Injection Molding Core Process Parameter Setting and Optimization Logic

Time：2026-11-06 08:02:37 / Popularity： / Source：

1. Principles for Setting Multi-Stage Injection Speeds

Gate Section (First Section): A slower speed is recommended to prevent "jetting marks" caused by melt suddenly entering open mold cavity.
Main Filling Section (Middle Section): High-speed filling is used to reduce melt temperature drop, ensure fluidity, improve weld line strength, and enhance surface gloss. High speed is especially necessary for thin-walled parts.
Final Filling Section (Last Section): When product is about to be filled, especially in areas with poor venting, speed should be reduced to allow sufficient time for gas to escape, preventing trapped gas and scorching.
Switching Points: Switching points between different speed stages should be set at locations where there are significant changes in product structure or wall thickness.

2. Golden Sequence for Setting Machine Parameters

Static Parameter Setting: First, set temperature parameters (barrel temperature, mold temperature) and wait for each part's temperature to reach and stabilize at set values. This is "static" preparation stage because machine has not yet started its cyclic operation.
Dynamic Parameter Setting: After temperature stabilizes, set and adjust "dynamic" parameters such as pressure, speed, time, and position.

3. Initial "Safety" Parameter Setting Method

Clamping Force: Set a relatively high initial value based on theoretical calculations or experience.
Injection Pressure: Set to 50-60% of machine's maximum value.
Holding Pressure: Set to 30% of machine's maximum value, or start from 0.
Injection Speed: Set to 30-40% of machine's maximum value.
Screw Speed: Set to approximately 50-60 RPM. Back Pressure: Set to 5-10 bar (0.5-1.0 MPa).
Metering Stroke: Calculate based on product's theoretical weight and set a slightly smaller value (e.g., 90%) for short shots.
Cooling Time: Set a significantly longer time.
Explanation of Data Conflicts: Regarding initial pressure settings, there are two strategies. One is conservative approach described above. The other, more aggressive approach, suggests setting injection pressure to maximum value, starting holding pressure from 0, and gradually increasing material volume to fully utilize speed setting. Choice of strategy depends on operator's experience and familiarity with mold.

4. Short Shot Analysis

One of the most basic and important machine adjustment methods. By intentionally reducing injection volume to produce incomplete products (short shots), melt flow path, filling sequence, weld line location, and venting within mold cavity can be visually observed. By starting with approximately 50% fill and gradually increasing material by 10% until full, injection speed and pressure of each stage can be systematically optimized.

Injection Molding Core Process Parameter

Observable Contents		Optimization Steps
Melt Flow Path	Observe melt flow path from gate to each area and final filling shape.	Start with approximately 50% fill volume	Set injection volume to approximately 50% and inject the first short-shot sample.
Filling Sequence	Determine order in which melt arrives at each area, identifying potential areas with trapped air or weld lines.	Increase fill volume by approximately 10% each time	Increase injection volume by approximately 10% each time, observing filling changes.
Weld Line Location	Confirm location, number, and direction of weld lines (bond lines), assessing their impact on appearance and strength.	Gradually fill product	Continue to increase fill volume until product is completely filled (100%). Record changes at key locations.
Ventilation	Observe whether venting is smooth and whether there are any signs of defects such as burning, silver streaks, or bubbles.	Optimize injection speed and pressure at each stage	Based on observation results, adjust injection speed, switching positions, and holding pressure at each stage to achieve the best filling effect and quality.

Tip: Short-shot analysis is simple, intuitive, and low-cost, making it a fundamental tool for machine adjustment and troubleshooting. It is recommended to use it first when changing molds or materials, or when filling abnormalities occur.

5. Optimization of Holding Pressure and Time

Holding Pressure Setting: Start with a relatively low pressure (or 50-70% of injection pressure), observe shrinkage at thick-walled sections of product, and gradually increase holding pressure until shrinkage is improved to an acceptable level. Excessive holding pressure will generate residual stress, leading to product warping or ejection difficulties.
Holding Time Setting: Classic method for determining holding time is "product weight method". Gradually increase holding time while weighing product. When product weight no longer increases with increasing holding time, it indicates that gate has solidified, this is optimal holding time. Further increasing holding time is meaningless, will only increase cycle time and cost.

