Taking Aim at Warpage With Windage

In marksmanship, the dictionary definition of Kentucky windage is “a rough but educated guess to account for factors like wind drift or a consistently misaligned sight.” In injection molding and moldmaking, it refers to an educated guess on warpage in a part and cutting the tool steel in the reverse direction for warp-free parts.

When Brad Harrison first saw the part design, he had his own educated guess about how they might turn out. “Once I saw the part design, I was like, ‘Hmm, that’s not good,’” Harrison recalls. Corporate technical services manager at the injection molder and thermoformer, Wilbert Plastic Services, Harrison was assessing a multipart job for a medical diagnostic machine, the largest component of which measured roughly 30 by 40 inches, weighed 12 lbs and would be molded from acrylonitrile butadiene styrene (ABS) on a 2,700-ton Haitian press utilizing a Synventive sequential valve-gate hot-runner system. The new molded design was conceived to replace a thermoformed version, which required a lot of secondary operations, including gluing on aluminum ribs for support.

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Here, the yellow portion is the segment expanded for shrinkage and morphed into the appropriate, toolable windage shape. Source (all): CAE Services

“I thought, ‘We’re going to have significant box warp for that particular part,’ and some of the other parts were scary as well,” Harrison says, noting some “unrealistic flatness tolerances” for several of the components. He described the system’s rear cover as essentially being an open-top box. “Anytime you see a box in plastics, you’re like, ‘That’s not going to work.’ The corners like to hold out and the centers come in. It’s not a good scenario.”

For the last seven years, when Wilbert has faced this scenario it’s turned to CAE Services Corp., Batavia, Illinois, which has developed a method for determining the best possible windage outcome using Moldflow simulations to determine an optimized reverse shape in a virtual space instead of hit-and-miss attempts in tool steel.

“We went to CAE; got our warp models done; cut them into the steel and the parts came out nearly perfect,” Harrison says. “The customer was over the moon happy with it. Most of those parts were switched from thermoforming to injection molding, and we actually had better dimensional stability than the thermoforming did.”

A Simulation Solution

CAE’s optimization of windage utilizing simulation and in-house experience has been an ongoing process over the last 15 years, according to Tim Lankisch, VP of research and development at the company, who notes that when it deploys windage, it’s because all other options have been exhausted.

“The kicker in all this is that you really don’t want to have to do windage,” Lankisch says. “It’s kind of a last resort to fixing a warpage problem.” This is especially true of the windage model where iterations were made in the tool vs. on a computer. “The old way of doing things was trial and error,” Lankisch says. “You’d mold some parts; you see some warpage; and the warpage is out of tolerance. Then you go fix the tool at the parting line — or wherever the warpage may be — a very expensive proposition. Sometimes, depending on how difficult the warpage would be to correct, you might see 10-12 tuning loops and those tuning loops are expensive not only in money but also in time.”

This shows how shrinkage from packing varies throughout the part due to part design, gate locations and process settings.

CAE’s answer to this cycle of inefficiency was to create a methodology in Moldflow that predicts the windage shape to cut into the tool, but only after exhausting all the other methods. “You can’t process it out; you can’t design it out; you can’t tool it out; what do you do?” Lankisch asks. “You do windage, but the windage prospect was always an expensive one. Using Moldflow to do this more efficiently is where we landed. We try to fix things without windage, but when we can’t, we have the capability to create a windage shape in-house that is toolable.”

Iterative Process

Because there is no automation to determine the initial warp shape, CAE’s methodology is based on the optimized gating, cooling and part modifications it determines will minimize warp. From there, Lankisch says they are able to reverse that shape in Moldflow to provide a starting point for windage.

“That’s where the automation lies,” Lankisch says, “in the reversal of the warped shape. We run our Moldflow, and it predicts that it’s going to warp say 5 mm in some direction. The temptation is to say let’s reverse that 5 mm in the opposite direction then run another Moldflow to see how close to flat that comes.”

This begins an iterative process to figure out how much windage to put in. “Is it 100%; is it 80%? Sometimes it’s 105%,” Lankisch says. “That’s rare, but the problem is when you bend something in the opposite direction, now it’s an arch, so it’s stronger. Its starting position is stronger and then it may not warp all the way back from 5 mm down to 0.” This determines the “windage factor,” which CAE provides to the molder and moldmaker.

This shows the out-of-plane warpage prediction in Moldflow used to create the windage shape. Roughly 3.2 mm of warpage was eliminated in the windage processs.

Alongside this discussion, the company also talks with the molder about how it wants to tool the part based on things like aesthetics and the impact of gate location, for instance. “Can we gate in certain areas and have a visible gate vestige? What are the parameters under which we need to work to design the tool first? It’s figuring out where the gate locations are going to be, what size molding machine is this going into and what is the pressure distribution throughout the part,” Lankisch says. “With the cooling system, we try to effectively get the least amount of warpage we possibly can before we decide whether or not this is a good candidate for windage.”

Much of that determination comes down to how a tool’s geometry and the process parameters impact the molecular orientation of the resin as it fills and cools in the cavity, he continues. “Our job as Moldflow engineers is to make moldmakers look good, and moldmakers make two big assumptions whenever they build a tool. The first one is that this part’s going to shrink a certain amount, so we need to expand the shape of the part by that shrinkage factor.” That factor is determined by myriad influences including, part geometry, wall thickness and how much pressure is applied in the packing phase. “This means there’s essentially an infinite number of shrinkage values throughout that part,” Lankisch notes. “Then the moldmaker has the dubious task of selecting one shrinkage value to represent that whole part, despite the fact that there are millions of different shrinkage values across that part.”

Another major hurdle that CAE’s windage process overcomes is the vital step of morphing the CAD geometry into a toolable shape. “While Moldflow is able to reverse the predicted warped shape, it’s only done at the simulation level,” Lankisch explains. “The moldmaker needs a CAD model to work with.”

This means that features that were once fine in die draw go would go into a die-locked condition if the windage shape from Moldflow was adopted directly. “Obviously, we don’t want that, so we must correct it,” Lankisch says. “Our tooling engineer morphs the CAD to create the final shape, expanded for shrinkage and with no features in a die-locked condition. The final product is something the moldmaker can immediately place into their mold design.”

Lankisch says best-case scenario is for a company like CAE to be involved in the very first design discussions to apply a design-for-manufacture (DFM) perspective and make sure the eventual process and design give molders a wide processing window for making acceptable parts.

“You can’t process it out; you can’t design it out; you can’t tool it out; what do you do?”

“Plastic parts warp because you have a lot of different wall thicknesses playing against one another, and you have a lot of different gate locations and all these different variables interplaying,” Lankisch says. “So if that warp is too much, then we have to correct it; it’s all interrelated. I like to fix problems upstream where I can. If I have to fix warpage with process, then I probably get a pretty tight molding window. Our job as Moldflow engineers is to try to produce a situation where there’s a lot of wiggle room at the press.”

Wilbert had wiggle room as well, but according to Harrison, it didn’t need it. “CAE gave us the process parameters that it ran the Moldflow with, which we translated to the machine setup,” Harrison says, “and it honestly we came out of the gate making good parts.”

The success of the project is why Wilbert goes back to CAE and applies windage, when necessary. “I’m not going to go to them to make a dog bowl,” Harrison says, “but we use them quite often. The customer tolerances keep getting tighter, and if it has to be perfect, we go to them. If we see characteristics in the part that are going to be warp prone, we go to them.”