Car Parts Mold Series

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In modern injection molding production, product defect rates, material waste, and production halts caused by mold defects are common pain points in the industry. According to industry statistics, the first-time mold trial pass rate of molds without mold flow analysis optimization is only 40%-50%, while common defects directly lead to a production efficiency decrease of over 30%. Based on computational fluid dynamics (CFD) and thermodynamics principles, mold flow analysis can accurately simulate the entire process of plastic melt in the mold cavity, providing a scientific basis for mold design and process adjustment. The spline test mold is a key carrier for verifying analysis results, and the combination of the two forms the core system for defect control.

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I. Common Defects of Injection Molds and Their Impact on Production
1. Types of Common Defects



Typical defects in injection mold production include flash, short shot, air bubbles, warpage deformation, weld lines, and sink marks. Among these, the occurrence rate of flash in thin-walled products is as high as 60%; weld lines are common in products with complex cavities; and warpage deformation is prominent in engineering plastic products such as ABS and PC.


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2. Impact on Production Efficiency and Product Quality



The rework rate caused by flash is approximately 15%-20%, with an average rework time of 3-5 minutes per piece; material waste caused by short shots accounts for 8%-12%; and the scrap rate of products with warpage deformation can be as high as 25%. In addition, the mold shutdown and debugging time due to defect handling accounts for 20%-25% of the total production time, severely restricting production capacity.


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II. Basic Principles and Key Data of Mold Flow Analysis
1. Basic Principle of Mold Flow Analysis




By establishing a 3D mold model and a plastic material database, it simulates the entire process of melt from injection to cooling and solidification. With the help of numerical calculations, it reproduces the distribution of temperature field, pressure field, and velocity field, and predicts the location and cause of defects.


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2. Interpretation of Key Data Indicators



Core data indicators include flow time, pressure distribution, temperature distribution, shear rate, and solidification time. The difference in flow time should be controlled within ±0.3s; the maximum injection pressure in the cavity should be less than 85% of the mold's allowable pressure (the allowable pressure of general engineering plastic molds is 150-200MPa); the uniformity error of temperature distribution should be ≤5℃; the shear rate should be controlled between 1000-5000s⁻¹; and the solidification time usually accounts for 70%-80% of the total cooling time.


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III. Core Methods of Mold Flow Analysis for Optimizing Injection Mold Defects
1. Optimization of Gate Design

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(1) Determination of Gate Position: Based on the simulation of the melt flow path, the gate is set at the farthest point of melt flow in the cavity or the position with the maximum wall thickness, avoiding the key stress-bearing areas of the product. For single-cavity products, the number of gates is usually 1-2.
(2) Optimization of Gate Size: Calculated based on material fluidity and product weight, the gate diameter for small PP products is 0.8-1.2mm, and for large products, it is 1.5-2.5mm.

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2. Optimization of Runner System



(1) Runner Layout Design: Priority is given to a balanced layout to ensure consistent melt flow distance and pressure loss in each cavity. The difference in runner length should be controlled within 5%. The diameter of the main runner is 1-2mm larger than that of the sub-runners, and the diameter of the sub-runners is 4-8mm.
(2) Optimization of Runner Size: Ensure that the pressure loss of the melt in the runner is ≤30MPa, and reduce the filling time difference of multi-cavity molds to within 0.2s.


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3. Optimization of Cooling System



(1) Cooling Water Channel Design: Follow the principle of "close to the cavity and uniform distribution". The distance between the water channel and the cavity surface is 15-25mm, and the spacing between water channels is 25-35mm. Conformal water channels are used for molds with complex curved surfaces, which can improve cooling uniformity by more than 40%.
(2) Selection of Cooling Medium: Industrial cooling water (temperature 20-25℃) with a flow rate of 1.5-2.5m/s is used for ordinary products; ice water cooling (temperature 5-10℃) is used for engineering plastic or thick-walled products, and the temperature fluctuation of the mold surface is ≤3℃.


