A veteran injection molding technician with ten years of experience knows that pressure, speed, and position are three core elements of injection molding, mutually constraining and interconnected. 90% of appearance defects (silver streaks, flash, shrinkage, weld lines, insufficient material, burning) originate from an imbalance in combination of these three parameters.
Many apprentices only know how to randomly increase pressure and adjust speed, without understanding logic of segmented position, relying entirely on luck for machine adjustment. This article explains principles, functions, segmented settings, matching logic, and troubleshooting in one go. Workshop technicians can directly save and apply this knowledge.
Four Key Injection Molding Parameters: Pressure, Speed, and Position – How to Control Them
Understanding adjustment of these four parameters is crucial to becoming an injection molding expert.
Basic Injection Molding Process

 

1. Position (Stroke, unit: mm)
Definition: Stroke distance of screw retracting/forward, used to divide the entire injection process into segments, serving as "boundary line" for all parameters.
Melting Position: The total stroke of molten material storage determines the total amount of molten material injected in a single shot. Multi-stage Injection Position: Divides filling process into three stages: runner, cavity body, and end filling. V/P Switching Position (Pressure Holding Point): The most critical position; when melt fills 95%~98% of cavity, control switches from speed control to pressure holding control. Buffer Position: Leaves a 5~15mm margin at injection endpoint to ensure stable pressure holding and shrinkage compensation. Core Function: Zoned control; different pressures and speeds are matched to different strokes to specifically improve local defects.
2. Speed (Flow Rate, Percentage/mm/s)
Definition: Speed at which screw advances controls flow rate and velocity of melt entering cavity.
Higher speed means faster melt propulsion and greater shear friction. Lower speed results in smoother material flow, sufficient venting, and lower internal stress.
Core Function: Controls material flow pattern, resolving jetting marks, weld lines, trapped air and scorching, and flow marks.
3. Pressure (Hydraulic Pressure, MPa/bar)
Definition: Ultimate thrust of hydraulic system driving screw, used to overcome material flow resistance; it is "power upper limit" of speed.
Sufficient pressure: Ensures set speed is reached. Insufficient pressure: Increased resistance causes screw to automatically decelerate, resulting in slow material flow and material shortages. Excessive pressure: Material flow forcibly squeezes mold, causing flash, mold bulging, and product stress cracking. Core logic: Speed is primary, pressure is secondary. During normal filling, machine prioritizes set speed, with pressure only acting as a limit. When cavity resistance exceeds set pressure, speed automatically decreases, entering pressure-dominated mode.

1. Resistance-Free Stage (Runner/Gate Section): Cavity is empty, resistance is low, set pressure > actual resistance, screw advances at a uniform speed according to set speed, pressure has no effect.
2. Mid-filling stage (product body): As molten material gradually fills cavity, resistance continues to rise. If pressure setting is too low, resistance will exceed pressure value, forcing screw to slow down, slowing down filling, and causing material shortage. At this point, pressure needs to be increased simultaneously to ensure stable speed.
3. End of filling stage (95% cavity full): Melt almost completely fills mold, and resistance spikes dramatically. Speed and pressure must be reduced in advance; otherwise, high-pressure, high-speed impact on parting line will directly generate flash. This position is V/P switching point to holding pressure. After switch, feeding is completely controlled by pressure, and speed is maintained at a very low flow rate.

Most products use a three-stage injection system, divided according to stroke position. Each stage has a fixed combination of pressure and speed:
Stage 1: Runner + Gate Stage (Location: Total melt stroke → Gate position, 5%~15% of total stroke)
Objective: Smoothly push cold material, avoiding jetting marks and damage to mold cavity; Speed: 20%-40%; Point gate/fine gate 20%-30%, side gate 30%~40%; Pressure: 40%~55% (low pressure, slow speed); Adjustment Points: If gate turns white or jetting marks appear, immediately reduce speed of this stage.
Second Stage: Cavity Main Filling Stage (70%-95% product filling, accounting for 60%-80% of total stroke)
Objective: Quickly fill main body of product, reduce weld lines, and improve surface gloss; Speed: 60%-90%; 80%-90% for thin-walled parts, 50%-70% for thick-walled large parts; Pressure: 60%-80% (medium-high pressure matched with high speed); Adjustment Points: For insufficient material or obvious weld lines, prioritize increasing pressure in this stage; increase speed gradually after slow filling.
Third Stage: End of Filling + Before V/P Switching (95%-98% cavity full, short stroke 515mm)
Objective: Slowly finish, release trapped air, prevent flash, smoothly switch to holding pressure; Speed: 10%-30% (significantly reduce speed); Pressure: 40%-60% (moderately reduce pressure); Adjustment Points: For flash on parting surface, burning at the end, or air bubbles, directly reduce pressure and speed in this stage.

