Medical Molding Series

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In the field of plastic injection molding production, short shots are among the most common and destructive quality defects. A short shot refers to a situation where molten plastic fails to fully fill the mold cavity during the filling process, resulting in problems such as material shortage, incomplete edges, and filling gaps in thin-walled areas of the molded product. These issues directly affect the structural integrity, assembly accuracy, and appearance quality of the product.


A case from a new energy vehicle manufacturer shows that a short shot problem in thin-walled bumpers alone caused the customer's production line to shut down for 8 hours, resulting in direct economic losses exceeding 500,000 yuan (Note: If converting to a foreign currency is more contextually appropriate, "over 70,000 US dollars" can be used, with the conversion rate based on the average market rate around 2024).


Starting from the essential mechanism of short shots, this article will systematically break down the four core causes, provide comprehensive solutions covering materials, molds, processes, and equipment, and illustrate the implementation paths with practical cases.

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I. The Nature of Short Shots: Dynamic Disruption of Flow Balance

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The formation of short shots is not caused by a single factor; its essence lies in the disruption of the dynamic balance between the "propulsive energy" and "flow resistance" of the molten plastic during the filling process. From a microscopic mechanism perspective, for the melt front to complete cavity filling, it must overcome three core types of resistance:
1. Viscous resistance of the melt itself: Generated by material viscosity and runner friction;
2. Pressure decay: The pressure loss of the melt from the gate to the end of the cavity increases exponentially with the length of the flow path;
3. Temperature gradient: Cooling of the melt leads to a sharp rise in viscosity—for every 10°C drop in temperature, the viscosity can increase by 2.3 times (per the Arrhenius equation).


When the sum of these resistances exceeds the propulsive force provided by the injection system, the melt front solidifies before fully filling the cavity, resulting in the short shot defect.


Typical manifestations of short shots include:
● Progressive material shortage (gradual thinning from the gate to the end of the part);
● Intermittent flow interruption (appearance of "bamboo joint-like lines" on the surface);
● Overall under-filling (insufficient filling of the entire cavity);
● Local material shortage (unfilled complex structures or mesh areas).
Different manifestations correspond to different combinations of causes.



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II. Four Core Causes: Full-Chain Tracing from Materials to Equipment

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(1) Material Properties: Innate Limitations on Flow Capacity
Materials are the fundamental factor affecting short shots, with core issues focusing on two aspects: flowability and purity.


High-viscosity materials (e.g., PC, PPO, PC + 20% mineral filler) exhibit high melt flow resistance. Particularly in thin-walled products (≤2mm) or long-flow-path products (flow ratio > 100:1), short shots are highly likely to occur due to insufficient flow. Impurities, unmelted particles, or excessive lubricants in raw materials further exacerbate the problem: impurities can block runners and form "flow obstacles," while excessive lubricants may cause melt slippage, reducing effective filling pressure.


In addition, for glass fiber-reinforced materials (e.g., PP + 30% GF), the reorientation of glass fibers increases flow resistance. Areas with concentrated long fibers may even form a network structure, hindering melt advancement.


(2) Mold Design: Structural Defects in Flow Paths
The rationality of mold design directly determines the smoothness of melt flow. Common design issues include:


● Improper runner and gate design (excessively thin runners, offset gate positions, or undersized gates), leading to excessive pressure loss;
● Poor cavity venting: Trapped air cannot escape, forming back pressure that obstructs melt filling;
● Unreasonable cooling system layout: Local overcooling causes the melt to solidify prematurely;
● Complex product geometry (e.g., dense meshes, narrow ribs): Increases flow resistance and easily forms flow stagnation zones.


For example, the mesh area of a notebook (NB) bottom cover has a large contact area and rapid heat dissipation—it is not only a high-risk area for short shots but also prone to flash defects due to the high pressure required for filling.


(3) Process Parameters: Imbalanced Matching of Molding Conditions
Process parameters are the most direct cause of short shots, with the core lying in the triangular matching of temperature, pressure, and speed.


● Insufficient injection pressure or speed results in inadequate melt propulsive force; conversely, excessively high speed may trap air, while excessively low speed causes the melt to solidify prematurely.
● Low melt temperature or mold temperature significantly increases melt viscosity. For instance, the flowability of PP material at 200°C is much lower than that in its optimal temperature range (220–240°C).
● Improper holding pressure settings (e.g., delayed switchover timing, insufficient pressure) fail to compensate for melt shrinkage, leading to material shortage at the part's end.