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4. Optimization of Injection Process Parameters

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(1) Injection Pressure and Speed: The injection pressure is set to 1.1-1.2 times the maximum pressure of the cavity. A segmented speed is adopted: 30-50mm/s in the initial filling stage, 60-100mm/s in the middle stage, and 20-40mm/s in the final stage.
(2) Holding Pressure and Time: The holding pressure is 60%-80% of the injection pressure. The holding time is determined by the product wall thickness—for every 1mm increase in wall thickness, the holding time is extended by 1-1.5s.
(3) Molding Temperature: The barrel temperature is 20-40℃ higher than the plastic melting point (200-240℃ for ABS materials, 260-300℃ for PC materials); for mold temperature, it is 40-80℃ for crystalline plastics and 60-120℃ for amorphous plastics.


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IV. Application of Spline Test Molds in Mold Flow Analysis
1. Overview of Spline Test Molds



A standard mold specially used to verify mold flow analysis results, adopting the size of ISO 527-2 standard tensile splines (170mm×15mm×4mm). It can be designed with single or multi-cavity, equipped with standard gates, runners, and cooling systems. By producing standard splines, it detects the consistency between material molding performance and analysis data.


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2. Key Design Points of Spline Test Molds



The mold core material is preferably S136 or H13 mold steel, with a hardness of HRC50-55 after heat treatment; the surface roughness of the cavity is Ra≤0.8μm; the ejection system uses a combination of ejector pins and ejector plates, with ejector pin diameter of 2-3mm and spacing of 30-40mm; and mounting holes for temperature sensors are reserved to monitor the cavity temperature in real time.


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3. Role of Spline Testing in Mold Flow Analysis



It serves as a "calibrator" for analysis results, correcting model parameters by comparing simulated and measured data. For example, if the mold flow analysis predicts a spline warpage of 0.5mm and the actual measurement is 0.52mm, the error can be reduced to within ±3% after adjustment. At the same time, it can verify process parameters in advance—such as testing the weld line strength of splines under different injection speeds to determine the optimal process range.


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V. Practical Case Analysis

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A company used ABS material to produce automotive door trim strips. The first mold trial showed severe weld lines and warpage deformation, with a defect rate of 12%. Mold flow analysis revealed that the single-gate design of the original mold resulted in an excessively long melt filling path, and uneven distribution of cooling water channels caused an 8℃ temperature difference in the cavity.


Optimization Plan: Add 1 auxiliary gate and adopt a balanced runner; adjust the spacing of cooling water channels to 30mm and add 2 conformal water channels. Spline testing showed that the tensile strength of the spline weld line increased from 18MPa to 25MPa, and the warpage decreased from 0.8mm to 0.3mm.


After applying the optimization plan, the weld line strength of the product met the standard, warpage deformation was controllable, the defect rate dropped to 2.5%, production efficiency increased by 28%, and material waste per batch decreased by 10%.

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VI. Development Trends of Mold Flow Analysis Technology
1. Integration with AI and Big Data



Moving towards intelligence, AI algorithms automatically identify design defects and parameter optimization spaces, and combine with big data to realize model self-learning and self-calibration. Some systems can complete the full-process analysis of complex molds within 10 minutes, improving efficiency by more than 50%.



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2. Multi-Physical Field Coupling Simulation



Strengthen the coupling analysis of flow field, temperature field, and stress field, simulate the interaction between melt flow and mold structure deformation, and combine with software collaborative simulation to realize full-chain digital verification from design to performance prediction.


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VII. Conclusion



Mold flow analysis is the core technology for optimizing injection mold defects, and spline test molds improve the reliability of optimization solutions. By optimizing design and processes, combined with spline test verification, the incidence of defects can be significantly reduced, and the first-time mold trial pass rate can be improved. With the development of technology integration, mold flow analysis will play a greater role in the field of precision injection molding, promoting the industry's transformation towards high efficiency, accuracy, and intelligence. Establishing a closed-loop system of "mold flow analysis - spline testing - mold optimization" is the key for enterprises to enhance their competitiveness.

