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Cracking is one of the most common and quality-critical defects in plastic injection molding production. Cracks, fine lines, fractures and brittle failure on molded parts not only cause unqualified appearance, but also significantly reduce structural strength, weather resistance and service life of plastic products. Such defects often lead to batch scrap, rework and delivery delays, making crack control a core priority for quality management in the injection molding industry. This article systematically introduces the classification, root causes, targeted solutions and long-term prevention strategies of injection molding cracks, providing professional guidance for precision injection production and quality optimization.

1. Common Types of Injection Molding Crack Defects
According to occurrence stage, location and morphological characteristics, injection cracks can be divided into four categories. Each type corresponds to different inducements and requires targeted troubleshooting and improvement.

1.1 Surface Fine Cracks
These fine shallow cracks usually appear on product surfaces, thin-wall areas and corner positions. Presenting as hairline lines or reticular crazing, they are inconspicuous at first and gradually expand under external force or environmental changes. Most surface fine cracks occur immediately after demolding.

1.2 Stress Cracking
As a latent defect caused by accumulated internal stress, stress cracking includes molding residual stress cracking and environmental stress cracking. Products show no obvious cracks after molding but crack during storage, assembly or service, triggered by external extrusion, temperature fluctuation or chemical corrosion. This defect frequently occurs in rigid plastics such as ABS, PC and PS.

1.3 Demolding Cracking
Demolding cracks and edge chipping emerge during or right after demolding, mainly on buckles, rib positions, deep cavities and thin-wall structures. The main causes include unbalanced demolding force, uneven ejection and insufficient mold draft angle.

1.4 Weld Line Cracking
Linear cracks formed at the fusion position of two plastic melt streams. The weak bonding strength of weld lines makes these positions prone to fracture under force. Beyond appearance defects, weld line cracking poses major hidden dangers to structural stability, especially for complex injection-molded parts.



2. Core Causes of Injection Molding Cracks
Injection cracking is rarely caused by a single factor but results from abnormalities in four key dimensions: materials, molding processes, molds and product structure. Comprehensive inspection is required to locate fundamental causes accurately.

2.1 Raw Material Factors
Raw material quality is the foundational factor of crack defects. First, insufficient drying leaves moisture in plastic pellets, which vaporizes under high temperature and forms internal voids, resulting in loose product structure and cracking. Second, excessive recycled materials or mixed impurities reduce material fluidity and toughness and increase brittleness. Third, mixed different materials or unstable batch quality leads to poor melt compatibility and uneven internal stress, inducing cracks.

2.2 Improper Injection Process Parameters
Unreasonable process parameters are the leading cause of molding cracks. In terms of temperature, low barrel and mold temperature lead to poor plasticization, insufficient filling and weak molecular bonding; excessive temperature causes material thermal degradation, aging and embrittlement. In terms of pressure and speed, overly high injection/holding pressure and fast filling speed generate severe shear stress and excessive residual internal stress. Conversely, insufficient pressure and slow speed result in incomplete filling and weak weld lines, forming fragile crack-prone areas. In addition, premature mold opening with inadequate cooling time causes deformation and cracking before product shaping and hardening.

2.3 Mold Design and Manufacturing Defects
Mold problems directly cause stress concentration and demolding damage. Insufficient draft angle and rough cavity surfaces increase demolding friction and produce tensile cracks. Unevenly distributed ejector pins and fast ejection speed lead to local overstress, resulting in whitening and cracking. Undersized gates and narrow runners increase melt flow resistance and generate excessive shear stress, leaving stress concentration areas prone to delayed cracking. Moreover, poor mold ventilation causes incomplete weld lines and local burning, further triggering crack defects.

2.4 Product Structural Design Defects
Unreasonable product structure is the root cause of frequent cracks. Uneven wall thickness leads to inconsistent cooling and shrinkage rates, producing huge internal stress. Right-angle and sharp-corner designs without rounded transitions form stress concentration points that crack preferentially under force or temperature change. Thin walls, deep cavities and slender ribs are structurally weak, with high risks of cracking and fracture during molding and application.



3. Targeted Improvement Solutions for Crack Defects
Based on actual production scenarios and crack causes, batch cracking problems can be effectively solved through four optimization approaches: material upgrading, process refinement, mold rectification and structural improvement.

3.1 Raw Material Management Optimization
Implement standardized drying procedures with temperature and duration tailored to different plastic materials to completely remove moisture and eliminate void-induced cracking. Standardize material proportioning, strictly control recycled material ratio, and prohibit the use of contaminated, expired or deteriorated materials to ensure purity and stability. For crack-prone materials, adopt high-toughness and stress-crack-resistant modified plastics to enhance product crack resistance fundamentally.

3.2 Refined Injection Process Adjustment
Optimize temperature parameters by appropriately raising barrel and mold temperature to ensure full plasticization, improve melt fluidity and molecular bonding, and eliminate cracks caused by poor plasticization, while avoiding excessive temperature-induced material degradation. Moderate injection/holding pressure and filling speed reduce melt shear stress and residual internal stress to prevent stress cracking. Extend cooling time appropriately to ensure complete product shaping before mold opening and demolding. For stress-crack-sensitive products, add annealing and post-baking processes to release residual internal stress.

3.3 Mold Optimization and Maintenance
Optimize the demolding system by increasing draft angles and polishing cavity surfaces to reduce demolding friction. Adjust ejector pin layout to achieve uniform and synchronous ejection and slow down ejection speed to avoid tensile cracking and top cracking. Optimize the gating system by expanding gates and optimizing runner design to reduce flow resistance and shear stress, and adjust gate positions to avoid key stress-bearing areas. Improve mold ventilation by adding exhaust grooves to eliminate poor welding and burning defects. Conduct regular mold cleaning, polishing and maintenance to sustain molding accuracy and stability.

3.4 Product Structural Optimization
Avoid structural defects in the design stage: maintain uniform wall thickness to ensure consistent cooling and shrinkage; replace sharp corners and right angles with rounded transitions to eliminate stress concentration points; optimize fragile structures such as thin walls, deep cavities and slender ribs, and add reinforcing ribs when necessary to improve structural strength and reduce cracking risks from the source.



4. Long-Term Prevention and Quality Control Measures
The core of crack management is prevention and full-process control. Establishing standardized production management systems can effectively avoid batch crack defects.

First, complete raw material inspection, equipment check and mold verification before production to eliminate hidden risks at the source. Second, solidify core process parameters during mass production, prohibit arbitrary adjustments, and implement full process recording and traceability. Third, implement strict first-piece confirmation, in-process inspection and final inspection, focusing on crack-prone areas such as corners, gates and ribs to detect latent defects timely. Fourth, conduct mold flow analysis for new products and new molds to predict risks of stress concentration and poor weld lines and optimize structure and processes in advance. Fifth, standardize warehousing, packaging and assembly management to prevent post-production stress cracking caused by external extrusion, chemical corrosion and sharp temperature changes.

5. Conclusion
Crack defects in plastic injection molding are systematic problems resulted from combined abnormalities in materials, processes, molds and product structures. For injection molding manufacturers, accurate classification of crack types and root cause analysis, together with refined process control, standardized mold maintenance and scientific structural design, can effectively eliminate crack defects, improve product yield, reduce production costs, and stabilize product quality, providing reliable quality assurance for high-precision injection molding mass production.

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.