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Analysis of Common Defects in Injection Molded Parts and Improvement Measures

2020-2-13

1. Short of material
Causes of defects:
The small parts and corners of the finished product cannot be completely formed. Due to insufficient mold processing or poor venting, the design is defective (insufficient wall thickness) due to insufficient injection dose or pressure.
Mould improvement measures:
Correct the missing part mold, adopt or improve the venting measures, increase the material thickness, and improve the gate (enlarge the gate, increase the gate).
Improved molding:
Increase injection dose, increase injection pressure, etc.

 

 


2. Shrink

Causes of defects:

It often occurs in the uneven wall thickness or material thickness of the molded product, which is caused by the different shrinkage of the hot melt plastic during cooling or solidification, such as the back of the rib, the edge with a side wall, and the back of the BOSS pillar.
Mould improvement measures:
Reduce the material thickness, but keep at least 2/3 of the material thickness; thicken the runner and increase the gate; add exhaust.
Improved molding:
Increase the material temperature, increase the injection pressure, and extend the dwell time.

3. the surface is bright

Causes of defects:

It often occurs on the back of the reduced BOSS column or rib, or the pressure mark is caused by the high design of the core and the ejector pin.
Mould improvement measures:
Lower the crater; modify the core and thimble; sandblast the mother mold surface to reduce the mold surface brightness.
Improved molding:
Decrease the injection speed and pressure.

4. Air marks

Defect reason:

Occurs at the gate, mostly because the mold temperature is not high, the injection speed and pressure are too high, the gate is not set properly, and the plastic encounters the spoiler structure during the gate
Mould improvement measures:
Change the gate, the runner is lightened, the runner cold area is enlarged, the gate is enlarged, and the surface is bitten (you can also adjust the line by adjusting the machine or repairing the mold).
Improved molding:
Increase mold temperature, decrease injection speed, reduce injection pressure, etc.

4. Flashing

Defect reason:

Occurs at the confluence of two streams, such as the confluence of the two inlets, the confluence of the bypass bypassing the core, due to the decline in material temperature and poor exhaust.
Mould improvement measures:
Change the gate, add cold material well, open the exhaust slot or bite the male mold surface.
Improved molding:
Increase material temperature, mold temperature, etc.

6. Burrs

Defect reason:

The joint of the male and female molds often occurs due to poor mold clamping or improper processing of the corners of the mold surface. In molding, due to insufficient clamping force, the material temperature and pressure are too high.
Mould improvement measures:
Correct the mold and re-clamp it.
Improved molding:
Increase the clamping force (check whether the tonnage of the injection machine is sufficient), reduce the material temperature, reduce the injection pressure, reduce the holding time, and reduce the holding pressure.

7. Deformation

Defect reason:

Slender pieces, large-area thin-walled pieces, or large finished products with asymmetric structures are caused by uneven cooling stress or uneven ejection forces during molding.
Mould improvement measures:
Correct the thimble; set tensioning pins, etc .; if necessary, adjust the deformation of the male mold with bite.
Improved molding:
Adjust the mold temperature of the male and female molds to reduce the holding pressure, etc. (The adjustment of small pieces of deformation mainly depends on the pressure and time, and the adjustment of large pieces of deformation generally depends on the mold temperature)

8. Unclean surface

Defect reason:

The mold surface is rough. For PC materials, sometimes the mold surface has residual glue and oil stains due to the mold temperature being too high.
Mould improvement measures:
Clean mold surface and polish.
Improved molding:
Reduce mold temperature, etc.

9. White mark

Defect reason:

It is likely to occur at the corners of thin-walled products or at the roots of thin-walled ribs. Due to poor force during demolding, improper setting of the ejector pin or insufficient draft angle.
Mould improvement measures:
Increase the R angle at the corner; increase the demolding angle; increase the ejector pin or increase its cross-sectional area; polish the die surface; ejector pin or oblique pin
Improved molding:
Reduce the rate of fire, reduce the injection pressure, reduce the holding pressure and time.

10. Drawing die

Defect reason:

It is manifested as poor demolding, mold damage, or pull-out, mainly due to insufficient draft angle or rough mold surface, and the molding conditions also have an influence.
Mould improvement measures:
Increase the draft angle; polish the die surface; add / change the pull pin when sticking to the die surface, pay attention to the horn diameter Φ when feeding the horns, and bite the male mold.
Improved molding:
Reduce the injection pressure, reduce the holding pressure and time.

10. Air hole

Defect reason:

The transparent finished PC material is easy to appear during molding. Because the gas is not exhausted during the injection molding process, improper mold design or improper molding conditions can affect it.
Mould improvement measures:
Increase the exhaust, change the gate (inlet gate increases), the PC runner must be polished.
Improved molding:
Strict drying conditions, increased injection pressure, reduced injection speed, etc.

11. Steps

Defect reason:

Occurs at the joints of male and female modules / sliders / inclined pins, etc., which appear as uneven levels of the joint surface, due to improper mold clamping or problems with the mold itself.
Mould improvement measures:
Correct the mold and re-clamp it.
Improved molding:
None

12. Beyond the dimensional tolerance

Defect reason:

Problems with the mold itself, or improper molding conditions caused inappropriate mold shrinkage.
Mould improvement measures:
Correct the mold, such as adding plastic, reducing plastic, and even reopen the mold in extreme cases (inappropriate shrinkage and excessive dimensional deviation).
Improved molding:

Generally, changing the holding pressure time and injection pressure (second stage) has the largest impact on the size. For example, increasing the injection pressure and increasing the pressure holding and shrinking effect can significantly increase the size, lowering the mold temperature, increasing the gate or increasing The gate can improve the adjustment effect
 
 
 
 
 
 

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