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Abstract：The existing research on shrinkage of the injecfion molded plastic part mainly focuses on various shrinkage ratios of the part
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dimensions，and the relevant experimental studies belong to mere dimension measurement after demouiding．Obviously,measuring after
the plastic part is demoulded from the cavity Call not offer shrinkage displacements of points on the plastic part．However,shrinkage
displacements of points on aninjection molded plastic part are essential for exposing the inner relation among shrinkage ratios of
various dimensions ofthe part．So visualization of the in-mold plastic part which can indicate the location relationship betweell the part
and the cavity is needed．In this paper,a visual injeetion mold was fabricated by adopting the half mold structure and light transmission
manner．With the visual mold,in-mold shrinkage images of injection molded plastic parts were photographed after the plastic part
stayed in the injection mold for 24 h．By means of digital image processing of the in-mold shrinkage images，the experimental data of
shrinkage displacements of points on injection molded parts were researched．From the experimental data，it is found that shrinkage
directions ofpoints on all injection molded part are related with both positions ofthe gate and ofthe part centroid,and either the gate or
the centroid will exert more influence on the shrinkage direction of some point which is closer．Furthermore，some pDint at the later
filled area has more shrinkage distance than the poim at the earlier filled area．Combination of shrinkage directions and shrinkage
distances of points on an injection molded part determine shrinkage ratios for various dimensions of the part，and shrinkage directions
are more influential to shrinkage ratios of dimensions．This experimental research of shrinkage displacements offers a unique approach
to understand the shrinkage principles of injection molded parts．Key words：injection molding，shrinkage displacement，visual mold,light transmission
1 Introduction 7268
During injection molding process，the plastic part will
shrink due to the phase change from liquid to solid，and the
shrinkage makes the plastic part smaller than the cavity．For
precise plastic parts，shrinkage during the injection molding
process needs to be given careful consideration．
Effects of processing parameters on longitudinal
shrinkage and perpendicular shrinkage were investigated by
POS玑～ⅥrA，et al【10 and LIAO，et al吲．POMERLEAU，et
al■compared shrinkages f-0r different mold geometries．
SPERANZA。et alt4j。found that shrinkage increased wim
increasing distance from the injection point．And effects of
plastic materialsl’J and mold materialsl01 on shrinkage have
been reported．Previously reported experimental studies
about shrinkage of injection molded plastic parts were
eonducted by measuring dimensions of the plastic part and
of the cavity after the plastic part was demoulded from the
cavity．Although shrinkage ratios for dimension of an
’Corresponding author．E-mail：zhutieli@dlut．edu．c11
This project is supported by National Key Technology R&DProgram of
omaaumgthelithFive-YearPlanPeriod(GrantNo．2006BAF04813)
injection molded part can be obtained by above—mentioned
method，the inner relation among different shrinkage ratios
for various dimensions of the plastic part is not known．The
underlying problem is that how some point of the molded
part moves owing to plastic shrinkage is not clear．
Obviously,shrinkage displacements of points on an
injection molded part determine shrinkage ratios for
various dimensions of the part．In order to know about
shrinkage principles of injection molded parts，shrinkage
displacements of points on an injection molded part should