Injection molding is a complex manufacturing process where every parameter plays a pivotal role in determining the quality of the final product. Among these parameters, pressure control stands out as a core element—it directly influences melt flow, mold filling, and the structural integrity of molded parts. Today, we’ll dive deep into four key pressure parameters: injection pressure, holding pressure, back pressure, and clamping pressure, exploring their definitions, functions, and optimal setting strategies.
Injection pressure refers to the melt pressure at the screw head (metering chamber) during the injection stage. Its primary functions are threefold: overcoming the resistance of melt flowing from the barrel to the mold cavity, providing the melt with a specific mold-filling rate, and compacting the melt. This pressure can be measured using sensors installed on the nozzle or hydraulic lines. Notably, there is no fixed value for injection pressure—the more challenging the mold filling (e.g., complex cavities), the higher the required injection pressure.
To achieve and maintain the set injection speed, the preset injection pressure acts as the upper limit of pressure during the injection process. In practice, the final set value should be 20% to 30% higher than the actual injection pressure. Why is this margin necessary? If the preset injection pressure is too precise, the pump cannot adjust quickly and accurately to match the set injection speed. Worse still, if the preset pressure is insufficient to support the set speed, a significant deviation will occur between the actual and desired injection speeds.
The consequences of improper injection pressure are straightforward:
• Low injection pressure: The melt fails to fully fill the mold cavity, leading to incomplete parts.
• High injection pressure: It not only causes defects like flash (excess material seeping out of mold gaps) and mold swelling but also introduces high internal stress in the product. Over time, this can damage both the mold and the injection molding machine.
During the injection process, increasing injection pressure brings additional effects: faster mold filling, longer melt flow length, higher weld line strength in the product, and increased part weight. For this reason:
• High injection pressure is recommended for large-sized, complex-shaped, or thin-walled products.
• It is also suitable for plastics with high melt viscosity and high glass transition temperatures (e.g., polycarbonate).
However, since internal stress in the product increases with injection pressure, parts molded at high pressure typically require annealing treatment during post molding operations to relieve stress and prevent deformation.
Holding pressure is the pressure maintained to compensate for material shrinkage in the mold and ensure continuous melt flow after the cavity is filled. It has a profound impact on the quality of molded products.
Similar to injection pressure, improper holding pressure leads to distinct defects:
• Insufficient holding pressure: Causes sink marks (depressions on the part surface), air bubbles, and excessive shrinkage.
• Excessive holding pressure: Results in over-packing (the melt is over-compressed in the cavity), high stress near the gate, and difficulty in demolding.
To optimize product quality, staged holding pressure control is widely adopted. The two most common approaches are:
1. Stepwise decreasing holding pressure: This avoids over-holding, reduces density differences between the gate area and the flow end, minimizes residual stress, and prevents deformation.
2. Low-to-high holding pressure: The first stage uses low pressure to prevent flash; the second stage applies higher pressure. Once the part’s surface has solidified, this higher pressure compensates for shrinkage and avoids surface sink marks.
Back pressure is the resistance the plastic encounters during the plasticizing process—it is the pressure the screw must generate and overcome before retracting. As a critical parameter for controlling melt quality and product quality, proper back pressure is key to improving part performance.
• Positive effects of increased back pressure: It compacts the material in the screw grooves, improves shearing efficiency, and drives out trapped gases (e.g., moisture or air). Higher back pressure also increases the screw’s retraction resistance, slowing retraction speed and extending the material’s heating time in the barrel—ultimately enhancing plasticizing quality.
• Negative effects of excessive back pressure: It increases melt backflow and leakage in the metering section’s screw grooves, reducing melt conveying efficiency. Moreover, excessive back pressure generates excessive shear heat and stress, causing material degradation (e.g., thermal decomposition or cross-linking) and intensifying colorant discoloration. It also accelerates mechanical wear of the pre-plasticizing mechanism and screw barrel, and may cause nozzle drooling (melt leaking from the nozzle).
Critical Note: In most cases, back pressure should not exceed 20% of the injection molding machine’s maximum rated injection pressure.
If back pressure is too low, the screw retracts too quickly. This leads to low-density plastic pellets (with high air content) entering the barrel from the hopper—during injection, part of the pressure is wasted on compacting and expelling this air. Worse, low back pressure combined with high screw speed results in poor plasticizing quality.
When back pressure is set correctly, it significantly improves part quality:
1. Better color mixing: The melt in the barrel is stirred for a longer time during pre-plasticization before being pushed to the screw head, resulting in more uniform color distribution compared to no back pressure.
2. Fewer defects: Back pressure creates higher pressure toward the barrel’s front end, enhancing gas expulsion from the plastic. This reduces silver streaks (caused by trapped gases) and air bubbles in the part.
3. Prevents material stagnation: Under consistent pressure, plastic in all parts of the screw grooves moves forward smoothly and continuously, avoiding local material stagnation in the barrel.
4. Efficient cleaning: When cleaning the barrel or changing materials/colors, increasing back pressure to an appropriate level enables fast and effective screw cleaning.
Clamping pressure—also called clamping force—is a key parameter of injection molding machines. It refers to the force the machine applies to clamp the mold. Along with injection volume, clamping force reflects the machine’s capacity to process parts and is often used as the main indicator of machine specifications.
Two factors are critical for calculating clamping force: projected area and cavity pressure.
• Projected area (S): This is the maximum area of the part visible along the mold’s opening/closing direction, calculated based on the part’s dimensions.
• Cavity pressure: It is influenced by multiple factors, including the number and location of gates, gate size, part wall thickness, plastic viscosity, and injection speed.
By mastering the selection and setting of these four pressure parameters, manufacturers can minimize defects, improve product consistency, and extend the lifespan of molds and machines. In our next blog, we’ll explore other core injection molding parameters—stay tuned!
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