Abstract:Residual density distributions are determined in injection‐molded poly(methyl‐methacrylate) (PMMA) with the aid of a quantitative Schlieren optical technique. The gapwise distributions typically show a maximum beneath the surface. The height of the maximum as well as the level in the core vary with distance from the gate of the mold. The density distributions and the influence of the molding parameters are explained by the pressure course measured in the cavity and the process of vitrification of the sample du… Show more
“…Again, the pressure does not affect the shape of the recovery curves. This is in clear contrast to, for instance, PMMA and PS, where recovery of residual compression leads to an expansion of the samples with time at room temperature. , 7 Volume recovery of PC samples at 23 °C vitrified under pressure and by injection molding. Injection molded: (◇) without packing pressure; (○) with packing pressure.…”
Section: Resultsmentioning
confidence: 99%
“…This is in clear contrast to, for instance, PMMA and PS, where recovery of residual compression leads to an expansion of the samples with time at room temperature. 16,17 The volume recovery of samples containing additives is shown for a few examples in Figure 8 together with a fast-cooled sample. (Note the vertical offset.)…”
The specific volume change of Bisphenol A polycarbonate was
measured at room temperature
for several years. The effects of formation conditions like
cooling rate and pressure, the addition of low
molecular additives, and injection molding and quenching were
investigated. The volume shrinkage at
long times was found to be much higher than extrapolated from short
term measurements. A general
pattern is observed in the volume recovery curves. They start with
a small constant slope on the
logarithmic time scale. After about 107 s a transition
is observed to a much steeper slope. Whereas in
the first section the characteristic increase in τeff is
observed, the volume recovery during the transition
can be fitted quite well with a single exponential. The sharp
transition suggests that there is a long
retardation time, which may represent the genuine α-relaxation.
It also means that the self-retardation
during physical aging is finite. The slope at long times
(>107 s) scales with the distance from
equilibrium,
as extrapolated from measurements close to T
g.
The cooling rate during vitrification affects the slope
in
the first zone only, while the formation pressure leads to a vertical
shift of the curves. Addition of low
molecular weight additives leads to a dramatic densification and
accelerates the volume recovery.
“…Again, the pressure does not affect the shape of the recovery curves. This is in clear contrast to, for instance, PMMA and PS, where recovery of residual compression leads to an expansion of the samples with time at room temperature. , 7 Volume recovery of PC samples at 23 °C vitrified under pressure and by injection molding. Injection molded: (◇) without packing pressure; (○) with packing pressure.…”
Section: Resultsmentioning
confidence: 99%
“…This is in clear contrast to, for instance, PMMA and PS, where recovery of residual compression leads to an expansion of the samples with time at room temperature. 16,17 The volume recovery of samples containing additives is shown for a few examples in Figure 8 together with a fast-cooled sample. (Note the vertical offset.)…”
The specific volume change of Bisphenol A polycarbonate was
measured at room temperature
for several years. The effects of formation conditions like
cooling rate and pressure, the addition of low
molecular additives, and injection molding and quenching were
investigated. The volume shrinkage at
long times was found to be much higher than extrapolated from short
term measurements. A general
pattern is observed in the volume recovery curves. They start with
a small constant slope on the
logarithmic time scale. After about 107 s a transition
is observed to a much steeper slope. Whereas in
the first section the characteristic increase in τeff is
observed, the volume recovery during the transition
can be fitted quite well with a single exponential. The sharp
transition suggests that there is a long
retardation time, which may represent the genuine α-relaxation.
It also means that the self-retardation
during physical aging is finite. The slope at long times
(>107 s) scales with the distance from
equilibrium,
as extrapolated from measurements close to T
g.
The cooling rate during vitrification affects the slope
in
the first zone only, while the formation pressure leads to a vertical
shift of the curves. Addition of low
molecular weight additives leads to a dramatic densification and
accelerates the volume recovery.
“…This actually produces high pressure gradients along the part, creating high internal stresses upon cooling. For an amorphous polymer it would show up as a high birefringence ( 18). There are reports (9, 10) on the dominant effect of internal pressure history on internal stresses in molded parts.…”
Section: Effect Of Packing Pressme and Gate Thickness On The Shrinkagementioning
Despite a significant number of publications and the increasing use of numerical simulation, there is still a debate about the optimum gate design and packing conditions in the molding industry. Shrinkage uniformity for unfilled polymers is dominated by the time dependent pressure distribution in the cavity and the resulting volumetric shrinkage; gate freeze‐off is obviously important and difficult to predict; and pressure gradients during the packing phase, depend on process and design parameters and are also affected by the mold elasticity. Molding trials have been conducted on an instrumented mold (fan gated rectangular slab, 2 mm thick) under a variety of processing conditions and with different gate thicknesses using HDPE (Solvay Eltex A1050). Pressure decay during the molding cycle at different locations along the flow path have been correlated with sample thickness distribution. Overpacking at moderate packing pressure is shown to be a direct consequence of mold elasticity and to be related to both filling flow rate and gate thickness. The decay to a finite residual pressure can be computed by coupling the mold elasticity with the PVT behavior of the polymer. The results highlight the importance of gate design and processing parameters on the dimensional accuracy of the part and low internal stress level. When dealing with thick gates, packing pressure profiling appears to be the best way to avoid gate area overpacking. Mold elastic deformation can play a significant role in the cavity pressure‐time history, even for a seemingly stiff mold construction.
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