Trial molding is core link between mold development and injection molding mass production of plastic parts. It verifies mold design and processing accuracy and is a crucial stage for adjusting injection molding process parameters. Effectiveness of defect handling during trial molding stage directly determines direction of mold rectification, establishment of injection molding process window, even affects product yield and production efficiency in mass production stage. In actual trial molding, many companies often fall into trap of "blindly adjusting parameters and treating symptoms but not root cause" when faced with common defects such as flash, shrinkage marks, and weld lines. They simply mask problem by adjusting injection molding parameters without finding root cause from dimensions of mold, material, and process compatibility, leading to repeated defects in mass production stage.

 

Solving defects in plastic part trial molding is not simply a matter of adjusting parameters or modifying mold. It requires combining injection molding process theory with root cause analysis methods from quality management, breaking down core causes of defects from four dimensions: equipment, mold, process, and materials. A systematic approach involving process optimization, mold correction, and material adjustment should be developed, along with a standardized trial molding optimization process to achieve a closed-loop solution for defects. This article combines injection molding process theory and quality management knowledge to conduct an in-depth root cause analysis of seven most common defects in trial molding, providing directly implementable solutions and trial molding optimization processes. It also proposes solutions to industry pain points in trial molding, making process more targeted and scientific.

Flash is one of the most common defects in trial molding, manifesting as excess plastic material edges at parting line and insert mating points. Its core essence is insufficient sealing accuracy of mold cavity, causing melt to overflow from sealing gap under pressure. Root causes involve four main aspects: equipment mold closing, mold structure, process parameters, and material properties. Core solution is to restore cavity sealing accuracy and match melt pressure with sealing capacity.
(I) Core Root Cause Analysis
Equipment Mold Closing Problems: Insufficient mold closing force, failing to meet matching requirements of melt pressure. Conventionally, mold closing force should be no less than 1.5 times injection pressure, which cannot offset melt pressure in cavity, leading to overflow; deviation in platen parallelism causes uneven loading during mold closing, resulting in excessively large local sealing gaps.
Mold design and manufacturing defects: Long-term grinding or wear on parting surface leads to poor fit; venting groove depth exceeds 0.03mm, losing its sealing function; gaps in mating parts such as inserts and sliders are too large (exceeding 0.02mm).
Inappropriate injection molding process parameters: Injection pressure and injection speed are too high, causing a sudden increase in melt pressure that exceeds cavity sealing capacity; holding pressure time is too long, and continuous shrinkage pressure causes melt to overflow from gaps; melt temperature is too high, and excessive material fluidity easily leads to overflow.
Poor material compatibility: Low-viscosity, high-flow-rate materials such as PA and PP are selected, which are prone to overflow from tiny gaps under normal process pressure.
(II) Systematic solutions
Equipment debugging and calibration: Increase clamping force according to mold requirements to ensure that clamping force can counteract melt cavity pressure; check and calibrate parallelism of mold plate, adjust clamping mechanism to avoid uneven loading, ensure uniform mold closing and sealing.
Mold Correction and Rectification: Polish and repair worn parting surfaces; re-inspect, adjust clearance of inserts and sliders to ≤0.02mm; correct excessive venting groove depth to standard range of 0.02~0.03mm, balancing sealing and venting requirements.
Injection Molding Process Optimization: Adopt a multi-stage injection mode to reduce injection pressure and injection speed, gradually and smoothly filling cavity; shorten holding time and lower melt temperature to reduce material fluidity while ensuring plasticization, minimizing possibility of melt overflow.
Material Adaptation Adjustment: If product performance allows, switch to high-viscosity materials such as PC or ABS; or add fillers such as glass fiber to existing material to increase viscosity and reduce possibility of melt overflow.

