A Brief Discussion on Scientific Injection Molding (Part 2)

Time：2026-04-16 08:26:23 / Popularity： / Source：

For previous reading, please refer to A Brief Discussion on Scientific Injection Molding (Part 1).
Core logic of scientific injection molding parameter derivation is "first define boundaries, then establish benchmarks, finally fine-tune and optimize." The entire process relies on quantitative testing of material properties and experimental verification of process boundaries, abandoning empirical estimations. Through a standardized "test-calibration-verification" process, precise parameters suitable for raw materials, molds, and equipment are derived. All steps are supported by quantifiable data, ensuring scientific validity, stability, and repeatability of parameters.

Core of this method is to first find acceptable critical boundaries of each parameter (lower limit is minimum requirement for acceptable molding, and upper limit is critical value at which defects begin to appear). Then, benchmark parameters are selected within safe boundaries, and finally, minor optimizations are made based on production needs. This method is applicable to all injection molding raw materials and plastic part types. Following is a step-by-step derivation process, including core testing methods, data judgment, and parameter calibration rules.

I. Preliminary Preparations: Establishing Foundation, Verifying Hardware, and Clarifying Derivation Prerequisites

This stage forms foundation for parameter derivation, eliminating inherent biases in raw materials, molds, equipment to prevent subsequent parameter derivations from becoming mere "compensation for hardware defects" and losing their scientific rigor. It also clarifies core testing indicators.
1. Basic Raw Material Property Testing. For raw materials used in actual production (including batches), test core rheological and thermodynamic properties to define theoretical ranges for subsequent temperature and pressure parameters. Avoid relying solely on raw material manuals (slight differences exist in characteristics of same type of raw material from different batches/manufacturers).
1.1 Mandatory Test Indicators: Melt Index (MI), Viscous Flow Temperature, Thermal Degradation Temperature, Crystallization Temperature (for crystalline plastics), Volumetric Shrinkage Rate;
1.2 Additional Tests for Modified Materials: Melt Viscosity, Influence of Filler (Glass Fiber/Carbon Black) Dispersion on Flow;
1.3 Outputs: Raw material plasticizing temperature range, thermal stability residence time (the longest time melt does not degrade in barrel), basic melt flow characteristics.
2. Mold and Equipment Hardware Verification Clearly define process execution boundaries of mold and equipment, determine hardware upper limit for parameter derivation, and avoid setting parameters beyond equipment capabilities or mold compatibility.
2.1 Mold Verification: Measure runner/gate dimensions, check cooling water circuit layout/venting efficiency, calculate cavity filling flow resistance, and determine mold's maximum filling capacity and cooling efficiency benchmark.
2.2 Equipment Verification: Verify injection molding machine's plasticizing capacity, injection pressure/speed adjustment accuracy, clamping force, and mold temperature controller temperature control deviation. Record equipment's rated injection pressure/speed and temperature control accuracy (e.g., ±1℃/±2bar).
3. Define Core Quality Requirements for Plastic Parts. Based on industry of plastic part (3C/Medical/Automotive/Packaging), determine core quality judgment standards (e.g., micron-level dimensional accuracy for 3C parts, burr-free for medical parts, and mechanical properties for automotive parts). Subsequent parameter boundary tests and benchmark selection will revolve around these requirements.

II. Core Steps: Boundary Measurement, Benchmarking, and Derivation of Core Molding Parameters

This is core step in scientifically deriving injection molding parameters. For four core parameters—temperature, pressure, speed, and time—critical boundaries are determined one by one through quantitative experimental testing. Then, benchmark parameters are calibrated within a safe margin range. Derivation of these four parameters is interrelated, prioritizing derivation of temperature parameters (basis of melt flowability), followed by pressure/speed (core of mold filling), and finally time (holding pressure/cooling, adapting to mold filling results).

(I) Temperature Parameter Derivation: Core principle is "fully plasticized melt without degradation, and flowability adapted to mold filling," encompassing barrel, nozzle, and mold.