The surface finish, mirror effect, and overall appearance integrity of injection molded plastic parts are a direct reflection of a brand's
product quality and market competitiveness. Surface scratches are a highly prevalent, easily overlooked, yet extremely impactful appearance defect in injection molding production. These include various types such as hairline scratches, hard scratches, demolding tears, mold marks, and friction scratches.
Surface scratches directly disrupt subsequent surface decoration processes such as pad printing, screen printing, spray painting, electroplating, and laser engraving: ink easily accumulates in the scratch grooves, resulting in broken lines, color differences, unevenness, poor adhesion, and peeling; scratches remain noticeable even after spray painting, requiring rework and polishing, which is time-consuming and material-intensive, and in severe cases, the entire batch can be scrapped, causing significant cost losses and delivery delays.
Main Causes of Scratches on Injection Molded SurfacesScratches originate from the entire chain, including mold, process, demolding mechanism, product structure, manual operation, and storage environment:
1. Mold Body Causes: Insufficient polishing of the cavity and core leaves tool marks and sand marks; insufficient hardness of the mold steel leads to burrs and wear over long-term production; excessive clearance in ejector pins, sleeves, inserts, and slides results in rough surfaces and misalignment; sharp corner burrs on the parting surface and carbon buildup and corrosion in the mold cavity can all transfer or scratch the product surface.
2. Injection Molding Process Causes: Injection speed too fast causes high-speed melt to scour the mold wall, forming flow marks and scratches; insufficient cooling time results in premature demolding before the product has set; excessive cooling causes the product to cling to the mold core, increasing demolding resistance and causing scratches; imbalance between mold and material temperatures leads to deformation, friction, and scratches.
3. Product Structure and Demolding Causes: Insufficient draft angle, thin ribs, excessive undercuts, and lack of proper rounded corners significantly increase demolding resistance, resulting in hard friction during ejection and producing strip-like scratches. 4. Human and Environmental Factors: Dust, adhesive residue, and metal shavings falling into the mold cavity cause scratches; employees using sharp tools to pry products; semi-finished products lacking protective films and rubbing against each other when stacked; lack of anti-collision barriers during handling and storage, resulting in secondary scratches.
Comprehensive Prevention and Solutions for Injection Molding Surface Scratches


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1. Mold Source Control: Use high-hardness, wear-resistant mold steel; perform mirror-finish nano-polishing of the cavity and core to remove tool marks and sanding marks; perform regular mold maintenance, rust and carbon removal, and hard chrome plating to improve wear resistance; optimize the fit precision of ejector pins and slides, chamfering and removing sharp corners to ensure smooth movement without scratches.

2. Injection Molding Process Parameter Optimization: Segmented speed injection to avoid high-speed erosion of the mold wall; match reasonable mold temperature, material temperature, and cooling time to ensure uniform product shaping; uniform and slow ejection to reduce demolding friction and scratches.

3. Pre-design Product Structure Optimization:** Increase draft angles and rounded corners during the design phase; avoid deep ribs, right-angle undercuts, and other easily damaged structures; use sliding core-pulling for complex structures to reduce forced demolding.

4. Standardized Workshop Operations:** Maintain a dust-free workshop and regularly clean mold cavity debris; prohibit the use of sharp tools for demolding, and uniformly use soft plastic tools; wrap semi-finished products individually with paper dividers for layered turnover, and add soft padding to turnover boxes to prevent friction and impact.

5. Graded Scratch Repair Solution:** Minor hairline scratches: fine polishing with fine sandpaper, oilstone, and diamond paste; moderate scratches: texturing, sanding, and spray painting for coverage; deep structural scratches: mold repair and machine adjustment are necessary to prevent batch recurrence from the root cause.

III. High-Quality Plastic Parts Guarantee for Pad Printing and Screen Printing:** Plastic shells, buttons, and panels requiring pad printing and screen printing must have a zero-scratch, highly flat base surface to ensure clear printing patterns, ink adhesion, and uniform color.

AAA MOULD integrates mold design, precision injection molding, defect rectification, pad printing, and screen printing surface decoration. It strictly controls surface quality throughout the entire chain from mold polishing, structural design, injection molding process, and production operation, avoiding molding defects such as scratches and weld lines in advance, and providing a perfect base for subsequent surface printing processes, helping customers produce high-quality, durable, and cost-effective plastic products.

