How to Set Injection Pressure for Thin Parts

Why Thin Parts Are Hard to Fill

Thin wall parts act like long, narrow tubes. The polymer must travel a distance before the gate closes. If the pressure is too low, the material cools too fast and the gate freezes before the mold fills. If it is too high, the part gets too much material, leading to sink marks or warpage.

The goal is to find the lowest pressure that consistently fills the mold. This minimizes stress and defects. You need a clear starting point and a method to move from there.

The physics of filling a thin wall part is driven by cooling. As the melt enters the cavity, the outer layer touches the cooled mold steel. This layer solidifies almost instantly. The core of the material remains liquid, but the growing solid skin acts as a barrier. If the injection speed is too low, the skin thickens before the material reaches the end of the cavity. The gate then seals with a thin layer of solidified plastic. The result is a short shot. The part looks incomplete, but the mold is actually full because the gate closed early.

Conversely, excessive pressure forces the melt into the mold with too much energy. When the part cools, it shrinks. Thick sections, such as bosses or ribs, shrink less than the thin walls. If the material is packed too hard, the thin walls are pulled inward to match the shrinkage of the thick sections. This creates sink marks. These dimples are visible on the surface and often trap air, leading to poor cosmetic quality.

A common misconception is that a full part is a good part. A part can be fully filled but still have high internal stress. This stress manifests as warpage after the part cools and shrinks. The mold holds the part in a stretched shape. When the clamp pressure is released, the part tries to return to its natural size. If the stress is too high, the part bends or twists. Therefore, the objective is not just to fill the cavity, but to fill it with the minimum force required to maintain part integrity.

Prerequisites Before You Touch the Dials

Do not start with a cold machine or an empty hopper. The process will be unstable and the data will be useless.

1. Stabilize the melt temperature. Run the machine for long enough so the barrel and die temperatures are constant. Check the melt temperature at the die. If it swings by more than a few degrees, your baseline is wrong. Thin wall parts are extremely sensitive to temperature changes. A variation of ten degrees can change the viscosity of the melt enough to alter the filling time significantly. You must wait until the melt temperature reading on the controller stabilizes. Do not move to pressure settings until the thermal mass of the machine is consistent.
2. Verify the mold condition. Look for wear at the gate and in the cavities. A worn gate restricts flow and forces you to use higher pressure. If the gate is not sealing properly, you may get flash, which is material leaking out of the parting line. If the gate is worn down, it may not seal at all, leading to continuous leakage. Inspect the gate area for nicks or deep scratches. These can create stress concentrators that cause cracking in brittle materials.
3. Clean the hopper. Old material can degrade and cause foaming or discoloration. This mimics pressure issues. Moisture in the resin causes steam bubbles during injection. These bubbles look like voids or short shots. If you are using hygroscopic materials like nylon or polycarbonate, ensure the dryer has run for the required time. Check the moisture content if possible. Dry material flows more predictably than wet material.
4. Set the injection speed. For thin walls, use a high injection speed. This pushes the melt through the long run before it cools. Do not use a slow, steady fill. The speed should be high enough to overcome the cooling barrier. However, if the speed is too high, you may cause shear heating. This can degrade the polymer or create internal stresses. Find a speed that is fast but stable. The pressure curve should show a smooth rise during the fill phase.

Step-by-Step: Setting Injection Pressure for Thin Parts

Follow this sequence to tune the process. Each step has a specific reason.

