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How to Determine Holding Pressure and Holding Time In Injection Molding?

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In the injection molding process, we often encounter three pressure-related parameters: injection pressure, holding pressure, and back pressure. We covered injection pressure in detail in the article “Injection Pressure and Injection Speed“. Today, we will focus on understanding holding pressure.

What are Holding Pressure and Holding Time?

We all know that pressure is used to overcome flow resistance and manifest speed. Generally speaking, injection pressure includes both pressure and speed, while holding pressure only involves pressure, not speed. Injection pressure refers to the pressure and speed used to fill the mold cavity with molten material up to 95% of its volume, after which it switches to holding pressure. Injection pressure typically transitions from high-pressure slow speed to high-pressure fast speed, then to low-pressure slow speed before switching to holding pressure.

Holding pressure is applied after the molten material fills the mold cavity to prevent voids caused by solidification shrinkage or weaknesses at the gate position, thereby ensuring sufficient part strength. It usually transitions from high-pressure low speed (time determined by product thickness) to low-pressure slow speed before switching to material storage.

Holding pressure means that the screw does not immediately retract after injection but continues to apply pressure to the molten material at the front end. During the holding phase, the plastic in the mold cavity shrinks due to cooling, and if the gate is not frozen, the screw slowly advances under the holding pressure, allowing more plastic to be injected into the cavity to compensate for shrinkage. Generally, holding pressure is less than injection pressure.

Holding time is the duration that ensures the product gate is completely frozen, preventing backflow. If this time is too short, shrinkage dents may form near the gate. If it is too long, it may cause excessive internal stress and gate protrusion. Determining whether the gate is frozen can be done using the weight measurement method.

Transition Point

A key concept here is the transition point. Generally, the transition point is when the product is 95% filled under zero holding pressure, switching from injection to holding. For thin-walled products, it is usually 98% filled. For unbalanced runners, it is generally 70%-80%, depending on the specific situation. Multi-stage injection with slow-fast-slow speeds is recommended.

The Impact of Holding Pressure and Holding Time on Injection Molded Parts

Impact of Holding Pressure:

  • Too Low: Results in short shots, sink marks, weld lines, etc. Insufficient holding pressure leads to insufficient compression of the plastic melt in the mold cavity, resulting in lower density and greater shrinkage upon cooling, causing the final volume to be less than the mold cavity volume.
  • Too High: Causes warping, flash, and severe cases of mold expansion. Excessive plastic melt compresses into the mold cavity, and the cooling shrinkage does not compensate adequately, leading to residual stress. In precision parts and those requiring electroplating, this situation should be avoided.
Adjustment of Holding Pressure
Adjustment of Holding Pressure

The optimal holding pressure is one that avoids short shots and sink mark defects while also eliminating residual stress. In other words, the plastic compressed into the mold cavity forms a high cavity pressure that just dissipates upon complete cooling.

Impact of Holding Time:

  • Too Long: Affects cycle time.
  • Too Short: Results in insufficient weight, internal voids, and smaller dimensions.

Note: Holding pressure affects holding time. The higher the holding pressure, the longer the holding time.

Setting Holding Pressure and Holding Time

Holding pressure is typically set between the minimum and maximum holding pressures, generally from low to high.

  • Minimum Holding Pressure: Based on the accurate transition point, it provides sufficient holding pressure to avoid underfilling.
  • Maximum Holding Pressure: Based on the accurate transition point, it provides holding pressure just before flashing occurs. This range indicates the safe processing window for the product.

Generally speaking:

  • PA Holding Pressure = 50% of Injection Pressure
  • POM Holding Pressure = 80% of Injection Pressure (can be 100% for high dimensional accuracy)
  • PP/PE Holding Pressure = 30-50% of Injection Pressure

Classic Steps for Setting Holding Time:

  1. Set the melt temperature to the mid-range of the manufacturer’s recommended values.
  2. Set the filling injection speed, transition point, and cooling time to safe values.
  3. Set the holding pressure (refer to the above steps).
  4. Set different holding times, noting that increasing holding time reduces cooling time, maintaining the same cycle. Weigh the parts under different holding conditions, enter the data into an Excel sheet, and plot the weight-holding pressure curve.
  5. Determine the time range where part weight does not change significantly. The time just before this range is the gate sealing time. Typically, set the holding time 0.5-2 seconds longer than the gate sealing time.
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