Temperature is foundation of melt flowability. First, a theoretical range is determined through thermal properties of raw material, then actual boundaries are verified through plasticization experiments. Core principle avoids a "one-size-fits-all" approach.
1. Barrel Temperature Derivation
1.1 Theoretical Range: Lower limit is raw material's viscous flow temperature, and upper limit is thermal degradation temperature. For crystalline plastics, temperature should be increased by 5-10℃ within this range (to ensure crystalline melting). For amorphous plastics, midpoint should be used.
1.2 Actual Boundary Verification: Gradually increase barrel temperature and observe melt state—Lower limit critical: Melt is uniformly plasticized, with no raw material or agglomerates, and can be smoothly extruded from nozzle into a continuous filament; Upper limit critical: Melt begins to yellow or develop silver streaks, and is prone to brittleness after extrusion (initiation of thermal degradation).
1.3 Benchmark Calibration: Within actual lower and upper limit range, select a value closer to midpoint (e.g., in an 80℃ range, take midpoint at 40℃). For modified materials (glass fiber/flame retardant), appropriately increase temperature by 3-5℃ (to improve fluidity and avoid filler agglomeration).
2. Nozzle Temperature Derivation
2.1 Baseline Calibration: Slightly lower than barrel front temperature by 3-5℃, to avoid drooling.
2.2 Boundary Verification: If nozzle temperature is too low, molten material condenses and clogs nozzle, increase it by 1-2℃; if drooling occurs, decrease it by 1-2℃, all within barrel temperature range.
3. Mold Temperature Derivation
3.1 Theoretical Range: Medium-high values are used for crystalline plastics (to ensure sufficient and uniform crystallization and improve dimensional stability); medium-low values are used for amorphous plastics (to shorten cooling time); mold temperature deviation for precision parts/thin-walled parts is controlled within ±1℃, and for large parts within ±2℃.
3.2 Actual Boundary Verification: Lower limit critical: No obvious warping or cold slug spots after demolding, and no obvious weld lines after filling; Upper limit critical: Sticking to mold after demolding, excessively long cooling time, or appearance of shrinkage cavities (over-crystallization).
3.3 Benchmark Calibration: Within actual range, select values based on part quality requirements—medium-high values for precision parts/thick-walled parts, and medium-low values for mass-produced thin-walled parts (balancing quality and efficiency).

(II) Derivation of Pressure Parameters: Plasticizing + Injection + Holding Pressure, with "Complete Mold Filling, Compensation for Shrinkage, and No Defects" as Core

Pressure parameters need to be derived based on a calibrated temperature baseline (fixed fluidity makes pressure testing easier and more accurate). Critical values are tested through mold filling boundary experiments. Core consists of three parts: plasticizing pressure, injection pressure, and holding pressure, all based on principle of "just meeting molding requirements," avoiding overpressure that could lead to flash and warping.
1. Derivation of Plasticizing Pressure
1.1 Test Method: Fix barrel temperature and screw speed, gradually adjust plasticizing pressure, and observe plasticizing effect of melt;
1.2 Critical Boundaries: Lower Critical Limit: Uniform plasticization of melt, no air residue in barrel (no bubbles in molded part); Upper Critical Limit: Screw plasticizing speed slows down, and melt stays in barrel for too long (degradation begins);
1.3 Baseline Calibration: Take lower value in actual range (reducing energy consumption and raw material degradation), appropriately increasing value for micro-parts/precision parts (removing microbubbles).
2. Derivation of Injection Pressure
2.1 Test Method: Inject at a fixed temperature and low speed (to eliminate speed interference), gradually adjust injection pressure, and observe cavity filling status.
2.2 Critical Boundaries: Lower critical limit (minimum filling pressure): Molten material just fills the entire cavity, with no short shots or uneven filling; Upper critical limit (maximum filling pressure): Molten material begins to produce flash from mold parting surface/gate.
2.3 Benchmark Calibration: Take 70% of upper and lower limit range (leaving a 30% safety margin to cope with minor fluctuations in raw materials/environment). For thin-walled parts/complex cavities, use a higher value; for thick-walled parts/simple cavities, use a lower value.
3. Derivation of Holding Pressure
3.1 Test Method: With a fixed injection pressure/temperature, switch to holding pressure after mold filling, gradually adjust holding pressure, and observe shrinkage of plastic part.
3.2 Critical Boundaries: Lower Critical Limit: No shrinkage cavities or surface depressions in thick-walled sections of plastic part, and shrinkage rate meets design requirements; Upper Critical Limit: Warping or dimensional deviations occur in plastic part, or overflow or internal stress cracking occurs at gate.
3.3 Reference Calibration: Take 50%-80% of injection pressure reference value (70%-80% for crystalline plastics, 50%-70% for amorphous plastics), using higher values for thick-walled parts and lower values for thin-walled parts.

