Demonstration of an Effective Design Validation Tool for 3D Printed Injection Molds (3DPIM)

Demonstration of an Effective Design Validation Tool for 3D Printed Injection Molds (3DPIM)

Injection molding, the process of injecting plastic material into a mold cavity where it cools and hardens to the configuration of the cavity, is one of the world’s most popular manufacturing processes. It is best used to mass produce highly accurate, and often complex, end-user parts.
To obtain a comprehensive and accurate assessment of a part’s functional performance or to run the safety tests on electrical or mechanical components, injection molded parts must be produced using the actual materials and injection molding process of the final production part. Therefore, 3D printed injection molds (3DPIM) are increasingly adopted to create prototype parts to detect issues in the part’s form, fit, function and validations(/certificates) if needed.
These molds are far less expensive than their steel (hard) counterparts with shorter lead time, sometimes up to 90%, but dedicated analysis tools for 3DPIM are not yet available. Therefore, Stratasys and Moldex3D joined together to perfect 3DPIM solutions with upfront simulation predictions. Using both solutions, one can develop the production tool much more efficiently with better results. Furthermore, customers can increase the longevity of the printed tool, improve the design and understand the process better.


WHAT STRATASYS CAN DO
3DPIM are able to create a prototype for a fraction of the cost and a matter of days compared to the weeks-long lead time associated with traditional tooling processes. For example, the price to create a small, straight-pull mold ranges from $2,500 to $15,000 with delivery usually taking 10 days to four weeks. This is an investment that most companies find difficult to justify for a few dozen test parts. 3DPIM have the capability to produce five to 100 parts in the same thermoplastic as production parts. They can be constructed in one or two days for a fraction of the cost of soft metal or steel tooling. Currently, 3DPIM are mostly used with thermoplastics injected up to 300 °C, with some limitation on part geometries and size relative to traditional metal tools. However, they show great benefit to customers where this method can be applied.
“Moldex3D is a powerful tool to help evaluate the moldability of 3D printed injection molds. Combining Stratasys with Moldex3D, customers have an enhanced solution for validating and testing the parts and molds for successful production.”

Benefits of Using 3DPIM:

• Average time savings of 50% - 90% for lead development
• Average cost savings of 50% - 70%
• Functional evaluation with production plastics
• Efficiency gains and automated tool-making with few steps
• Early validation on part performance, mold design and thermoplastic selection

The printed mold needs to bear the resin being injected at high temperature and high pressure. Moreover, high shear stress exists and can ruin the mold when ejecting the part. The amount of successful shots depends on the injected material (flowability, viscosity and melting temperature) and the mold geometry. To optimize the performance of a particular mold geometry, it recommended for users to follow the Stratasys design guidelines (TAG – Technical Application Guide [1]). This document information will help 3DPIM users to:

• Evaluate the mold with a printed replacement
• Revise the printed mold design such as the gate locations or number of gates
• Use metal inserts for critical features

WHAT MOLDEX3D CAN DO
Moldex3D is a process CAE (Computer Aided Engineering) simulator that evaluates the effect of material properties, process conditions and part/mold design on the process dynamics and part quality. The mold filling, packing, cooling and post-molding warpage analysis provide valuable information in the design phase as well as in the trouble-shooting of the existing process/ design. Moldex3D also predicts the process characteristics during the injection molding cycle and shrinkage behavior of the molded part according to the selected material and process conditions. It helps to quickly evaluate, verify, and further optimize the design parameters.
Fig. 1 - True 3D numerical simulation technology.

Moldex3D simulates the entire injection molding process using true 3D solvers, thus, there is no need to manually simplify geometry models for the simulation. For 3DPIM users the “Moldex3D Professional Package” or “Moldex3D Advanced


Package” is the most suitable package for 3DPIM defect prediction and design optimization (Fig. 2).

Fig. 2 - The simulation process of Moldex3D.
Moldex3D can generate full 3D solid mesh with enough boundary layers intuitively to guarantee prediction accuracy. After solid mesh generation, users can easily define process conditions and follow the basic operation procedures to perform the analysis. According to the analysis results, part/mold dimensions and layout can be optimized considering the rheological, thermal, and mechanical properties.

USING MOLDEX3D TO DETECT POTENTIAL 3DPIM DEFECTS
The product in this showcase is a test part designed by Stratasys® to test several common design features that appear in injection molded parts while using a printed mold (i.e 3DPIM process). Past experience indicates feature cracking is a critical issue which has to be avoided to ensure product quality and prototype mold life requirements. Stratasys applied Moldex3D to predict potential flow-induced defects and cracking. This showcase demonstrated the value of early defect diagnosis for improving 3DPIM performance (Fig. 3).

Fig. 3 - The 3DPIM with towers
Challenges

• The towers are heated and softened due to low thermal resistance, and tend to break during injection or ejection (Fig. 4).
• The mold surface temperature of the specific area is significantly higher after part ejection.

Fig. 4 - The towers tend to break off after 2 to 6 shots.
Solutions
The molding condition data are provided as follows:

Part	ABS Terluran GP-35
material
3DPIM material	Digital ABS
CUSTOMIZED 3DPIM MATERIAL PROPERTIES
Maximum machine	80 MPa	Packing pressure:
pressure		20 MPa
Filling time	2.4 seconds	Cooling time:
		70 seconds
Packing time	2.5 seconds	Mold-open time:
		100 seconds
VP switch	98%

Moldex3D Designer BLM (boundary layer mesh) and MCM (multiple component molding) analysis technologies are utilized to observe the flow behavior and deformation of 3DPIM. In this case, the 3DPIM of core and cavity molds are set as two “inserts” of a plastic mold in Moldex3D analysis (Fig. 5). We then can apply Moldex3D Core Shift analysis to predict the insert deflection and stress results caused by non-uniform pressure distribution during the filling stage (Fig. 6).

Results
The comparison of simulated melt front and a short shot sample from real molding at 1.24sec (Fig. 7) demonstrates the feasibility of using Moldex3D to evaluate flow behavior inside a 3DPIM. The tower roots are under higher von Mises stress by the unbalanced flow fronts around the towers, implying greater stress subjection which may easily lead to fracture. We can clearly observe the towers broke off at the same locations in real molding (Fig. 8).
Comparison of the simulated mold temperature distribution and thermal image from the real molding further validates the accuracy of Moldex3D thermal analysis. The red area indicates elevated 3DPIM surface temperature