The "one-size-fits-all" constant-speed injection strategy in traditional processes cannot adapt to the pressure decay law of long-flow-path products, which is also a key cause of progressive short shots.


(4) Equipment Performance: Insufficient Precision in Energy Transfer
The performance of injection molding machines directly affects the stability of "energy supply," and short shot occurrences vary significantly across machines of different brands. For electric injection molding machines, key influencing factors include pressure transfer accuracy, flow rate stability, and temperature field uniformity:


● Fanuc machines have a high flow rate decay rate (up to 60%), which easily leads to progressive material shortage;
● Response delays in the holding pressure switching valve of Sumitomo machines may cause intermittent flow interruption;
● Speed fluctuations in Haitian machines result in pressure instability, leading to overall under-filling.
In addition, equipment defects such as empty hoppers, blocked feed inlets, and worn check valves can cause a drop in actual injection pressure or pressure leakage, further worsening short shot issues.


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III. Comprehensive Solutions: A Systematic Strategy from Prevention to Eradication

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(1) Material Optimization: Enhancing Flow Capacity at the Source
The core of material adjustment is to reduce flow resistance, which can be achieved through three approaches:
1.Replace with resin grades of better flowability: Select materials with higher Melt Flow Rate (MFR) to reduce the injection pressure required for filling;
2.Raw material modification: Improve melt flowability by adding plasticizers, reducing the proportion of fillers, or optimizing the length distribution of glass fibers;
3.Raw material pretreatment: Strictly control impurity content and fully dry hygroscopic materials to avoid viscosity fluctuations caused by moisture.


For glass fiber-reinforced materials such as PP + 30% GF, the screw compression ratio and the length of the mixing section can be adjusted to reduce glass fiber breakage and lower flow resistance.


(2) Mold Optimization: Building a Smooth Flow System
Mold improvement requires precise optimization combined with CAE mold flow analysis:
● Runner and gate design: Expand runner size, optimize gate position (e.g., changing from side gating to center gating), and balance the filling time of each cavity. A certain automotive component achieved a 20% increase in filling rate through this method;
● Ventilation system: Add venting grooves (depth ≤ 0.02mm, width 5-10mm) at the end of the cavity and air accumulation areas, or use porous steel inserts to eliminate the impact of back pressure;
● Cooling system: Avoid local overcooling; for long-flow-path products, additional heating rods can be installed at the end to maintain melt temperature;
● Complex-structured products: Optimize product geometry through CAE simulation, such as thickening thin-walled areas or simplifying flow paths, to reduce flow resistance.


(3) Process Optimization: Achieving Precise Process Control
The key to process adjustment is to establish a dynamic matching relationship among "temperature, pressure, and speed":
● Temperature control: Appropriately increase melt temperature (e.g., raising PP temperature from 200°C to 220-240°C) and mold temperature to reduce melt viscosity, while avoiding material degradation caused by excessively high temperatures;
● Pressure and speed control: Adopt a segmented injection strategy and dynamically adjust parameters based on flow path length. For example, use high pressure and high speed in the initial stage to quickly establish a pressure gradient, maintain pressure in the middle stage to compensate for pressure decay, and reduce speed in the final stage to avoid flash. At the same time, use a gradually decreasing holding pressure to prevent sudden pressure changes;
● Parameter adjustment principle: Follow the "small and gradual" principle—adjust injection pressure by 5%-10% each time to avoid new defects caused by drastic parameter changes.


(4)Equipment Optimization: Ensuring Stable Energy Transfer
Equipment-level optimization includes:
● Regularly calibrate injection pressure and flow rate accuracy to ensure the accuracy of pressure transmission;
● Inspect and replace worn check valves to prevent pressure leakage;
Develop differentiated parameters based on the characteristics of machines from different brands. For example, Fanuc machines need to focus on compensating for flow rate decay, Sumitomo machines require optimizing holding pressure switching timing, and Haitian machines need to control speed fluctuations;
● For long-flow-path and thin-walled products, select equipment with higher stability in injection volume, pressure, and speed to avoid short shots caused by insufficient equipment capacity.