In injection molding production, the mold gating system serves as the core fluid channel connecting the injection molding machine and the mold cavity, acting as the "delivery artery" for plastic molding. Its design quality directly determines the melt filling effect, product molding precision, surface quality and production yield, making it an indispensable core part of injection mold structural design. This article comprehensively analyzes the structural composition, working principle, mainstream types and core design principles of injection mold gating systems, helping customers fully understand the core process logic of injection molding.



I. Core Definition and Functions of Gating Systems
An injection mold gating system refers to a series of specially designed channel structures from the nozzle of the injection molding machine to the mold cavity, dedicated to transporting, diverting, stabilizing pressure and cooling plastic melts. In short, its core function is to deliver the high-temperature molten plastic melt from the injection molding machine to each mold cavity in a stable, uniform and balanced manner, completing the entire process of filling, pressure holding and shrinkage compensation to form intact plastic products.

A high-quality gating system achieves three core values: first, stable material delivery to avoid molding defects such as melt turbulence, splashing and air bubbles; second, balanced filling to ensure consistent molding quality of products in multi-cavity molds; third, material saving to reduce residual gating waste and lower production loss and subsequent processing costs.

II. Basic Structural Composition of Gating Systems
A complete injection gating system consists of four core components: main sprue, runner, gate and cold slug well. Each component performs its own function and cooperates with others to form a complete melt delivery system.



1. Main Sprue
The main sprue is the inlet channel of the gating system, directly connected to the nozzle of the injection molding machine and serving as the first passage for melt entering the mold. It is generally designed as a tapered structure wider at the top and narrower at the bottom, which facilitates the demolding and removal of solidified sprue slugs, reduces melt flow resistance, and prevents melt retention, temperature drop and channel blockage. Undertaking the tasks of receiving all melt and preliminary flow guidance, the structural precision of the main sprue directly affects the initial flow state of the melt.

2. Runner
As an extended branch of the main sprue, the runner is mainly used to divert and distribute the melt from the main sprue to each independent cavity, widely applied in multi-cavity molds and molds for large and complex plastic parts. According to mold structural requirements, runners can be designed with circular, trapezoidal or semicircular cross-sections, among which the circular cross-section features the minimum flow resistance and the highest melt delivery efficiency. The core design logic of runners is to ensure uniform melt distribution, consistent melt pressure, flow rate and temperature in each cavity, and eliminate product defects such as dimensional deviation, short shot and sink mark.

3. Gate
The gate is a tiny connecting port between the runner and the mold cavity, as well as the final passage for melt entering the product molding area, making it the most critical precision structure of the gating system. With a miniature size, the gate can accelerate, stabilize pressure and throttle the melt, enabling the melt to fill the cavity at a high speed and in a stable state. It also effectively prevents melt backflow and ensures molding stability during the pressure holding stage. After cooling and solidification, the tiny gate slug can be easily separated from the product, greatly reducing subsequent trimming and polishing processes.

4. Cold Slug Well
Cold slug wells are reserved small storage grooves arranged at the end of the main sprue and the corners of runners. At the initial stage of injection molding, a small amount of low-temperature solidified cold slugs exist at the nozzle port. If these cold slugs directly enter the cavity, they will cause surface spots, weld lines and other defects on products. Cold slug wells intercept cold slugs and impurities in advance, filter unqualified melt, ensure pure and uniform-temperature melt entering the cavity, and effectively improve product appearance and quality.

III. Complete Working Principle of Injection Gating Systems
The working process of an injection mold gating system is a continuous and closed-loop melt delivery and molding auxiliary process, which runs synchronously with the entire injection molding procedure. It is specifically divided into four stages:



1. Melt Introduction Stage
The injection molding machine melts plastic particles into fluid melt at high temperature, and pushes the melt into the main sprue of the mold through high pressure via the machine nozzle. The tapered main sprue quickly receives the melt, reduces flow resistance, guides the melt to flow stably into the mold, and avoids melt leakage and retention.

2. Flow Division and Pressure Stabilization Stage
After entering the runners, the melt is evenly distributed to each branch channel through the preset flow path. During this process, the runner structure stabilizes the melt flow rate and pressure, eliminates melt turbulence and uneven pressure, and ensures consistent melt flow and pressure in all channels of multi-cavity molds.

