In the injection molding process, there are three pressure related parameters: injection pressure, holding pression and back pressure. Injection molding holding pressure is the key parameter that ensures the quality and precision of custom molded plastics part.This article provides an in-depth exploration of injection molding holding pressure, including its definition, working principle, importance, parameter relationships, common problems, and solutions.
What is Holding Pressure in Injection Molding?
Injection molding holding pressure refers to the pressure applied after the mold cavity is filled with molten plastic. The holding pressure focuses on filling the cavity rapidly, holding pressure maintains a consistent force to compensate for material shrinkage and ensure the mold is fully packed. It helps to prevent defects such as sink marks, voids, and warpage by maintaining pressure until the gate solidifies, effectively locking the shape of the part.
The Working Principle of Holding Pressure
In the overall injection molding cycle, holding pressure follows the filling phase and precedes cooling injection molding and ejection. When the high pressure rapidly fills the mold cavity to about 95% capacity, the system switches to holding pressure in injection moulding, applying a lower pressure, typically 50-80% of the injection pressure, to pack the remaining material. The basic mechanism of pack pressure injection molding involves maintaining cavity pressure through continued forward movement of the screw or plunger, which pushes a small amount of additional melt into the mold. The screw advances slowly, compensating for shrinkage by injecting extra material until the gate freezes, usually at a temperature below the polymer’s solidification point.
Injection molding holding pressure can be derived from the formula P = F/A, where P is pressure, F is force on the screw, and A is the screw area. This ensures uniform density distribution.
Why Holding Pressure is Important to Product Quality？
If you learn about injection moulding working principle, the packing pressure compensates for material shrinkage during cooling and directly influences the final part quality.
- Dimensional Stability: Proper pack pressure injection molding reduces shrinkage rates to below 1%, maintaining tolerances within ±0.1%. Insufficient pressure allows uneven cooling, resulting in warped or undersized parts.
- Surface Quality: Holding pressure in injection moulding eliminates sink marks and voids by packing material densely.
- Structural Strength: By expelling trapped air and gases, the holding pressure in injection molding increases the part’s density and tensile strength.
- Consistency Across Batches: Optimized packing pressure in injection molding minimizes weight variations (aim for ±0.5% part-to-part).
- Material Efficiency: Effective holding pressure reduces energy consumption in the cycle, as it shortens cooling time by promoting uniform solidification.
The Relationship Between Pressure, Time, and Position in Holding Pressure
The parameters of pressure, time, and position in injection molding holding pressure are closely linked, determining the effectiveness of the packing phase. These elements must be optimized to ensure consistent part quality and minimize defects.
Holding Pressure
Holding pressure, typically set at 50-80% of injection pressure, fills the remaining 2-5% of the mold cavity to compensate for shrinkage. For materials with high fluidity, like polycarbonate (PC) or polypropylene (PP), the switch to holding pressure occurs at 85-90% cavity fill, with pressure set at 60-80% of injection pressure. For less fluid materials, like PMMA, switching happens at 95% fill, with higher pressure (70-85%) to ensure complete packing. Multi-stage holding pressure, gradually reducing from 80% to 50%, is often used to balance packing and prevent overpressurization.
Holding Time
Holding time extends until the gate freezes, typically 0.5-2 seconds beyond the gate’s cooling time, depending on wall thickness. Thin-walled parts (e.g., 2mm) require shorter times (1-3 seconds), while thicker sections demand longer durations. Excessive holding time increases cycle duration without benefit, while insufficient time causes backflow, leading to dimensional inconsistencies.
Holding Position
Position-based switching, triggered when the screw reaches a set point (e.g., 95% fill), is the most reliable method, as it accounts for consistent cavity volume regardless of melt variations. In contrast, time-based switching depends on a predetermined duration, making it susceptible to fluctuations in viscosity or hydraulic pressure, which can affect part weight and size. Pressure-based switching uses cavity sensors for precision, ideal for small, high-accuracy parts, but requires advanced equipment.
Common Problems with Improper Holding Pressure and Solutions
Improper holding pressure leads to defects in injection molding. Common issues and their solutions include:
Short Shots and Incomplete Filling
When holding pressure is too low or holding time is too short, the molded part may not fill completely, resulting in short shots or incomplete filling.
Solution: Increase holding pressure and extend holding time to allow the molten plastic to fully pack the cavity until the gate solidifies. Also, raise injection pressure or material temperature if needed to improve flow.
Sink Marks and Voids
Insufficient holding pressure causes volumetric shrinkage during cooling, creating sink marks, hollow spots, or voids on the part surface or interior.
Solution: Maintain adequate holding pressure throughout the holding time to compensate for shrinkage and ensure dense packing of material. Optimize the pressure level and time accordingly.
Flash and Overpacking
Excessive holding pressure may cause flash, where molten plastic seeps beyond the mold cavity, or internal stresses that lead to warpage.
Solution: Reduce holding pressure to a suitable level. Ensure proper clamping force to prevent mold opening under high pressure. Balance pressure and clamp force carefully to avoid defects.
Warpage and Distortion
Improper holding pressure settings can cause uneven cooling and internal stresses, leading to warpage or distortion of the finished part.
Solution: Fine-tune holding pressure and time, and consider multi-stage pressure profiles to balance cooling and packing evenly. Adjust mold temperature uniformity and cooling channels if needed.
Gate Freeze and Premature Solidification
If holding time is too short, the gate may solidify too early, preventing material from being packed, causing shrinkage-related defects.
Solution: Extend holding time just enough to keep the gate open until packing is complete. Use sensors or process controls to optimize gate freeze timing.
Conclusion
Injection molding holding pressure is a core stage of the process, enabling effective compensation for material shrinkage to deliver parts with precise dimensions and minimal defects. Zhongren provides expert injection molding services and eliminates common defects based on experience in parameter setting. Welcome to contact us and get a tailored injection molding solution.
