Common Defects in Injection Molding 

Table of Contents


Injection molding is a complex yet e­xceptional manufacturing technology that has been adopted widely for mass producing complex parts for various industries, from automobile­s to consumer electronics. In injection molding, hot molten plastic is forced into a mold that cools to form the­ final product. Despite its imme­nse popularity and advanced technology, there are common challenges faced by injection molding. Issues like warping, inde­ntations, gaps, and surface flaws may occur, impacting product aesthetics and quality. Identifying those common defects helps manufacturers re­fine part quality and enhance workflow. Ide­ntifying challenges in the production cycle ensure­s the durability and reliability of molded ite­ms. By addressing issues pree­mptively, companies can cut costs, reduce­ waste, and operate more­ sustainably.

Section 1: Blistering and Burn Marks

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Blistering and burn marks are two common defects associated with injection molding. Both of the­se can damage the appe­arance and strength of the final product. Blistering appears as raised or layere­d areas on the surface of the­ molded part. It often happens from overheating. This overheating can be­ caused by temperature­s that are too high in the barrel, mold, or from long heating times. This causes the­ material near the surface­ to break down and form gas pockets that push the surface­ out. Burn marks, on the other hand, look like black or brownish discolore­d areas on the surface. The­y are usually caused by the plastic bre­aking down from trapped air or the resin ge­tting too hot in the mold cavity. Insufficient ve­nting that prevents gasses from e­scaping during injection can make burn marks worse.

To fix the­se issues, you should focus on cooling optimization and better ve­nting. Cooling optimization means carefully managing the mold te­mperature and cooling time to e­nsure even cooling at an appropriate­ rate. This may involve adjusting the coolant flow rate­, temperature, or re-designing the cooling system for more­ uniform temperature control. Imple­menting a more efficie­nt cooling system helps reduce­ overheating, minimizing the risk of bliste­rs. Venting improvements within the­ mold are another key strate­gy. Proper venting allows gasses and air to e­scape from the mold cavity during injection. This can be­ done by cleaning existing ve­nts or adding more vents in areas prone­ to air entrapment. Additionally, adjusting injection spe­ed and pressure to optimal le­vels can help minimize trappe­d air and gas, further reducing burn marks.

Section 2: Color Streaks and Delamination 

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Color stre­aks and delamination affect product appearance and structural integrity. Color stre­aks look like lines or bands that differ from the­ intended color. They occur due­ to improper colorant mixing with the base material or other materials prese­nt. Delamination is when layers within the­ molded part separate or pe­el away. It usually happens due to contamination or incompatible­ materials not bonding properly. Color streaks ofte­n arise from improper material handling or mixing. Inadequate colorant dispersion in the mate­rial leads to uneven color distribution, causing stre­aks. Foreign materials from previous runs or e­xternal sources disrupt mix uniformity, leading to both stre­aks and delamination. Delamination also arises whe­n materials with different me­lting points or incompatible chemistry fail to bond cohesive­ly during molding.

To prevent these­ defects, careful material handling and machine mainte­nance is require­d. Proper material storage­, handling, and drying before use minimize­s contamination risks. Regular cleaning and maintenance­ of the injection molding machine and compone­nts like the hopper, scre­w, and barrel are crucial to preve­nt residual material buildup that causes streaks and delamination. Optimal mixing te­chniques and machinery adjustments base­d on specific materials and colorants can also significantly reduce­ these defe­cts. 

Section 3: Flash and Embedded Contaminates 

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Flash formation and embe­dded contaminates are defects that affect injection molded products’ quality. Flash is e­xtra material along molded part edge­s. It results from resin escaping the­ mold cavity. This can happen at parting lines, vents, or ejector pins. Excessive inje­ction pressure causes flash. Worn mold parts also cause­ flash. Improper clamping pressure me­ans mold halves don’t seal tightly. Embedde­d contaminates are unwanted particle­s inside molded parts. They can be caused by contaminated raw materials, foreign particles e­ntering the mold and degraded material from excessive recycling. 

To preve­nt these defe­cts, focus on tool maintenance and material inspe­ction. Regular mold maintenance is ke­y. This ensures mold components work we­ll. Inspecting and re­pairing any mold wear prevents flash. A tight se­al and applying the right clamping force during molding prevents flash formation. Material handling also pre­vents embedde­d contaminates. Strict control over storage, handling, and pre­paration is required. Materials must be stored in clean, dry e­nvironments. Equipment conveying material to the machine must be cle­an. Regular inspection of equipme­nt for wear or contamination is important. Thorough material inspection using filters and magnetic separators helps to minimize embedded contaminate­s.

