Injection Molding Trouble Shooting--03 - Ningbo Jinyi Precision Machinery Co., Ltd.

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Injection Molding Trouble Shooting--03

9) Crazing, Cracking

Crazing is a network of fine cracks that can form on or under the surface of injection molded products.

It occurs when the resin in the mold cools and hardens on the surface quicker than it does in the center.

Residual stress remains when a boundary forms and hardens between the center and the surface, and if this stress exceeds the strength of the product, then crazing will appear. When the process continues, the crazing will turn into cracking.

To solve the problem, eliminate the stress by lowering the pressure and reducing the amount of resin in the mold.

Cause		Solution
Residual stress is generated by high injection pressure.	Lower the injection pressure.
Residual stress is generated by high Packing pressure.	Reduce the Packing pressure.
Too much resin is in each shot.	Reduce the amount.
The mold temperature is low.	Raise the mold temperature.
Causes exist for deformation in the product structure.	Redesign the product to eliminate sharp corners and rapid increases in thickness.
Problems are caused be the solvent in the separation compound coating the mold surface.	Stop using the separation compound, or switch to another product for crazing prevention.
The products do not separate well from the mold; stress collects in specific parts of the product.	Adjust the releasing slop. Use caution to ensure that the ejector pin is now cracked when using an undercut. Use the appropriate separation compound. Repair the mold surface.

What is environmental stress crack resistance (ESCR)?

- When polyethylene and polypropylene are in contact with certain chemical compounds, cracks can appear under stress that is lower than the original elastic limit.

ESCR is an index for measuring resistance to cracking in the external environment. ESCR decreases as density and melt index increase

10) Black Streaks

The cause of black streaks that appear on the surface of molded products can be determined by their location or shape. (1) When the streaks radiate from the gate, air in the cylinder is not completely discharged through the hopper, generating high temperature from adiabatic compression that causes thermal decomposition. The thermal decomposition can also result from high friction resistance in the nozzle and gate. (2) When black spots appear inside the molded product, pellets are being caught in the cylinder and remaining too long there, resulting in thermal decomposition. (3) When black streaks are found on the weld line or molded product ends, air inside the mold is pressurizing suddenly as the resin flows in, creating high temperature.

Cause		Solution
Feed zone temperature is high.	Lower the cylinder temperature.
Resin stays in the cylinder for a long time.	Remove scratches from the cylinder walls and remove any foreign objects from inside the cylinder.
Air is not discharging adequately from the mold.	Enlarge the air discharge port. Clean the discharge port.
The cylinder and screw are filled with gas.	Keep the bottom of the hopper sufficiently cooled. Modify the cylinder and screw assembly.
Excessive lubricant is burning.	Reduce the amount of lubricant used.

Wait a minute! Before you suspect that the resin is decomposing from heat...

- After all work is done, make sure that purging has been carried out sufficiently. This is particularly important with color master batch has been used

11) Voids , Pin Holes

Cause		Solution
Volatile elements exist.	Dry the resin sufficiently. Reduce the amounts of lubricant and mold releasing agent used. Lower the cylinder temperature and shorten the time that the resin spends in the cylinder, reducing the generation of volatile elements that lead to thermal decomposition. Increase the packing pressure and ensure that the hopper bottom is adequately cooled.
Air is not discharged from the mold adequately.	Install discharge equipment in the mold
Pressure is insufficient.	Increase the injection pressure. Provide adequate packing time. Slow the injection speed. Increase the diameter and shorten the length of the sprue, runner and gate.

Let's discover the cause of the problem by examining where thevoids occur!

- Sometimes, voids will be found on the thick parts of the molded product, particularly inside. This is a sink mark, which occurs when shrinkage is not filled in as the resin cools down. Solving this problem requires taking the last step above. By contrast, when the (voids are not limited to the thick part of the molded product, you can suspect they are the result of volatile elements

12) Gloss, Transparency Defects

A milky film unlike the original resin gloss appears on the surface of the molded product. This is usually because the mold is not polished well enough or because excessive lubricant or mold releasing agent is vaporizing, preventing a tight seal between the mold and the resin.

Cause		Solution
The mold polish is poor.	Polish the mold and plate with chrome.
Too much lubricant or releasing agent is being used	Reduce the amounts used.
The resin flowability is inadequate.	Raise the cylinder temperature and increase the injection speed.
There is volatile matter.	Refer to the corrective action for volatile matter in the section of bubbles and pin holes.
The mold temperature is improper.	The resin grade requires a specific temperature for a good gloss. Raise the temperature..

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