In injection molding industry, cycle time is a matter of life and death. Accurate estimation leads to competitive pricing, sufficient production capacity, and visible profits; inaccurate estimation results in lost orders or losses. Today, I will guide you through a step-by-step breakdown of deep logic behind cycle time.

Injection molding cycle time refers to the total time elapsed from the moment mold closes until it closes again. It is one of the most critical efficiency indicators in injection molding production, directly determining three things:
Production Rate: How many parts can be produced per unit time.
Single-Piece Cost: Cost of machine hours, labor, and energy allocated to each part.
OEE (Operational Equipment Effectiveness): Whether machine is producing products or idling

 

Typical Cycle Time Range: Varies from 10 seconds to several minutes, depending on complexity, size, material, and mold design of parts.
Optimizing cycle time = Maximizing output × Maximizing profit; this is balance every injection molding plant continuously strives for.

In Request for Quotation (RFQ) stage, you face core challenge:
Extreme lack of information (potentially no detailed 3D model, no definitive mold design, no actual measurement data), but you must provide a competitive quote.
Without cycle time estimation, relying solely on guesswork often leads to dissatisfaction on both ends:

Key insight: Even when drawing on experience from similar projects and products, it's still recommended to validate data through forward calculations, where possible.

A complete injection molding cycle consists of five stages linked together. Understanding characteristics of each stage is the first step to accurate estimation.
① Injection Stage
Time Range: 1–5 seconds
Process: Molten plastic is injected into mold cavity under high pressure, filling every corner of cavity.
Influencing Factors: Injection speed, Material Flowability (MFI), gate design, part wall thickness
Estimation Points: Thin-walled parts require higher injection speeds, while precision parts require slower, more controlled filling.
② Holding/Packing Stage
Time Range: 2–15 seconds
Process: After injection, a certain pressure is maintained to push more material into mold cavity to compensate for cooling shrinkage.
Influencing Factors: Material shrinkage rate, part geometry, holding pressure curve
Estimation Points: Semi-crystalline materials (such as PP, PA) have high shrinkage rates, and holding time is usually longer than that of amorphous materials (such as PC, ABS).
③ Cooling Stage – "Largest Part" of Cycle
Time Range: 10–60+ seconds (up to several minutes)
Process: Part cools and solidifies within mold until it has sufficient rigidity to be ejected.
Percentage: 50–70% Total Cycle Time
This is the most critical optimization target in the entire cycle and also area with the greatest potential for cost reduction.
④ Mold Opening/Close Stage
Time Range: 3–10 seconds
Process: Mold opens, allowing space for ejection stroke; after ejection, mold closes again.
Influencing Factors: Mold size, clamping mechanism stroke, safety protection devices
Estimation Points: Larger molds have longer opening and closing strokes, resulting in correspondingly longer times. Note that operating time of an electric motor is shorter than that of a hydraulic press.
⑤ Ejection Stage
Time Range: 1–3 seconds
Process: Ejection mechanism pushes cooled and solidified part out of mold.
Influencing Factors: Draft angle of part, ejection mechanism design, presence of automatic gripper
Estimation Points: If a robotic arm or robot is used for part retrieval, ejection time needs to be coordinated with automation rhythm.

 

Following data is based on industry experience and is applicable to early estimation and quotation stages. Note: Actual cycle times need to be accurately calculated and adjusted based on specific materials, mold design, and equipment conditions.

Above range is for reference only and should not be used directly for quotations. Actual cycle time must be determined comprehensively based on material property tables, mold flow analysis, and equipment parameters. It should also be noted that slider operation, robotic part removal, insert placement are additional time costs and must be included separately in cycle calculation.

5.1 Why does cooling account for such a large proportion?
Essence of injection molding is: heating plastic to a molten state, injecting it into a mold, and then allowing it to cool back to a solid state. Heating and injection are both short, but cooling takes time. Plastic must cool from a molten state to below its heat distortion temperature within mold cavity before it can be ejected without deformation.