3. Cavity Filling Stage
The melt after flow division and pressure stabilization is further accelerated and pressurized through the tiny gates, then injected into the mold cavity at a high and stable speed to fully fill the entire molding space. The throttling effect of the gate avoids splashing and air bubbles caused by excessive melt inflow speed, as well as short shots and sink marks caused by insufficient flow speed, ensuring full and uniform cavity filling.

4. Pressure Holding, Cooling and Slug Separation Stage
After cavity filling is completed, the gating system maintains continuous pressure to compensate for shrinkage caused by plastic cooling volume reduction, ensuring product dimensional accuracy. After the product and the melt in the gating system are completely cooled and solidified, the mold opens to eject the product, and the solidified gating slugs are ejected together with the product. Finally, the gate waste is separated from the finished product manually or by automated equipment, completing one full molding cycle.

IV. Common Types and Application Scenarios of Gating Systems
According to differences in product structure, appearance requirements and production efficiency, gating systems are divided into two main categories to meet diverse production needs:



1. Conventional Gating System (Cold Runner System)
As the most widely used traditional gating system, the melt in the main sprue and runner cools and solidifies together with the product, and the runner waste needs to be trimmed after molding. Featuring simple structure, low mold cost and wide adaptability, it is suitable for most conventional plastic products and medium and small-batch production scenarios, serving as the mainstream choice for general injection molds.

2. Hot Runner Gating System
The hot runner system is equipped with heating devices to keep the melt in the runner at a constant temperature at all times, maintaining the plastic in a molten state without forming solid slugs. Only finished products are taken out after molding with no runner waste generated. Its advantages include high material utilization, no need for subsequent trimming, high molding efficiency and excellent product consistency. It is mostly applied to high-precision, high-appearance standard and mass-produced plastic products, such as electronic accessories and precision medical device parts.

V. Core Design Principles of Gating Systems
The design quality of the gating system determines mold quality and product yield. Professional mold design must follow the following core principles:

1. Smooth Flow Principle: Adopt a concise runner layout with rounded corners to minimize melt flow resistance and avoid melt retention, degradation and burning.

2. Balanced Filling Principle: For multi-cavity molds, ensure symmetrical and uniform dimensions of all runners and gates to realize synchronous filling and eliminate product dimensional and weight deviation.

3. Quality Priority Principle: Reasonably set the position and size of gates to avoid product appearance surfaces and stress surfaces, preventing molding defects such as weld lines, sink marks, air bubbles and warpage.

4. High Efficiency and Energy Saving Principle: On the premise of meeting molding quality, simplify runner length and volume to reduce material waste, shorten the molding cycle and improve production efficiency.

5. Easy Demolding Principle: The structure of runners and gates is designed to adapt to demolding requirements, ensuring smooth removal of solidified slugs without jamming or product surface damage.

VI. Conclusion
Essentially, the injection mold gating system is a complete process system that realizesstable melt delivery, balanced filling, pressure-stabilized molding and efficient demolding through scientific channel structure design. As the core "delivery system" of injection molds, its structural design directly affects product precision, surface quality, production efficiency and production cost.

A well-designed gating system must adapt to the flow characteristics of different plastic materials and fit product structures and production requirements, balancing molding quality, production efficiency and economic benefits. It is the core key to the research and development and design of high-precision injection molds.

Injection mold dimension calculation is a core process of mold design, which directly determines the dimensional accuracy and mass production qualification rate of plastic products. Plastic products shrink after cooling and forming. Therefore, mold dimensions cannot be directly copied from product drawings. Scientific formulas are required to compensate for shrinkage, combined with tolerance, structure and production conditions for correction. This article briefly introduces the standard calculation logic, core formulas and practical key points of injection mold dimensions, providing references for the design and manufacturing of various injection molds.



1. Core Principle of Dimension Calculation

Plastic melt undergoes volume shrinkage during high-temperature injection molding and normal-temperature cooling. If the mold is manufactured according to the nominal product size, the final product size will be generally smaller. The core logic of mold dimension calculation is to enlarge the mold forming size in advance based on the plastic shrinkage rate to offset the cooling shrinkage of products, and make corrections in combination with product tolerance, machining error and mold wear to ensure the finished product dimensions are within the qualified range.