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IV. Practical Cases: The Path from Mass Short Shots to 95% Yield

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Case 1: Solution to Short Shots in Thin-Walled Bumpers for New Energy Vehicles
A certain automotive manufacturer produced thin-walled bumpers made of PP + 30% GF (wall thickness: 1.8mm, flow ratio: 120:1). All three electric injection molding machines experienced mass short shots, with a material shortage of 2-5mm at the end of the parts. The traditional method of increasing injection pressure only improved the issue by 15% and caused severe flash.

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Solution: A coupled optimization strategy of "equipment characteristics - material behavior - process parameters" was adopted:
● For Fanuc machines: Segmented injection was implemented (initial stage: 140MPa/200mm/s → middle stage: 120MPa/150mm/s → final stage: 100MPa/100mm/s). Additional heating rods were installed at the end of the runners to increase the temperature at the end from 185°C to 220°C, reducing viscosity by 48%;
● For Sumitomo machines: The holding pressure switching timing was optimized to shorten the response delay to 0.05 seconds, and the R-angle at the runner bends was expanded to 1.5mm;
● For Haitian machines: The length of the screw mixing section was adjusted to reduce the uneven dispersion of glass fibers, and the speed fluctuation was controlled within ±1rpm.


Ultimately, the short shot defect was completely resolved, the yield increased from 70% to 95%, and the flash volume decreased by 80%.


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Case 2: Collaborative Solution to Short Shots and Flash in the Mesh Area of NB Bottom Covers
A certain notebook (NB) bottom cover (wall thickness: 1.2mm, PC + 20% mineral filler) had coexisting short shots and flash due to dense meshes. Three adjustments to the gate showed no significant improvement. CAE mold flow analysis revealed that the mesh area had high flow resistance and concentrated pressure.

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Solution:
● The mesh was optimized using an equivalent wall thickness correction model, adjusting the wall thickness of the mesh area to 1.05mm;
● The gating system was redesigned: changing from four-point gating to two-point symmetric gating, and expanding the diameter of the cross-runner to 8mm;
● Micro venting grooves (depth: 0.015mm) were added in the dense mesh area;
● Process parameters were adjusted: increasing the melt temperature from 260°C to 275°C, and adopting the "low-speed filling - high-pressure holding" strategy.


After optimization, the short shot defect was eliminated, and the flash defect rate dropped from 35% to 2%.

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V. Conclusion: Core Logic of Short Shot Prevention and Control

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The short shot problem in injection molding is essentially a mismatch between the flow system, energy transfer, and material properties. Preventing and controlling short shots requires establishing a closed-loop system of "prevention - diagnosis - optimization":
● Potential risks are prevented in the design stage through CAE mold flow analysis;
● The causes are accurately diagnosed based on defect manifestations;
● A radical solution is achieved through the collaborative optimization of materials, molds, processes, and equipment.


With the increasing popularity of precision injection molding today, short shot prevention and control has evolved from simple parameter adjustment to a multi-dimensional systems engineering. Only by mastering its core mechanism and combining it with practical experience can we effectively improve product yield and reduce production costs.

In the injection molding process, the runner system is the "bridge" connecting the injection machine nozzle and the mold cavity. It is responsible for transporting molten plastic to each cavity stably and uniformly, directly affecting the molding quality, production efficiency and cost control of plastic parts. As the core components of the runner system, runners and sub-runners have clear division of labor and work together, which are the key links to ensure stable and efficient injection molding production. Below is a detailed analysis of their definitions, structures, functions and core differences.



I. Runners: The "Main Channel" of the Runner System
In injection molding, the runner is the first channel that molten plastic flows through after entering the mold from the injection machine nozzle. It is also the "starting end" and "main delivery pipe" of the entire runner system. It is directly connected to the injection machine nozzle and undertakes the core task of transporting high-temperature and high-pressure molten plastic from the machine to the sub-runners or directly into the cavity (for single-cavity molds). It is the part with the largest diameter and the largest flow capacity in the runner system.

1. Structural Characteristics of Runners
The structural design of the runner needs to adapt to the size of the injection machine nozzle and the flow characteristics of the plastic. It usually adopts a conical structure with a taper of 2°-5°. This design can not only reduce the flow resistance of molten plastic, but also facilitate the smooth removal of runner scrap during mold release. One end of the runner is the feed port, which is closely attached to the injection machine nozzle. The diameter of the feed port is slightly larger than that of the nozzle to avoid material leakage, material accumulation and other problems; the other end is connected to the sub-runners or the cavity to realize the smooth transition of plastic.

