Cycle time is a key metric in plastic injection molding that affects efficiency, cost, and quality. For companies that fabricate rapid molds for injection molding prototypes, like Protoshop Inc., optimizing cycle time and part cost may not be the primary goal but should be considered early in the design phase to ultimately meet cost and quality targets for production. This article will go through the stages of injection molding cycle time, its impact on efficiency and cost, and practical ways to reduce it for better productivity.
What is Injection Molding Cycle Time?
Injection molding cycle time is the total time it takes to complete one full cycle in the plastic injection molding process. Each cycle has multiple stages that affect the efficiency and quality of the molded part, including injection, cooling, dwelling, ejection and mold opening/closing. For prototype molding, there can be an additional few steps before and after mold opening/closingthat increase cycle time, if the part requires side action to form some of the part features.
Why Cycle Time Matters for Efficiency and Cost
Reducing cycle time means reducing per-part cost. Shorter cycle times mean more parts are produced in the same time frame, which means less labor and operational costs, allowing companies to meet deadlines and demand more effectively.
Breaking Down the Stages of Cycle Time in Injection Molding
1. Injection Time
Injection time is the time it takes to inject molten plastic into the mold. Material flow rate and injection speed affects this stage. Reducing injection time means optimizing material flow and high speed injection where possible. Proper venting of the mold to allow the trapped air to escape allows the plastic to flow into the mold more quickly with fewer molding issues like knit lines and air traps.
2. Cooling Time
After injection, the material must cool and solidify in the mold. This stage is critical for part stability and accounts for the largest portion of cycle time. Adjustments to cooling channel design and temperature control can reduce this stage without sacrificing part quality.
3. Dwelling Time
During the dwell or holding phase, fill pressure is increased as the part cools to prevent shrinkage and warping and to keep part dimensions consistent. While necessary, reducing holding time by fine-tuning the required pressure can reduce overall cycle time.
4. Ejection Time
Once the material has cooled and solidified, the part is ejected from the mold. Ejection mechanisms like automated ejectors or robotic arms can help speed up this phase, especially for high-volume production.
5. Mold Opening/Closing Time
This stage involves the mold’s mechanical movement to open and close for the next cycle. Simpler part designs can be molded by opening the core and cavity on a single axis. Clever mold designers can utilize the opening of the mold to remove slides or core pins for off-axis features without adding time to the molding cycle. Adjustments to mold complexity and clamping systems can improve this time to get faster cycles.
In prototype molding, the off-axis part features are often created with non-mechanized core pins. The extra time required to manually place and remove these core pins increases the cycle time and cost of prototype parts.
How to Calculate Cycle Time in Injection Molding
To optimize cycle time, you need to calculate it accurately. The basic formula for cycle time is:
Injection Molding Cycle Time = Injection Time + Cooling Time + Dwelling Time + Ejection Time + Mold Opening/Closing Time
Measuring each stage accurately helps manufacturers see where to make adjustments to reduce time and improve productivity.
Factors that Affect Cycle Time in Plastic Injection Molding
Cycle time is affected by many factors, primarily mold design, material and process parameters. Each of these can be optimized to reduce cycle time without sacrificing quality.
- Mold Design: Cooling channels, runner and gate placement and number of cavities can impact cycle time. Mechanisms that form features that are off-axis from the direction of pull, such as slides, collapsing cores, lifters, and/or rotational mechanisms, may add to the de-molding timeline.
- Prototype tooling often involves non-mechanized core pins or pick-outs that must be manually placed into the mold before molding and manually removed from the part after ejection. These manual mold components decrease the complexity of the tool, reducing mold cost and timeline but increasing the cycle time (and cost) to mold the parts.
- Material Choice: Choosing materials with good flow and cooling properties can reduce cycle time, especially for prototypes that must replicate production parts.
- Process Parameters: Fine-tuning injection speed, mold temperature, and injection pressure according to material and mold specifications can yield faster cycles while maintaining product quality.
