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James Anderton


Injection Mold Design: Why Simulation Is Essential

Complex mold designs need software to match

Published: Thursday, August 20, 2020 - 12:02

Injection mold making used to be a relatively simple business: machine the cavities and runners, polish, cross-drill for cooling, then shoot resin. Today, however, relentless pressure for lower part cost and higher productivity have led to bigger, faster machines with molds to match.

Multilayer stack molds, gas-assisted molding, overmolding, co-injection, advanced hot-runner systems, and other technologies have collapsed the cost of high-volume commodity resin parts. At the other extreme, a new generation of functional fillers and special-purpose engineering resins are allowing very large, special-purpose part making for industries such as automotive, aerospace, and medicine. In every application, cycle time, dimensional stability, and surface finish are paramount; hiding a sink mark under a trim plate just doesn’t cut it anymore.

Despite all the technical understanding of making injection molds, a surprising number of manufacturing engineers know very little about the complex dynamics that happen inside an injection mold.

Advanced simulation is changing the injection molding industry

Changes in the industry have been dramatic, and many of those changes can be traced back to the advent of advanced simulation software to model the flow of resin within the complex runners, gates, and cavities of modern high-productivity molds.

Plastics engineer Jennifer Schmidt has seen these changes firsthand as a mold and hot-runner designer, an application engineer, and now as a senior Autodesk Moldflow simulation instructor at the American Injection Molding Institute located in Erie, Pennsylvania.

“People who think it’s simple have never had formal education in plastics fundamentals because that couldn’t be further from the truth,” says Schmidt. “It’s probably one of the most complex industries, or one of the most complex ways to make a part.”

The complexity starts with the unusual properties of thermoplastic resin as a raw material. Engineers and nonengineers alike tend to think of solid substances as materials that melt at a defined temperature, and whose viscosity decreases linearly with temperature. To material scientists, this is called Newtonian material behavior. Although many crystalline materials do behave this way, most commodity thermoplastics do not.

Transient cooling of injection molds is complex. Advanced simulation makes it possible to resolve complex systems like this. (Image courtesy of Autodesk.)

“If you are specifying or designing a mold, the material properties affect the approach to mold design, and its cost and performance,” Schmidt declares. “Add in modern mold accessories such as hot-runner systems, and the problems become even more complex. For the better mold designers, that knowledge has to be there. But for many who I’ve interfaced with over the years, I’m not sure that they really do have an understanding of the non-Newtonian flow and what impact some of their decisions have on the part.”

Although a smooth-running injection molding operation looks as simple as heat, fill, cool, and eject, those non-Newtonian material properties must be understood in order to design a productive mold capable of quality part production.

Most mechanical engineers can design fluid power systems with simple assumptions about fluid viscosity and compressibility. However, resin in the melt has flow properties that change with temperature, pressure, and shear, meaning the speed and acceleration of the screw matters during mold filling and packing. The length, geometry, surface, and cross-sectional area of runners all matter, as does the geometry of cavities.

Mistakes or suboptimal mold design were once common, and press operators learned to alter machine parameters to compensate. Altering temperature was the most common way to get more melt into the cavities faster, but at the risk of degrading the resin. Much depended on factors outside the tool technology, and good operators were at a premium.

Iterating to success with simulation

Who owns the tool? In many industries, the customer owns the mold, and job shops may not have been involved in the mold design process. Bidding on those jobs requires experience and conservatism to maintain margins when running jobs that are complex, high-volume, or use specialty resins. If the job is captive, the ability of the mold designer to think forward about the equipment the job will be run on and the needed cycle time is a major advantage, says Schmidt.

“If the mold designer and the molder are the same company, I think perhaps a lot more of that understanding is there because they feel the pain a little bit more,” explains Schmidt. “In my experience, especially from the hot-runner side, they’ll get the hot-runner supplier involved early from the mold design standpoint. But in the end, the molder is the one that has to live with the results, and they are not always included in a lot of the decision-making process that they could or should be. That’s where a simulation tool like Moldflow would come in handy to help you make better-informed decisions. Some of them use it; some of them don’t.”

Why simulate? Under perfect conditions, an experienced mold designer can render an efficient mold that works first shot, every shot. Those perfect conditions generally include good symmetry in the mold both in and between cavities, with consistent and equal runner lengths and perfectly uniform cooling throughout the mold. Hot-runner systems help, but the real world is rarely so perfect.

Take family molds, for example. It’s attractive for many manufacturers to shoot all component parts of an assembly at once, but it plays havoc with mold design when quick, consistent filling is essential.

Schmidt describes the problem. “The best rule would be not to design family tools at all because once you put two dissimilar parts in the same mold, you automatically have filling imbalances and balance issues between the cavities. It’s best to just not do that, but for cost and sometimes assembly reasons, decisions are still being made to design family tools. If you have to do it, the best way is to at least balance the filling. Again, Moldflow can help you size the runners to get balanced filling. At that point, it’s all about the pressure.”

For the production manager, reducing the variables to a single setpoint such as pressure is a dream scenario, but in the real world it’s a lot more complex. Neither the mold nor the press “knows” what they are making; they simply respond to the controller setpoints and transducer feedback. In a mold that is unbalanced by design, runners designed to compensate can be very complex.

