雙面微結構超薄件微射出成型之實驗探討
Date Issued
2004
Date
2004
Author(s)
黃勝田
DOI
zh-TW
Abstract
This study is devoted to investigating the micro-injection molding of ultra-thin parts. The thin rectangular (36 mm × 60 mm) sheets with thicknesses of 500, 300, 200, 100 and 60 μms are molded with a 15-ton injection molding machine. There are several aspects in this study: First of all, with aid of short shots, the shapes of melt fronts are observed to trace the process during filling an H-type symmetric four-cavity mold and a symmetric eight-cavity mold. The effects of thickness, mold temperature, injection speed and melt temperature on filling patterns are systematically investigated. Secondly, the moldability is defined and the operation windows of different melts (PS, PMMA, PC) in molding thin parts of different thicknesses are compared. Also the moldability in molding parts with features and without features on part surfaces are investigated. The effects of processing parameters on transcription ratio (TR) of micro features, and on the distribution of orientation and residual stress of the injection parts are studied. Finally, the cycle times of the molding process using a mold with rapid mold-heating and cooling system are measured.
Based on the short shots, it is noticed that when filling the symmetrical 4-cavity mold for the cavity of 100μm thick in the mold at low mold temperature (80℃), the melt fronts in the four cavities are not asymmetrical and bends outward. The major cause is the hesitation effect of flow between thin-walls. When the mold temperature is raised or the thickness of runners and cavities is increased, the melt fronts become symmetrical. When filling the symmetrical 8-cavity of 100μm thick in the mold at low mold temperature, hesitation phenomenon occurs in the 4 inner cavities; but the filling of the 4 outer cavities continues with symmetric melt fronts, overtaking the 4 inner cavities. Afterwards, the inner 4 cavities experience a pierce-through and are filled again with the asymmetric melt fronts bending outward. Similarly, after raising the mold temperature or increasing the cavity and runner thickness, conventional filling pattern are observed; the inner 4 cavities are filled with symmetrical melt fronts, overtaking the melt fronts in outer 4 cavities.
Moldability is defined as the “molding area” on the mold temperature-injection speed plane. The thinner the cavity thickness is, the smaller the operation window is. The operation windows are different in size, area and shape for three materials (PS, PMMA and PC). The required injection speed for PS is the lowest. In this study, the thinnest part ever successfully molded is the ultra-thin part of 30μm thickness. Molding parts with or without micro-features make no significant difference if the feature size is small compared to the thickness. But molding micro ultra-thin (60μm) parts with micro-lenses (25μm radius, 2.8μm deep) and grating (3μm pitch, 1μm deep) features on surfaces demands much higher injection speed than other thicker parts.
The transcription of micro-features is satisfactory as long as the operation conditions is inside the molding area, since the micro-features is of low aspect-ratio. The molecular orientation and residual stress are compared with aid of birefringence observed with polarized light, it is found that the moldings molded with high mold temperature yields least orientation and residual stress. For injection molding ultra-thin parts with lowest cycle time, a rapid mold-heating and cooling system must be employed, and lowest mold temperature and highest demold temperature allowed should be used.
Subjects
雙面微結構
多模穴
薄件
微射出成型
Thin part
Molding parts with micro-features on both surfaces
Micro-injection molding
Multi-cavity
Type
thesis
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