Frontline technicians in injection molding know that daily machine adjustments always revolve around five core parameters: speed, pressure, position, time, and temperature. Everyone understands literal meaning of each parameter, but real challenge lies in inherent logic of their interaction.
Especially underlying relationship between injection speed and injection pressure. Most people adjust these parameters haphazardly based on experience, only looking at surface parameters without understanding principles behind molding curve. This ultimately leads to problems such as flash, insufficient filler, inconsistent flow lines, dimensional fluctuations, inconsistent injection times, and repeated defective products. Adjusting machine by guesswork will never make you a true process expert.
Many people focus only on speed and V-P switching position when adjusting machine, neglecting fact that pressure is hidden driver of speed stability. This article uses plain language from a workshop insider, combined with real-world waveform examples and process data, to thoroughly explain relationship between speed and pressure, relationship between V-P switching pressure and speed stability. It includes both theoretical principles and practical examples and machine adjustment methods, allowing readers to directly apply knowledge to process establishment and stable mass production. (Previous article discussed injection curves, but some colleagues felt it wasn't fully explained.)

First, remember this lifelong ironclad rule of injection molding: In injection filling stage, speed is goal, and pressure is guarantee; pressure works for speed, and pressure serves speed.
Injection system of an injection molding machine is essentially a pressure-speed closed-loop control: We set injection speed, and machine automatically adjusts output pressure to push screw forward at set speed. When molten plastic flows in runner and mold cavity, it encounters multiple resistances: decreased material temperature leading to increased viscosity, changes in runner width, differences in product wall thickness, mold cavity venting and extrusion, and a surge in back pressure during later stages of filling.
To ensure set flow rate is accurately achieved without drift, sufficient injection pressure is required to overcome all in-mold flow resistance.
To illustrate, imagine driving a car. Setting a constant speed of 80 km/h allows for easy and stable driving on flat roads; however, when encountering a hill with increased resistance, insufficient throttle will cause speed to drop. Speed is desired target speed, pressure is throttle force, mold and material are road resistance. Material changes and temperature adjustments alter resistance, requiring synchronized speed and pressure matching; otherwise, process will be disrupted.
A common misconception among experienced technicians is treating pressure as a sole means of fixing product defects, while speed is merely a secondary adjustment—a misplaced focus. Many also habitually set very high speeds and very low pressures, rendering these parameters ineffective. They fail to support actual flow rate, leading to disordered filling, abnormal flow curves, a multitude of subsequent appearance and dimensional defects.

Injection filling involves pushing glue forward and controlling flow rate. However, once V-P switch position is reached and holding pressure stage begins, logic immediately reverses: Holding pressure is primarily driven by pressure; speed is merely secondary, does not play a decisive role.
Core value of holding pressure lies in compensating for shrinkage, stabilizing pressure, and maintaining mold cavity pressure. This compensates for melt cooling and shrinkage, resolving issues such as product shrinkage, surface depressions, internal voids, and bubble cracking. This stage does not require rapid melt flow; only a constant holding pressure is needed for slow replenishment and stabilization.
Therefore, when adjusting machine, don't worry too much about holding pressure speed. Focus on controlling three key elements: V-P switch position, holding pressure, and holding time. This will ensure a smooth transition from filling to compensating for shrinkage.

Case 1: Pressure Reaching Upper Limit, Speed Completely Out of Control

 

Process Settings: Three-stage injection pressure 210MPa, segmented speeds 15, 5, 30, 25, 20mm/s;
Actual Working Conditions: Peak filling pressure reached 209.8MPa, almost reaching upper limit of set pressure;

 

Results: Filling time fluctuated between 2.278s and 2.308s, process was extremely unstable.
A more intuitive comparison with injection waveform curve: Speed set line and actual speed line completely separated in latter half, actual speed continued to decline; pressure curve directly flattened out at the top, machine entered a pressure limiting protection state.
Principle Breakdown: As mold cavity becomes increasingly full in later stages of filling, melt back pressure continues to rise. Set pressure is insufficient to maintain set speed, machine pressure stops rising, and screw is forced to slow down by melt back pressure. With speed fluctuating, filling volume per mold is inconsistent, and product weight, size, appearance naturally fluctuate accordingly.
Case 2: V-P Switching Pressure Setting Too Low Causes Complete Process Collapse

 

- Incorrect Setting: V-P switching pressure set to only 60MPa, speed remains constant; Actual operating V-P injection pressure requires 114.8~118.8MPa, far exceeding set value. Speed cannot reach set standard, making set speed unreliable;
- Correct Setting: Adjust V-P pressure to 130MPa, greater than actual required 114.8MPa;
Consequence: Injection time locked at 1.086s, perfect consistency across molds, extremely stable mass production.
Core Conclusion: Setting pressure lower than actual speed requirement will inevitably lead to insufficient screw power and speed drop; only by leaving a margin in set pressure can set speed be stabilized and process parameters solidified.
Case 3: Ample Pressure Margin, Rock-Solid Speed and Cycle
High-speed process case perfectly demonstrates speed-pressure matching logic:

 