“Even if I get the filling balanced between the cavities, now you have different-sized runners that allow me to balance that filling,” says Schmidt. “So, you’re going to have different shear histories, different thermal histories, and that means different viscosity and rheological properties that go to each cavity. You still end up with pressure differences and volumetric shrinkage differences and warpage differences. It’s still a challenge to get that to work. But people do it.”

Controlling warpage is a time-honored problem in part design for injection molding. The primary responsibility lies with the part designer, but there are circumstances where it’s just not possible to maintain consistent wall thicknesses or radii in the part where it’s needed to relieve stress as the part cools. This is when the mold designer must step up to compensate.

Simulation is a major time and money saver here, and can replace expensive, iterative programs that may use single-cavity aluminum tools for part validation.

Simulation of cooling channels with Moldflow. (Image courtesy of Autodesk.)

Even when the balance issues are under control, hot runners and gating must be considered for most volume applications. Temperature monitoring is critical, but all systems show only an approximation of the actual temperature profile inside a system. Thermocouple placement is very important, as is heater calibration. The problem is magnified as the number of cavities increases.

“If the thermocouple placement isn’t consistent from, say, a 128-cavity or 144-cavity hot runner, where you have a manifold and then a submanifold, it causes filling imbalances or just imbalances in general in the whole hot runner system,” says Schmidt. “It’s amazing what you can do with them, but it does add another level of complexity that you have to manage. A lot of that is heater draw, and some of it is design.”

“In the automotive industry, for example, they frequently use multigated, nonsymmetrical parts, and certain hot-runner suppliers won’t always concentrate on supplying a balanced manifold layout,” she adds. “They will give a pipeline layout with just legs coming off. Well, at that point, that’s like the family tool version of a part, even though it’s one part.”

Gating is another issue. Hydraulic, pneumatic, and servomotor control is now common and adds another tool to control the fill, but it also adds another set of parameters that must be controlled. A well-simulated, well-designed mold minimizes the need to tinker with gating to achieve a balanced fill with short cycle times. Part designers frequently and inadvertently add issues by designing parts for which efficient gating is difficult or impossible.

Of course, cooling is also critical. Heat flux through the tool has traditionally been considered a mold designer’s black art, but simulation here can dramatically reduce surface finish and dimensional problems—especially when cavities or runners require special heat strategies to achieve a balanced fill.

Experienced mold designers consider these factors as well as the known properties of the material, in order to work up a good mold design the first time. However, with the huge proliferation of thermoplastic resin types, master batches, and engineering resins, simulation software that includes a large materials database also saves time.

Even nonengineers should consider simulation

Normally, part designers have little or no experience in mold design, and manufacturing managers tend to see molds as a capital expense or overhead expenses with little thought about how simulation can save time and money. However, choosing a mold maker that uses advanced simulation can significantly improve the likelihood of project success.

“The best simulation projects get us involved really early so that we can have some input into part design,” says Schmidt. “But even if the part design is locked and you get us in early, we can give you a couple of different gate-location options, and steer you away from problems. Then our customer will typically go away for a week or two, bring us back a rough layout of cavitation and spacing, and then we can help them size their runners, and work on cooling. Normally they’ll give us their first draft of cooling, and we can make comments, maybe a baffle here, or you should put an insert there, and it would go back and forth a couple of times. Those are typically the best ways to use simulation; those are the best projects. They have the best outcome with that concurrent constant interaction.

For engineers using advanced mold simulation software such as Moldflow, it’s possible to fix an imperfect design earlier and at lower cost.

“Sometimes you build a mold and you’re at the mold trial, and now something goes wrong,” explains Schmidt. “You have an air trap in the middle of a cavity, or it’s warping more than it should. I have examples where people who never ran Moldflow up front who ask, ‘Can you help us fix this?’ That’s still a good use of Moldflow because we can play the virtual what-if game, but it’s not a best time to get it in.”

There can be other issues as well. “Sometimes we’re contracted in the mold-design phase where people want us to help size runners or optimize their cooling or reduce warpage before they finish machining the mold,” says Schmidt. “The best place to get Moldflow or an analyst involved would be when the part design is still fluid. A lot of the issues, a lot of the warpage that I end up seeing, is typically caused by part design in general. I always joke that, if people followed the golden rule of part design, which is maintaining uniform wall thickness, then I wouldn’t have a job.”

For mold designers, simulation is about representing the problem with 3D or dual domain meshing, whether to mesh runners with beams or 3D, which solvers to use, and how to use them. For manufacturing or production managers who are purchasing molds, however, it’s about delivering production-ready tools that are run off and qualified on time and on budget. Choosing a mold design team that uses advanced simulation software such as Moldflow is good insurance when deadlines are short, margins are tight, and mistakes are expensive.

First published July 6, 2020, on engineering.com.


About The Author

James Anderton’s picture

James Anderton

James Anderton is the Director of Content for engineering.com. Anderton was formerly editor of Canadian Metalworking Magazine and has contributed to a wide range of print and on-line publications, including Design Engineering, Canadian Plastics, Service Station and Garage Management, Autovision, and the National Post. He also brings prior industry experience in quality and part design for a Tier One automotive supplier.