Holding pressure in injection molding is a critical phase in the injection molding process that occurs after the initial injection of molten plastic into the mold cavity. It influences the quality and consistency of molded parts. In this post, we will explore the holding pressure definition, impacts on molded parts, and the differences with other pressures in injection molding.

What is Holding Pressure in Injection Molding?

Holding pressure, also known as packing pressure, is maintained to compensate for the shrinkage of the material as it cools and solidifies. Holing pressure in injection molding is a continuous pressure. Typically, holding pressure is lower than the initial injection pressure and is applied for a specific duration to ensure that the molded part achieves the desired density and dimensional stability.

Purpose of Injection Molding Holding Pressure

Compensating for Shrinkage: As molten plastic cools, it contracts. Holding pressure helps to fill any voids that may form due to this shrinkage, ensuring that the part maintains its intended shape and size.

Preventing Defects: Adequate holding pressure is essential for avoiding common defects such as short shots, sink marks, and weld lines. Insufficient holding pressure can lead to increased shrinkage and dimensional instability, while excessive pressure can cause issues such as flash and internal stress in the molded part.

Enhancing Part Strength: By applying holding pressure, the molecules in the resin are encouraged to align more densely, which increases the overall strength and stability of the final product. This is particularly important for thin-walled sections that require careful filling to avoid weaknesses.

Facilitating Gas Escape: Trapped gases within the mold need time to escape through vents. Holding pressure allows for this escape, reducing the risk of defects associated with trapped air.

Quality Control: Holding pressure plays a critical role in ensuring consistent part quality. It is one of the key parameters that influence the final dimensions and mechanical properties of the molded part.

Difference Between Injection Pressure and Holding Pressure

In injection molding, injection pressure, and holding pressure are two distinct parameters that serve different purposes in the manufacturing process.

Injection pressure is the pressure used to fill the mold cavity with molten plastic. Its primary function is to overcome the flow resistance of the material and ensure that the cavity is filled. Injection pressure is typically applied at a high speed to facilitate rapid filling of the mold. Injection pressure is used to fill the mold cavity up to approximately 95% of its volume.

Holding pressure, on the other hand, is applied after the mold cavity has been filled with molten plastic. Its main objectives are to prevent shrinkage and ensure the strength of the final part. During the holding phase, the injection screw does not retract immediately but continues to apply pressure to the molten material at the front end. This sustained pressure helps to compensate for the shrinkage that occurs as the plastic cools and solidifies.

Impact of Holding Pressure on Molded Part Quality

Holding pressure plays a significant role in determining the quality of molded parts in injection molding. The correct application of holding pressure can enhance part integrity, while insufficient or excessive holding pressure can lead to various defects.

Insufficient Holding Pressure

Short Shots: This occurs when the mold cavity is not filled, leading to incomplete parts. Insufficient holding pressure fails to compensate for the shrinkage of the material as it cools, resulting in voids.

Sink Marks: These are depressions that form on the surface of the part due to uneven cooling and insufficient material to fill the cavity as it shrinks. Low holding pressure can exacerbate this issue, especially in thicker sections.

Weld Lines: Weld lines occur when two flow fronts meet but do not bond properly. Insufficient holding pressure can prevent proper packing of the material at these junctions, leading to weak spots.

Lower Density and Greater Shrinkage: Insufficient pressure results in lower overall density of the molded part, making it more susceptible to defects and reducing its mechanical properties.

Excessive Holding Pressure

Warping: Excessive pressure can cause uneven stress distribution within the part, leading to warping as the part cools and solidifies.

Flash: This occurs when excess material is forced out of the mold cavity, resulting in thin, unwanted projections on the part. High holding pressure can push material into areas where it should not be.

Mold Expansion: High holding pressure can exert excessive force on the mold itself, potentially leading to mold damage or deformation over time.

Residual Stress: Excessive holding pressure can introduce residual stresses within the molded part, which may lead to issues such as cracking or premature failure during use.

How to Set the Ideal Holding Pressure in Injection Molding

Setting the ideal holding pressure in injection molding is crucial for achieving high-quality molded parts. The correct holding pressure helps to compensate for material shrinkage, prevent defects, and ensure the structural integrity of the final product. Here are the key steps and considerations for setting holding pressure effectively.

1. Understand the Transition Point

The transition point is the moment when the mold cavity is approximately 95% filled with molten plastic, signaling the switch from injection pressure to holding pressure. For thinner-walled products, this may be closer to 98%. Accurately determining this transition point is essential, as it dictates when to apply holding pressure to effectively compensate for shrinkage.

2. Determine Minimum and Maximum Holding Pressure

Minimum holding pressure should be set just high enough to prevent short shots and ensure that the cavity is fully packed. It typically ranges from 50% to 65% of the injection pressure.

Maximum holding pressure should be limited by the machine’s capabilities and the mold design. Excessive holding pressure can lead to defects such as flash, warping, and residual stress in the molded part.

3. Setting Holding Time

Holding time in injection molding is the duration for which holding pressure is applied after the cavity is filled. It ensures that the gate is completely frozen to prevent backflow. A general guideline is that the holding time should last until the gate solidifies, often around 30% of the total cooling time.

4. Monitor and Adjust Parameters

Continuously monitor the holding pressure and holding time during production. Adjustments may be necessary based on the specific material properties and part geometry.

5. Consider Material Properties

Different materials react differently under pressure. Understanding the specific requirements of the material being used, such as its viscosity and cooling characteristics, is essential for setting the ideal holding pressure.

What Factors Should I Consider When Setting Holding Pressure?

When setting holding pressure in injection molding, several factors must be considered to ensure optimal part quality and process efficiency. Here are the key factors to take into account:

1. Material Properties: The viscosity and shrinkage rate of the different molten plastics will affect the holding pressure.

2. Wall Thickness: Thicker sections may require higher holding pressure to prevent sink marks and ensure proper filling. Conversely, thinner sections may need lower pressure to avoid warping.

3. Gate Size and Location: The size and position of the gate and the mold’s cooling system will influence the required holding pressure. A smaller gate may need higher holding pressure and a well-designed cooling system can allow for lower holding pressures.

4. Injection Parameters such as injection pressure and injection speed will affect how much pressure is needed during the holding phase. Faster injection speeds may require adjustments in holding pressure to compensate for rapid filling.

Conclusion

Holding pressure is a critical parameter that requires careful consideration and optimization in injection molding. Insufficient holding pressure can lead to issues such as short shots, sink marks, and weld lines, while excessive pressure can cause warping, flash, and residual stress. Factors such as material properties, part design, mold design, and injection parameters can all influence the optimal holding pressure settings. By understanding its impact, setting appropriate pressure levels, and employing proper control techniques, manufacturers can produce high-quality, consistent parts that meet the demands of their customers.