2. Core Parameter: Plastic Shrinkage Rate

The calculated shrinkage rate is uniformly adopted in mold design and serves as the basic basis for dimension verification. The formula is as follows:

Calculated Shrinkage Rate (S) = (Mold Dimension at Room Temperature – Product Dimension at Room Temperature) / Product Dimension at Room Temperature × 100%



Reference shrinkage rates of common plastics:

- Low shrinkage (precision parts): ABS, PC, PMMA, 0.3%~0.8%

- Medium shrinkage (general parts): PP, PE, PS, 1.0%~2.0%

- High shrinkage (structural plastics): POM, PA6, PA66, 2.0%~3.5%

- Glass fiber reinforced modified materials: 0.2%~1.0%

Value Selection Principles: Select a slightly lower value for precision appearance parts to reserve fine-tuning space; select a slightly higher value for thick-walled and large-size parts. For modified materials and special working conditions, prioritize the parameters provided by material manufacturers or test mold data.



3. Standard Calculation Formulas for Core Mold Dimensions

Mold dimensions are divided into three categories: cavity dimension, core dimension and center distance dimension, with corresponding independent calculation formulas. The mold manufacturing tolerance (δ) is uniformly set to 1/3~1/4 of the product tolerance.

3.1 Cavity Dimension (Product External Dimension)

Applied to external dimensions such as product shells, outer contours, length, width and outer diameters, adapting to the negative tolerance rule of product external sizes.

Formula: A = (A0 + A0×S - 3/4Δ) ± δ

Parameters: A = Mold cavity dimension; A0 = Nominal product dimension; S = Plastic shrinkage rate; Δ = Product dimensional tolerance; δ = Mold manufacturing tolerance.



3.2 Core Dimension (Product Inner Hole/Inner Groove Dimension)

Used for internal dimensions such as product holes, clamping grooves and inner cavities, adapting to the positive tolerance rule of product internal sizes to compensate for the shrinkage and reduction of inner holes after molding.

Formula: B = (B0 + B0×S + 3/4Δ) ± δ

The parameter definitions are consistent with those of cavity dimensions.



3.3 Center Distance Dimension (Hole and Pillar Spacing)

The center distance features uniform shrinkage and symmetrical tolerance, mostly used for the design of screw hole spacing and positioning pillar spacing.

Formula: C = (C0 + C0×S) ± δ/2



4. Practical Dimension Correction Key Points

- Structural Correction: Appropriately increase the shrinkage rate for thick-walled products, and finely adjust local areas such as thin walls and rib glue accumulation positions to improve shrinkage and deformation.

- Wear Correction: Reserve a wear allowance of 0.02~0.05mm for molds with an annual output of more than 100,000 pieces to extend mold service life.

- Process Correction: Products formed under high pressure holding and high mold temperature have lower shrinkage, allowing a slight reduction of the shrinkage rate value.

- Precision Part Correction: Avoid applying standard shrinkage rates directly for high-precision products; revise mold dimensions reversely based on actual shrinkage data from test molding.



5. Common Calculation Errors

- Adopting a single fixed shrinkage rate for the entire mold without zoning adjustment according to wall thickness and structure;

- Only calculating theoretical dimensions while ignoring mold machining tolerance with no fine-tuning allowance;

- Mixing up cavity and core calculation formulas, resulting in unqualified internal and external dimensions and assembly failure;

- Applying shrinkage rates of raw materials directly without adapting to glass fiber reinforced, flame retardant and other modified plastics.



6. Conclusion

The core of injection mold dimension calculation is taking the shrinkage rate as the basis, adopting differentiated calculations for cavity, core and center distance dimensions, and making corrections combined with product structure, tolerance and mass production conditions. Accurate dimension calculation is the key to ensuring product precision, reducing defect rates and realizing stable mass production. In design practice, it is necessary to combine theoretical formulas with actual test molding effects to continuously optimize dimensional parameters.