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Injection molding cycle profile optimization: pressure, hold and cooling steps

2025/09/26 By le zhan

Injection molding cycle 1-2

Injection molding cycle profile optimization, specifically the coordinated control of pressure, holding, and cooling steps, is the most effective tool for production engineers to improve part quality, reduce cycle time, and lower unit cost. Whether you operate a single-unit injection molding line or manage multiple injection molding machines in a single molding facility, optimizing the cycle profile can deliver significant benefits: fewer sink marks, lower warpage, higher first-pass yields, reduced scrap rates, and shorter ramp-up times after changeovers. An optimized cycle profile can improve profitability while maintaining product performance.

The Practical Significance of Filling, Holding, and Cooling in Injection Molding

Before making adjustments, you must clearly define the actual role of each cycle step on the injection molding machine. The injection molding cycle consists of three interrelated phases: rapid filling, holding, and cooling. The settings for each phase are interdependent, and changing one setting requires revalidating the others.

During the filling process, the melt flows from the barrel/nozzle into the runners and cavity. The injection profile determines the fill front velocity, shear heating, and initial direction. Using S-curve acceleration can reduce shear peaks and maintain the progression of the laminar flow front. On many injection molding machines, it is recommended to configure multi-segment velocity profiles to manage long flow lengths or thin ribs. Monitor injection pressure at the nozzle and cavity pressure to detect front lag or excessive packing. The typical goal is to achieve rapid filling, minimizing packing time while avoiding ejection and air entrapment.

Once the cavity is substantially complete, packing compensates for volumetric shrinkage as the part cools. Adequate packing reduces shrinkage and voids, but overpacking can lead to flash and internal stresses. You can switch from velocity control to pressure control at the calibration point to initiate the packing process. After packing, the clamps remain closed until the part cools, which is typically the longest portion of the cycle. Cooling must remove sufficient heat to achieve ejection stiffness. Cooling time depends on the thermal properties of the polymer, part thickness, mold temperature, and the effectiveness of conformal/internal cooling channels. Optimize cooling by balancing mold temperature and cooling time to achieve optimal results.

The Practical Significance of Filling, Holding, and Cooling in Injection Molding 1

Analysis of Pressure Control in Injection Molding

Pressure is the primary control variable in injection molding because it transmits force to the melt front and subsequently holds the part in place. Effective pressure profile control means controlling injection pressure during filling and then applying a measured packing/holding pressure strategy that matches the part’s gating and cooling behavior.

During filling, initially set the mode to velocity control to prevent sudden pressure spikes and maintain control over shear and orientation. Switch to pressure control at a point determined by cavity pressure or injection position. Avoid switching position alone, as changes in material viscosity or mold temperature can lead to over- or under-packing. When purchasing an injection molding machine today, select a controller that supports pressure-based switching and features a near-real-time cavity pressure loop.

Use a stepped packing profile with a high initial packing pressure to push the melt into the edge area, followed by a reduced packing pressure to maintain volume compensation as the gate freezes. This reduces internal stress and flash. A typical approach is as follows: Pack 1 = approximately 90-95% of the maximum pressure for a short period; Pack 2 = 60-70% of the remaining pressure. Calibrate the packing duration using the cavity pressure profile. Furthermore, overpressure should be avoided; pressure alone cannot replace a properly designed gate.

Gate Freeze Logic, Time, and Pressure Control for Packing Optimization

Packing is a delicate balance. It can compensate for volumetric shrinkage, but if misused, it can become a source of sink marks, internal stresses, and dimensional drift. Determine the gate freeze point through simple experiments. Maintain a constant packing pressure and record the increase in cavity pressure and part weight as the packing time increases. When the gate freezes, the weight reaches a steady state. This steady state is used as the minimum required packing time, plus a safety margin to account for variations in the process.

This is still common in the absence of sensors. Use injection monitoring to record the time of gate freeze and set the packing time to 80% to 120% of the observed time, taking into account process variations. For conservative production, choose the upper end of the range to avoid underpacking. However, longer packing times increase cycle time and cost. During this process, you need to set an initial packing pressure and then gradually reduce it to maintain the packing pressure as the cavity cools. Implement a decaying pack to reduce internal stress concentrations: initially apply a high-pressure pulse and then gradually reduce the set value until the gate solidifies. This technique typically reduces overall packing time because it avoids the extension force required to compensate for a single high-pressure burst.

How is cooling performed?

Cooling often dominates cycle time, and intelligent cooling design can reduce ejection time without compromising dimensional stability. Uniform cooling design ensures evenly distributed cooling, eliminating thermal gradients that can cause warpage. Place cooling channels close to thick-walled parts and parallel to the runners for consistent heat dissipation. Where direct cooling is impractical, use baffles and bubblers. For complex geometries, consider conformal cooling to reduce cycle time and hot spots. Integrating a temperature controller ensures a reasonable flow rate and precise, constant temperature control.

For crystalline materials, controlling mold temperature affects crystallinity and shrinkage. For cavities requiring different mold temperatures, separate circuits should be used. Additionally, avoid undersizing water channels, as this can result in insufficient flow. Calculate water channel diameters based on the heat load, use turbulence enhancers where necessary, and ensure that the feed and return manifolds balance the flow to each cavity. Reduce adequate cooling time by improving heat transfer efficiency within material limitations, increasing mold surface contact area, or employing conformal cooling.

How is cooling performed

Implementing Cycle Optimization Profiles

Optimizing the pressure, packing, and cooling profiles of the injection molding cycle is a technique with clear and measurable benefits. Start by installing instrumentation to collect reliable baseline data, then conduct targeted Design of Experiments (DoEs) to determine the optimal combination of injection speed, packing pressure sequence, and cooling time. Whenever possible, use cavity pressure as the primary signal, as it directly links machine action to part results. When conventional injection molding machines cannot support the required control fidelity, consider upgrading to models from the machine manufacturer that offer closed-loop pressure control, high-frequency data acquisition, and convenient recipe management.

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