(III) Derivation of Speed Parameters: Segmented Injection Speed, with "Smooth Mold Filling, No Turbulence, No Defects" as Core

Speed and pressure work together to determine mold filling effect. Based on calibrated temperature + injection pressure benchmark, segmented injection testing is adopted (adapting to needs of different stages of mold filling), rejecting a single speed throughout the entire process. Core principle is "low-speed mold opening, medium-to-high-speed mold filling, and low-speed mold closing."
1. Segmentation Principle: Mold filling process is divided into 3 segments—initial injection (molten material enters gate/runner, occupying 10%-20% of cavity), mid-injection (molten material fills main body of cavity, occupying 20%-80%), and late injection (molten material approaches full mold, occupying 80%-100%).
2. Speed Derivation for Each Segment
2.1 Initial Injection Stage: Upper limit of test threshold is "no sprue marks in molten material, no wear on gate." Baseline calibration is low speed (20%-30% of equipment's rated speed), with smooth core mold opening.
2.2 Mid-Injection Stage: Lower limit of test threshold is "no condensation in molten material, no weld lines," and upper limit is "no turbulence in molten material, no air bubbles in cavity." Baseline calibration is medium-high speed (50%-80% of equipment's rated speed). Higher values are used for thin-walled parts/complex cavities, and medium values are used for thick-walled parts.
2.3 Late Injection Stage: Upper limit of test threshold is "no flash, sufficient air expulsion from cavity." Baseline calibration is low speed (20%-40% of equipment's rated speed). Core mold is closed at low speed to avoid impacting cavity.
3. Overall Verification: Trial molding is performed at segmented speeds. If material shortage occurs, mid-term speed is appropriately increased; if air bubbles/sprue marks appear, initial/mid-term speeds are reduced, all without exceeding critical boundaries of each segment.

(IV) Derivation of Time Parameters: Holding Pressure + Cooling + Plasticizing, with "Part Shaping, No Deformation, and Optimal Efficiency" as Core

Time parameters are matching and adaptation of preceding temperature, pressure, and speed. Derived based on trial molding results of preceding parameters, core principle is to reject "experience-based time estimation." Critical times are determined through shaping experiments, balancing part shaping and production efficiency.
1. Derivation of Holding Pressure Time
1.1 Test Method: Fix holding pressure and temperature, gradually adjust holding time, observe gate solidification state and part shrinkage;
1.2 Critical Boundaries: Lower Critical Limit: Gate just solidifies, and part has no post-shrunk shrinkage (no dimensional change after 24 hours of demolding); Upper Critical Limit: Excessive holding pressure time leads to excessive internal stress in part (warping/cracking after demolding);
1.3 Reference Calibration: Take lower critical limit value + 1-3 seconds (allowing for shaping margin). For thick-walled parts/crystalline plastics, extend time appropriately; for thin-walled parts/disposable packaging parts, take lower critical limit value.
2. Cooling Time Derivation
2.1 Test Method: Fix all preceding parameters, gradually shorten cooling time, and check deformation/temperature of plastic part after demolding;
2.2 Critical Boundaries: Lower Critical Limit: Surface temperature of plastic part after demolding is ≤ heat distortion temperature of raw material, with no warping, no sink marks, and no sticking to mold; Upper Critical Limit: Excessive cooling time results in a meaningless increase in molding cycle;
2.3 Benchmark Calibration: Take lower critical limit value (optimal efficiency). For precision parts/large parts, appropriately extend time by 1-2 seconds. For large batches of small parts, directly take lower limit.
3. Plasticizing Time Derivation
3.1 Benchmark Calibration: Match total time of injection + holding pressure + cooling (achieving "synchronous plasticizing and molding," with no waiting time). If plasticizing is too slow, appropriately increase screw speed (without exceeding critical boundary of plasticizing pressure).