In addition, the runner is usually set at the center of the mold (corresponding to the center of the injection machine nozzle) to ensure uniform force when the plastic enters the mold, reduce pressure loss and temperature loss during the flow process, and is especially suitable for the runner layout of multi-cavity molds.

2. Core Functions of Runners
Transportation function: Quickly and stably transport the molten plastic injected by the injection machine to the sub-runners or cavities, ensuring sufficient plastic flow to meet the molding needs.

Transition function: Alleviate the pressure difference and temperature difference between the injection machine nozzle and the mold cavity, so that the molten plastic can smoothly transition from a high-pressure and high-speed state to a state suitable for cavity filling, reducing defects such as shrinkage marks, bubbles and material shortage in plastic parts.

Guiding function: Provide a clear flow direction for molten plastic, avoid disorderly flow of plastic in the mold, ensure the filling process is carried out in an orderly manner, and improve the dimensional accuracy of plastic parts.

II. Sub-Runners: The "Branch Pipes" of the Runner System
Sub-runners are branch channels connecting the runner and the mold cavity, mainly used in multi-cavity injection molds. After the runner transports the molten plastic to the designated position, the sub-runners will evenly distribute the plastic to each cavity, ensuring that the filling speed, pressure and temperature of each cavity remain consistent, thereby ensuring the dimensional consistency and quality stability of multi-cavity plastic parts.

1. Structural Characteristics of Sub-Runners
The structural design of sub-runners needs to be flexibly adjusted according to the number of cavities, layout and plastic part size. Common cross-sectional shapes include circular, trapezoidal, rectangular, etc., among which the circular cross-section is the most widely used—it has the smallest flow resistance and uniform heat dissipation, which can minimize the energy loss of plastic during the flow process; trapezoidal and rectangular cross-sections are convenient for mold processing and the removal of runner scrap.

The diameter of sub-runners is usually smaller than that of the main runner, and the length needs to be reasonably designed according to the cavity layout, trying to shorten the flow path and reduce bends, so as to avoid excessive pressure loss and too fast temperature drop during the plastic flow process. At the same time, the layout of sub-runners must follow the principle of "equal distance and equal diameter" to ensure consistent flow resistance of each branch and realize uniform distribution of plastic.

2. Core Functions of Sub-Runners
Distribution function: Evenly distribute the molten plastic transported by the runner to each cavity, ensuring that the filling volume and filling speed of each cavity are consistent, and avoiding material shortage in some cavities and material overflow in others.

Pressure reduction and stabilization function: Further reduce the pressure and flow rate of molten plastic, so that the plastic enters the cavity in a more stable state, reducing defects such as flow marks and gate marks on the surface of plastic parts, and improving the appearance quality of plastic parts.

Auxiliary heat dissipation function: The surface area of sub-runners is relatively large, which can assist in dissipating part of the heat of molten plastic, allowing the plastic to cool gradually during the filling process, and avoiding plastic part deformation, uneven shrinkage and other problems caused by excessive temperature.

III. Core Differences Between Runners and Sub-Runners
Although both runners and sub-runners belong to the runner system, they have obvious differences in function, structure and design requirements. The specific comparison is as follows:




Comparison Dimension
Runners
Sub-Runners


Core Function
Connect the injection machine to sub-runners/cavities and mainly transport plastic
Connect the runner to cavities and distribute plastic to each cavity


Structural Shape
Mostly conical with a taper of 2°-5°
Circular, trapezoidal, rectangular, etc., with uniform cross-section


Size Characteristics
Large diameter, the thickest part in the runner system
Diameter smaller than the runner, adjusted according to cavity layout


Application Scenario
Required for both single-cavity and multi-cavity molds
Mainly used for multi-cavity molds; can be omitted for single-cavity molds


Design Focus
Adapt to nozzle size, reduce material leakage and pressure loss
Equal distance and equal diameter to ensure uniform plastic distribution




IV. Key Design Points of Runners and Sub-Runners
The rationality of the design of runners and sub-runners directly affects the efficiency of injection molding production and the quality of plastic parts. The core design points are as follows:




Match plastic characteristics: Adjust the diameter, length and taper of the runner according to the fluidity, melting point and other parameters of the plastic—for plastics with poor fluidity (such as PC, ABS), the runner diameter needs to be increased to reduce flow resistance; for plastics with good fluidity (such as PE, PP), the runner diameter can be appropriately reduced to reduce raw material loss.
Optimize layout design: The runner must be aligned with the center of the injection machine nozzle, and the sub-runners must follow the principle of "equal distance, equal diameter and shortest path" to reduce bends and excessively long flow paths, avoiding pressure loss and temperature drop.
Facilitate mold release and cleaning: The runner design must consider the convenience of taking out the runner scrap. The conical runner and smooth inner wall of the runner can reduce the adhesion of runner scrap and lower the cleaning cost.
Control cost loss: On the premise of ensuring molding quality, try to reduce the runner size to reduce the loss of molten plastic (runner scrap needs to be recycled and reused), thereby reducing production costs.

V. Summary
Runners and sub-runners are two indispensable core parts of the injection molding runner system. They work together to form a complete transportation path for molten plastic from the injection machine to the cavity. As the "main channel", the runner undertakes the core transportation task; as the "branch pipes", the sub-runners are responsible for uniformly distributing plastic. The rationality of their design directly determines the quality, production efficiency and cost of plastic parts.

In actual injection molding production, it is necessary to optimize the structure and layout of runners and sub-runners in combination with the plastic part structure, the number of cavities, plastic characteristics and injection machine parameters, so as to achieve efficient, stable and high-quality injection molding production and escort product quality.

In the injection molding production process, blooming is a common defect that affects product quality. It is characterized by white powdery, foggy deposits or waxy layers appearing on the surface of plastic parts. This not only seriously affects the appearance of the product but also may reduce the mechanical properties, surface adhesion, and service durability of the plastic parts, bringing troubles to enterprises such as increased production costs and delayed delivery. To help industry partners accurately identify and efficiently solve this problem, this article comprehensively analyzes the core causes of blooming problems based on years of practical experience in injection molding processes, and provides implementable and replicable solutions to help enterprises improve product qualification rates and reduce production losses.



I. Core Definition and Hazards of Blooming in Injection Molding
Blooming in injection molding, also commonly referred to as "whitening" or "powdering" in the industry, is essentially a phenomenon where additives inside the plastic (such as lubricants, plasticizers, stabilizers, vulcanizing agents, etc.) reach saturation in the resin and cannot be stably accommodated by the resin matrix. During production, processing, finished product storage, or use, these additives slowly diffuse and precipitate from the inside of the plastic part to the surface, forming visible deposits.

The hazards of blooming problems are mainly reflected in three aspects: first, affecting the appearance, causing the surface of the plastic part to turn white, foggy, and lose luster, failing to meet the quality requirements of appearance parts and being directly judged as unqualified products; second, damaging performance, the precipitated additives will destroy the integrity of the plastic part surface, reduce its wear resistance, weather resistance, and bonding performance with other components, and shorten the product service life; third, increasing costs, the rework and scrapping of unqualified products, as well as the loss of materials and time in the process of troubleshooting problems, will lead to an increase in enterprise production costs and affect production efficiency.

It should be particularly noted that blooming is easily confused with plastic part aging "whitening" and surface contamination, which can be distinguished by a simple method: the deposits from blooming can temporarily restore the original color of the plastic part after wiping with a clean cloth, and there will be powder or oily residues after wiping; while aging "whitening" cannot be restored after wiping, and there are no obvious residues.



II. Analysis of Core Causes of Blooming Problems in Injection Molding
The occurrence of blooming problems is not caused by a single factor, but by the combined effect of various factors such as formula, process, raw materials, and environment. Combining industry practice and technical research, the core causes can be summarized into four categories to accurately locate the root cause of the problem.

(1) Formula Factors: Unreasonable Selection and Proportion of Additives
The formula is the core root cause of blooming problems. Most blooming phenomena are directly related to the improper selection and proportion of additives, which is also the most easily overlooked link:

