Cause analysis and solution of the bad phenomenon

2022-08-13
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Cause analysis and Countermeasures for the defects of formed products

mainly analyze and judge the reasons for the defects of formed products at present, and provide reference factors in molding machines, molds and raw materials, so as to effectively control the occurrence of defects and reduce production costs

1 sore: (silver stripes)

on the surface of the finished product, there are many silver stripes centered on cate, which are basically produced along the flow direction of raw materials. This phenomenon occurs after the accumulation of many adverse conditions. Sometimes it is difficult to grasp the real reason

1.1 if there is water or other volatile components in the raw materials and they are not fully dried, a lot of silver bars will be produced on the surface

1.2 when other raw materials are accidentally mixed into the raw materials, it will also form sores, which are in the shape of mica or needle points, which are easy to be different from the sores caused by other reasons

1.3 this situation is also easy to happen when the raw materials or material pipes are not clean

1.4 the injection time is long, and the temperature of raw materials injected into the mold cavity at the initial stage is low. As a result of solidification, volatile components will not be eliminated, especially for temperature sensitive raw materials

1.5 if the mold temperature is low, the raw material solidifies quickly and is prone to (1.4) conditions, so that the volatile components will not be discharged

1.6 when the mold exhaust is poor, the gas is not easy to be eliminated when the raw material enters, and it will produce sores. In this situation, the top of the finished product is often burnt black

1.7 if water is attached to the mold, the heat brought by the filling raw material will evaporate and fuse with the molten raw material to form a sore in the form of protein fog

1.8 there is cold material or small in the cold material nest of the glue channel. When it is injected, the cooled raw material is brought into the mold cavity, and part of it will solidify rapidly to form a thin layer. When the mold temperature is low at the beginning of production, it will also open into sores

1.9 during the filling process, the raw materials form a thin layer due to the rapid cooling of the contact part of the mold cavity surface, which is melted and decomposed by the following raw materials to form a white or stain shape, which is mostly seen in thin shell products

1.10 when filling, the raw material can form turbulent flow, so that the flow path of the raw material is extended, and friction is caused by the influence of the structure in the mold cavity. In addition, the filling speed is faster than the cooling speed of the raw material, and the gate position is at the muscles and bones or small, which is easy to cause sores, and the place where the meat thickness of the finished product is sharply increased is also easy to cause sores

1.11 gate and flow channel are small or deformed, filling speed is fast, and friction occurs instantly, causing the temperature to rise sharply, causing the decomposition of raw materials

1.12 raw materials contain recycled materials, which are not fully dried and decompose during injection, resulting in sores

1.13 raw materials stay in the tube for a long time, causing partial overheating and decomposition

1.14 insufficient back pressure and air entrainment (insufficient compression ratio)

blistering: Table 1

2 glue line

glue line is a small line produced by raw materials at the confluence, which is produced due to incomplete fusion. There are thin lines on the same part on the front and back of the finished product. If the temperature of one side of the mold is high, the glue line in contact with it is shallower than the other side

1 increasing the temperature of raw material and increasing the injection speed will reduce the glue line

2 increase the mold temperature and increase the fluidity of raw materials in the mold, then the temperature of the raw material when it meets perpendicular to the axis is higher and the glue line is reduced

3 the position of the cate determines the position of the glue line. Basically, the positions of the glue line are in the same direction

4 if there is oil or other non-volatile components in the middle of the mold, they are concentrated in the joint and are not fully fused to form the glue line.

5 due to the influence of the mold structure, it is impossible to completely eliminate the glue line, so it is more important not to restrict the removal of the glue line when adjusting the machine, but to minimize the adverse phenomena caused by the glue line

glue line: Table II

the gap generated inside the wall thickness of the finished product. Opaque products cannot be seen from the outside, but must be planed before they can be seen

the center of the wall thickness is the place where the cooling is the slowest, so the rapidly cooled and rapidly shrinking surface will pull up the raw materials to produce voids and form bubbles

1 the injection pressure should be as high as possible to reduce the shrinkage of raw materials

2 when the thickness of meat on the molded product changes sharply, the cooling speed of each part is different, and it is easy to produce bubbles

3 bubbles are produced due to stagnant air

4 if the gate is too small, the thickness of the finished meat changes quickly

5 sufficient pressure must be maintained before gate curing

bubbles: table III

4 warpage:

when the resin in the mold is injected, the internal stress is generated due to high pressure. After demoulding, the two sides of the finished product appear deformation and bending, and the products formed by thin shell are prone to deformation

1 when the molded product has not been fully cooled, it will be ejected, and the surface will be pressurized by the thimble, so it will cause warpage or deformation

2 when the cooling speed of each part of the molded product is uneven, the cooling speed is slow, the shrinkage is increased, the raw materials with strong radiation effect in the thin-walled part are cooled rapidly, the viscosity is increased, and the most important thing is the high material selection and heat treatment, which causes warpage

