One of the biggest challenges in molding process of thick-walled plastic products is problem of dimensional accuracy. In this project, plastic hook (Figure 1) warped during the first mold trial; even if process parameters were adjusted, warpage still existed.
School of Materials Science and Technology (MTF STU) of Slovak University of Science and Technology uses Moldex3D to study causes of warpage and find feasible solutions. Finally, according to results of Moldex3D's warpage analysis, mold design was optimized to successfully solve warpage problem and avoid unnecessary heavy work.

 

Picture 1: Plastic hook in this case

Main challenges faced in this case were "must control and reduce excessive warpage" and "must find a feasible solution in a short time." For above mentioned challenges, MTF STU team assisted tool makers to find the most suitable solution to modify mold base design based on Moldex3D warpage analysis. Benefits are as follows:
• Meet size requirements;
• Avoid cost waste caused by tool manufacturing errors and heavy work;
• Shorten process of correcting problems.

In manufacturing process of thick products, the biggest problem is how to achieve required dimensional accuracy. Goal of this case is to solve warpage problem of this plastic hook. Product's tolerance for dimensional error is ± 1.5 mm, but warpage of original design in hook area reaches 1.86 mm (Figure 2). For such cases, general procedure is to optimize process parameters first; however, after modifying process parameters many times, warpage results have not been improved. Therefore, tool manufacturers sought assistance from MTF STU to verify possible solutions by means of Moldex3D's numerical analysis technology.

 

Figure 2: Original design and position prone to warping
Follow-up with Moldex3D for following research projects:
• Verification of waterway system after design changes;
• Verification of application of gas-assisted molding technology;
• Modification of mold design to achieve required geometric and dimensional accuracy of molded product.
In this stage, several cooling water channels and fountain water channels are added to mold base (Figure 3, Figure 4), and Moldex3D is used to evaluate design changes of water system. However, this still cannot solve problem of product warpage.

 

Figure 3: Original waterway system

 

Figure 4: Waterway system after adding fountain-style waterway
Moldex3D cooling analysis results show that cooling effect of original water system is quite sufficient, and there is no need to add additional water channels. Generally speaking, if gas-assisted molding technology is used, it can effectively improve warpage of thick products; therefore, MTF STU then simulated several gas-assisted methods (Figure 5). However, simulation results show that using these methods, gas flow in mold is not optimized (all produce gas fingerprint effects), filling, pressure holding, and cooling stages cannot achieve sufficient balance. If process is to be optimized, it must be carried out. Large-scale mold modifications are contrary to original intention.

 

Figure 5: Gas is injected from (a) runner system, (b) left gate, (c) right gate and (d) front end of hook.
In the end, tool manufacturer agreed to change design of mold cavity based on simulation results, but compared to traditional way of maintaining outer contour of product and reconfiguring thickness and ribs, they only reversed warpage history and made geometric design changes based on the results of Moldex3D's warpage analysis (Figure 6).

 

Figure 6: Mold design change process. Gray is original mold design, blue is reverse mold design.
Results show that after modification of this mold, required size of hook can be reached (Figure 7). Table 1 shows comparison of measurement results of original and modified molds.

 

Figure 7: Verification of warpage results. These are (a) original design and (b) product after reverse warpage

Table 1: Comparison of maximum dimensional deviation values between original and mold design changes
This case presents Moldex3D's ability to predict warpage of thick products, so as to compensate warpage by means of reversal warpage to correct warpage problem. Final product reaches required dimensional accuracy, satisfies tolerance of geometric deviation, and solves warpage problem.