1.Excessive additive dosage: Each type of additive has an upper limit of solubility in the resin. If the addition amount of lubricants, plasticizers, stabilizers, etc., exceeds their saturated solubility in the resin, the excess additives cannot be fully wrapped by resin molecules and will gradually precipitate in the subsequent process to form blooming. For example, the solubility of sulfur in natural rubber is about 2%, and excessive addition is likely to cause powdering blooming; excessive use of stearic acid and paraffin is likely to cause wax blooming.
2.Poor compatibility of additives: The mismatch between the polarity of the resin and the additives will lead to insufficient compatibility between the two. Even if the additive dosage does not exceed the standard, migration and precipitation will occur because they cannot be stably dispersed in the resin matrix. For example, using polar plasticizers for non-polar resins or non-polar paraffin for polar resins is likely to cause blooming; some accelerators have extremely low solubility in specific resins, and a slightly larger dosage may cause blooming. For example, TMTD and TMTM have very low solubility in BR, IIR, and EPDM, but a slightly larger dosage in NBR is not easy to cause blooming.
3. Abnormal interaction between additives: Some additives will undergo chemical reactions to form insoluble substances, or the precipitation of one additive will drive other additives to precipitate together. For example, if the amount of antioxidant is too high (mostly p-phenylenediamine type), its precipitation will drive the residual sulfur and accelerators to spray out together; the reaction between accelerator M and antioxidant MB generates insoluble salts, which will also precipitate to form blooming.

(2) Process Factors: Improper Control of Injection Molding Process Parameters
The precise control of injection molding process parameters is the key to avoiding blooming. Deviations in parameters such as temperature, pressure, and time during the production process will directly lead to blooming problems, which is also the most easily adjustable link in front-line production:

1.Unreasonable temperature control: Both too high and too low temperatures can cause blooming. If the barrel temperature is too high, it will accelerate the volatilization and migration of additives, and some additives will precipitate in advance at high temperatures and attach to the surface of the plastic part; if the mold temperature is too low, the cooling speed of the plastic part is too fast, and the additives inside the resin cannot diffuse evenly, which will crystallize and precipitate on the surface to form foggy blooming. In addition, if the temperature of the rubber compound is too high during mixing, it will also lead to local high concentration of additives, laying hidden dangers for blooming.
2.Improper pressure and holding pressure parameters: Insufficient injection pressure and too short holding time will lead to insufficient compactness inside the plastic part with tiny gaps, and additives are likely to migrate to the surface along the gaps; on the contrary, too high pressure and too long holding time will over-compress the resin molecules, and additives will be squeezed out to the surface to form blooming.
3.Improper mixing and feeding process: If the equipment speed is too low, the mixing time is insufficient, or the feeding order is messy during mixing, the additives will not be evenly dispersed in the resin, and the local concentration of additives will be too high, forming saturated precipitation conditions; if additives such as sulfur are damp and agglomerated due to improper storage and are put into use without crushing, it will also lead to local high concentration and cause blooming.
4. Improper storage of rubber compound and plastic parts: If the mixed rubber compound does not enter the injection molding process in time and is stored for too long, the additives will slowly diffuse inside the resin and eventually precipitate; if the plastic parts are not cooled to room temperature in time after molding, or are placed in a high-temperature and high-humidity environment after cooling, it will also accelerate the precipitation of additives and cause blooming.

(3) Raw Material Factors: Insufficient Raw Material Quality and Processing
The quality of raw materials and the effect of pre-treatment directly affect the quality of injection molding, and are also important incentives for blooming problems:

1.Insufficient raw material purity: The resin raw material contains too many impurities, low molecular volatile substances, or the proportion of recycled materials added is too high. These impurities and low molecular substances will be precipitated during injection molding, forming a phenomenon similar to blooming, which is easily misjudged as blooming.

2.Insufficient drying of raw materials: Resin raw materials (especially hygroscopic resins such as PA and PC) are not fully dried before injection molding and contain moisture inside. During injection molding, the moisture evaporates, which will drive the additives to precipitate together to form foggy blooming; some fillers have high water content, which will be higher in humid weather. During high-temperature injection molding, they will dissolve out with the evaporation of moisture, and white powder will be left after the moisture evaporates, forming powdering blooming.

3. Poor quality of additives: The used lubricants, plasticizers and other additives have insufficient purity or contain volatile components, which are easy to volatilize and precipitate during injection molding to form blooming; some fillers (such as alkaline precipitated silica) are unstable in nature and are also easy to cause powdering.

(4) Environmental Factors: Improper Control of Production and Storage Environment
The temperature, humidity, cleanliness, etc., of the production and storage environment will indirectly affect the occurrence of blooming problems, which are easily overlooked:

1.Temperature and humidity fluctuations: High temperature and high humidity in the production environment will accelerate the migration and precipitation of additives; drastic temperature fluctuations in the storage environment, the solubility of additives increases at high temperatures, and drops sharply at low temperatures, the excess components precipitate, and repeated alternation of high and low temperatures will further accelerate blooming.