3 the position distribution of mold cooling water path is uneven, so the temperature must be changed or adjusted by using multiple mold temperature machines

4 for molds with more mold waterways, it is best to use the mold temperature machine to control by sections, and the ideal temperature has been reached

warpage: Table IV

5 flow marks:

when the raw material flows in the mold cavity, there is a small Zou pattern with gate as the center on the surface of the finished product

1 increase the temperature of raw materials and mold to make the raw materials flow easily

2 if the filling speed is slow, the temperature drops during the filling process, and this phenomenon occurs

3 if the nozzle is too long, the temperature at the nozzle will drop. Therefore, the cooled raw material will be emitted first, and the pressure will drop, resulting in flow marks

4 the cooling pit is small, and at the initial stage of injection, the raw materials with low temperature are filled first, resulting in flow marks

flow marks: table V

the finished product is not completely filled, and there is a part of the missing shape energy. As the reason, it is considered that there are the following points:

1 the area of the finished product is large, and the injection capacity of the machine is insufficient. At this time, the machine with large capacity should be selected

2 the exhaust effect of the mold is poor. If the air in the mold cavity is not eliminated at the time of injection, the filling will be incomplete due to the residual air, and sometimes charring will occur

3 in the mold cavity, if the raw material flow distance is long, or there is a thin-walled part, it will be cooled and solidified before the completion of raw material filling

4 low mold temperature is also easy to cause meat shortage, but increasing the mold temperature will prolong the cooling time, resulting in the extension of the molding cycle time. Therefore, it must be considered to determine the appropriate mold temperature from the perspective related to production efficiency

5 the temperature of molten raw material is low or the injection speed is slow, and the raw material solidifies before filling the mold cavity, resulting in the phenomenon of short injection

6 if the nozzle diameter is small or the nozzle length is long, the nozzle temperature should be increased, which is usually expressed in pounds per inch (lb./in.) To reduce the flow resistance, the selection of nozzle should be as short as possible. If the nozzle with small aperture or long nozzle is selected, the friction resistance of its flow will not only increase, but also slow down due to the effect of resistance, resulting in the early solidification of raw materials

7 the number of finished mold holes is large, and the flow is unbalanced. It is necessary to set the size of the entire gate to control. Large resistance of small gate mold holes often leads to meat shortage. If there is a hot rubber channel system, you can also adjust the temperature of a certain meat shortage mold hole separately to control it

8 the ejection pressure is low, resulting in insufficient filling

lack of meat: Table 6

the phenomenon of excess plastic in the finished product mostly lies in the mold closing place, thimble, slider and other activities

1 if the sliding block and positioning block are worn, burrs are easy to appear

2 burr will also appear when foreign matters are attached to the mold surface

3 the clamping force is insufficient, and the mold is opened at injection, resulting in burrs

4 if the raw material temperature and mold temperature are too high, the viscosity will decrease, so it is easy to produce burrs on the only gap of the mold

5 too much material is supplied, and the excess raw materials are ejected to produce burrs

rough edge table VII

due to volume shrinkage, the surface raw materials at the wall thickness are pulled in, and there are dents on the surface of the finished product during chemical processing. Shrinkage is the most undesirable phenomenon on the surface of finished products, mostly at the wall thickness. Generally, if the pressure drops, the shrinkage probability will be greater

1. When designing the mold, it is necessary to consider removing unnecessary thickness. Generally, the wall thickness of the molded product must be as uniform as possible

2. if the molding temperature is too high, shrinkage is easy to occur at the wall thickness, ribs or the opposite side of the bulge, because the places that are easy to be cooled are solidified first, and the raw materials of the parts that are difficult to be cooled will move there. Try to control the shrinkage in a place that does not affect the quality of the finished product

3. generally reduce the molding temperature and mold temperature to reduce the shrinkage of raw materials, but it is bound to increase the pressure

shrinkage table VIII

9. it is not easy to demould (crown)

when the mold is opened, the finished product is attached to the moving mold for demoulding, and when the mold is ejected, the finished product is broken or crown. If the mold is bad, it will stick to the static mold

1. the mold is not smoothly demoulded due to poor exhaust or no exhaust slot (the exhaust slot is not in the right position or the depth is not enough)

2. If the injection pressure is too high, the deformation will be large and the shrinkage will be uneven, so demoulding will be carried out

3. adjusting the mold temperature is effective to prevent unsmooth demoulding. After cooling and shrinking the formed product, it is easy to demould. However, if the shrinkage is excessive, it is not easy to demould on the moving mold. Therefore, the best mold temperature must be maintained. Generally, the mold temperature of the moving mold is about 5 ℃ - 10 ℃ higher than that of the static mold, depending on the actual situation

4. If the center of the nozzle and the rubber port is not aligned, the hole offset or the nozzle hole diameter is larger than the rubber channel hole diameter, it will cause unsmooth demoulding

unsmooth demoulding table IX

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