Maximum injection pressure is set to 130MPa, but actual peak pressure across the entire range is only 113.9MPa, indicating ample pressure margin; injection time is stable at 0.49s without any fluctuations, and speed executes exactly as set.
Case 4: Fixed Pressure Setting, Actual Pressure Changes with Speed

 

Injection curve verifies rationality of setting:
- Actual injection pressure is consistently lower than set value of 135MPa, indicating no "hard top" in process, sufficient pressure margin, and actual pressure changes with speed.
- Speed curve highly overlaps with set value, indicating stable speed control during injection stage and a stable melt filling process without significant fluctuations or lag. Who is controlling speed?
- Holding pressure curve is clearly reversed. Speed does not follow set value at all, while holding pressure follows set value.
In summary: Sufficient pressure leads to stable speed; stable speed leads to stable injection time; stable injection time ensures stable product quality and mass production. Often, blindly adjusting speed or position is less effective than ensuring adequate V-P switching and injection pressure margin; all fluctuation issues will be resolved.

A common practice in the field is to immediately reduce injection pressure upon seeing blinders or flash. However, this approach has prerequisites and should not be done blindly.
1. If measured injection time is stable, it indicates that there is sufficient pressure margin; reducing speed is optimal, not pressure.
2. If measured injection time is unstable, it indicates insufficient pressure. In this case, reducing speed must be significantly reduced until injection time stabilizes to be effective; reducing pressure will further destabilize the process and is not the best choice.
Root cause: Blinders are result of combined effect of speed-pressure matching and mold cavity pressure. Without understanding logic of pressure serving speed during filling stage, simply adjusting a single parameter will never stabilize process or create reproducible standard parameters.

Injection molding machines are divided into ordinary hydraulic presses and servo motor presses. Difference in damage caused by improperly setting injection pressure is significant:
- Ordinary Hydraulic Presses: Many people habitually set injection pressure to maximum, when high pressure is actually unnecessary for molding. Oil pump continuously outputs high pressure, and excess pressure can only be released through oil overflow, wasting electricity, wearing down hydraulic components, and contributing nothing to process stability.
- Electrically Controlled Servo Injection Molding Machines: Intelligent on-demand output; it outputs only pressure actually needed. With proper pressure matching, energy savings are significant.
Remember: Injection molding machines do not automatically increase pressure intelligently; they execute only set pressure. If pressure is insufficient, machine will not actively compensate, inevitably leading to speed drops, filling imbalances, and frequent defects. Precisely matching pressure parameters can stabilize quality, reduce defects, save electricity, reduce machine load, and lower production costs.

Based on case studies, here's a set of on-site adjustment methods that can be directly applied:
1. Read Actual Peak Pressure: After producing several stable molds during normal production, read peak filling pressure and the highest pressure during V-P switching phase from machine waveform diagram or actual process measurement data. Take average of several molds; this is actual pressure required for melt.
2. Leave a 10%~20% Safety Margin: When initially adjusting, set pressure below 80% of maximum pressure for speed matching. After ensuring product has no appearance defects, adjust injection pressure and pressure before V-P switching. Set value must be 10%~20% higher than actual peak pressure. For example, if actual peak pressure is 120MPa, a setting of 135~145MPa is recommended, avoiding jamming or being too low, handling small fluctuations in material and mold temperature to prevent triggering pressure limiting.
3. Verify Curing Using Waveform Curves: Examine two curves: Does speed curve closely match set value, without any drop in speed in latter half? Does pressure curve not peak at the top, leaving a margin? Only when curves are normal can process be considered truly cured.
4. Pressure and Speed Mismatch: If pressure and speed are mismatched, speed must be reduced while pressure is increased to match curves. Adjust repeatedly according to product characteristics to achieve optimal results.

Process aims for reasonable margins and precise matching, not simply high pressure. A measured margin of 10%~20% is optimal range for balancing stability, quality, and energy consumption.

1. Injection Speed: Specifically affects product appearance, resolving flow lines, air bubbles, weld lines, scorching, uneven gloss, etc., controlling injection speed in segments according to location.
2. Injection Pressure: Its sole core function is to overcome in-mold flow resistance, ensuring normal operation of set speed; it is strong support for speed.
3. Multi-segment Injection Positions: Precisely segmented according to product wall thickness, ribs, and injection structure, switching speeds avoids filling dead zones and appearance defects.
4. V-P Switching Position: Precisely delineating critical points of filling completion and holding pressure start, achieving a smooth transition between high and low pressure, eliminating shrinkage and stress marks caused by switching fluctuations.
5. Temperature and Time: Coordinated with high pressure and position, stabilizing material viscosity and cooling rhythm, locking in molding cycle, making process replicable and mass-producible.

Today, injection molding workshops are mainly automated mass production. Core work of technicians is no longer about putting out fires, but about quickly converting molds, establishing standard processes, solidifying replicable and stable mass production parameters.
Only by truly understanding relationship between speed and pressure, matching logic of V-P switching, linkage principle of five major parameters, then analyzing them in conjunction with injection waveform curves and real-world case studies, can one avoid pitfalls of haphazardly adjusting based on experience. A qualified injection molding process technician's true skill lies in using principles to determine parameters and logic to stabilize process, rather than relying on intuition or guesswork, to achieve standardized molding conditions with low defects, stable cycles, energy savings, and reproducibility.