Injection Pressure in Plastic Molding: Peak, Pack, and Hold Explained
Injection Pressure in Plastic Molding: Peak, Pack, and Hold Explained
Injection pressure in injection molding is the single most influential process variable controlling part weight, dimensional accuracy, and cosmetic quality. Get it wrong by even 10% and you see short shots, sink marks, or flash. Get it right and you cut scrap rates from a typical 8% down to under 2%. This guide breaks down every pressure stage in the cycle and gives you the numbers to set them correctly.
Why Injection Molding Pressure Matters More Than Any Other Variable
Temperature gets most of the attention in process setup, but pressure is what actually drives material into every corner of the cavity. Melt temperature controls viscosity; pressure controls flow rate and cavity fill. You cannot compensate for a pressure deficit with heat alone without degrading the resin.
The injection molding pressure curve is the fingerprint of your process. A healthy curve shows a sharp, controlled rise during fill, a brief pressure spike at transfer, a plateau during pack, and a gradual decay through hold. Any deviation from that shape tells you something is wrong before you pull a single part off the press.
Cavity pressure sensors give you the most direct read on what is happening inside the tool. According to RJG Inc., parts molded with closed-loop cavity pressure control show dimensional variation 60% lower than those run on open-loop screw position control alone. That number justifies the sensor cost on any multicavity family tool.
The Four Pressure Stages in Every Injection Cycle
Every injection cycle has four distinct pressure stages. Confusing them is one of the most common process setup errors we see when we audit offshore tools coming into US production.
- Fill stage (injection pressure): The screw drives forward and fills the cavity to roughly 95 to 98% full. Injection pressure here is set high enough to fill the cavity within the target fill time, typically 1 to 3 seconds for most consumer and industrial parts.
- Transfer point: The press switches from velocity control to pressure control. This happens at a set screw position, cavity pressure, or fill time. Poor transfer timing causes either a short shot or a pressure spike that flashes the parting line.
- Pack pressure (pack stage): Immediately after transfer, pack pressure pushes additional material into the cavity to compensate for volumetric shrinkage as the part cools. Pack pressure is typically 50 to 80% of peak injection pressure and lasts 1 to 5 seconds depending on wall thickness.
- Hold pressure (hold stage): Hold pressure maintains a lower, steady pressure on the melt until the gate freezes off. Gate freeze time determines how long you hold. Once the gate is frozen, additional pressure does nothing useful and just wastes cycle time.
These four stages are not the same thing with different names. Each one has a specific function, and each requires an independent setting on the press controller.
Peak Injection Pressure: What It Is and What It Should Be
Peak injection pressure is the maximum hydraulic or electric screw pressure recorded during the fill stage. It is not a setpoint; it is a result. You set fill speed and cavity geometry dictates the pressure required to achieve that speed. If peak injection pressure is hitting your machine limit, you have a problem with either material viscosity, gate restriction, or fill speed.
Typical peak injection pressure ranges depend heavily on material and part geometry. The table below gives representative ranges for common engineering resins, measured at the machine nozzle. Cavity pressure runs 30 to 50% lower than nozzle pressure due to runner and gate pressure drop.
| Resin | Typical Peak Injection Pressure (psi) | Notes |
|---|---|---|
| ABS | 8,000 to 15,000 | Wide processing window, gate restriction drives upper end |
| Polypropylene (PP) | 6,000 to 12,000 | Low viscosity; thin walls push pressure higher |
| Nylon 66 (PA66) | 10,000 to 20,000 | Hygroscopic; dry material runs lower pressure |
| Polycarbonate (PC) | 14,000 to 22,000 | High viscosity; requires adequate barrel capacity |
| Acetal (POM) | 10,000 to 18,000 | Sharp melt point; overpressure risks flash instantly |
| Glass-filled Nylon (30% GF) | 14,000 to 24,000 | Fiber orientation sensitive to fill speed/pressure |
If your peak injection pressure is consistently above 80% of machine capacity, you are running with no safety margin. A 10% viscosity increase from a bad lot of resin will push you into short shots. The fix is larger gates, a higher-tonnage press, or a material change.
Pack Pressure and Hold Pressure: Where Dimensional Control Lives
Most part quality problems trace back to incorrect pack pressure or hold pressure settings, not fill. Sink marks, warpage, and part weight variation are almost always a pack and hold issue.
Pack pressure directly controls part density. Too low and the part shrinks away from the cavity wall, creating sinks over ribs and bosses. Too high and you overstuff the cavity, creating internal stress, warpage, and sticking on the core side at ejection. The correct pack pressure is the lowest pressure that produces a fully packed, sink-free part at nominal wall thickness.
For most semi-crystalline resins like PP and PA66, pack pressure runs 50 to 65% of peak injection pressure. For amorphous resins like PC and ABS, pack pressure typically runs 60 to 80% of peak. These are starting points, not absolutes. You validate with a pressure study, not with trial and error.
