Plastic products are cracked, don't blame our molds

Time：2020-04-17 09:05:57 / Popularity： / Source：

Cracking, including filamentary cracks, microcracks, whitening, cracking on the surface of part, or trauma crisis caused by die sticking of part and sticking die of runner. According to cracking time, it is divided into mold cracking and application cracking. Main reasons are as follows:

1. Processing:

(1) Excessive processing pressure, too fast speed, more filling, longer injection and holding time will cause excessive internal stress and cracking.
(2) Adjust mold opening speed and pressure to prevent cracking of demolding caused by rapid and strong drawn parts.
(3) Properly increase mold temperature to make part easy to demold, and appropriately lower material temperature to prevent decomposition.
(4) Prevent cracking due to mechanical strength from being lowered caused by weld marks and degradation of plastic.
(5) Appropriate use of mold release agent, eliminate aerosol and other substances attached to mold surface.
(6) Residual internal stress of part can be eliminated by annealing heat treatment immediately after forming to reduce generation of cracks.

2. Mold:

(1) Ejection should be balanced. For example, number of ejectors and cross-sectional area should be sufficient. Mold release slope must be sufficient. Cavity surface must be smooth enough to prevent cracking due to residual stress concentration caused by external force.
(2) Structure of part must not be too thin. Transition part should use arc transition as much as possible to avoid stress concentration caused by sharp corners and chamfers.
(3) Minimize use of metal inserts to prevent internal stress from increasing due to difference in shrinkage between insert and product.
(4) Appropriate demolding air inlet channels should be provided for deep bottom parts to prevent formation of vacuum negative pressure.
(5) Mainstream channel is big enough for ambassador's gate material to be demolded before it is cured, which is easy to demold.
(6) Joining of main stream bushing and nozzle should prevent drag of cold hard material and make product stick to fixed mold.

3. Material:

(1) Content of recycled material is too high, which causes strength of product to be too low.
(2) Excessive humidity causes chemical reactions between some plastics and water vapor, reducing strength and causing ejection cracking.
(3) Material itself is not suitable for environment being processed or quality is poor, and pollution will cause cracking.

4. Machine:

Plasticizing capacity of injection molding machine should be appropriate. If it is too small, it will not be fully mixed and brittle. If it is too large, it will degrade.

