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Case Analysis of Molding Defects in Injection Products That Automotive Engineers Must Understand

Date: Mar 04 2021 | By:LC Rapid
Tag:Injection Molding,

As an automotive engineer, you master a lot of knowledge and skills, but you still feel troublesome when you encounter some actual injection molding defects of auto parts. Don't worry, this article conducts professional analysis on 3 cases of automotive injection molding product molding defects, and comes with solutions. This article deserves your several minutes.


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Ⅰ. Ejector pin mark on the injection molding product, car door


A car generally contains four door panels, which are left, right, front and rear assemblies, and PP is generally used as the material, which requires high apparent quality.


1. Analysis of possible causes and improvement measures


1) Possible reasons: The main reason for the ejector pin mark is that the ejector pin exerts too much force, which causes the apparent defect of the ejector pin position of the product.

2) Improvement measures: Through the optimization of mold structure and injection mold process, the force exerted by the ejector pin is effectively reduced, and the possibility of ejector mark is reduced. The solutions adopted in this case are considered from the following two aspects:

① In terms of mold structure: When the draft angle of the mold is small or the polishing effect of the ribs is not good, the mold release resistance of the part will be increased. The force exerted by the ejector rod on the workpiece will increase, resulting in white or convex top. The improvement method is to improve the polishing of ribs or increase the draft angle of the part.

② Injection molding process: when the holding pressure is too large or the holding time is too long, it will cause the parts to be overfilled. At this time, the covering force of the part on the core is very large, resulting in the ejection force of the ejector rod acting on the part is also very large, and finally the phenomenon of topping will appear. The improvement method is to reduce the holding pressure or reduce the holding time.


2. Key tips

Some injection molded parts cannot have ejector mark because of the application requirements. Such as transparent appearance parts, if there is a pin print, it will affect its appearance characteristics. Or because the wall thickness is too thin, etc., there is no place to set the ejector rod, which can be solved by using a hidden ejector rod.


Ⅱ. Splay mark on the injection molding products, automobile bumper


The automobile bumper is a safety device that absorbs and relaxes the external impact and protects the front and rear of the body. Generally choose PP with better mechanical strength. The flower-like cold material near the car bumper not only affects the apparent quality of the part, but also affects its mechanical properties. How to deal with it?


1. Analysis of possible causes and improvement measures


1) Possible reasons:

① In terms of auto parts mold: The mold used in the product in this case contains a hot runner and a cold runner, but there is no cold slug well on the cold runner, which is directly connected to the part.

② Injection molding process: For the injection molding process, it is observed that some needle valve gates are not sealed tightly, there is residual cold material on the gate, and the air pressure for closing the needle valve is very low, which can not play the role of sealing glue.


2) Improvement measures: In this case, the air pressure of the needle is increased by using a pneumatic booster pump, and the cold material disappears after the hot gate no longer leaks.


2. Key tips


In the structure of a typical hot runner nozzle, it can be seen that the front end of the nozzle is in contact with the template. Although the contact area is limited, the heat at the front end of the nozzle will be transferred to the cooler template; in addition, due to the need for cooling, there is sometimes cooling around the nozzle circuit, which further increases the possibility of low temperature at the tip of the nozzle. The temperature at the front of the nozzle is low, resulting in a small amount of cold material at the front of the nozzle. Especially when the needle sealing is not tight, the melt will overflow and it is easier to form cold material. Therefore, when the hot runner nozzle is switched to the cold runner, it is recommended to install the cold slug well on the cold runner.


Ⅲ. Peeling of the injection molding products, car bumper


The surface of automobile bumper parts is large, the defect rate is low, and it is difficult to find the peeling defects, but this defect seriously affects the appearance quality of the parts. The bumper surface appears delamination and peeling, and the defect rate is 3-5%, which randomly appears on the surface of the part. Due to the large surface of the part and the low defect rate, it is difficult to find, but it seriously affects the appearance quality of the part.


1. Analysis of possible causes and improvement measures

1) Possible reasons:

In terms of injection molding process: the judgment of the product in this case may be caused by cold material.

Through inspection, it is found that the temperature of the material and the hot runner are normal, and the open hot runner used by the mold is also normal. Due to manual pickup during production, when the pickup takes a long time, different degrees of salivation will occur in each hot nozzle. After shortening the pick-up time, it was found that the “salivation” phenomenon of the hot nozzle was effectively improved.

Therefore, it is possible that the cold material produced by the salivation was washed into the cavity and caused delamination and peeling, and it was found on the site that it was the length of the worker's pick-up time that determines whether the next product will have delamination and peeling.


2) Improvement measures:

In terms of injection molding process: the ejection distance of the injection molding machine is changed from 16mm to 20mm, the hot nozzle no longer has salivation, and there is no more delamination and peeling on the part.


2. Key tips


The cold material rushing into the cavity may cause delamination and peeling. The cold material may be caused by the followings: the material temperature is too low, the nozzle temperature is too low, the runner is not equipped with a cold slug well or the length of the cold slug well is insufficient, the temperature of the hot runner is too low, and the mold temperature is too low.

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