2. Poor environmental cleanliness: Excessive dust and oil stains in the production workshop will attach to the surface of plastic parts, forming an illusion similar to blooming. At the same time, dust may mix into the raw materials, affecting the dispersion of additives and indirectly causing blooming.

3. Light exposure and medium contact: During the storage or use of plastic parts, long-term exposure to sunlight, ultraviolet rays will damage the resin structure and promote the migration of additives; contact with acid-base media, alcohol and other solvents will also accelerate the precipitation of additives and cause blooming.

III. Targeted Solutions to Blooming Problems in Injection Molding
In view of the above causes of blooming, combined with industry practical experience, the idea of "accurately locating the root cause + targeted rectification + long-term prevention" is adopted to formulate implementable solutions. No complex testing equipment is needed, and front-line operators can directly refer to and apply them to quickly solve the blooming problem.

(1) Optimize the Formula: Reduce Blooming Hidden Dangers from the Root
Formula optimization is the core of solving blooming problems. It is necessary to scientifically select additives and adjust the proportion according to the resin characteristics to avoid additive precipitation from the root:

1.Reasonably adjust the additive dosage: According to the type of resin, strictly control the addition amount of various additives to ensure that it does not exceed their saturated solubility in the resin. For example, for products with silica as the main filler, the amount of PEG can be reduced to 5% to 6% of the amount of silica to avoid wax blooming; the dosage of sulfur and accelerators needs to be determined through experiments to avoid excessive addition.
2. Select additives with good compatibility: Prioritize additives that match the polarity of the resin and have good compatibility to reduce the risk of migration and precipitation. For example, use non-polar lubricants for non-polar resins and polar plasticizers for polar resins; select accelerators with strong reactivity with the resin as the main accelerator to reduce the residue of free additives; add plasticizers or homogenizers such as coumarone resin and phenolic resin to improve the dispersion stability of additives in the resin by using their solubilizing effect.

Silver streaks are one of the most common and typical surface defects in injection molding. Characterized by silvery, needle-like, striped or cloudy marks along the melt flow direction on plastic parts, these flaws not only compromise surface flatness and glossiness but also degrade local mechanical properties. They frequently cause batch rework, product rejection and customer complaints, severely affecting production yield and brand reputation. This article comprehensively elaborates on the classification, root causes, targeted solutions and routine prevention measures of silver streaks, providing practical technical guidance for injection molding manufacturers.



1. Definition and Common Types of Silver Streak Defects
Silver streaks refer to silvery strip defects formed on the part surface when internal moisture, air or impurities fail to escape completely during mold filling and solidification, and are stretched and squeezed within the molten plastic. Based on formation mechanisms and morphological features, the four most prevalent types in mass production are summarized as follows:


Moisture-induced Silver Streaks: Fine and uniform random streaks widely distributed on part surfaces, primarily caused by excessive moisture in raw materials. This is the most common type of silver streak defect.
Air-trap Silver Streaks: Intermittent streaks concentrated at melt flow ends, dead corners and wall thickness transition areas, resulting from insufficient mold venting.
Degradation-induced Silver Streaks: Coarse, opaque and heavy silvery marks often accompanied by slight scorching near the gate, generated by gas released from overheated material degradation.
Impurity-induced Silver Streaks: Irregular streaks with uneven width and scattered white spots, caused by contaminated raw materials or excessive impurities in recycled resin.



2. Root Causes of Silver Streaks
Silver streaks are rarely caused by a single factor but result from the superposition of abnormalities in materials, processes, molds and equipment. Accurate cause identification is critical for effective defect resolution.



2.1 Material Factors (Primary Cause)
Most engineering plastics (ABS, PC, PA, PMMA, modified PP, etc.) are hygroscopic and absorb ambient moisture during storage and transportation. Insufficient drying before production leads to instant water vaporization under high melting temperature. The vapor trapped in the melt is stretched during filling and forms visible silvery stripes after cooling and solidification. In addition, dust, oil contamination, mixed foreign materials or excessively degraded recycled resin will produce volatile gas and impurities at high temperatures, further triggering silver streaks.