Ways to Reduce Cycle Time
Injection Stage Optimization
High-speed injection, optimized pressure, and creative venting can reduce the time to fill the mold without sacrificing accuracy. A balanced flow rate will avoid defects, so adjustments should ensure material flows smoothly to all parts of the mold.
Cooling Stage Optimization
Cooling is the best way to reduce cycle time. Optimizing cooling channel placement and design will control temperature evenly across the part, which reduces cooling time and shrinkage. Using cooling materials with high thermal conductivity also helps to reduce cycle time. Protoshop has the capability of adding cooling to prototype molds for large molding orders if needed.
Dwelling Stage Optimization
While dwelling prevents defects, using only the required pressure and time can reduce cycle time without sacrificing the final part’s stability. A precise approach will balance time and quality.
Ejection Stage Optimization
Fast ejection systems and automated ejectors can speed up the part ejection. Robotic systems can be very useful for larger molds, to eject quickly and accurately without damaging the part or mold.
Mold Opening/Closing Optimization
Quick clamping systems and smooth mold movement can reduce the time to close the mold. Efficient mold opening and closing can streamline the cycle and is very beneficial for high volume production. Simple part designs that require fewer molding mechanisms, such as sliders, lifters, etc., offer reduced cycle times and less wear and tear on the molds.
Accelerate Your Production with Protoshop’s Expert Injection Molding Solutions
In plastic injection molding, cycle time is a critical factor that affects efficiency, cost, and productivity. Manufacturers can get shorter cycle times and lower costs per part by understanding and optimizing each stage, from injection to mold closing.
Although Protoshop focuses on producing high-quality prototype parts quickly while minimizing mold fabrication costs, our expertise in rapid prototyping and tooling can help clients think through factors to ultimately reduce cycle time through mold design, material, and process parameter adjustments.
Frequently Asked Questions
What Is The Standard Industry Formula For Cycle Time?
The industry standard formula simplifies cycle time into three main components: t = td + ti + tc.
In this formula, td is intermediate time (all mold movements, ejection, and auxiliary actions), ti is injection time (including both the filling and dwelling phases we discussed), and tc is cooling time.
This standard approach helps manufacturers benchmark and communicate across the industry, but the breakdown we provided gives you more insight into optimization opportunities.
What Are Typical Time Ranges For Each Component Of The Cycle?
Injection time (ti) is very short, usually 1-2 seconds for most parts, regardless of size. This includes both the filling phase and the dwelling/holding phase we mentioned earlier. Cooling time (tc) dominates most cycles and can be 10-60 seconds or more, depending on part thickness, material properties and cooling system efficiency. Intermediate time (td) can be short with modern machines – 2-5 seconds – but can be longer for complex parts that require manual core pin placement or removal, as we discussed in our prototype molding section.
Why Does Cooling Time Dominate The Overall Cycle Time?
Cooling time is usually 60-80% of the total cycle time because plastic parts must solidify before ejection to prevent deformation. Unlike injection which happens under high pressure and speed, cooling is a thermal process that depends on heat transfer through the plastic material. Thick sections cool slower than thin sections and the cooling rate is limited by the thermal properties of both the plastic material and the mold material. That’s why optimizing cooling channel design and placement, as we mentioned, offers the biggest opportunity for cycle time reduction.
How Can Manufacturers Balance Cycle Time Optimization With Part Quality?
The key is to understand that each stage serves a specific quality function that can’t be eliminated, only optimized. Injection time ensures the mold is filled completely without defects like short shots or flow lines. Dwelling time prevents sink marks and dimensional variations from material shrinkage. Cooling time ensures the part maintains its shape and properties. Rather than reducing times, successful optimization is about improving efficiency within each stage – better cooling channel design for faster heat removal, optimized injection parameters for complete filling at higher speeds and precise pressure control during dwelling to use only the minimum time required for quality parts.