Hold pressure is set lower than pack pressure, typically 40 to 60% of peak. Its job is simply to keep the gate from sinking back as the cavity cools. A hold pressure that is too high after the gate has frozen does nothing for part quality and adds residual stress. Gate freeze time studies, where you step down hold time in 1-second increments and weigh parts until weight stabilizes, are the correct way to set hold time. We run this study on every new tool qualification in our shops before we ship the mold to the customer.
Reading the Injection Molding Pressure Curve to Diagnose Problems
The injection molding pressure curve displayed on your press controller or a process monitoring system is the fastest diagnostic tool you have. You do not need a cavity sensor to get value from it, though a sensor gives you more.
A steep, early pressure rise during fill indicates a restriction. Check gate diameter first. A minimum gate diameter of 50 to 60% of the nominal wall thickness is the standard starting point per Moldflow simulation guidelines. Gates undersized relative to that threshold create both excessive peak injection pressure and shear heat that degrades the resin.
A pressure curve that drops sharply at transfer and then climbs again during pack indicates underfill at transfer. You are transferring too early by position. Move the transfer point forward or reduce pack pressure to avoid overpacking after the correction.
A pack pressure plateau that decays faster than expected before hold time ends indicates gate freeze-off is happening earlier than your hold time setting. This is common in cold runner tools with small gates or in high-crystallinity resins like POM. Reduce hold time to match actual gate freeze time and recover 1 to 2 seconds of cycle time with no quality impact.
Injection Pressure Settings for Offshore-Sourced Tooling
When a tool is built in China and first sampled there, the T1 sample report should include documented process conditions including fill time, peak injection pressure, pack pressure, hold pressure, melt temperature, and mold temperature. Without that documentation, your team is starting from scratch when the tool lands at your US molder.
In our experience auditing hundreds of offshore tools, roughly 40% arrive without complete first-article process documentation. That forces your US molder to re-develop the process, which costs you time and press hours. At a typical press rate of $85 to $150 per hour for a 300-ton to 500-ton machine, a 2-day re-process development run costs $1,360 to $2,400 in press time alone, before labor or scrap.
Our project managers require Chinese toolmakers to submit a process data sheet with every T1 and T2 shot. The sheet must include the injection molding pressure curve as a saved graph, not just peak numbers. This single requirement has cut re-qualification press time by an average of 30% across our active tool programs. Require it on your purchase order, not as a verbal request.
Also confirm that the Chinese molding press used for sampling is proportionally matched to your US production press. A tool sampled at 600 psi hydraulic on a Chinese press may transfer to 8,500 psi plastic pressure on one machine brand and 10,200 psi on another with a different intensification ratio. Intensification ratios typically run 8:1 to 12:1 on hydraulic presses. Always document and transfer plastic pressure, not hydraulic pressure.
Frequently Asked Questions
What is the difference between injection pressure and clamp force?
Injection pressure is the force per unit area the screw applies to the melt, measured in psi at the nozzle. Clamp force is the force in tons that holds the mold halves closed against the cavity pressure trying to push them apart. They are related because higher cavity pressure requires more clamp force, but they are controlled independently. A rough rule of thumb is 2 to 5 tons of clamp per square inch of projected part area, varying by material and cavity pressure.
How do I know if my pack pressure is too high?
The clearest indicators are parts that stick on the core side at ejection, witness marks from ejector pins, or parts that measure oversize on critical features. A formal pressure study where you run parts at incremental pack pressure levels and measure weight and dimensions at each step gives you a definitive answer. Overpacked parts also show elevated residual stress, which you can see with polarized light on transparent materials like PC or PMMA.
Can peak injection pressure damage a mold?
Yes. Sustained overpressure events crack cores, deform thin cavity walls, and cause parting line wear that leads to flash. P20 steel cavities are rated for roughly 20,000 psi cavity pressure before elastic deformation becomes a concern. H13 and S7 at full hardness handle higher pressures but are more brittle. If your process routinely hits machine limit pressure, the mold is at risk. Fix the gate, runner, or process before the tool is damaged.
What causes a pressure spike at the transfer point?
A pressure spike at transfer is almost always a response time lag in the switchover from velocity to pressure control, combined with a fill speed that is too high near end of fill. The screw is still decelerating when the switchover happens and the remaining 2 to 5% of cavity volume gets packed at fill pressure instead of pack pressure. Reduce fill speed in the last 10 to 15% of stroke and make sure your decompression and transfer settings are matched to your press response time.
How often should I audit my injection molding pressure curve during production?
Every production run, at minimum at startup and end of shift. Statistical process control on peak injection pressure with control limits set at plus or minus 5% of the nominal value catches resin lot changes, wear on check rings, and gradual nozzle buildup before they cause scrap. SPC on cavity pressure is better still. RJG and Kistler both publish guidelines recommending real-time cavity pressure monitoring on any tool running a critical-to-function part.