injection molding defects Archives

PC/ABS injection molding defects and solutions

March 9, 2020November 18, 2019 by Jackie

Seven Common Defects and Solutions for PC/ABS injection molding Products

Flow Lines

The flow lines are caused due to the poor fluidity of the material. Unlike silver streaks, it is not caused by moisture or material decomposition, thus resulting in a different look.

Solution: The flow lines can be eliminated by raising the material temperature so as to improve the fluidity. Appropriate increase of mold temperature and decrease of injection speed can also solve the problem.

Pitting

The shrinkage cavities are caused by short shot in the mold cavity.

Solution: Increase mold temperature and material temperature appropriately to improve material fluidity; extend the injection / pressure holding time, and raise the injection speed to improve the mold filling, or increase the size of the gate and heat the sprue to reduce and eliminate the shrinkage cavities on the products.

If the product is not appropriately designed and the material temperature is too low, not only the shrinkage cavities, but also pitting will occur. If the material temperature and the mold temperature are too high, the melt will shrink excessively during cooling, resulting in pitting.

Solution: Appropriately increase the injection speed.

Warpage / Deformation

Solution:

The warpage / deformation of PC/ABS injection molding product is caused by the unreasonable product design, the improper gate location and the inappropriate conditions of the injection molding process, so that the stress is generated inside, leading to the ununiform or excessive shrinkage. If the mold temperature is too high or uneven, it will cause the difficult product ejection, or ununiform cooling, which will also lead to warpage / deformation.

Solution:

Molding Process: Extend the injection molding cycle, lower the injection molding temperature, appropriately adjust the injection pressure and the injection speed, reduce the ejection speed, increase the ejection area, and balance the ejector force.

Product Design: Increase product wall thickness, add ribs and fillet reinforcements to reduce warpage / deformation.

Silver Streaks

Silver streaks are the most common defects with the PC/ABS injection molding material, which refers to the appearance of the silvery-white silk-like stripes on the surface of the molded product along the flow direction.

The main cause is the interference of gas, which is mainly comprised of three components:

Air: The air drawn in during the melting and injection stages;

Moisture: The moisture contained in the material itself;

Pyrolysis Gas: Gas generated by high temperature hydrolysis / thermal decomposition.

Solution: First of all, check whether the material is fully dried; after the material is confirmed to be dry, the silver streak defect can be improved by adjusting the injection molding process. At the same time, this defect is also correlated to mold venting.

Matting

Solution:

Poor dispersion: add dispersant or oil, raise temperature, and increase backpressure.

Pay attention to the mold problem. Try some other materials.

Whether the injection temperature and time are sufficient.

Adjust the mold temperature.

Product Delamination

The delamination defect of PC/ABS injection molding products is closely related to the rupture of the melt caused by high shear force. Under low shear stress or velocity conditions, small disturbances caused by various factors are suppressed by the melt, while at high shear stress or velocity, disturbances in the melt are difficult to suppress and thus develop into an unstable flow;

When a critical shear force is reached, the fracture of the fluid will be caused.

Solution:

Material: The two components of PC and ABS are partially compatible, so it is necessary to add proper compatibilizer to improve the compatibility of the two components in the process of modification. Of course, the first thing to do is to eliminate the delamination caused by the mixture.

Mold: The principle of mold design should follow the direction of minimizing shear. Generally speaking, products with densely striped surfaces are more prone to delamination (caused by the friction shear between melt and inner wall of cavity during high-speed filling). At the same time, the gate size designed too small will lead to excessive shear when the melt is flowing through the gate, and then lead to product surface delamination.

Injection Molding Process: The main purpose is to avoid excessive shear. The high-speed & high-pressure injection can be used to improve the product filling difficulty, and therefore, in the actual injection molding process, the flow resistance in the actual filling process can be reduced, so as to avoid excessive shear caused by high-speed and high-pressure injection molding process

Stretch Marks

Stretch marks usually occur due to melt rupture when the melt expands into the mold cavity during high-speed injection.

Solution:

Molding Process: This defect can be reduced by raising the material temperature and the nozzle temperature, and slowing down the injection speed, etc.

Warpage injection molding defects causes and solutions

March 9, 2020November 5, 2019 by Jackie

Warpage as one of the most common defects in a plastic injection molded product, refers to the situation in which the shape of the injection molded part deviates from the shape of the mold cavity. The following is a brief analysis of the factors that affect the warpage of the injection molded products.

How the mold structure influences the warpage of an injection molded product

With regard to the mold, the main factors that affect the deformation of a plastic part are the gating system, the cooling system and the ejection system

Gating System

The gate location, type and quantity of a plastic injection mold affect the filling status of the plastic in the mold cavity, leading to changes to the plastic part. For some flat plastic parts, if only one central gate is employed, the molded plastic part will be distorted, as the shrinkage rate along the diameter is greater than that along the circumference; if multiple pin-point gates or the film gate is used, product warpage defects can be effectively prevented.

