How to use less plastic in injection molding

By Bruce Geiselman?

Lightweighting plastic products and reducing plastic waste can protect the environment and save manufacturers money.?

But there is no single solution to using less plastic. Manufacturers that are sensitive to how parts are designed, and mindful of processing parameters and the training needs of their machine operators, can reduce waste and save money.?

Today’s sophisticated processing machinery comes with built-in tools that can help if they are fully utilized, but that is only part of the solution.??

Two injection molding experts — Jason Robinson, a senior consultant and trainer with RJG Inc., a training, consulting and technology company, and David Hoffman, director and instructor of plastics education and training at the American Injection Molding (AIM) Institute — shared suggestions with?Plastics Machinery & Manufacturing.??

Design with less in mind?

From material selection to design, the amount of resin a part consumes is determined, in part, before it’s ever manufactured.??

“Don’t fix the design and then choose a material,” Hoffman said. “The design and material selection should go hand in hand.”?

To determine what material is appropriate for their project, part designers and manufacturers can perform a cost analysis and then design for the material selected.?

For example, Hoffman said, people might consider making a plastic shelf from either PP or glass-filled nylon. Their choice will have implications for how much material they end up using.?

While glass-filled nylon by weight is more expensive than PP, a shelf made from glass-filled nylon can be produced with much thinner sections and less material.?

“Optimizing the design based on the modulus of the material will allow us to reduce thickness, material use and cycle time and potentially mold a less-expensive part in the end versus the less-expensive material when only considering cost per pound,” Hoffman said.?

How that shelf — or any other part — is designed also is important.?

“A lot of times part designers are not very well-educated in plastic part design,” Hoffman said. “They may not understand volumetric shrinkage, for example. Designing with plastics is drastically different than designing a part out of metal. It is fine to have thick sections of a part made from metal, but plastic parts should avoid those chunky sections.”?

Thinner walls or hollowed-out sections can save material, while chunky sections can cause sinks and voids and even warpage, which leads to scrap. Part designers need to learn the fundamentals of plastic part design, shrinkage and warpage, he said.?

By designing parts with the correct rib-to-wall ratio and coring out thicker sections, manufacturers can make parts lighter and reduce sinks and voids, Robinson said.??

“There are some guidelines that tool designers or part designers can use to determine what thickness the intersecting wall or ribs need,” he said.??

Molding matters?

Like part design, mold design has significant influence on the amount of resin that’s used.?

Simulation software can help mold designers determine runner size, gate location and cooling designs, Hoffman said. With it, designers can ensure the injection molding machine (IMM) will have enough pressure to fill the mold.?

“Optimize the mold design before cutting steel because if I don’t pick the right gate, if I don’t design the right cooling system, that’s going to lead to scrap on the molding floor,” he said.?

Runner design and the choice between cold and hot runners are other important considerations.?

For example, Hoffman said, mold makers should evaluate whether an H-, I- or X-branching runner design would be more efficient when they’re designing a four-cavity mold.?

“Simple spreadsheets help to evaluate the material volume based on diameters and lengths that each design option requires,” he said. “Each one needs to be evaluated on its own. There’s not a one-solution-fits-all scenario here.”?

One technology AIM Institute recommends is the?MeltFlipper,?developed by its CEO and CTO, John Beaumont. The patented runner design, which works with most molds, influences plastic flow to promote uniformity and higher quality in molded products, Hoffman said. The result is a more-efficient mold that produces less-variable parts and less start-up scrap.?

"It is used in single- and multi-cavity molds to control plastic flow for filling, packing, cooling and warpage," Hoffman said.

He and Robinson put forth varying views on the use of cold runners versus hot runners. While both technologies provide a flow path to the mold cavities, hot-runner systems allows the plastic to remain in a molten state, reducing the amount of waste.??

Robinson was unequivocal, saying, “The next [tip] is a pretty simple one: Just use hot runners whenever possible versus a cold runner.”?

With hot runners, manufacturers can reduce cycle times and produce more parts. They also can streamline operations, he said.

“Hot runners can eliminate secondary operations being done to remove the runner, streamlining the entire production process. Sometimes, the runner will need to be trimmed; the runner will have to be ground ,stored or trashed,” he said.

But Hoffman expressed some reservations, especially considering the upfront costs of hot runners versus cold-runner systems.?

"Yes, a hot runner will save the cost of the plastic from the cold runner. But that hot runner costs money," he said.?

