Hot Selling Medical Device Plastic Molding

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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.

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I. Potential Hazards of Excessive Wall Thickness in Injection Molded Parts. Potential Hazards of Excessive Wall Thickness in Injection Molded Parts



Excessive wall thickness is a common issue in the design or production of injection molded parts. It not only impairs product quality but also increases production costs and reduces production efficiency. The specific hazards are mainly reflected in the following aspects:


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1. Frequent Molding Defects: Excessive wall thickness leads to uneven cooling rates of the melt, making it difficult for internal heat to dissipate. This easily causes defects such as sink marks, depressions, bubbles, and warpage. For example, when the thick-walled area shrinks during cooling, the surface material cannot be replenished in time, resulting in obvious sink marks; the incompletely cooled melt inside continues to shrink, which may generate internal bubbles and affect the structural integrity of the product.

2. Low Production Efficiency: Increased wall thickness prolongs the melt filling time and cooling cycle, significantly reducing the efficiency of injection molding. Under the same production conditions, the cooling time of thick-walled parts may be 2-3 times that of parts with reasonable wall thickness, leading to a substantial drop in output per unit time and increased production energy consumption.

3. Material Waste and Rising Costs: Excessive wall thickness means an increase in raw material usage, directly raising material costs. Meanwhile, the extended cooling time leads to higher energy consumption and accelerated mold wear, further driving up the overall production costs.

4. Poor Mechanical Properties: Excessive wall thickness may cause uneven crystallization inside the plastic and the occurrence of stress concentration, reducing the mechanical properties of the product such as impact strength and toughness, and affecting the service life and safety of the product.


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II. Core Solutions to Excessive Wall Thickness in Injection Molded Parts. Potential Hazards of Excessive Wall Thickness in Injection Molded Parts



To address the issue of excessive wall thickness, a comprehensive approach is required from multiple aspects such as design source optimization, process adjustment, and material adaptation, aiming to achieve the goal of "reducing thickness without compromising quality".



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(I) Optimize Product Structure Design: Control Wall Thickness at the Source

Product design is the fundamental link to solve excessive wall thickness. On the premise of meeting usage requirements, unnecessary wall thickness should be reduced through scientific design while ensuring structural strength.

1. Adopt Uniform Wall Thickness Design: The ideal wall thickness of injection molded parts should be consistent, avoiding localized thick-walled areas. During design, a reasonable wall thickness range should be determined based on the product's application and stress conditions (typically, the reasonable wall thickness for thermoplastics is 1-4mm, which needs to be adjusted according to specific material properties). For parts of the product subject to high stress, methods such as adding reinforcing ribs and fillet transitions can be used instead of increasing wall thickness, which not only ensures strength but also avoids localized excessive thickness.

2. Incorporate Hollow Structures and Lightening Holes: For large-volume injection molded parts with no internal functional requirements, hollow design or lightening holes can be adopted to reduce the wall thickness of the core area. For example, for products such as home appliance casings and automotive interior parts, grid-like hollow structures can be arranged in non-stressed areas, which not only reduces material usage but also facilitates melt flow and cooling.

3. Optimize Transition Zone Design: The transition between areas of different wall thicknesses in the product should be smooth, avoiding right-angle or acute-angle abrupt changes. This prevents melt stagnation and accumulation during flow, while reducing defects caused by uneven cooling. The inclination angle of the transition zone is recommended to be controlled between 30°-60° to ensure smooth melt flow.

(II) Adjust Injection Molding Process Parameters: Adapt to Molding Requirements After Thickness Reduction

After optimizing the product structure, it is necessary to adjust the injection molding process parameters to ensure the smooth molding of the injection molded parts with reduced thickness and avoid problems such as insufficient filling and flash.

1. Improve Melt Fluidity: Appropriately increase the barrel temperature and mold temperature to reduce melt viscosity and enhance its flow capacity, ensuring that the melt can quickly fill the cavity with reduced thickness. The barrel temperature should be adjusted according to the plastic material (e.g., the temperature for PP material is usually 180-220℃, and for ABS material is 200-250℃), and the mold temperature is generally controlled at 40-80℃. This avoids material degradation caused by excessively high temperatures or poor fluidity due to excessively low temperatures.

