In injection molding, melt flow is core process that determines product shrinkage, bubbles, silver streaks, dimensional fluctuations, color differences, glass fiber float. 90% of process operators only know how to adjust parameters, but don't understand internal forces, melt flow, and screw mechanical transmission logic of back pressure, melt speed, and release/retraction.
This article breaks down process into four layers: screw structure, melt pressure field, hydraulic servo force, and physical changes in plastic, thoroughly explaining underlying principles of these three key factors. After reading this, you can accurately solve most melt-related defects. Suitable for mold makers, process engineers, and workshop managers.

 

Summary: Molten plastic is foundation, back pressure is key, and retraction is guarantee. Only through proper coordination of these three factors can high-quality, high-efficiency injection molding be achieved.

Injection screw consists of three sections: feeding section, compression section, and metering section. The entire melting process is divided into three continuous actions:
Melting (Storing) Action: Screw rotates, plastic is sheared and melted, melt is pushed forward, screw is pushed backward by melt pressure until it reaches set storage position and stops rotating;
Continuous Back Pressure: During screw's backward movement, hydraulic cylinder continuously applies reverse resistance to control melt compression density;
Retraction (Extraction) Action: After screw stops rotating, it retracts slightly backward a short distance, releasing static pressure of melt at the front of barrel.
These three actions are interconnected and mutually restraining. Any change in any parameter will alter melt pressure, density, and amount of bubble formation inside barrel.

1. Mechanical Force Logic
During melting, screw rotates, and molten plastic continuously accumulates in metering zone at screw's front end. Melt generates a forward thrust, pushing screw backward. Screw's tail end connects to an injection cylinder. Hydraulic oil in cylinder provides reverse resistance. This reverse oil pressure is converted into pressure on melt, which is back pressure.
Force Balance Formula: Forward Thrust of Melt = Back Pressure Resistance of Cylinder + Rotational Friction Resistance of Screw
The greater back pressure: the greater cylinder resistance, the slower screw's backward movement, and melt is continuously compressed under high pressure.
The smaller back pressure: the smaller cylinder resistance, the faster screw retreats, and melt becomes loose with internal voids.
2. Changes in Internal Pressure Field of Melt
Low Back Pressure: Melt is loose with large intermolecular gaps. Moisture, decomposition gases, and air within raw materials cannot be expelled, easily leading to silver streaks, bubbles, and pores.
High Back Pressure: Melt in metering section is compacted under high pressure, molecular chains extend, pigments and glass fibers mix evenly, color difference is stable, and floating fibers are reduced. However, excessive pressure has two side effects: Intense shear friction causes heating, leading to decomposition and blackening of heat-sensitive plastics (PVC, POM, PC+ABS); Screw and check ring are subjected to high pressure for extended periods, resulting in accelerated wear, poor sealing, and dimensional fluctuations.
3. Impact of Back Pressure on Screw Components
Check ring (medium) is component directly subjected to back pressure: During melting process, melt pushes check ring forward. The higher back pressure, the more continuous pressure on contact surface between check ring and screw head. Prolonged high back pressure can lead to medium wear, sealing failure, melt backflow during injection, and inconsistent product weight.

Melt speed = screw rotation speed, determining dynamic shear strength of plastic. It is a dynamic force, complementary to static compressive force of back pressure.
1. Dual Forces of Screw Rotation
Plastic Shear Resistance: Screw's agitation of solid and molten plastic generates reverse torque through material friction.
Melted Material Extrusion Force: Accumulated melt at front continuously pushes backward against screw.
Servo Motor Output Torque Overcomes Dual Resistance to Complete Storage:
High Melt Rotation Speed: High shear rate, high frictional heat generation, fast plasticizing speed;
Slow Melt Rotation Speed: Gentle shearing, low frictional temperature rise, suitable for heat-sensitive, low-viscosity transparent materials.
2. Logic of Melt Speed and Back Pressure Linkage
Scenario 1: High Back Pressure + High Melt Speed
High screw retraction resistance + high-speed shearing, dual heat generation, melt temperature significantly higher than barrel set temperature, suitable for difficult-to-mix materials and high-glass fiber materials; however, PC and PVC are easily yellowed and decomposed.
Scenario 2: High Back Pressure + Low Melt Speed: Gentle shearing, relying solely on static pressure to compact melt, controllable temperature rise, suitable for standard processes in transparent optical components and medical micro-injection molding.
Scenario 3: Low Back Pressure + High Melt Speed: Rapid screw retraction prevents melt from being compacted in time, resulting in air entrapment and a high incidence of defects such as bubbles and shrinkage. Only suitable for temporary use on thick-walled, low-requirement products.
3. Advantages of Segmented Melt Speed Storage
Many high-end all-motor systems support segmented melt speeds: Low-speed, gentle plasticizing before metering avoids significant gas entrainment; appropriately increased speed after metering improves efficiency, maintaining stable pressure throughout and resolving issues of large weight variations in products of varying lengths.

