to 6.5 μm. It was also found that the flow marks formed by gas bubbles on the part surface can be removed completely at a mold temperature of 160• C. Further increases in the mold temperature show slight improvements in the surface roughness up to 180• C, at which point the surface roughness starts to level off at 5 μm. This surface roughness value reflects an 80% improvement without a significant increase in cycle time over parts molded at a mold temperature of 60• C using water heating. Higher melt temperature and faster injection speed will also improve the surface quality of microcellular injection molded parts but not as significantly. The usefulness of a variable mold temperature in improving part surface quality during microcellular foam injection molding has been successfully demonstrated.

Structure and mechanical properties of polystyrene foams made through microcellular injection molding via control mechanisms of gas counter pressure and mold temperature

“…Several ways has been proposed to improve the surface roughness of polymer foamed products, including control of SCF content [14], material modification, combined with In Mold Decoration (IMD) process to have decorated film on part surface, and co-injection to produce a solid skin and microcellular foamed core [15]. Further possibilities are the use of coated surfaces or film-insert mold surface to provide heat insulation, leading to a higher mold temperature during melt-filling stage [16][17][18]. But, among these methods, besides IMD and co-injection molding, MuCell can prevent part surface from defects at high expense.…”

Effects of cavity surface temperature on reinforced plastic part surface appearance in rapid heat cycle moulding

“…For CMTC of CIM, the cavity surface temperature during filling process should be much lower than the glass transition temperature or melt temperature of the plastic resin to ensure a short moulding cycle and competitive production efficiency. The low cavity surface temperature makes the polymer melt freeze prematurely and consequently a frozen layer will formed during filling process at the interface between the hot polymer melt and the cold mould cavity, which leads to a series of defects of the final moulded parts, such as flow mark [11,12], weld mark [13,14], swirl mark [15][16][17], roughness [18,19], low gloss [20][21][22], and low replication accuracy [23,24]. For DMTC of RHCM, the cavity surface temperature during filling process has much smaller negative influence on the moulding cycle because the mould can be cooled as soon as possible after filling.…”

Gas-assisted mold temperature control for improving the quality of injection molded parts with fiber additives

“…Consequently, many new injection molding technologies have been investigated [1][2][3][4][5]. In traditional injection molding, the cavity surface did not require heating.…”