Shrinkage marks often appear in thick-walled areas, ribs, and bosses, manifesting as surface indentations and internal shrinkage cavities. Core reason is insufficient shrinkage compensation during cooling process of plastic melt, or uneven cooling leading to excessive local shrinkage differences. The key solution is to achieve precise melt shrinkage compensation and uniform mold cooling, matching shrinkage characteristics of material.
(I) Root Cause Analysis
Deficiency in holding pressure shrinkage compensation process: Insufficient holding pressure and too short holding time prevent melt from fully filling shrinkage space before gate solidifies; single holding pressure parameter settings fail to provide targeted shrinkage compensation for thick-walled areas.
Inadequate mold cooling system design: Thick-walled areas lack dedicated cooling water channels, resulting in much slower cooling speeds compared to thin-walled areas, leading to excessive local shrinkage; uneven water channel layout causes overall mold cooling deviation, resulting in inconsistent shrinkage and shrinkage marks.
Mold Product Structural Design Defects: Thickness of ribs and boss pillars exceeds 0.6 times main wall thickness. When melt cools and shrinks in thick-walled areas, it cannot effectively compensate for shrinkage from thin-walled areas, resulting in localized shrinkage marks.
Material Shrinkage Characteristics: High-shrinkage materials such as PP and PE were selected. Conventional pressure compensation parameters cannot match their shrinkage requirements, easily leading to obvious shrinkage marks.
(II) Systematic Solutions
Precise Injection Molding Process Optimization: Adopt a stepped pressure holding mode, increasing pressure in core pressure compensation stage while extending holding time until gate is completely solidified; reduce melt temperature, optimize cooling time to allow melt to shrink evenly and reduce excessive localized shrinkage; calibrate holding pressure parameters through actual product weight measurement to avoid insufficient pressure compensation.
Mold Cooling and Structural Rectification: Add conformal cooling water channels in areas prone to shrinkage marks, such as thick-walled areas and ribs, to achieve rapid and uniform cooling in key areas; modify product's glue thickness, adjusting thickness of ribs and boss pillars to within 0.6 times main wall thickness to structurally avoid shrinkage dead zones.

 

Material property adaptation and adjustment: Switch to low-shrinkage materials such as ABS and PC to reduce pressure and time required for shrinkage compensation; or add mineral fillers to existing material to reduce the overall shrinkage rate, decrease probability of shrinkage marks.

Weld lines are linear marks formed when multiple melt fronts meet in mold cavity without complete fusion. They not only affect product's appearance but also lead to a decrease in mechanical strength of fusion area. Core reasons are insufficient temperature and pressure when melt fronts meet, or poor venting in fusion area hindering fusion. The key to solving this is to increase temperature and pressure during melt fusion, optimize venting conditions in fusion area.
(I) Core root cause analysis
Insufficient melt fusion conditions: If melt temperature and mold temperature are too low, melt fronts will have cooled significantly upon meeting, preventing sufficient diffusion and fusion between molecules; if injection pressure is insufficient, pressure at melt front meeting will be inadequate, resulting in a loose bond at fusion surface.
Defects in mold venting system: Weld line area lacks venting channels or insert venting structures, preventing gas from escaping from cavity and hindering sufficient contact between melt fronts, resulting in loose weld lines.
Inappropriate mold gate design: Multiple gate layouts lead to excessive melt splitting, or improper gate placement causes large-angle melt splitting, resulting in multiple melt fronts meeting unnaturally in cavity and poor fusion.
(II) Systematic Solutions
Injection molding process parameter optimization: Increase melt temperature and mold temperature to prolong melting state of melt fronts, providing sufficient time for molecular fusion; appropriately increase injection speed to enhance pressure when melts meet while avoiding flash; adopt an "overflow well" process design to guide weld line to non-appearance, non-stress areas of product, avoiding quality impact.
Mold design and processing optimization: Add venting channels or use insert gap venting in weld line area to quickly expel cavity gas; re-optimize gate position and number to reduce melt splitting angle, allowing melt fronts to meet naturally and smoothly in cavity, improving fusion. Material compatibility adjustment: Use materials with better flowability, such as PA66+GF, to improve flowability of melt front, allowing for more complete contact and fusion of melt upon contact; or add toughening agents to material to improve mechanical strength at weld line, compensating for strength loss caused by fusion defects.

