The cover lens cavity of the injection molding tool is shown in Fig。 1。 This tool has been designed with a flash (film) gate, which is preferred for parts with large areas, and a cold material trap that is useful to avoid the ingress of solidified material through the mold cavity, which would cause imperfections in the part。

The photoelasticity technique will be described in the next section, used to analyze how the streamline behavior occurs in the gate of a PCL during qualitative analysis。 Fig。 2 shows the cover lens analyzed in this investigation which has been produced using an amor- phous PC。 Fig。 3 shows a section of a PCL extracted for analysis, where cracks have been presented in PCLs。 This sample is under polarized light, allowing the monochromatic lines to be observed and associated with the corresponding fringe order values found

Fig。 3。 Sample subject to analysis under polarized   light。

in the literature。 The dimensions of the samples shown in Fig。 3 are a length of 120 mm, a width of 95 mm, and a uniform wall thick- ness of 2。8 mm。

2。2。 Photoelasticity technique

The photoelasticity technique is a nondestructive test method applied to analyze transparent or translucent plastic materials with- out harming or altering the properties of the samples。 It is described in ASTM D4093-95, which consists in the determination of resid- ual stress by the effect of double refraction, where the measurements of the direction of principal strains using a compensator belong to the scope of this test method。

Photoelasticity is based on the phenomenon of birefringence, which denotes anisotropy due to strained transparent material and as a consequence the index of refraction, n, it becomes directional。 In this case the transmission of light through strained materials obeys the following two laws, which form the basis of photoelastic stress   determination:

1。 The light is polarized in the direction of the principal-stress axes and is transmitted only on the planes of principal   stress。

2。 The velocity of the transmission on each principal plane is dependent on the intensities of the principal stresses in these two planes and the difference of the refraction index given by the following   equations:

δ1 ¼ n1−n0 ¼ Aσ1 þ Bσ2 。

δ2  ¼ n2−n0  ¼ Aσ1 þ Bσ2

where δi  refers to the refraction index on the I axes,

n0 is the refractive index of unstressed  material, n1 the refractive index on principal plane no。  1, n2 the refractive index on principal plane no。  2, σ1  and σ2      the principal  stresses,

A and B    the photoelastic constants of the  material。

In this case, the difference between the refractive indices on the principal planes is given by the  equation

δ1−δ2  ¼ n1−n2  ¼ ðA−BÞðσ1−σ2Þ ¼ CB ðσ1−σ2Þ ð2Þ

where CB  is the differential-stress optical constant。

As a result of their velocity difference, the waves vibrating along two principal planes will emerge out of phase, and their relative distance, or retardation, δ, is given by:

δ ¼ ðn1−n2Þt ¼ CBtðσ1−σ2Þ ð3Þ

where t is the thickness of material crossed by the light and λ is the wavelength of light (570 nm for plastic according to ASTM standards); where the Eq。 (4) expresses the stress-optical   law。