This chapter will focus on traditional methods used in production practice to determine holding pressure parameters (mainly pressure and time). These methods form basis of injection molding process setting and are still widely used today. We will discuss setting strategies for holding pressure, holding time, V/P switching point, and analyze underlying principles, steps, advantages, and disadvantages.
Setting Holding Pressure
Setting holding pressure is core task of holding pressure stage, as its magnitude directly determines driving force of shrinkage compensation.
Traditional setting methods mainly include rules of thumb, experimental optimization, and preliminary estimation based on theoretical formulas.

 

Based on long-term production practice and summaries of specific material behavior, these provide process engineers with a quick and convenient initial setting value. A common rule of thumb is to set holding pressure as a percentage of maximum injection pressure (or peak pressure at the end of injection).
In most cases, initial holding pressure setting is recommended to be between 30% and 80% of maximum injection pressure. A more commonly used conservative starting range is 50% to 60%, or 1/2 to 1/3 of peak pressure.
Material-specific settings: Different materials have significantly different shrinkage characteristics and viscosities, therefore their holding pressure requirements also differ.
Crystallic plastics: Due to significant volume shrinkage during crystallization, higher holding pressures are typically required.
Polyoxymethylene (POM): High shrinkage, good flowability; holding pressures are usually set higher, reaching 80% of injection pressure, and even 100% for products requiring high precision.
Polyamide (PA, Nylon): Generally recommended to be around 50% of injection pressure.
Amorphous plastics: Relatively small and gradual shrinkage; holding pressures are typically lower.
Low-shrinkage general-purpose plastics: Polypropylene (PP) / Polyethylene (PE): Good flowability, shrinkage rate is greatly affected by pressure, and holding pressure is usually set at 30% to 50% of injection pressure.
Advantages: Simple and quick, providing a reasonable starting point for initial mold trials and reducing time spent on blind debugging.
Disadvantages: Empirical values are very rough, not considering specific conditions such as mold structure, product wall thickness, and gate size. They can only be used as an initial reference, and subsequent fine adjustments must be made based on actual conditions.

Experimental optimization method is a systematic approach to find optimal holding pressure through a series of controlled experiments. Its results are more reliable and accurate than empirical method. Common experimental methods include product weight method and visual observation method.
Product Weight Method (or Weighing Method):
Principle: Under premise of a sufficiently long holding time (ensuring gate is frozen), final weight of product directly reflects its internal density. The higher holding pressure, the more material is injected, and the greater weight of product. When pressure increases to a certain level and weight of product no longer increases significantly, it indicates that mold cavity has been fully compacted. This pressure is optimal holding pressure.
Steps: First, set a sufficiently long holding time. Start production with a low holding pressure (e.g., 30% of peak injection pressure). After process stabilizes, take a set of products (e.g., 5-10 pieces), weigh them accurately, and record average value. Gradually increase holding pressure (e.g., 5-10 MPa each time).
Plot holding pressure against corresponding average product weight. Pressure corresponding to inflection point of curve or starting point of plateau zone is usually considered optimal holding pressure. Continuing to increase pressure contributes little to weight but increases risk of flash, internal stress, and energy consumption.
Advantages: Method is scientific and objective, using quantitative data (weight) as basis for judgment. It has good repeatability and is one of industry-recognized standard methods for finding optimal holding pressure.
Disadvantages: Requires multiple sets of experiments, consuming time and materials.
Visual Inspection Method: Principle: Adjusting holding pressure directly by observing macroscopic defects in product.
Start production with a lower holding pressure. Inspect product surface, especially thick-walled areas and roots of ribs, for obvious shrinkage or depressions. If shrinkage is present, gradually increase holding pressure until shrinkage is just eliminated. Simultaneously, check for burrs (flash) on parting surface. If burrs appear, pressure is too high and needs to be reduced appropriately. Optimal holding pressure is usually within process window between "just eliminating shrinkage" and "about to produce burrs."
Advantages: Intuitive and fast, especially suitable for products with high appearance requirements.
Disadvantages: Judgment criteria rely on subjective human vision; insensitive to internal quality (such as voids) and minute dimensional changes; results have poor accuracy and consistency.

