When a product decides to use injection molding as its production process, engineers must consider both product design and mold design. Product design includes thickness and structural design; while mold design includes configuration and size setting of gates, runners and cooling channels. Good product design can make strength and rigidity of product meet requirements, while good mold design can shorten product molding time and reduce defective rate. The two are interdependent and complement each other.
In design and manufacturing process of general products, product design part rarely considers mold, so that mold repair and mold trial are carried out repeatedly, delaying delivery time, which is a practice of putting cart before horse. The best process is to gather material engineers, mold engineers, process engineers, quality control engineers, etc. during product design to discuss problems and solutions that may be encountered during molding, so as to design and manufacture "optimized molds". In this way, we can shorten delivery time, improve product quality and productivity.
Main functions of injection molding molds can be summarized as follows:
a. Good gate and runner configuration and size setting, so that molten plastic can be evenly filled in mold.
b. Good cooling channel configuration and size setting, so that cooling of molten plastic is both uniform and fast.
c. Shape and size of product meet customer's requirements.
d. Consider subsequent processing and assembly of molding.
For molding result, product design is more important than mold design. This report will discuss them together. In order to complete optimized mold design.
Following is based on analysis report of C-MOLD and Moldflow, predicting problems encountered in future product production. And based on analysis of various output results, determine the best change plan to develop an optimized mold.

Product appearance and description
This product is a front panel of a desktop computer (Desktop PC), product appearance and size are as follows.

Figure 1. PC panel size diagram
Since this product must be matched with structural steel frame of computer, the closer finalized dimensions in X, Y, and Z axis directions are to design dimensions, the better. Product is an appearance part, and appearance requirements must be met.
Analysis software uses C-MOLD 2000.1 and MoldFlow Plastic Insight 1.1 in parallel, analyzes based on analysis results to propose reasonable and feasible solutions, then uses software to verify results.
Plastic material
Chi Mei Industry ABS POLYLAC PA-757
Process conditions
Filling time: 2.5 seconds
Pressure holding time: 10 seconds
Cooling time: 30 seconds
Mold opening time: 5 seconds
Product design
Nominal thickness of product is 3.0mm, rib thickness is 1.0mm, 1.5mm and 1.8mm. Maximum thickness of product is 6.0mm. Product thickness distribution and local thickness distribution are as follows:

 

Figure 2-1. Product thickness distribution (front)

 

Figure 2-2. Product thickness distribution (back)

 

Figure 2-3. Local thickness distribution of product

I. Original design
Gate and runner configuration of mold will affect molding shrinkage and molecular orientation of product. From results of filling and holding analysis, it can be judged whether filling of melt in mold is balanced. Generally speaking, shrinkage rate of each part of product with balanced filling is relatively close, residual stress is low, and it is not easy to warp.
Table 1 Product molding analysis table

 
Figure 3-1. Original design C-MOLD wavefront advancement diagram and joint line position diagram

Figure 3-2. Original design MoldFlow wavefront advancement diagram and joint line position diagram
According to analysis results of C-MOLD and Moldflow, it is known that original gate and runner design will lead to uneven filling - top side area is filled first, and bottom side area is filled later. Due to different condensation time on both sides, volume shrinkage rate on both sides is very different, which is prone to warpage. Whether filling is balanced is a must-consider in mold design.

Figure 3-3. Volumetric Shrinkage Distribution of Original Design (Volumetric Shrinkage)
Product thickness is unevenly distributed, and thickness on ground side is 6.0mm. This is last filling area, with the lowest pressure and the largest volume shrinkage rate. According to analysis results of C-MOLD and MoldFlow, it can be known that volume shrinkage rate here is about 5.5%, which is much larger than volume shrinkage rate of the rest of parts (2% ~ 3%).

Key to configuration of cooling channels is to reduce temperature difference between male and female molds, remove hot spots, and shorten cooling time. When temperature difference between male and female molds is large, thermal stress (thermal induced stress) is asymmetric to the center surface of product. When hot spots exist, volume shrinkage rate at that location is large, which will lead to large warpage. Cooling time accounts for two-thirds of injection molding cycle time. Optimized cooling design can not only improve yield rate, but also shorten cycle time and improve productivity.

Figure 4-1. Original design cooling system
Right figure is temperature difference between male and female molds. Maximum temperature difference is about 23℃, which is close to gate. This report only discusses improvement of product design and gating system design. Changes to cooling system are not considered - that is, original design and modified design use same cooling system design.
Following is a warpage analysis. Figure 5-1 shows inward displacement of outer wall on ground side in X-axis direction. Figure 5-2 shows inward displacement of outer wall on ground side in Z-axis direction.
 

 

Figure 5-1. Original design C-MOLD warpage analysis (ground side X axis)
 

 

Figure 5-2. Original design C-MOLD warpage analysis (ground side Z axis)
Gray part in figure is original mold cavity position, and deformation is magnified 10 times.
From above, it can be seen that uneven distribution of product thickness or improper design of gate and runner will cause warpage of molded plastic parts, causing problems in subsequent product assembly.
2. Modified design
According to filling and holding results analysis of original design, runner at upper window should be removed to improve filling balance, and ground side thick ribs should be thinned (6.0mm → 3.5 mm) to improve uniformity of thickness.
Runner and thickness distribution of modified design are as follows.

 

Figure 6-1. Modified design thickness adjustment diagram

 

Figure 6-2. Modified design runner configuration diagram

After flow channel design is changed, filling balance requirement can be achieved.
Table 2 Product molding analysis table

Figure 7-1. Corrected design C-MOLD wavefront advancement and joint line position diagram

Figure 7-2. Corrected design MoldFlow wavefront advancement and joint line position diagram

Figure 7-3. Corrected design volume shrinkage diagram
Maximum volume shrinkage at ground side is reduced due to thinning of thickness - from 5.5% of original design to 3.0%.

Figure 8-1. Original design cooling system
Right figure is temperature difference diagram of male and female molds. Since cooling design has not been changed, there is not much impact on temperature difference. Maximum temperature difference still occurs near gate.
Following two figures are results of warpage analysis. Figure 8-2 shows inward displacement of inner edge of ground side rib in X-axis direction, and Figure 8-3 shows inward displacement of outer wall of ground side in Z-axis direction.

 

0.84mm
Figure 8-2. Corrected design C-MOLD warpage analysis

 

0.12mm
Figure 8-3. Corrected design C-MOLD warpage analysis

Table 3 Comparison of deformation between original design and corrected design

Correct product design and mold design can reduce product defect rate, increase output and profit. "Simultaneous engineering" should be implemented at initial stage of design to maximize its effectiveness. A good mold must also consider following factors:
1. High manufacturing accuracy. In addition to high processing accuracy of parts that make up mold, comprehensive accuracy (combination accuracy) of parts after assembly must also be high.
2. Mold itself must have sufficient rigidity to withstand cavity pressure caused by injection pressure, pressure caused by clamping, etc., to avoid excessive deformation.
3. Whether cooling of molded product is balanced.
4. Design of gates and runners affects directionality of melt flow and volume shrinkage of products.
5. Movement of each part of mold can be carried out accurately in each molding cycle.
6. Mold parts will wear out after long-term operation, which will affect accuracy and must be under control.