2. Optimize Injection Pressure and Speed: Increase the injection pressure and injection speed to shorten the melt filling time and prevent insufficient filling caused by the reduced cavity wall thickness. The injection pressure is recommended to be increased by 10%-20% compared with the original parameters (adjustments should be made in combination with the mold bearing capacity). The injection speed can adopt segmented control: rapid filling in the early stage and slow pressure holding in the later stage, so as to reduce melt impact and flash generation.

3. Adjust Pressure Holding and Cooling Parameters: The cooling speed of injection molded parts with reduced thickness accelerates, so the pressure holding time and cooling time need to be adjusted accordingly. The pressure holding time can be shortened by 10%-15% to avoid product deformation caused by excessive pressure holding; the cooling time should be optimized according to the actual molding situation to ensure that the product temperature is lower than the heat distortion temperature when demolded, thereby reducing warpage.

(III) Select Suitable Materials and Improve Molds

1. Choose High-Flow Materials: For injection molded parts with thinner walls, it is recommended to select plastics with good fluidity, such as PP, PE, and ABS, or modified plastics (e.g., plasticizer-added modified PVC) to reduce molding difficulty. Avoid using materials with poor fluidity (such as PC and POM) for thin-walled products unless molding requirements can be met through process optimization or mold improvement.

2. Optimize Mold Structure: Improve the mold’s gating system by increasing gate size and shortening runner length to reduce melt flow resistance. For example, adopt pin-point gates or fan gates to enhance melt filling efficiency; install cold slugs in the runner to prevent cold material from entering the cavity and affecting molding quality. Meanwhile, the mold cavity surface should be polished to reduce melt flow friction and improve filling effectiveness.

3. Add Venting Systems: After wall thickness reduction, gas inside the cavity is more difficult to discharge, which may easily cause defects such as bubbles and burn marks on the product. Vent grooves should be arranged at the end of the mold cavity, dead corners of melt flow, and other positions. The recommended width of the vent groove is 0.01-0.03mm, and the depth is 0.5-1mm, ensuring timely gas discharge during the molding process.

(IV) Strengthen Quality Inspection and Iterative Optimization

1. Establish Wall Thickness Inspection Standards: Regularly inspect the wall thickness of injection molded parts during production using tools such as calipers and ultrasonic thickness gauges to ensure the actual wall thickness meets design requirements and avoid localized excessive thickness caused by production deviations.

2. Conduct Mold Testing and Process Iteration: After adjusting the product structure and process parameters, conduct mold testing to verify the product’s filling condition, surface quality, and mechanical properties. If problems such as insufficient filling or sink marks occur, further optimize the structural design or process parameters until the qualified standard is achieved.

3. Long-Term Monitoring and Feedback: Continuously monitor product quality during mass production, collect data on molding defects, and regularly analyze the causes to optimize the scheme. For example, if localized sink marks occur persistently, there may still be room for optimizing the wall thickness in that area, requiring re-adjustment of the structural design.

III. Precautions and Summary

To solve the problem of excessive wall thickness in injection molded parts, the principle of "design priority, process adaptation, and material matching" must be followed. The core is to minimize unnecessary wall thickness to the greatest extent on the premise of meeting product functions and mechanical properties. Meanwhile, the following points should be noted:

1. When optimizing the structure, do not excessively reduce the wall thickness. It is necessary to verify whether the product meets the usage requirements through strength tests to avoid quality problems such as product fracture and deformation caused by overly thin walls;

2. The adjustment of process parameters should be carried out step by step. Avoid drastic modifications at one time to prevent new molding defects such as flash and mold sticking;

3. Mold improvement should be combined with product structure and material characteristics. Avoid blindly adding gates or vent grooves, which may affect mold service life and product appearance.

In conclusion, solving the problem of excessive wall thickness in injection molded parts is a systematic project. It requires collaborative optimization from multiple aspects including design, process, materials, and molds. This not only addresses existing defects but also balances production efficiency and cost control, ultimately achieving a win-win situation for product quality and economic benefits.