Hot runners also add complications.

“A hot runner has heaters, thermocouples, blind drilled holes that need to line up, thermal expansion issues, interface issues with a controller, leak points and so on," Hoffman said. "There are a lot of chances for things to go wrong in a hot-runner mold, so they should be used, designed and processed with caution."

Evaluations that take into account part volumes and costs can help a processor determine whether a hot sprue or a hot runner system is justified. It’s a financial investment, Hoffman said.?

“It should be looked at that way,” he said. “If I'm making 100,000 parts a year, maybe I can't justify a hot runner. If I am making 10 million parts a year, maybe I can justify a hot runner. It should be looked at accordingly. … Not every project should necessarily go to a hot runner, but they are a good benefit in many, many cases,” he said.?

Optimizing the machine?

Machine variables that can affect material usage include injection speeds, cavity pressure and part variation. Molders also can improve their processes by performing molding studies and collecting data — but that approach is only helpful with the appropriate follow-through.?

Using faster injection speeds is “kind of a mantra at RJG,” Robinson said.??

“There are lots of quality benefits from filling at high rates of speed,” he said. “When the part is good, it provides some consistency.”?

In many cases, RJG has found quicker mold filling results in a part being made with less weight.??

“What happens is your higher rate of filling allows you to pack before the time to freeze happens inside the cavity,” Robinson said. “The faster we can get it in there, we can distribute the material through the cavity more evenly.”?

To control the packing phase of the molding process,?RJG’s CoPilot system?uses cavity pressure sensors in the mold. With the system, molders can reduce the number of bad parts they produce.?

“We use the machine to pack the part out to a very specific requirement, and then there’s a feedback loop from the cavity pressure transducers through the CoPilot system, and then we tell the machine when to stop packing,” he said. “It’s real time, on the fly, as the parts are made.”?

A gate-freeze study, also known as a gate-seal study or a hold-time study, tells a processor when material in the gate solidifies.?Taking too much time can allow the runner system to fill more heavily, creating more waste, Robinson said.?

But, Hoffman warned, studies actually can waste resin, if the data’s not being used.?

“When folks learn processing … they’re always taught to do various studies,” Hoffman said. “Several years ago, we started picking some of those studies apart and doing more research on them. One of those is a pressure-drop study.”?

The AIM Institute found that most processors don’t do anything with the data. And each shot during a pressure-drop study wastes material.?

“Essentially, what we found is there’s very little value in that study in how it is typically done today,” Hoffman said. “Now, we still do a pressure-drop study, but we’ll do the 95 percent full part, and then an air shot out of the nozzle, and that’s it. We’re not going to do right after the gate, right before the gate, through the sprue, through the runner, because we saw no value in it, and in our research, it wasn’t giving us good data.”?

Planning for resin conservation?

Choices outside the IMM also influence how much resin is used. Here, maintenance and changeover strategies matter.??

As an example, Robinson said, when one or two cavities on a multi-cavity mold are damaged, some plastics processors will block off the bad part instead of repairing it. Blocking one or two cavities on a mold not only reduces productivity, but it can also impact the stability of the process and lead to higher scrap rates, he said.??

When the cavities are blocked in a cold-runner system, unnecessary scrap plastic is still being produced, he said.?

“Ideally, you should fix the broken cavity and not block it,” Robinson said.?

In addition, he recommended going from light to dark material, rather than the other way around when planning for changeovers. Transitioning from dark to light parts takes a considerable amount of purging or cleaning, he said.?

“That is a big source of waste,” he said. "It’s a big waste of time and money.”?

Training to use less?

The ability to save resin and optimize designs and process parameters relies on people, and Robinson and Hoffman echoed each other in asserting the importance of training.?

Starting up molds efficiently can save wasted material during start up and from poor execution of the start-up process, he said.???

“A lot of folks that are doing this job, often times don’t understand the whys of what they’re doing, and why the various process settings are important,” Hoffman said. “They’re just told to get the mold started up.”?

He recommends processors embrace critical thinking rather than using memorized guidelines.??

“This will get to the root cause [of the problem] and reduce scrap versus tweaking and fighting the issue over and over again,” he said.?

AIM’s Molding 1 class teaches processors a methodology to efficiently start up a mold using an existing process sheet, but teaches them what the process values on the controller mean, why they are important and how to set them properly.?

“You can’t emphasize enough the importance of training people to know how to do their jobs,” Robinson said.?