To provide theoretical guidance for setting holding pressure, some researchers have attempted to establish mathematical models. One of mentioned formulas is: P = (1 + S) × (P1 + P2).
Formula Explanation: P: Theoretically required holding pressure (Bar). S: Volume shrinkage rate of plastic (%), representing proportion of volume reduction from molten state to solid state at room temperature, which is core objective that holding pressure needs to compensate for. P1: Injection pressure (Bar), representing basic pressure required to push melt to fill mold cavity. P2: Minimum pressure required for mold filling (Bar), which can be understood as minimum pressure required to overcome flow resistance of mold runner and cavity.
Significance and Limitations of Formula: Significance: This formula, based on physical essence, clearly points out positive correlation between holding pressure (P) and intrinsic properties of material (volume shrinkage rate S) and molding conditions (pressures P1, P2). It emphasizes that volume shrinkage rate is core factor determining holding pressure requirement, which provides a good theoretical perspective for understanding holding pressure.
Limitations: While formula is theoretically inspiring, it is rarely used directly for precise calculations in actual production for following reasons:
Difficulty in Obtaining Parameters: Parameters in formula, especially volumetric shrinkage rate (S) and minimum filling pressure (P2), are variables influenced by various process conditions, making them difficult to obtain accurately and independently. Value of S varies with pressure and cooling rate, while P2 is closely related to mold geometry, melt temperature, etc.
Process Simplification: This formula simplifies complex dynamic holding pressure process into a static mathematical relationship, failing to consider dynamic effects such as time, temperature field changes, solidified layer development, and pressure attenuation in viscous fluids.
Narrow Applicability: It is more suitable for theoretical analysis and teaching understanding than for guiding complex and variable actual production. Conclusion: In traditional methods, scientific setting of holding pressure is usually a process of "starting with rules of thumb and optimizing through experiments." Weighing method, due to its objectivity and quantitative characteristics, is considered the most reliable method for determining optimal holding pressure.
Setting Holding Time: Holding time determines effective duration of compensation effect. Its core principle is that it must continue until gate is completely frozen.

 

Gate Freezing Principle: Gate is a narrow channel connecting runner and cavity, and is also "throat" of mold cavity. During cooling process, due to its smallest cross-section, it will cool and solidify (freeze) first. Once gate freezes, mold cavity is isolated from injection system, holding pressure can no longer be transmitted, and mission of holding pressure stage ends.
Too Short Holding Time: If holding pressure is withdrawn before gate freezes, melt, still in a high-temperature and high-pressure state inside mold cavity, will flow back into runner system under action of internal pressure. This will lead to insufficient shrinkage of product, resulting in serious shrinkage and dimensional instability problems. Too Long Holding Time: Continuing to maintain holding pressure after gate freezes has no effect on product quality, because pressure can no longer enter mold cavity. This will only needlessly prolong molding cycle, reduce production efficiency, and increase energy consumption. Therefore, optimal holding time is gate freezing time.
Experimental Determination Method (Weighing Method)
Similar to determining holding pressure, weighing method is also gold standard for determining optimal holding time.
Steps: First, set a reasonable holding pressure. Start production with a relatively short holding time. After process stabilizes, take a set of products, weigh them accurately, and record average value. Gradually increase holding time (e.g., 0.5-1 second each time). Plot holding time against corresponding average weight of products. Time corresponding to inflection point of curve or starting point of entering plateau zone is gate freezing time, which is optimal holding time. After this time point, product weight no longer increases.
Empirical and Estimation Methods: When weighing experiments are inconvenient or rapid estimation is required, some empirical formulas can be used.
Relationship with Cooling Time: It is generally believed that holding time usually accounts for 30% to 40% of the total cooling time. Calculation Formula Based on Wall Thickness: Cooling time is proportional to square of product wall thickness. Some simplified empirical formulas are used to estimate cooling time, then extrapolate holding time. For example, one formula proposes a cooling time t = 1.5 × s², where s is maximum wall thickness of part (in mm) and t is cooling time (in seconds). Another formula is T = s² / (4α), where α is thermal diffusivity. These formulas typically calculate the total cooling time, with holding time being a subset of it. CAE software prediction: Modern mold flow analysis (CAE) software (such as Moldflow, Moldex3D) can predict gate freezing time quite accurately, providing important preliminary guidance for setting holding time.
V/P switching point setting: V/P switching point is critical point for switching from a speed-controlled filling stage to a pressure-controlled holding stage. Setting this point directly affects pressure state at the end of filling phase and starting conditions of holding stage, which is crucial for preventing flash, shrinkage, and stress concentration.
Setting principle: Switching should occur when mold cavity is almost completely filled but not completely filled, typically when cavity volume is 95% to 99% full.
Common Setting Methods:
Screw Position Switching: This is the most common and reliable method. Screw position when mold cavity is just filled is determined through trial molding. Switching position is then set a small distance (e.g., 1-3mm) before this point.
Time Switching: A fixed filling time is set, and switching occurs when time is up. This method is significantly affected by material viscosity fluctuations and oil temperature changes, resulting in poor stability.
Pressure Switching: A pressure threshold is set, and switching is triggered when injection pressure reaches this value. Suitable for applications requiring mold protection or process stability.
Impact of Switching Point Setting:
Switching Too Early: Entering holding pressure before mold cavity is fully filled causes melt front to complete final filling at low speed, potentially resulting in flow marks, poor weld lines, or even short shots.
Switching Too Late: Screw impacts end of mold cavity at high speed, causing a sudden surge in cavity pressure (pressure spike). This easily leads to flash, trapped air, scorching, and creates significant residual stress inside product, while also impacting mold and machine.