The Influence of Structural Design of Even Wall Thickness on Injection Molding
How to design a proper Wall Thickness?
The even wall thickness refers to the even of cooling and hardening time of the Wall Thickness in the mold system during injection molding. If some parts of the plastic have been cooled and hardened, but some have not. Then, it will cause the thick parts to collapse and shrink.
What is the purpose of designing stiffener?
When designing the structure, the first step of shell design is to confirm the shell. First, determine the basic wall thickness of the shell. Because the wall thickness of the shell is very important for many key characteristics of the part, including structural strength, appearance, molding and cost.
The designed optimized shell thickness can reduce the risk of subsequent reliability testing, the cost of mold repair, and the molding difficulty.
To put it simply, for a flat section, for every 10% increase in wall thickness, the rigidity of the part will increase by about 33%. For a simple plastic surface, a 25% increase in thickness can double the rigidity of the shell. However, increasing the thickness will have a negative impact on the appearance of the product, the molding time of the parts, the cost and the weight of the entire plastic product.
Why does the plastic product shrink after being molded?
The shrinkage refers to the phenomenon that due to the volume shrinkage of the product during the cooling and hardening process, the surface plastic at the wall thickness area are pulled in, so that the sunken marks appear on the surface of the product.
Shrinkage is usually related to the following factors:
(1) Injection molding process setting problems, such as low injection pressure, insufficient pressure-holding time, high mold temperature, insufficient cooling time, etc.
(2) Structural mold design problems (uneven thickness of wall, improper ratio of rib width to wall thickness, excessive wall thickness, etc.)
(3) Mold processing problems (the gate is too small or the runner cross-section is too small, etc.).
(1) It is relatively easy to solve the problems caused by the injection molding parameter setting.We only need tomake corresponding adjustments according to the investigated reasons.
(2) When it is caused by poor mold structure, it is necessary to modify the mold. Generally speaking, the thickness of the rib should not exceed 2/3 of the thickness of the wall.
(3) If there is a mold problem, adjust and modify it accordingly.
Let’s analyze how to design according to the schematic diagram.
【01】L-shaped corners with even Wall Thickness design
According to the picture above, all wall thicknesses are 1.7mm, so which design is right, A or B?
At B, the Wall Thickness should be designed to be 1.7*0.6 (the rib position cannot be greater than 2/3 of the main thickness) = 1.0mm, otherwise the appearance will shrink.
【02】U-shaped ribs with balanced rubber thickness design
According to the picture above, the balanced Wall Thickness at A and B is 1.7mm. While the Wall Thickness at C is 1.0mm. Which design is correct?
Answer: A and C.
The Wall Thickness at B should be designed to be 1.7*0.6 (the rib position cannot be greater than 2/3 of the main thickness) = 1.0mm
 Wall Thickness design of the screw column
According to the picture above, the screw posts at A and B are all with a uniform thickness of 1.0mm, so which design is correct?
According to the overall large thickness of the D line, it can be seen that the thickness of the screw column A is uneven at the position C. If the thickness exceeds 1.0mm, it will shrink here.
Someone will certainly ask, how to solve the problem that the screw cannot be driven at B for it is too narrow, and A shrinks again?
As shown in the figure above, the screw column is wall here. After ensuring the thickness of 0.8mm from the glued surface to the appearance surface, it is necessary to ensure that there is a mold steel thickness of 0.6mm or more here, and then make a rib connection to ensure strength.
 The design of the screw column crater
Question 1 : According to the above picture, the thickness of the screw post has exceeded 1.7mm * 2/3 of the large thickness. But to ensure the strength of the screw post and the screw does not slip, it must be such thick. Then, what should be done?
Answer : You can use a bridge design or a bottom crater design.
Then how to design the crater?
According to the above figure, A and B need to have a thickness of 1.0mm or more to prevent the overly thin thickness of the wall. Or else it will cause difficulty in melt plastic running. The mold strength needs to be ensured at C, and the width should be at least 0.8mm.
Question 2 : Why can digging out the crater improve the shrinkage here?
Answer : Because it can quickly reduce the cooling and hardening of the plastic and prevent it from being too thick to collapse and shrink.
Question 3 : Except for the screw posts, the crater can be used to remove glue to prevent shrinkage. Is the same method possible for other parts?
Answer : Yes, such as very thick ribs. However, it should be noted that the crater is only used for improvement. And in many cases, the problem cannot be completely solved. If the appearance of the product is made of material, it is not recommended to use the crater to improve the shrinkage. It is recommended to use a bridge.