1. Finding Standard Procedure
Set holding pressure and speed to 0; Inject at low speed and low pressure, gradually decreasing injection endpoint until product is just slightly short; Move this position forward 2-5mm to find standard holding pressure transition position (cavity 95% filled).
2. Dangers of Switching Too Early/Too Late
Switching too early: Switching to holding pressure before cavity is fully filled results in significant product shrinkage, smaller dimensions, and material shortage; Switching too late: Continuing high-speed, high-pressure impact after complete filling leads to severe flash, mold cracking, high internal stress in product, and demolding deformation.

Step 1: Determine Position (Segmented Stroke)
Calculate the total amount of glue in product, set melt storage position, and reserve an 8-15mm buffer; Divide injection stroke into three segments, with second segment covering most of product cavity; Find V/P to holding pressure transition position and fix it.
Step 2: Matching Speed (Setting Flow Rate by Segment)
Low speed in flow channel segment, high speed in main body segment, and slow down in final segment; Adjust speed of this segment for appearance defects, without changing parameters of other segments.
Step 3: Matching Pressure (Providing a Power Upper Limit for Speed)
Incomplete speed operation or longer injection time → Increase pressure of corresponding segment; Flash or rough edges appear → First reduce pressure of final segment, then reduce pressure of main body segment; Pressure should always be adjusted according to speed; Do not blindly increase pressure alone.
Step 4: Fine-tuning Holding Pressure Parameters (Pressure Control Stage)
Holding pressure relies entirely on pressure compensation; speed is set to a very low flow rate (5%~15%); Holding pressure is generally 60%~80% of second-stage injection pressure. Segmented decreasing holding pressure can significantly improve warpage deformation.

1. Insufficient Material, Inadequate Filling
Location: Increase second-stage injection stroke, delay V/P switching point; Speed: Increase second-stage filling speed; Pressure: Increase second-stage injection pressure.
2. Product surface shrinkage and dents
Location: Fine-tune V/P switching point, delaying it by 2-3mm; Speed: Low speed at the end to reduce impact; Pressure: Increase holding pressure and extend holding time.
3. Parting surface flash and burrs
Location: Advance V/P switching position and shorten high-pressure filling stroke; Speed: Reduce end speed of third stage; Pressure: Reduce injection pressure of second and third stages, decrease holding pressure.
4. Gate spray marks, water droplets, and silver streaks
Location: The first stage stroke should only cover runner, not extend into cavity; Speed: Significantly reduce speed of the first gate stage; Pressure: Reduce pressure of the first stage and ensure sufficient material drying.
5. Obvious weld lines and poor strength
Location: Extend second stage filling stroke, placing weld point in high-speed zone of second stage; Speed: Increase injection speed of main stage; Pressure: Increase pressure of second stage to increase fusion of melt at weld point.
6. Burning, bubbles, and trapped air at the end of product's stroke:
Position: Shorten second high-speed stroke and enter third low-speed stroke earlier; Speed: Reduce speed of third stroke at the end; Pressure: Reduce pressure in all three strokes and optimize mold's venting channels.

Divide stroke into sections, first set rotation and hold pressure; Slow in the beginning, fast in the middle, and always lower at the end; Use speed pipe for flow lines and venting, and pressure pipe for full burrs; If speed can't keep up, increase pressure; if burrs appear, reduce pressure first; Hold pressure at 95% full, early reduction and late burrs are ironclad rules; Adjust parameters in sections, if one section has a problem, only modify that section, don't make random adjustments.
This article comprehensively covers principles and practical operation of three elements of injection molding. Whether you are a novice apprentice or a machine operator, understanding logic of pressure, speed, and position will allow you to quickly solve most appearance defects.