2.2 Improper Injection Process Parameters
Unbalanced process parameters aggravate gas generation and entrapment. Excess barrel and nozzle temperature, insufficient screw back pressure and fast plasticizing speed lead to material thermal degradation and uneven melt mixing, making internal gas difficult to discharge. Excess injection speed and pressure cause turbulent flow and air entrapment, wrapping air inside the melt. Insufficient holding pressure and short holding time fail to compact the part surface, allowing internal gas to precipitate and form streaks.

2.3 Mold Design and Venting Abnormalities
Poor mold venting is the main cause of localized silver streaks. Shallow, blocked or improperly positioned vents fail to exhaust trapped air and melt gas at flow ends, rib positions, pillar areas and thin-thickness transitions. Low mold temperature accelerates melt cooling, freezing gas inside the part before it escapes. Undersized gates and narrow runners increase flow resistance, causing local overheating, material degradation and gate-area silver streaks.

2.4 Equipment and Operational Issues
Worn screws and barrels result in poor plasticization and material retention, causing local overheating and degradation. Incomplete barrel cleaning leaves residual old materials that generate volatile contaminants. Unsealed hoppers, open material storage and inaccurate dryer parameters cause secondary moisture absorption and insufficient drying, continuously inducing silver streak defects.

3. Targeted Solutions for Silver Streak Defects
Practical and mass-production-friendly solutions are proposed covering material control, process optimization, mold improvement and equipment management to eliminate silver streaks effectively.



3.1 Material Control: Eliminate Moisture and Impurities at the Source
Follow standardized drying specifications for different resins: dry ABS and PP at 70-80℃ for 2-4 hours; dry highly hygroscopic materials including PC, PMMA and PA at 90-120℃ for 4-6 hours to meet moisture requirements. Keep hoppers and barrels sealed during production to prevent secondary moisture absorption. Reject damp, contaminated or deteriorated raw materials, and control the proportion of recycled materials with sufficient filtration and re-drying. Regularly clean hoppers, loaders and dryers to avoid impurity contamination.

3.2 Process Optimization: Reduce Gas Generation and Air Entrapment
Optimize thermal parameters by properly lowering barrel front and nozzle temperatures to avoid thermal degradation. Increase screw back pressure and reduce plasticizing speed to achieve uniform melt mixing and full gas release. Adopt low-speed filling to avoid turbulent flow and air entrapment. Appropriately increase holding pressure and extend holding time to compact the part surface and suppress gas precipitation. Shorten material residence time in the barrel to prevent long-term high-temperature degradation.

3.3 Mold Optimization: Improve Venting Performance
Optimize the venting system by deepening and widening vents at gas-prone positions including flow ends, ribs and pillars, with a standard vent depth of 0.02-0.05 mm for smooth gas discharge. Regularly clean mold parting surfaces and vents to remove carbon deposits and resin residues. Moderately increase mold temperature to slow down cooling and reserve sufficient time for gas escape. Optimize gate and runner designs to reduce flow resistance and avoid local overheating.

3.4 Equipment Maintenance and Standardized Operation
Establish periodic equipment maintenance schedules to inspect and repair worn screws and barrels for stable plasticization. Thoroughly clean barrel residuals before production and complete purging during material changes. Calibrate dryers and temperature control devices regularly to ensure accurate drying parameters. Standardize operational procedures: avoid open material storage and seal hoppers after shutdowns to eliminate human-induced defects.

4. Routine Prevention and Mass Production Control
The core of silver streak management is prevention and closed-loop control. Implement incoming material inspection to test moisture content and purity, rejecting unqualified batches. Standardize drying and injection parameters for each material grade and form unified process specifications to prevent arbitrary parameter adjustment. Develop daily vent cleaning and weekly full mold maintenance plans. Sustain regular equipment calibration and maintenance to ensure stable production.

5. Conclusion
As a frequent surface defect in injection molding, silver streaks involve the entire production chain including materials, processes, molds and equipment. The core root causes are residual moisture, trapped air, material degradation and poor venting. Most silver streak issues can be thoroughly resolved through four key measures:precise material drying, venting system optimization, process parameter calibration and standardized equipment maintenance. For injection molding enterprises, refined and standardized full-process control is essential to reduce defect rates, improve surface quality, stabilize mass production, guarantee on-time delivery and enhance market competitiveness.