Cooling System

During the plastic injection molding process, the ununiform cooling rate of the plastic part will also cause ununiform shrinkage in the part, which leads to the generation of the bending moment, and therefore warpage of the plastic injection molded part.

During the plastic injection molding of a flat plastic part (such as the mobile phone battery case), if the temperatures in the mold cavity and the core differ greatly, the molten plastic close to the surface of the cold mold body is quickly cooled down, and that close to the surface of the hot mold cavity continues to shrink. Such an ununiform shrinkage will cause part warpage. In addition to considering the temperature balance inside and outside the plastic part, it also needs to be ensured that the temperature on each side of the plastic part is consistent, i.e., during mold cooling, the temperatures of mold core and the mold cavity should be kept as consistent and uniform as possible, so that an uniform cooling rate throughout the plastic part is ensured to effectively prevent the occurrence of warpage.

Ejection System

The design of the ejection system also directly affects the warpage of a plastic injection molded part. If the ejection system is not designed with a balanced layout, it will cause an imbalance in the ejection force, which leads to plastic product warpage. The cross-sectional area of the ejector pin should not be too small, so as to prevent the plastic part from being deformed by excessive force per unit area (especially when the temperature is too high during ejection). The ejector pins should be placed as close as possible to the area with a large ejection resistance.

How plasticization process influences product warpage

Plasticization refers to the process in which the plastic is converted from vitreous granules into a viscous melt. During this process, the temperature differences of the polymer along the axial and the radial directions (relative to the screw) will cause stress in the plastic material; in addition, the injection pressure, speed and other parameters of the injection molding machine have a great impact on the degree of molecular orientation during plastic filling, which causes warpage, or deformation.

How plastic filling and cooling influence product warpage

The effect of temperature on product warpage is reflected in the following aspects:

The temperature difference between the upper and lower surfaces of the plastic part may cause thermal stress and therefore thermal deformation;
The temperature difference between different areas of the plastic part will cause uneven shrinkage between different areas;
Different temperature conditions influence the shrinkage of a plastic part.

How product ejection influences product warpage

The imbalanced ejection force, unsteadiness in the movement of the ejection mechanism, or the inappropriately designed ejection area easily causes product deformation . At the same time, the stress that is “frozen” in the plastic part during the filling and cooling processes will be released in the form of “warpage” due to the loss of external constraints, resulting in product deformation.

How product shrinkage influences product warpage

The immediate cause of warpage defects of an injection molded product is the uneven shrinkage of the product. If the influence of shrinkage during the filling process is not taken into consideration during the mold design stage, the geometry of the product will vary greatly from the design requirements. For warpage analysis, shrinkage itself is not important, but what is important lies in the shrinkage difference.

How residual thermal stress influences product warpage

During the plastic injection molding process, residual thermal stress is an important factor that causes product warpage, and thus exerts a great influence on the quality of an injection molded product. Since the influence of residual thermal stress on product warpage is very complicated, it will not be detailed here.

How the metal insert influences product warpage

With regard to plastic injection molded products with inserts, it is easy to cause distortion (even cracking), since the shrinkage of plastic is much higher than that of metal; to improve this, the metal part needs be preheated (generally at a temperature of no lower than 100°C), before being put into production.

Conclusion

There are many factors that affect the warpage defects of an injection molded product. The structure of the mold, the thermophysical properties of the plastic material, as well as the conditions and parameters of the molding process all exert different influences on the product warpage defects. Therefore, when handling the warpage of a molded product, we must take all the above factors into consideration.

Injection Molding Defects’Causes & Countermeasures

March 10, 2020April 6, 2018 by Jackie

The causes and countermeasures described below are aimed to solve the commonly seen injection molding defects that might occur in ordinary circumstances. The examples are just based on my own personal work experience, so if there is anything inappropriate, you are welcome to correct me!

Short shots (incomplete filling)

(1) Short shots (incomplete filling): The molten plastic fails to fill up each and every corner in the entire molding space (the cavity).

(2) Causes and Countermeasures

Flash

(1) Flash: Extra plastic material in the shape of films or burrs, which appears on the parting surface, around the runner or in the insert crevice.

(2) Causes & Countermeasures (See the table below)

*Note: Flash also tends to occur when the injection pressure/speed are too high, but molding time is too long and mold temperature is too low.

Silver streaks

(1) Silver streaks (silver lines): The radial silver white streaks on or around the product surface, which is formed along the plastic flow direction.

(2) Causes & Countermeasures (See the table below)

Low product glossiness

(1) Low product glossiness means the surface glossiness of the product does not meet the quality standard – the surface is not dioptric.

(2) Causes & Countermeasures (See the table below

Deformation

(1)Deformation includes diagonal twists and warpage along parallel edges.They are the irregular curves that occur in the plastic injection molding process.

(2)Causes & Countermeasures (See the table below)