Incomplete shots manifest as parts of mold cavity not being filled by melt, resulting in incomplete molding. Core reason is insufficient melt filling capacity, or gas in cavity hindering melt flow, preventing melt from reaching cavity end. The key solution is to improve melt's flow and filling capacity, eliminate venting obstacles during melt filling.
(I) Core Root Cause Analysis
Insufficient injection volume: Incorrect injection stroke settings, insufficient barrel discharge, resulting in insufficient total melt volume to fill cavity; unreasonable injection endpoint settings, stopping injection before melt has completely filled cavity.
Insufficient melt flow: Low melt temperature and insufficient plasticization result in high material viscosity and poor flowability; slow injection speed causes melt to continuously cool during filling, losing its flowability by time it reaches cavity end.
Mold venting and runner defects: Inadequate mold venting system design prevents timely gas removal from cavity, creating air resistance and hindering melt filling; overly narrow runner design and small gate size lead to excessive pressure loss during melt flow, reducing filling capacity.
(II) Systematic Solutions
Injection Molding Machine Parameter Calibration: Check and adjust injection stroke and metering stroke, increase injection volume to ensure the total melt volume fills cavity; recalibrate injection endpoint to ensure melt fully fills cavity end.
Injection Molding Process Optimization to Improve Flowability: Increase melt temperature to ensure sufficient plasticization and reduce viscosity; increase injection speed to reduce temperature loss during melt filling; appropriately increase injection pressure to compensate for pressure loss during melt flow.
Mold Venting and Runner Optimization: Add venting grooves at cavity end and in final melt filling area, or use vacuum venting technology to eliminate air resistance; appropriately increase runner diameter and gate size to reduce pressure loss during melt flow and improve filling capacity.

Warpage deformation manifests as product deviating from designed shape after demolding. It is a complex defect that is difficult to solve in trial molding. Core reason is uneven cooling and shrinkage of different parts of product, or release of internal stress generated during injection molding after demolding, causing product deformation. The key to solving this is to achieve uniform cooling of mold and reduce accumulation of internal stress during injection molding.
(I) Core Root Cause Analysis
Mold Cooling System Design Defects: Uneven water channel layout, with one side of product cooling faster and the other slower, resulting in excessive differences in shrinkage rates among different parts, causing warpage after demolding; inconsistent cooling rates between thick-walled and thin-walled areas, leading to localized deformation due to differential shrinkage.
Inappropriate injection molding process parameters: Excessive holding pressure leads to excessive compressive stress within product; insufficient cooling time results in premature demolding and uneven shrinkage during subsequent natural cooling; excessive injection speed causes excessive melt shear force, leading to uneven molecular orientation and internal stress.
Product and mold structural design defects: Significant differences in product wall thickness without a transition structure cause stress concentration during cooling and shrinkage; an unreasonable mold ejection mechanism design results in uneven force during ejection, causing product deformation upon demolding.
(II) Systematic Solutions
Mold cooling system optimization: Re-adjust the water channel layout, adding water channels to cooling dead zones to ensure consistent cooling speed across all parts of product; add conformal water channels in thick-walled areas to improve cooling efficiency and reduce cooling difference between thick and thin-walled areas.
Adjusting Injection Molding Process Parameters to Release Internal Stress: Reduce holding pressure using a stepped pressure reduction and holding method to minimize internal pressure accumulation; extend cooling time to allow product to cool fully within mold to ejection temperature, reducing subsequent natural shrinkage deformation; employ a slow-fast-slow multi-stage injection pattern to reduce melt shear force and internal stress caused by uneven molecular orientation.
Product and Mold Structure Rectification: Optimize product structure, adjust wall thickness, and add transition structures to avoid uneven shrinkage caused by abrupt changes in wall thickness; redesign mold ejection mechanism, optimizing ejector pin arrangement to ensure uniform force distribution during ejection and prevent demolding deformation.