1. Core Mechanical Problems Caused by Retraction: Static High Pressure After Storage Completion
At instant melting stops, melt at screw tip is in a high-pressure sealed state under back pressure, melt in barrel nozzle and flow channel is continuously under pressure:
When mold is not closed, high-pressure melt drips from nozzle (drooling); During injection delay stage, high-pressure melt continues to press forward, resulting in an excessively heavy first injection and unstable product dimensions; Melt is continuously compressed under high pressure, and after cooling, internal stress is released, causing product warping and denting.
Mechanical Force Logic of Retraction: After melting is complete, screw stops rotating, and injection cylinder drives screw to move slightly backward, instantly increasing volume of metering section and rapidly releasing melt pressure, achieving purpose of decompression.
Force Change: High pressure at the end of storage material → Negative/normal pressure of melt after retraction, eliminating static residual pressure.
2. Two Stress Resulting from Different Retraction Distances
1. Insufficient Retraction Distance: Insufficient pressure relief, resulting in nozzle drooling, large weight fluctuations per mold, and shrinkage at the front end of thick-walled parts.
2. Excessive Retraction Distance: Excessive screw retraction creates negative pressure in metering section, allowing air to be drawn back into melt from nozzle and barrel gaps, resulting in numerous silver streaks and internal bubbles after molding. Negative pressure will also slightly pull the check ring back, increasing seal gap and exacerbating injection backflow.
3. Hidden Stress Details in Retraction Speed
Retraction should not be too fast: Rapid screw retraction creates momentary vacuum bubbles inside melt, allowing tiny bubbles to mix into plastic, resulting in haze and crystals on transparent parts.
Standard Process: Slow and gradual retraction releases pressure, avoiding vacuum suction.

Complete force chain of the entire melting process: Screw rotation (melting speed provides shear torque) → Plastic melts and accumulates forward; Melt generates a backward thrust, which balances with back pressure resistance of hydraulic cylinder, compacting melt; Upon reaching storage endpoint, screw stops rotating, leaving high pressure residue in melt at the front end; Screw retracts backward, increasing metering volume, releasing static pressure of melt, and completing the entire storage process.
Summary of Force Logic of Three Classic Process Combinations
1. Precision Transparent Optical Components (PC, COC): High back pressure, low melting speed, moderate retraction
Logic: Low shear prevents yellowing of plastic at high temperatures, high back pressure compacts and removes air bubbles, moderate retraction prevents drooling, and no negative pressure draws in air.
2. Thin-walled high-speed packaging (PP thin-walled lunch boxes): Low to medium back pressure, high melt speed, slight retraction.
Logic: High-speed melting improves circulation efficiency, low back pressure reduces shear heating, and short retraction prevents negative pressure air intake, suitable for ultra-short cycle production.
3. Fiberglass reinforced engineering materials (PA66+30%GF): High back pressure, segmented melt speed, standard retraction.
Logic: High back pressure breaks up fiberglass agglomerates, low speed at the beginning reduces fiberglass shearing, increased speed at the end maintains production capacity, and retraction releases pressure to prevent drooling.
4. Thermosensitive and easily decomposed materials (POM, PVC): Low back pressure, extremely low melt speed, minimal retraction.
Logic: Reduces dual heating from shear and static pressure; retraction involves only a slight backward movement to prevent negative pressure air intake and accelerated decomposition.

1. Product weight varies greatly per mold
Root Cause: Excessive back pressure wears down anti-reverse ring; excessive retraction creates negative pressure for air intake; inconsistent melt speed leads to unstable melt density.
2. Silver streaks on the product surface, internal air bubbles
Root Cause: Insufficient back pressure results in inadequate melt compaction; excessive retraction distance creates vacuum suction; excessively fast melt speed entraps air.
3. Uneven color difference, obvious fiber floating
Root Cause: Insufficient back pressure leads to low melt compression and poor pigment/glass fiber dispersion; excessively low melt speed results in insufficient shear mixing.
4. Continuous drooling and stringing at nozzle
Root Cause: Insufficient retraction distance prevents release of residual high pressure in stored material; overall back pressure setting is too high.
5. Yellowing, decomposition, and black spots on plastic
Root Cause: High back pressure + high melt speed result in combined shear friction heat, leading to localized overheating and degradation of melt.
6. Product Warpage and High Internal Stress
Root Cause: Excessive pressure at melt back end, lack of proper pressure release, high pressure stored in melt, and deformation after stress release during molding.

Back pressure control for density, speed control for temperature, pressure control for release; High pressure and dense material easily heat up, low pressure and loose material trap air bubbles; Fast rotation generates shear friction heat, slow rotation protects material and prevents decomposition; Too short a release will cause drooling, too long a release will draw in air; Adjust these three in a coordinated manner to eliminate all melt defects.

Back pressure, melt speed, and release are not three independent parameters, but a mutually balancing pressure-shear-pressure release system. Adjusting only a single parameter is not a fundamental solution. Understanding force logic between melt, screw, and cylinder is crucial for accurately matching process according to material, product wall thickness, precision requirements, ensuring stable mass production and reducing defect rates.