Cracks require metallographic analysis of mold

Name	Definition	Feature
Austenite	Solid solution of carbon and alloy elements dissolved in γ-Fe, still maintaining face-centered cubic lattice of γ-Fe	Grain boundaries are relatively straight and regular polygons; retained austenite in quenched steel is distributed in spaces between martensite needles.
Ferrite	Solid solution of carbon and alloy elements dissolved in a-Fe	Slow-cooled ferrite in hypoeutectoid steel is lumpy and grain boundaries are relatively smooth. When carbon content approaches eutectoid composition, ferrite precipitates along grain boundaries.
Cementite	A compound of carbon and iron	In liquid iron-carbon alloy, cementite (primary cementite) that crystallizes separately is massive, angle is not sharp, eutectic cementite is skeletal. Carbides (secondary cementite) precipitated along Acm line during cooling by eutectoid steel are network-like. Eutectoid cementite is flaky. When iron-carbon alloy is cooled below Ar1, cementite (tertiary cementite) is precipitated from ferrite, and it is in the form of discontinuities on secondary cementite or grain boundaries
Pearlite	Mechanical mixture of ferrite and cementite formed by eutectoid reaction in iron-carbon alloy	Distance between pearlite pieces depends on degree of supercooling during austenite decomposition. The greater degree of undercooling, the smaller distance between pearlite flakes formed. Pearlite flakes formed at A1 to 650℃ are thicker, parallel wide strips of ferrite and thin strips of cementite can be distinguished at a metallographic microscope and zooming more than 400 times. which are called coarse pearlite, flake pearlite, and pearlite for short. Pearlite formed by pearlite at 650 ~ 600℃ is magnified 500 times with a metallographic microscope. Only a black line is seen from pearlite cementite. Layer that can be resolved only by magnification of 1000 times is called soxite. Pearlite formed at 600 ~ 550 ℃ is magnified 500 times with a metallographic microscope. Pearlite flakes cannot be distinguished. Only black pellets are seen. Flakes that can only be resolved by magnification of 10,000 times with an electron microscope are called bainite bodies.
Upper bainite	A mixture of supersaturated acicular ferrite and cementite, with cementite between ferrite needles	Phase change product of supercooled austenite at medium temperature (about 350 ~ 550 ℃), its typical form is a bunch of ferritic slabs with an approximately parallel azimuth difference of 6 ~ 8od, with short carbide rods or small pieces arranged along long axis direction of slabs; typically upper bainite is feather-shaped, grain boundaries are axes of symmetry. Due to different orientations, feathers can be symmetrical or asymmetrical, ferritic feathers can be needle-shaped, dot-shaped and block-shaped. If it is high-carbon and high-alloy steel, needle-shaped feathers cannot be seen clearly; medium-carbon medium-alloy steel, needle-shaped feathers are clearer; low-carbon low-alloy steel, feathers are very clear, and needles are thick. During transformation, upper bainite is first formed at grain boundaries, and grows into grains without penetrating grains.
Lower bainite	Same as above, but cementite is inside ferrite needle	Transformation products of supercooled austenite at 350℃ ~ Ms. Its typical form is biconvex lens-shaped ferrite containing supersaturated carbon, and there are distributed unidirectional carbide flakes in it; Needle-shaped in crystal, needle leaves do not cross, but can be connected. Different from tempered martensite, martensite can also be divided into layers, and lower bainite has same color. Carbonized material point of lower bainite is thicker than tempered martensite, which is susceptible to erosion and blackening. Tempered martensite is lighter in color and less susceptible to erosion. High-carbon high-alloy steel has higher carbide dispersion than low-carbon low-alloy steel, and needles are finer than low-carbon low-alloy steel.
Granular bainite	Large-scale or strip-shaped ferrites with multiple islands	Transformation products of supercooled austenite at the uppermost part of bainite transformation temperature zone. When it was first formed, it consisted of lumped ferrite and small island-shaped carbon-rich austenite combined from strip ferrite. Carbon-rich austenite may be retained as residual austenite in subsequent cooling process. It may also be partially or completely decomposed into a mixture of ferrite and cementite (pearlite or bainite); It is most likely partially transformed into martensite and partially retained to form a two-phase mixture called M-A structure.
Carbide-free bainite	Single-phase structure of lath-shaped ferrite, also called ferrite bainite	Formation temperature is at the uppermost part of bainite transformation temperature region. Between lath ferrite is carbon-rich austenite, carbon-rich austenite has a similar transformation in subsequent cooling process. Carbide-free bainite generally occurs in low-carbon steels, and is also easily formed in steels with high silicon and aluminum content.
Martensite	Supersaturated solid solution of carbon in a-Fe	Lath Martensite: It is formed in low and medium carbon steel and stainless steel. It consists of a number of parallel laths in a lath bundle. One austenite grain can be transformed into several lath bundles (usually 3 to 5). Lamellar martensite (acicular martensite): common in high, medium carbon steels and high Ni Fe-Ni alloys, a stitch in needle leaves divides martensite into two halves. Due to different orientation, it can be needle-shaped or block-shaped. Needles are arranged at an angle of 120°. Needle boundaries of high-carbon martensite are clear, fine needle-shaped martensite is cloth-like, known as cryptocrystalline.
Lycite	Eutectic mixture of austenite and cementite	Dendritic austenite is distributed on matrix of cementite
Tempered martensite	Martensite decomposition to obtain a very fine transition carbide and supersaturated (low carbon) a-phase mixed structure	It is formed by tempering martensite at 150 ~ 250 ℃. This kind of structure is extremely susceptible to corrosion. It has a dark black needle-like structure (maintaining quenched martensite orientation) under optical microscope, which is similar to lower bainite. Very fine carbonized material spots can be seen only under a high-power electron microscope.
Tempered bainite	Carbide and a-phase mixture	It is formed by tempering martensite at 350 ~ 500 ℃. Its microstructure is characterized by extremely fine granular carbides distributed in ferrite matrix. Needle-like morphology has gradually disappeared, but it is still faintly visible. Carbides cannot be distinguished under optical microscope. Only dark tissues is observed, two phases can be clearly distinguished under electron microscope. It can be seen that carbide particles have grown significantly.
Tempered sorbite	With ferrite as matrix, uniform carbide particles are distributed on matrix	It is formed by high temperature tempering of martensite at 500 ~ 650 ℃. Its microstructure is a multiphase structure composed of equiaxed ferrite and fine-grained carbides. Traces of martensite flakes have disappeared, shape of cementite has been clearer, but it is also difficult to distinguish under light microscope, and cementite particles visible under electron microscope are large.
Granular pearlite	Composed of ferrite and granular carbides	It is formed by spheroidizing annealing or tempering of martensite in temperature range of 650 ℃ ~ A1. It is characterized by carbides distributed in granular form on ferrite
Weiss organization	If austenite grains are relatively coarse and cooling rate is more appropriate, first eutectoid phase may be in the form of needle-like (flaky) morphology mixed with flaky pearlite, which is called Weiss microstructure.	In hypoeutectoid steel, ferrite in Weiss structure has flaky, feathery, or triangular shapes, and coarse ferrite is distributed in parallel or triangular shapes. It appears at austenite grain boundaries, and grows into grain at the same time. In hypereutectoid steel, morphology of cementite in Wei’s structure is needle-shaped or rod-shaped. It appears inside austenite grains.

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