In the injection molding industry, weld lines (weld marks) are one of the most common, troublesome, and impactful surface defects affecting the quality of high-end plastic parts. When molten plastic flows through the mold cavity at high temperatures, it encounters ribs, pillars, holes, or multi-gate structures. The melt is divided into two or more streams, which then converge and fuse again after passing around. The interface between the two streams cannot be completely fused, resulting in a visible linear mark after cooling. This is what the industry commonly refers to as a weld line, seam line, or cold joint.


Weld lines not only severely damage the smoothness and consistency of the product's appearance but also significantly reduce the structural strength, impact resistance, and bending resistance of plastic parts. Especially suitable for products requiring both high-end appearance and durability, such as electronic casings, button panels, automotive interior parts, medical device components, and home appliance casings. Minor welds can affect subsequent surface treatments like pad printing, screen printing, spray painting, and laser engraving; severe welds can directly lead to batch scrap, rework, and repairs, increasing production costs and delaying delivery.


I.Core Causes of Injection Molding Welds



Weld formation is not caused by a single factor, but rather by the combined effect of five dimensions: mold design, plastic raw materials, injection molding process, product structure, and production environment:
1. Mold Design Factors: Excessive number of gates, unreasonable gate placement, uneven material flow paths; insufficient or blocked venting channels in the cavity, poor venting at the parting line; uneven cooling water distribution, and large local temperature differences in the mold can all lead to lower temperatures at the material flow convergence point, trapping gas and forming noticeable welds.
2. Plastic Raw Material Factors: Low melt flow index and poor fluidity of raw materials; excessive addition of fillers such as glass fiber and talc; inadequate drying of raw materials; excessive moisture content; excessive mixing of recycled materials; and mixing of multiple grades of materials all increase melt viscosity and reduce fusion performance.
3. Injection Molding Process Factors: Low barrel temperature, mold temperature, and nozzle temperature; slow injection speed and insufficient injection pressure; short holding time and insufficient back pressure; rapid cooling before melt convergence leads to poor fusion.
4. Product Structure Factors: Significant differences in wall thickness, excessively dense or thick ribs, numerous openings, and lack of rounded corners at right angles easily alter the material flow direction, artificially creating points of divergence and convergence.


II. Comprehensive Improvement Solution for Injection Molding Welds

To improve or even eliminate injection molding welds, a comprehensive solution must be adopted, encompassing early-stage design prevention, mid-stage process debugging, and post-stage surface finishing:
1. Mold Front-End Optimization: Optimize the gate design and layout, employing hot runners and valve needles for sequential injection, controlling the material flow convergence point; increase venting grooves and optimize venting angles in the weld line area; evenly distribute cooling water channels to stabilize mold temperature and delay melt cooling.
2. Standardized Raw Material Control: Select high-flow-compatibility raw materials; strictly control the filler ratio; thoroughly dry and dehumidify the material; limit the proportion of recycled material and eliminate the mixing of impurities.
3. Fine-tuning of Injection Molding Process: Appropriately increase material temperature, mold temperature, and nozzle temperature; increase injection pressure and mid-stage injection speed; increase back pressure and extend holding pressure and shrinkage compensation time to ensure full fusion of the material flow under high temperature and high pressure.
4. Pre-emptive Product Structure Avoidance: Design uniform wall thickness and use rounded corners; guide welds to non-visible surfaces, hidden side areas, and non-stressed areas.
5. Post-processing Finishing and Masking Minor, unavoidable weld seams can be treated with fine mold polishing, texturing, and spray painting for masking. Alternatively, they can be cleverly concealed using pad printing and screen printing patterns, without affecting the overall aesthetics or functionality.


III. Value of Professional Mold and Injection Molding One-Stop Service AAA MOULD specializes in precision mold design, injection molding, and surface decoration services. We intervene in weld line risk prediction from the early stages of product development, optimizing across all dimensions from product structure and mold runners to venting and cooling, and injection molding processes. This reduces weld seam formation at the source and allows for aesthetic enhancement through post-processing techniques such as pad printing, screen printing, and spray painting, balancing product structural strength, appearance, and mass production stability. This helps customers reduce scrap, save costs, and maintain consistent quality.

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