1. Holding Pressure Setting
Purpose: To eliminate shrinkage in thick-walled areas and avoid residual stress caused by excessive pressure by setting a reasonable holding pressure.
Start with a lower pressure: Set holding pressure to 50-70% of injection pressure as starting point. Observe product shrinkage: Observe product for defects such as shrinkage, depressions, and bubbles in thick-walled areas. Gradually increase holding pressure: Gradually increase holding pressure by 5-10% each time, and continuously observe improvement effect. Determine optimal pressure: Optimal holding pressure is when shrinkage in thick-walled areas is improved to an acceptable level.
Note: Excessive holding pressure will generate residual stress, leading to product warping or difficulty in ejection. A lower holding pressure should be used as much as possible while meeting quality requirements.
2. Holding Time Setting (Product Weight Method)
Purpose: To determine optimal holding time, avoiding shrinkage due to too short a time or increased cycle time and cost due to too long a time.
Product Weight Method Steps: Set initial holding time: Set a relatively short holding time to begin test. Gradually increase holding pressure time: Increase time by a certain amount each time (e.g., 1-2 seconds). Weigh product: Weigh and record product weight after each injection molding. Plot a weight-time curve: Plot product weight as a function of holding pressure time. Determine optimal holding pressure time: When product weight no longer increases with increasing holding pressure time, it indicates that gate has solidified; this is optimal holding pressure time.
Summary: Holding pressure time determined by "product weight method" ensures product quality, improves production efficiency, and reduces costs.

6. Balance between Back Pressure and Screw Speed

Back Pressure Selection: Transparent products (e.g., PC, PMMA): Require higher back pressure to compact melt, expel gases, and reduce gas streaks.
Precision-sized products: Require stable back pressure to ensure consistent melt density, thereby ensuring stable injection volume. Heat-sensitive plastics (e.g., PVC) or shear-sensitive materials (e.g., long glass fiber reinforced materials): Lower back pressure is preferable to reduce shear heat and fiber damage.
Screw Speed Selection: Choose the lowest possible speed while ensuring that plasticizing time is less than cooling time. Lower speeds reduce shear heat, prevent plastic degradation, and ensure uniform melt temperature.
Choosing right back pressure and screw speed is crucial for ensuring plasticizing quality, improving product quality, extending equipment life, and reducing energy consumption.

I. Back Pressure Selection		II. Screw Speed Selection
	1. Transparent Products (e.g., PC, PMMA) Requires higher back pressure to compact melt, expel gas, and reduce gas streaks. Recommended: Higher back pressure.	Screw speed should be selected as low as possible, provided that plasticizing time is less than cooling time. Low speed reduces shear heat, prevents plastic degradation, and ensures uniform melt temperature.
	2. Precision-Dimensional Products Requires stable back pressure to ensure consistent melt density, thereby ensuring stable injection volume. Recommended: Stable medium back pressure.
	3. Heat-Sensitive Plastics (e.g., PVC) Preferably uses lower back pressure to reduce shear heat and prevent material degradation. Recommended: Lower back pressure.	Key Points: Plasticizing time < Cooling time (ensuring production cycle time). Select lower value within recommended speed range whenever possible. Balancing Plasticization Quality and Production Efficiency.
	4. Shear-Sensitive Materials (e.g., long glass fiber reinforced materials) Preferably uses lower back pressure to reduce shear heat and fiber damage. Recommended: Lower back pressure.
III. Balancing Back Pressure and Screw Speed
		General Principles: Set back pressure comprehensively based on material properties, product requirements, and equipment conditions. Minimize shear heat while ensuring plasticization quality. Regularly verify and optimize to maintain process stability.

Practical Recommendations: Determine back pressure range based on materials and products → Start trial operation with a lower screw speed → Check plasticization quality (appearance, density, bubbles, etc.) → Fine-tune back pressure and speed to achieve optimal balance → Record parameters and solidify process conditions.

7. Significance and Control of Buffer Material Amount

Function: It ensures that screw can always apply effective pressure to melt during holding pressure stage, transmitting holding pressure effect. If buffer amount is 0, screw will "lock up," and holding pressure will completely fail.
Setting: Buffer material amount is usually set to about 10% of screw diameter, or a distance of 3-5 mm. During production, stability of buffer position should be monitored, as its fluctuations directly reflect stability of plasticizing and injection processes.

1. Significance (Function) of Buffer Material Amount It ensures that screw can always apply effective pressure to melt during holding pressure stage, transmitting holding pressure effect. If buffer material amount is 0, holding pressure will be completely ineffective when screw is "locked in".
With Buffer Material Amount (Correct): Screw is not locked in, and there is a melt cushion layer, which can effectively transmit holding pressure.	Buffer Material Amount is 0 (Incorrect): Screw is locked in, making rigid contact with front end, and cannot transmit holding pressure, resulting in holding pressure failure.
2. Setting Buffer Material Amount Empirical	3. Monitoring and Stability of Buffer Position
Setting Value	During production process, stability of buffer position should be monitored, as its fluctuations directly reflect stability of plasticizing and injection processes.
		Stable State Small fluctuations indicate stable plasticizing and injection processes, and guaranteed product quality. Unstable State: Large fluctuations in plasticizing or injection process can lead to product defects (such as weight fluctuations, dimensional inconsistencies, etc.).
Note: Setting value needs to be adjusted appropriately for different machines, molds, materials to ensure holding pressure effect and product quality.

Summary: Appropriate buffer material amount is key to effective pressure holding. Setting a reasonable amount and maintaining its stability is the foundation for achieving stable production and excellent quality.

Gud Mould Industry Co., Ltd