Cycle time refers to the total time it takes to produce one plastic part. That's the time elapsed from the moment the mold closes, through the injection process and cooling time, to the point at which the part is ejected from the mold and the cycle starts all over again for the next shot.

"Cooling time is almost always the major part of the cycle," explains Christophe Beauve, Tooling Manager at Rompa. "If your part takes 20 seconds to mold, often 12 of those are spent waiting for the part to cool and harden."

Longer injection molding cycle time means fewer parts per hour and a higher cost per part. When you're producing hundreds of thousands (or millions) of parts, shaving off just a few seconds from the cycle time can make a significant difference to your bottom line. Even with lower volume projects, it can make a big impact.

The good news is that you can make a difference to your cooling time to save time and money. Here are three common design factors, and how to optimize them. 

1. Wall Thickness: Less Can Be More

One of the biggest contributors to long cooling time is wall thickness. Thicker walls take longer to cool - sometimes exponentially longer, because the heat has to dissipate from the center outward. 

Many designers assume that thicker = stronger. But that's not always the case with plastic. In fact, overly thick walls can cause shrinkage (where the plastic pulls in on itself as it cools), or air bubbles that weaken the part. 

"It's a common mistake," says Christophe. "We often recommend switching from a thick solid shape to something like a cross-section or ribbed structure. You get the same strength, faster cooling, and lower material costs." 

2. Smart Use of Cut-Outs

If parts of the geometry don't carry load or play a functional role, those areas can often be hollowed out or removed without affecting the part's performance. 

This reduces the amount of plastic in the mold, which often simplifies the design of the tool that's needed, as well as: 

• Speeding up cooling 

• Shortening cycle time 

• Reducing part weight (with the potential for a bonus cut to transportation carbon footprint) 

"A classic example of this was a recent Rompa customer whose product team added clips and tabs to the part late in the design stage," says Christophe. "These amendments more than doubled the complexity of their mold and would have nearly doubled their cycle time. By suggesting strategic cut-outs and reshaping some internal features, we were able to help the cycle time and cost back down to the original levels, without compromising on the extra functionality." 

3. Cooling Access: Think Ahead

Some parts, especially those with deep holes or complex internal structures, are difficult to cool evenly. Hot spots can form if your design doesn't allow room for cooling channels in the mold where water can circulate to cool the plastic. As the name suggests, hot spots take even longer to solidify. 

"If we can't reach a hot area with a water channel, it will stay warm longer, and that holds up the whole process," Christophe explains. "By making space for proper cooling, we can cut seconds off each cycle." 

Rompa's experienced injection molding engineers often help customers adjust their designs to allow better cooling access. There are several ways you can do this:  

• Changing parting lines. The parting line is where the two halves of the mold separate. By adjusting its location, engineers can improve access to internal features, allowing cooling channels to be added more effectively around critical areas. 

• Amending the placement of inserts. Inserts are metal components placed inside the mold to shape or reinforce certain features of the part. Moving them slightly can open up space for cooling channels or reduce the distance heat must travel to escape. 

• Adjusting the wall geometry. Simplifying or thinning certain wall sections can reduce thermal mass and make cooling more efficient, especially when it allows for more uniform wall thickness or removes areas where heat would otherwise get trapped. 

You might be thinking: isn't this something for the tooling engineer to worry about? And in many cases, it is. But if your partner isn't reviewing your part before tooling begins, you might be missing out on major savings.

It's true that your injection molding partner should catch these issues and suggest improvements. But that only works if you involve them early enough in the process. If the design is already locked - and especially if the tool is already cut - it's often too late to optimize.

That's why Rompa's team includes both tooling and production experts who review your part design as part of our quotation process. We work with your team to find small, smart changes that speed things up and avoid costly surprises later.

"Let's say your part has a 20-second cycle time, and you produce 1 million parts per year," Christophe explains. "That's about 5,500 production hours. If we can reduce that to 18 seconds through better cooling design, you save over 500 hours of injection molding machine time every year. That frees up production capacity, shortens lead times, and reduces your cost per part. It's not just engineering, it's good business."

As we've seen, cutting cycle time doesn't always require a major redesign. Often, it's a matter of reviewing your wall thickness, adding a few cut-outs, or adjusting the part geometry to allow for better cooling. Of course, there are other things we can do to reduce cycle time – such as increasing the number of cavities per mold. 

These are the kinds of improvements that come from working with experienced partners who know the trade-offs and are willing to ask the tough questions before the mold is made.  

Want to talk to one of our experts about how you could cut the cycle time on your next plastic injection molding project?  '

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