In injection molding, holding pressure stage is a crucial step that determines final quality (dimensions, weight, appearance, mechanical properties) and molding efficiency of product. Incorrect holding pressure settings can lead to a series of problems such as shrinkage, flash, warpage, and excessive internal stress. This article will systematically explain how to scientifically and efficiently determine correct holding pressure and holding time, laying foundation for stable production.

Holding pressure occurs after mold cavity is almost filled (approximately 95%-99%). At this point, melt begins to cool and shrink within mold cavity. Core function of holding pressure is to continuously replenish material into cavity before melt solidifies and seals gate (mainly through gate solidification) to compensate for shrinkage, thereby obtaining a dimensionally stable and structurally dense product.
Therefore, fundamental goal of holding pressure settings is: to achieve complete volume compensation with minimal internal stress and to shorten cycle time as much as possible.

Before discussing holding pressure itself, it's crucial to ensure timing of entering holding pressure stage is correct. This timing is "injection-holding pressure switching point." Switching too early results in an incomplete cavity, insufficient material, or inadequate dimensions; switching too late leads to an overfilled cavity or even flash, making further holding pressure application counterproductive.
Setting Method (Recommended Position Switching Method): Temporarily set holding pressure and holding time to zero, perform pure injection.
Gradually increase injection stroke (or decrease switching position setting), injecting one mold at a time.
Observe product and find injection endpoint where product is just filled (no obvious shrinkage marks, and no flash).
Set this position as holding pressure switching point. Typically, at this point, cavity fill rate is 95%-99%, providing an optimal basis for applying holding pressure.
Key Point: Switching point should ensure repeatability and consistency for each injection. For advanced models, using "cavity pressure" or "screw position + pressure" as switching condition is more precise.

Higher holding pressure is not always better. Insufficient pressure leads to inadequate shrinkage, while excessive pressure causes flash, mold bulging, difficulty in demolding, generates significant molecular orientation and internal stress, resulting in product warping and performance degradation.
Scientific Pressure Determination Process:
1. Set Initial Value: Start with a lower pressure, such as 30%-50% of maximum injection pressure. Simultaneously, set a significantly insufficient holding time (e.g., 2-3 seconds) to observe pressure effect.
Perform a "Pressure Scan": Keep pressure constant for a short period.
Gradually increase holding pressure in small increments (e.g., 5-10 bar each time).
Observe product after each adjustment, once production mold has stabilized.
2. Find "Minimum Effective Holding Pressure": Focus on the areas most prone to shrinkage marks: near gate, areas with thicker walls, and behind reinforcing ribs.
When holding pressure is increased to just eliminate these major shrinkage marks, record this pressure value (P1). This is "minimum effective pressure" under current conditions.
3. Check pressure limit: Continue to increase pressure slightly and observe whether burrs (flashes) appear on mold parting surface, ejector pin holes, and slide blocks.
Once burrs appear, it indicates that pressure has reached or exceeded mold's tolerance limit. Record pressure value (P2) before the burrs appear.
4. Determine working pressure range: Your ideal holding pressure working range is between P1 and P2.
Initial setting can be slightly higher than P1 to ensure shrinkage compensation effect and leave room for subsequent optimization.

Holding pressure time refers to duration of continuous pressure applied to cavity from switching point. If time is too short, gate will not solidify before material replenishment, resulting in product shrinkage and insufficient weight; if time is too long, gate will solidify, holding pressure will fail, wasting energy, prolonging cycle, and potentially increasing internal stress due to prolonged high pressure.
The most objective and scientific method for determining this is "product weighing method."
Operating Procedures:
Fixed Parameters: Set holding pressure to a stable value slightly higher than P1, as determined in previous step.
Setting Starting Point: Set a short holding time (e.g., 3 seconds), and produce molds until process stabilizes.
Weighing and Recording: Produce one mold of product, remove it, and immediately cool it to room temperature.
Weigh the total weight of product using a precision electronic scale (accuracy 0.01 grams or higher) and record it. To ensure accuracy, you can weigh a single major component or the total weight of all components.
4.1 Incrementing Time: Increase holding time by one step (e.g., 0.5 seconds or 1 second; step can be larger for thicker wall products).
After process stabilizes (approximately 3-5 molds), produce another mold, weigh and record weight.
4.2 Plotting a Curve and Finding Inflection Point: Continue repeating step 4 until product weight no longer increases with increasing holding time.
Plot a curve with "Holding Time" as x-axis and "Product Weight" as y-axis.
4.3 Determining "Saturation Time": Initially, weight increases rapidly over time.
Then curve flattens out, eventually forming a horizontal line.
Time corresponding to point where curve just becomes a horizontal line is theoretical "sufficient holding pressure time."
Principle: Once gate has completely solidified, cavity is physically sealed, and holding pressure can no longer force material in, thus product weight reaches its maximum and remains constant.
4.4 Setting Working Holding Pressure Time: In actual production, for safety, 0.5-1 second is usually added to "sufficient holding pressure time" as final setting to compensate for minor process fluctuations. This time is your optimized holding pressure time.

 

After obtaining initial (pressure, time) combination, final fine-tuning is needed to balance quality, efficiency, and cost.
5.1 Quality-Prioritized Optimization (Reducing Internal Stress)
Try slightly reducing holding pressure while ensuring no shrinkage.
Due to reduced pressure, holding time may need to be appropriately extended (fine-tuning using weighing) to ensure sufficient shrinkage compensation before gate solidifies.
Advantages: Lower internal stress means better dimensional stability, less warpage, and higher product strength.
5.2 Efficiency-First Optimization (Shorter Cycle Time)
Try slightly increasing holding pressure while ensuring no shrinkage.
Increased pressure increases shrinkage compensation efficiency, potentially allowing for a shorter holding time (verification by weighing).
Risks: Increased pressure may increase internal stress and flash risk, requiring close monitoring.
Advantages: Reduced cooling waiting time, improved production efficiency.
5.3 Multi-Stage Holding Pressure
For complex products with uneven wall thickness, multi-stage holding pressure can be used.
First Stage: Higher pressure, shorter time, quickly compensating for shrinkage near gate (note difference: shrinkage in the area near gate, not freezing at gate location).
Subsequent Stages: Gradually reduce pressure, maintain or slightly longer, compensating for shrinkage away from gate area and allowing pressure to release smoothly, reducing internal stress. Setting of each pressure and determination of time must still follow principles of "pressure scanning" and "weighing method" mentioned above.

Determining correct holding pressure and time is a rigorous experimental process, not guesswork. Please follow these rules:
1. Start with basics, then adjust. Ensure stability of basic processes such as injection speed, temperature, cooling, and accurately set holding pressure switching point.
2. Start with pressure, then time. Find effective pressure range for eliminating shrinkage marks within a short time.
3. Weigh to determine time. Objectively determine the shortest sufficient holding time using product weight saturation method.
4. Fine-tune to find balance. Based on product requirements (quality/efficiency), find optimal solution on (pressure, time) curve.