 

Cooling speed is constrained by three factors:
Thermal conductivity of material itself—different materials have vastly different thermal conductivity coefficients; Thermal conductivity efficiency of mold metal—thermal conductivity of mold steel; Design of cooling system—waterway layout, flow rate, cooling medium temperature.
5.2 Exponential Influence of Wall Thickness
This is the first rule worth noting: T ∝ t²
Under same material and mold temperature, cooling time is directly proportional to square of maximum wall thickness of plastic part, serving as an engineering reference rule. Product cycle time is determined by the thickest point in a localized area of product.
For example: 1mm wall thickness → approximately 2 seconds; 2mm wall thickness → approximately 8 seconds (4 times); 4mm wall thickness → approximately 32 seconds (16 times).
This explains why "uniform wall thickness" is emphasized in product design phase—a locally thick wall can increase the entire cycle time several times over.
5.3 Comparison of Material Thermal Properties
Different materials have different thermal diffusivity, directly determining their cooling rate:

Semi-crystalline polymers (PP, PA, POM) need to release latent heat of crystallization during cooling, therefore their cooling time is usually longer than that of amorphous polymers (ABS, PC, PS).
5.4 Quick Suggestions for Cooling Optimization
Prioritize uniform wall thickness in design phase – this controls cycle time from source; Optimize cooling water channel design – recommended distance between water channel and cavity surface is 1.5–2 times water channel diameter; Consider conformal cooling – especially effective for complex cores, reducing cooling time by 30–50%; Focus on thermal conductivity when selecting materials – prioritize materials with better thermal conductivity while meeting mechanical requirements; Use mold flow simulation software – Moldflow, Moldex3D, etc., can provide cooling time predictions before trial molding.
Data Warning: Underestimating cooling time by 20% may result in a 30% or more reduction in profit margin.

Having mastered cycle time, next step is to convert it into capacity data for production scheduling and cost accounting.
Core Formula: Hourly Output (PPH) = 3600 seconds ÷ Cycle Time (seconds) * Number of Holes (The following example uses 1*1 hole)
Example Calculation (Cycle Time = 45 seconds)

Quick Reference Table

This calculation represents theoretical maximum output. Actual production capacity must consider factors such as machine malfunctions, mold changeover time, quality inspections, and is typically multiplied by an OEE factor (75–85%).

Error 1: Ignoring Cooling Time Analysis
Manifestation: Failure to carefully calculate cooling stage, or simply applying an empirical value.
Impact: Severely underestimated cycle time; Production results in output falling far short of expectations; Frequent defects such as part warping and shrinkage; Mold modifications are necessary, leading to losses in both cost and delivery time.
Solution: Perform cooling simulation analysis during design phase. Optimize wall thickness distribution and cooling water channel design using DFM tools and mold flow analysis software.
Error 2: Ignoring mold complexity and mechanical movements
Manifestation: Treating complex molds as three-platen molds in cycle calculations, without including time for slides, lifters, thread release, and robot part removal.
Impact: Actual cycle time is 15-40% longer than estimated. Production scheduling is completely disrupted, and machines are downtime; If action times are not synchronized, it may also damage mold.
Solution: Communicate fully with mold designer during estimation stage to clearly define all core pulling, slide, ejection, part removal actions, and quantify time for each.
Error 3: Directly applying general industry data
Manifestation: Using a "typical injection molding cycle time" found online or in manuals without distinguishing specific material grade, machine tonnage, and number of cavities in mold.
Impact: Prices are either too high (losses) or too low (losses). Production capacity planning is completely inaccurate, leading to wasted or insufficient resources.
Solution: Use specific material property data sheets, actual machine injection speed and response time, and historical production statistics for similar products to support estimates. Establish your own historical database and continuously calibrate.
Error 4: Ignoring post-processing and other auxiliary time
Manifestation: Only in-mold cycle time is calculated, forgetting out-of-mold processes—gate removal, packaging, annealing, etc.
Impact: Overall production time is underestimated. Labor costs and auxiliary equipment investment are not included.
Solution: Include post-processing time and equipment usage in the total cost accounting.

Project Launch: Project Name / Number / Date; Estimated Annual Demand
Material Information: Material Type: Amorphous / Semi-crystalline / Reinforced / Filled; Material Grade / Melt Flow Index (MFI).
Geometric Analysis
Wall Thickness Characteristics: Thin Wall (<1mm) / Uniform Wall Thickness / Existence of Thick Wall Areas
Complexity: Complex Geometry / Requires Slider/Undercut / Contains Threads/Inserts
Critical Wall Thickness (mm) / Maximum Size (mm)
Time Estimation Table for Each Stage

Utilizing a professional automated calculation model

 

 

 

Cooling Priority—Place cooling analysis first, as it accounts for 50-70% of the cycle time.
The more specific data, the better—Use actual material property tables + actual machine parameters + historical data from similar products.
Don't forget mold actions—slider, ejector, insert placement, robot part removal…every action must be quantified and included.
Continuous Calibration—Collect cycle time data from each actual production run and use it to improve estimation model for next cycle. Cycle time estimation is a combination of art and science, requiring data-driven tools and continuous feedback from practical experience.