Flow marks often appear near gate, manifesting as striped or serpentine textures on product surface, affecting appearance quality. Core cause is spraying of melt as it enters cavity from gate, or excessive velocity changes during flow leading to unstable melt flow. The key solution is to ensure smooth melt entry into cavity, avoiding spraying and flow fluctuations.
(I) Root Cause Analysis
Mold Gate Design Defects: Using a direct-pump gate, molten metal enters wide cavity from a small gate at excessive speed, causing jetting and forming serpentine flow marks. An undersized gate causes a sudden increase in molten metal velocity, also leading to jetting.
Inappropriate Injection Molding Process Parameters: Excessive injection speed prevents molten metal from spreading smoothly upon entering cavity, causing it to jet directly onto cavity wall and form flow marks upon cooling. Insufficiently low melt temperature and high viscosity cause rapid cooling upon contact with cavity wall, resulting in noticeable flow marks.
(II) Systematic Solutions
Mold Gate Structure Optimization: Replace direct-pump gate with a fan-shaped gate, overlapping gate, or pin-point gate to allow molten metal to spread smoothly upon entering cavity, avoiding jetting. Appropriately increase gate size to reduce molten metal velocity through gate and decrease probability of jetting.
Injection Molding Process Parameter Optimization: Adopt a slow-fast-slow multi-stage injection pattern to reduce injection speed of melt through gate, allowing melt to smoothly enter cavity before increasing speed for filling; appropriately increase melt temperature and mold temperature to reduce material viscosity, making melt flow more smoothly and reducing formation of flow marks.

 

Bubbles manifest as bulges on product surface or hollow bubbles inside. Core reason is that gas in cavity is not discharged in time, forming residue, or material is not sufficiently dried and contains moisture, which vaporizes at high temperatures to form bubbles. The key solution is to ensure proper material drying and optimize mold venting system to eliminate residual gas.
(I) Core Root Cause Analysis
Mold Venting System Defects: No venting channels are provided in gas residue area of cavity, or venting channels are blocked by plastic material, preventing timely gas discharge and causing gas to be trapped by melt to form bubbles; unreasonable runner design allows air to be entrained during melt flow, forming bubbles.
Inadequate material pretreatment: Hygroscopic materials such as PA, PET, and PC are not dried sufficiently as required, leaving moisture in material. At high temperatures in barrel, this moisture vaporizes into water vapor, forming bubbles as it enters mold cavity with melt.
Inappropriate injection molding process parameters: Excessively high melt temperatures cause thermal degradation of material, generating gas; excessively fast injection speeds cause melt to rapidly fill mold cavity, entraining a large amount of air that cannot be expelled in time.
(II) Systematic Solutions
Optimization of mold venting system: Add venting channels in areas with residual gas in mold cavity and at the end of runner; regularly clean venting channels to prevent blockages; use vacuum venting technology to quickly extract gas from mold cavity, fundamentally eliminating residual gas; optimize runner design to prevent air entrainment during melt flow.
Standardized material drying treatment: Based on hygroscopic characteristics of material, strictly follow standards for drying treatment. For example, PC materials need to be dried at 120℃ for 4 hours, while PA and PET need to be dried at 80~120℃ until moisture content meets standard; hygroscopic materials should be transported and stored in vacuum packaging to prevent moisture absorption.
Injection Molding Process Parameter Optimization: Lowering melt temperature avoids gas generation from material thermal degradation; employing a multi-stage injection mode reduces melt filling speed, allowing sufficient time for gas to escape from cavity and reducing air entrapment.

Resolving mold trial defects is not a matter of random trial and error adjustments, but requires establishing a standardized optimization process. This process combines injection molding process principles with traceability requirements for quality management, making adjustments and rectifications more targeted. Simultaneously, it allows for advance prediction of potential defects, reducing trial and error costs. Core process consists of three steps:
Scientific Adjustment of Process Parameters by Priority: Following order of adjusting injection speed/pressure first → then temperature → finally holding pressure and cooling. Injection speed/pressure directly affects melt filling effect and is key to resolving filling defects such as insufficient fill, flash, and weld lines. Temperature affects melt flowability and cooling shrinkage, crucial for resolving defects such as flow marks, shrinkage marks, and warpage. Holding pressure and cooling are final steps for refined shrinkage compensation and stress elimination, preventing new defects from arising after initial parameter adjustments due to improper holding pressure and cooling.
Optimize parameter recording and establish a stable process window: After each adjustment of process parameters, promptly record parameter values and defect changes. Filter through multiple sets of test data to determine parameter range that can produce qualified products, establishing a preliminary process window. Simultaneously, record location, method, and effect of mold modifications, creating a trial molding data archive to achieve process traceability and provide a basis for setting mass production process parameters.
Predict defects in advance using mold flow analysis: Before trial molding, use mold flow analysis software such as Moldflow to simulate filling, cooling, and shrinkage process of melt in cavity. Predict location of potential defects such as weld lines, insufficient glue, and warpage. Optimize mold structure and process parameters before trial molding to reduce defects at source and improve trial molding efficiency.