III. Verification and Optimization: Establishing a Baseline, Conducting Verification, and Minor Optimization to Mass Production State

Parameters calibrated through above steps serve as baseline parameters. Their stability needs to be confirmed through continuous mold trials. Then, minor optimizations are made within safe boundaries to ultimately form optimal process parameters directly applicable to mass production. This stage adheres to principle of "single-variable adjustment and quantitative verification" to avoid confusion regarding defect causes due to simultaneous changes in multiple parameters.
1. Baseline Parameter Verification
1.1 Produce 30-50 molds continuously according to baseline parameters, inspecting quality (appearance, dimensions, mechanical properties) of each molded part. A 100% pass rate and no batch variations are required.
1.2 If minor defects occur, pinpoint specific parameter (e.g., shrinkage marks → adjust holding pressure/time), make minor corrections within their critical boundaries, and then re-verify.
2. Mass Production Parameter Optimization
2.1 Optimization Principle: Quality first, while considering efficiency and cost; optimization is only performed under premise of acceptable quality.
2.2 Optimization Direction: Gradually shorten cooling time (1 second each time), increase injection speed in the middle stage (5% each time) until approaching critical lower limit; or appropriately reduce barrel temperature (2℃ each time) to reduce energy consumption and raw material degradation.
2.3 Certification Standard: After optimization, if continuous production of more than 100 molds is achieved with stable quality and normal equipment operation, parameter is solidified as official mass production process parameter.

IV. Key Supporting Elements: Establish Files, Define Scope, and Ensure Parameter Traceability and Stability

Parameters derived from scientific injection molding are not static. Parameter files and fluctuation control ranges need to be established to address minor changes in raw material batches and environmental temperature and humidity during production, while achieving full-process traceability.
1. Establish standardized process parameter files, recording raw material batches, mold numbers, equipment numbers, baseline and critical boundary values of four core parameters, trial molding verification data, and plastic part quality inspection results;
2. Set allowable fluctuation ranges for parameters (e.g., barrel temperature ±2℃, injection pressure ±5bar, mold temperature ±1℃). During mass production, if parameter fluctuations exceed this range, system will automatically issue a warning and provide minor compensation (e.g., increasing barrel temperature by 1℃ when ambient temperature drops);
3. When changing raw material batches/molds/equipment, derive parameters again according to above process to avoid directly applying old parameters.

Core Summary

The key to scientifically deriving molding parameters for injection molding is transforming "experience-based judgment" into "data testing," and "fuzzy estimation" into "boundary quantification." Its essence is not simply parameter adjustment, but rather ensuring that parameters are fully adapted to inherent characteristics of raw materials, molds, and equipment through standardized testing procedures.
Compared to traditional empirical methods, this method yields more stable parameters and a lower defect rate. Furthermore, it does not rely on experienced technicians; new employees can derive qualified parameters simply by following testing procedures. This also lays a data foundation for subsequent intelligent manufacturing and closed-loop control.
Parameter derivation process is consistent across all industries; only testing focus needs to be adjusted according to industry characteristics (e.g., for 3C parts, focus on testing holding pressure/mold temperature boundaries related to dimensional accuracy; for medical parts, focus on testing injection speed/pressure boundaries related to burr-free properties; for automotive parts, focus on testing mold temperature/holding pressure boundaries related to mechanical properties).

Gud Mould Industry Co., Ltd