Virtual Prototyping Through Mold Simulation

With so many grades of engineering resins available, and many not characterized for simulation, similar alternatives are often used. Results, in most cases, will be sufficiently accurate in understanding fill pattern, last-to-fill, gas traps and knit lines. The next level of process simulation and forecasting, pack/hold/warp, will require resin characterization and a relatively sophisticated mold design. Source (All images) | Dynamic Tool Corp.

In nearly every instance of quoting an injection mold build, we rely on our experience in similar circumstances and the lessons learned. For a moldmaking project manager and their team of resources, that means identifying and researching past projects of a similar scope — primarily part geometry and cavitation / projected production volumes. After identifying the project(s) that fit the targeted profile, an experienced tooling engineer will evaluate the mold layout, any enhanced thermal management techniques at play, the bill of materials, the shop worksheets and will begin creating a budget and basic timeline. Several meetings and follow-ups later, the quote is formalized and sent to the customer.

An approach to part design that balances in-use part performance with high-productivity manufacturing is the ultimate NPD success.

Only when the project is formally awarded will a comprehensive project plan with a milestone-targeting timeline be established. Likely, the original request for quote (RFQ) was first evaluated several or many months prior to formal project launch. Now we need to assess our current scheduled workload in design, programming, the required machining processes, mold assembly and sampling to meet the agreed delivery date. Transparent communication with the customer on the identified project “unknowns” is especially critical in new product development (NPD). Overcoming challenges is what a moldmaker does every day — and forecasting those resolutions accurately becomes the newest challenge.

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In the world of new product development, it is practically expected that at least one element of part design will provide a challenge — whether it be filling a certain feature or a flatness specification or “you can’t gate it there” or “we can’t have a parting line,” something is very likely going to surface that requires some educated speculation and experimentation. But budgets and timelines seldom allow for multiple concurrent mold design builds to determine which engineering options are the best solutions.

Taking experience and tribal knowledge to the next level, let’s have a look at the design impact, optimization and progress advancement opportunities provided by virtual prototyping through mold simulation.

Virtual Prototyping

Four target areas that impact the quality and productivity performance of an injection mold are gate location(s), processing parameters, pack / hold / warp challenges and cycle time optimization.

1. Gating

Gate location is very often a matter of geometric experience. Every seasoned designer, toolmaker and project engineer will quickly know where gating is likely to be, based on the fundamental part design. Gating strategy is one of the more straight-forward and expedient simulation activities, as it does not require a mold layout – part design data can be used to experiment with various gate locations and techniques. Basic simulations at this stage will allow us to see fill patterns and last-to-fill location(s).

Additional information will be generated on gas traps (the need for venting and possible vacuum assist), back fill, knit lines, thick wall impact and thin sections / shorts requiring excessive fill pressure. As a best practice, make certain your simulation has achieved steady state in its molding cycles, as this is reflective of the production environment.

2. Processing Parameters

Experimenting virtually with process estimations (based on experience with the target resin and familiarity with the geometry of the design) facilitates comparisons of a range of injection pressures and the effect of various mold temperatures. Simulation will also reveal any hot spots in the mold and the impact of thick wall sections.

Simulation based solely on part design provides usable data at an early stage of development. For example, we can audition various gate locations and dimensions within hours or days without the expense, resource dedication (personnel and equipment) and time investment of building actual prototype tooling. 

3. Pack / Hold / Warp

When we examine the potential for pack / hold / warp challenges through process simulation, the quality of the material / resin characterization will come into play. We will also require a high resolution of the mold design for these virtual prototypes.

Simulation software will let you evaluate any process inputs, even those that are not realistic.  We need to have a base of reality for the process we ask the application to simulate. For example, with regard to injection speed, do not test filling parameters beyond the working limits that any conventional press can achieve. Thin wall sections resulting in the need for high injection pressure typically lead to warp issues. 

In many injection molding applications, the pursuit of optimized cycle time is a priority. This is especially true in high-volume programs and products with a projected extended lifecycle. Using process simulation to sample different cycle time lowering techniques can be a significant time saver as we develop the high cavity production tooling system.

4. Cycle Time

The governing factor in the final production molding cycle time is our ability to successfully manage the thermal environment of the part-surrounding steel. Water temperature is an obvious variable to test. A comparative analysis of the effects of standard cooling channels vs. conformal cooling channels (perhaps with an optional configuration or two) vs. selective use of thermally conductive mold components (steel versus alloy) is a valuable exercise in cycle time reduction. This phase of experiments also facilitates the identification of hot spots in the mold that may impact cycle.

If an optimized cycle is the absolute critical factor, consider testing alternative part designs, if possible. Is there a design feature that is forcing cooling channels away from key surface areas? An approach to part design that balances in-use part performance with high-productivity manufacturing is the ultimate NPD success.

As a best practice, make certain your simulation has achieved steady state in its molding cycles, as this is reflective of the production environment.

As well-intended as the previous content is, there needs to be an element of caution when using simulation data in place of actual development tooling.  Simulation can sometimes provide inaccurate results.  Several factors can impact the validity of a simulation:

• The flow characterization of the resin
• Molding process inputs selected for the simulation
• Design resolution of the mold components used to build the simulation

This is the moldmaker’s target — designing and building an injection mold that correctly and repeatedly produces the customer’s component per the specifications and noted aesthetics in their part design. Simulation enables the rapid evaluation of mold design options, assisting in identifying issues and ultimately the best path forward.

While the list above is far from comprehensive, it names those elements that need to be carefully reviewed.  The software providers and engineering service companies that focus on flow and process simulation technology will possess a much deeper understanding when executing a simulation that has the potential for invalid results. 

For certain, it is an excellent tool for understanding the direction and impact of several variables.  The cost is low and the lead time is quick compared to any physical tooling and subsequent sampling efforts.  We value it for screening our thoughts and potential solutions to the identified challenges prior to the first physical mold build. Running a virtual prototype mold can assist in targeting the most productive and cost-effective solution.