1. Start with a low injection pressure. Set the pressure to a low value that will not damage the mold. This gives you a baseline for short shots. You want to see where the material stops moving. Start with a pressure that results in a visible short shot. This ensures you are not starting from a point of overfilling. If the part is short, you know the limit. If the part is full at the start, you are starting in the wrong zone and must back off.
2. Run a cycle and check the gate. Look at the gate area. If the part is short, the gate likely froze early. If it is full, the pressure is too high or the speed is too slow. Examine the gate seal. A good gate seal should be flush with the part surface. If there is a visible step or a rough edge, the gate closed before the cavity was full. This is a sign that the fill time was too long relative to the cooling rate.
3. Increase pressure in small increments. Add a small amount of pressure at a time. Wait for the melt temperature to stabilize before the next run. This prevents thermal shock and keeps the data comparable. Do not rush this step. Each increment should be followed by a full cycle and a physical inspection of the part. If the short shot moves slightly closer to the end of the cavity, you are on the right track. If the part becomes full but develops sink marks, you have passed the optimal zone.
4. Monitor cavity pressure if available. Cavity pressure tells you the actual force inside the part. Barrel pressure is only a guess. If you have sensors, watch them. The pressure curve should show a steady rise, not a spike. Cavity sensors are the most accurate way to monitor the filling process. They show when the material actually enters the cavity and when the gate closes. If the cavity pressure drops before the fill is complete, the gate is sealing early.
5. Check for sink marks. If the part is full but has dimples near thick sections, the pressure is too high or the hold time is too long. Reduce the pressure slightly. Sink marks are a direct result of packing. When the melt cools, it shrinks. If the pack pressure is too high, the material is forced to fill the voids left by the thicker sections. This creates the dimple. The goal is to use the minimum hold pressure that prevents flash but does not create sink marks.
6. Adjust hold pressure. Hold pressure packs the part and prevents shrinkage. For thin walls, keep hold pressure low. Too much hold causes warpage. Use the minimum hold pressure that prevents flash. Start with the hold pressure equal to the injection pressure. If flash appears, reduce the hold pressure. If the part shrinks too much, increase it slightly. The difference between the two pressures should be small for thin wall parts. A large difference creates shear stress that can lead to warpage.
7. Run a stability check. Run ten consecutive parts. Measure the weight and the critical dimensions. If the values drift, the process is not stable. Check the melt temperature again. Weight variation is a quick indicator of process stability. If the weight is consistent, the melt is flowing at a constant rate. If the weight fluctuates, check for moisture, material degradation, or machine mechanical issues.

Understanding the Pressure Curve

The pressure profile tells you what is happening inside the mold.

Phase	What happens	What to watch
Fill	Melt moves into the cavity	Pressure rises steadily. Sudden spikes mean restriction.
Pack	Material flows into the gate	Pressure peaks. This is where sink marks form.
Hold	Material stays packed	Pressure stays flat. Too high causes warpage.
Release	Mold opens	Pressure drops to zero.

If the fill phase has a steep slope, your speed is too low. If the pack phase is very high, you are using too much pressure. A smooth, gradual rise during fill is the target.

The fill phase represents the movement of the melt through the mold cavities. A steep slope indicates that the material is struggling to move. This could be due to low speed, high viscosity, or a restricted gate. If the slope is too steep, the material cools before it reaches the end of the cavity. The result is a short shot. A smooth slope indicates that the material is flowing freely.

The pack phase occurs after the cavity is full. The mold closes, and the gate seals. The pressure rises as the material is packed into the cavities. This is where sink marks form. If the peak pressure is too high, the material is forced into the thick sections, causing shrinkage voids. If the peak pressure is too low, the part may shrink too much, leading to dimensional issues. The peak pressure should be the lowest value that prevents flash and maintains part dimensions.

The hold phase maintains the pressure after the pack. This phase ensures that the part does not shrink during cooling. If the hold pressure is too high, the part may warp as it cools. If it is too low, the part may have voids or dimensional instability. The hold pressure should be just enough to keep the material packed without causing stress.

Common Mistakes to Avoid

Engineers often make these errors when tuning thin walls.

