The occurrence of surface roughness in gas assist injection molded nylon composites

Abstract

Gas assist injection molding has increasingly become an important industrial process because of its tremendous flexibility in the design and manufacture of plastic parts. However, there are some unsolved problems that limit the overall success of this technique. The purpose of this report was to study the surface roughness phenomenon occurring in gas assist injection molded thermoplastic composites.The materials used were 15 % and 35% glass-fiber filled nylon-6 composites. Experiments were carried out on an 80-ton injection molding machine equipped with a high-pressure nitrogen-gas injection unit. Two "float-shape" axisymmetric cavities were used. After molding, the surface quality of molded parts was measured by a roughness meter. Various processing variables were studied in terms of their influence on formation of surface roughness: melt temperature, mold temperature, melt filling speed, short-shot size, gas pressure, and gas injection delay time.Scanning electronic microscopy was also employed to characterize the composites.It was found that the surface roughness results mainly from the exposure of glass fiber in the matrix. The jetting and irregular flows of the polymer melt during the filling process might be factors causing the fiber exposure.

Influence of injection molding on the flexural strength and surface quality of long glass fiber-reinforced polyamide 6.6 composites

“…We also note that the presence of glass fiber increases the roughness of the composites molded from that material approximately five times compared with the sample of pure PA 6.6. The roughness values and their relationship with the process parameters are compatible with the literature [13,20,33].…”

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.…”

“…Researchers both in academia and industry have tried to improve the surface aesthetic of the reinforced plastic parts moulded by CIM. Liu and Chang [18,34] studied the influence of process parameters on the surface roughness of the short glass fibre reinforced plastic (SGFRP) part in gas-assisted injection moulding (GIM) process. It was found that the exposure of the fibre from the resin matrix should be responsible for the rough surface.…”

Parameters affecting the full‐shot molding of gas‐assisted injection‐molded parts

“…A pressure-controlled nitrogen gas injection unit (Gas Injection Model PPC-1000, UK) was attached to the machine. 7 A plate cavity with an exchangeable gas channel of three different geometries (trapezoid, semicircle, and rectangle) across the center was used in this study. Figure 3 shows the dimensions of the cavity and the exchangeable mold inserts.…”