In actual project management, efficiency and effectiveness of trial molding work are often limited by two core pain points: insufficient knowledge accumulation and a lack of multi-skilled personnel. These are root causes of recurring trial molding defects and low rectification efficiency. Targeted solutions are needed to overcome industry development bottlenecks:
(I) Analysis of Core Pain Points
Insufficient ability to summarize and accumulate knowledge after problem-solving: Most engineers, after resolving trial molding defects, do not conduct systematic reflection and summarization, may even be unable to clearly describe problem or break down solution process. This leads to recurrence of similar defects in subsequent trials, preventing companies from forming a reusable knowledge system and hindering improvement of trial molding efficiency.
Severe shortage of multi-skilled technical personnel: On-site machine adjustment personnel are mostly trained from front lines and possess rich practical experience, but lack theoretical knowledge of mold flow analysis and scientific injection molding; while design engineers often master mold flow analysis but lack practical on-site machine adjustment capabilities. Multi-skilled personnel who understand machine adjustment techniques and are proficient in mold flow analysis and scientific injection molding methods are a scarce resource in industry.
(II) Targeted Solution Strategies
Establish standardized problem-solving files to strengthen knowledge accumulation: Develop a "Trial Molding Defect Resolution File Template," requiring engineers to clearly record defect phenomena, root cause analysis, solutions, effect verification, and experience summaries, forming a company-specific trial molding defect knowledge base; regularly organize team case sharing sessions to break down resolution process of typical defects, allowing all employees to share experience, avoid repeated trial and error with similar problems.
Build a tiered talent development system to cultivate multi-skilled talents: Develop a talent development path of "on-the-job training + theoretical learning + project practice," encouraging university students majoring in related fields to start from front-line machine adjustment positions, first accumulating on-site practical experience, then systematically learning theoretical knowledge such as mold flow analysis, scientific injection molding, and quality management, finally combining theory and practice through actual projects; establish an internal training and technical exchange mechanism to enable on-site machine adjustment personnel and design engineers to learn from each other, improving team's comprehensive technical capabilities.

 

Core of resolving common defects in plastic part trial molding lies in moving beyond a single-parameter adjustment mindset. A root cause analysis must be conducted across four dimensions: equipment, mold, process, and materials. This, combined with principles of injection molding, requires developing a systematic solution strategy. Mold is fundamental, determining upper limit of defect resolution; process is crucial, enabling precise control of melt filling, shrinkage compensation, and cooling; and materials are a prerequisite, ensuring compatibility between mold and process. Simultaneously, establishing standardized trial molding optimization processes, meticulously recording parameters and establishing process windows, combining mold flow analysis to predict defects in advance can significantly improve trial molding efficiency and reduce trial-and-error costs.
Ultimate goal of trial molding is to achieve "first-time trial success and rapid transition to mass production." Overcoming industry pain points in trial molding hinges on two key aspects: firstly, ensuring knowledge accumulation so that companies can develop reusable defect resolution systems; and secondly, cultivating well-rounded professionals who are proficient in both on-site machine adjustment and mold flow analysis and scientific injection molding. Only by combining scientific nature of root cause analysis, systematic nature of solution strategies, continuous nature of knowledge accumulation, professionalism of talent cultivation can trial molding truly become an efficient bridge connecting mold development and injection molding mass production, laying a solid foundation for stable product quality and improved production efficiency in mass production stage.