• Using barrel pressure only. Barrel pressure does not equal cavity pressure. The barrel has friction and compression. It can be high while the cavity is low. Use cavity sensors if you can. Barrel pressure is affected by the friction in the barrel and the compression of the screw. It does not accurately reflect the force on the part. Relying on barrel pressure can lead to overpacking or underpacking. Cavity pressure is the direct measure of the force on the part.
• Changing too many variables at once. Do not change pressure, temperature, and speed in the same cycle. You will not know what caused the change. Always change one variable at a time. If you change the injection speed and the melt temperature simultaneously, you will not know which change caused the improvement or the defect. This makes troubleshooting difficult and extends the tuning time.
• Ignoring the gate. A frozen gate looks like a short shot. But if you increase pressure, you might get a full part with a bad gate seal. Check the gate physically. The gate is the most critical part of the mold for thin wall parts. If the gate is too small, the material cannot flow fast enough. If the gate is too large, it may not seal properly. Inspect the gate for wear, nicks, or debris. A clean, well-designed gate is essential for a consistent fill.
• Forgetting the cooling time. If the cooling time is too short, the part may have internal stress. This causes warpage later. Do not run the machine faster than the cooling cycle. The cooling time is determined by the thickness of the part and the mold temperature. If you reduce the cooling time, the part may not be fully cooled when the mold opens. This leads to warpage and dimensional instability. Ensure the cooling time is sufficient for the part to reach the mold temperature.

Final Verification Step

After you set the pressure, do not just look at the part. Run a full verification.

1. Measure the parts. Use a micrometer or calipers. Check the thickness at the thinnest section. If it is outside the drawing tolerance, the pressure is wrong. Use the correct measurement tool for the part. For thin wall parts, a micrometer is more accurate than calipers. Measure the part at multiple locations to ensure uniformity. If the thickness varies, the mold may be warped or the cooling is uneven.
2. Check the weight. Run ten parts and weigh them. The weight should be consistent. If it varies, the melt is unstable. Weight is a quick indicator of process stability. If the weight fluctuates, check for moisture, material degradation, or machine mechanical issues. A consistent weight ensures that the melt is flowing at a constant rate.
3. Look for cosmetic defects. Check for sink marks, flow lines, and short shots. A full part with sink marks is not a good part. Inspect the part under good lighting. Look for sink marks near thick sections, flow lines at the gate, and short shots at the end of the cavity. A good part should have a smooth, uniform surface with no visible defects.
4. Review the log. Save the process parameters. If you stop and restart, you need to know exactly what you had. Do not rely on memory. Save the process parameters to a file or log. This ensures that the process can be reproduced if the machine is restarted or if a different operator takes over.

When to Call the Mold Maker

If you have tuned the machine and the part still does not fill, the problem is likely the mold. A mold with a too-small gate or a restricted channel will not fill properly. You can force it with high pressure, but that causes defects. Ask the mold maker to check the gate size and the channel design. Do not keep pushing the machine harder.

If the machine settings are optimized and the part still has defects, the issue is likely with the mold. A mold with a small gate or a restricted channel will not fill properly. You can force the material through with high pressure, but this causes defects such as sink marks and warpage. Do not keep pushing the machine harder. This will only stress the part and the mold.

Contact the mold maker to review the design. Ask them to check the gate size, the channel design, and the mold temperature. They can provide a solution that addresses the root cause of the problem. Do not try to solve a mold design issue with machine settings. This is a waste of time and can lead to further defects.

Materials Matter

Not all polymers behave the same. High-flow grades fill thin walls easier. They have lower viscosity. Standard grades need more pressure. If you switch materials, you must restart the tuning process. Do not assume the same injection pressure settings work for different resins. Check the material data sheet for the recommended melt temperature and viscosity.

The material properties of the resin have a significant impact on the injection molding process. High-flow grades have lower viscosity and fill thin walls easier. They require less pressure and speed. Standard grades have higher viscosity and require more pressure and speed. If you switch materials, you must restart the tuning process. Do not assume the same injection pressure settings work for different resins. Check the material data sheet for the recommended melt temperature and viscosity.

The melt temperature and viscosity of the resin are critical factors in the filling process. A higher melt temperature reduces the viscosity and allows the material to flow more easily. However, too high a temperature can cause degradation. A lower melt temperature increases the viscosity and requires more pressure. Find the optimal melt temperature for the material and the part.

Conclusion

Setting injection pressure for thin parts is a balance. You need enough force to fill the mold, but not enough to cause defects. Start low, move slow, and check the data. The process will stabilize when the weight and dimensions are consistent. If they are not, go back and check the variables. The goal is a stable, repeatable process that produces a good part every time.