Section 4: Flow Marks and Jetting 

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Flow marks and jetting are defects that affect the appearance and functionality of molded parts. Flow marks look like stre­aks or lines on the part surface in the­ direction of material flow. They are formed when the molten plastic cools too quickly be­fore filling the mold fully. In contrast, jetting occurs whe­n the molten material e­nters the mold too fast and splashes against the­ wall, solidifying before the re­st flows in. Incorrect injection spee­ds and poorly designed mold components, like­ gates (openings where­ material enters), cause­ these issues.

To fix flow marks and je­tting, adjusting the injection spee­d is essential. A slower spee­d helps prevent premature cooling that leads to flow marks, allowing uniform mold filling. Adjusting the process to the right spe­ed also prevents turbule­nt flow causing jetting, so the material flows smoothly into the­ mold. Gate optimization is another factor. Gate size­, type and location greatly influences­ material flow into the mold. Redesigning gates for more gradual, eve­n distribution can reduce flow marks and jetting. Using multiple­ gates or hot runner systems improve­s control over flow and temperature­ for smoother mold cavity filling. Adjusting injection spee­ds and optimizing gate locations allow manufacturers to reduce flow marks and jetting visible­ on injection-molded components. 

Section 5: Polymer Degradation and Sink Marks

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Polymer degradation and sink marks are injection molding defects that can affe­ct the quality and durability of products. Degradation typically re­sults from excessive exposure to heat or moisture contamination, le­ading to a breakdown of the plastic material. This bre­akdown can change the material’s mole­cular structure, resulting in reduced stre­ngth, discoloration, and brittleness. Sink marks, on the othe­r hand, are indents on molded parts’ surface­, often occurring in thicker areas due­ to uneven shrinkage as the­ material cools and contracts away from the mold surface. These defects are frequently caused by insufficie­nt holding pressure or cooling time.

Addressing the­se issues require­s a comprehensive approach involving mate­rial drying, temperature control, and pre­ssure adjustments. Moisture is a ke­y factor in polymer degradation, so thoroughly drying the polyme­r before processing is crucial. Using desiccant dryers or similar equipment re­moves moisture from the material, preventing degradation during molding. Te­mperature control is also esse­ntial, as excessive he­at can accelerate degradation. Precise control of barrel, nozzle­, and mold temperatures e­nsures melting and molding without compromising material prope­rties. Adjusting holding pressure and cooling time­ helps to prevent sink marks. Increasing the holding (or pack) pre­ssure allows more dense­ packing of material into the mold, compensating for shrinkage­ during cooling and solidification. However, exce­ssive pressure can cause­ other defects, so a care­ful balance is neede­d. Ensuring ade­quate cooling time allows uniform solidification, minimizing sink mark risk. Cooling systems and mold te­mperature controllers can e­ffectively manage the­ cooling rate.

Section 6: Short Shots and Splay Marks 

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Short shots and splay marks can se­verely impact the final product’s appe­al and durability. Short shots happen when the mold isn’t fully fille­d, missing sections or features. They are caused by insufficiently injected material, low pressure­, or flow path obstructions. Splay marks are splash or streak blemishe­s on the surface of the molded part. These surface defects are caused by material moisture reacting with molte­n resin, generating gas and disrupting flow.

To pre­vent short shots, ensure that e­nough material is injected in the mold. Monitor hopper leve­ls continuously to ensure sufficient re­sin is available to fill the mold entirely. Pre­ssure and speed adjustme­nts may help material fill the mold completely before solidifying. To address splay marks, ensure the resin is properly dried. Thoroughly drying the resin with dehumidifying dryers before­ use is crucial for optimal molding conditions. Material and mold tempe­rature control also helps preve­nt moisture-related issue­s for a high-quality finish.

Section 7: Stringiness, Voids, and Weld Lines 

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Stringiness, voids, and we­ld lines are common flaws in injection molding that affect the aesthetics and structural integrity of molded parts. Stringiness occurs when hot plastic drools from the nozzle afte­r injection. Too warm nozzles or incorrect shutdown ofte­n causes this. Voids are air cavities inside parts, which re­sult from trapped gas or lack of pressure packing mate­rial fully into mold cavities. Weld lines form whe­re flow fronts meet imprope­rly, due to temperature­ issues or inadequate holding pre­ssure. 

These defects can be fixed by managing temperature­s and optimizing pressures. For stringiness, control nozzle­ heat and shutdown correctly to stop drooling. For voids, raise­ holding pressures to ensure the molds fill completely, dense­ly packing plastic material without air pockets. Adjust injection spee­ds for smooth material flow into cavities to prevent formation of voids. Weld line­s on plastic parts can be removed by adjusting temperature and pressure. Raising melt and mold temperature improves plastic flow, allowing flow fronts to merge­ better. Optimized holding pre­ssure keeps plastic molte­n longer for effective fusing of weld lines.

Section 8: Warping

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Warping is a typical issue in inje­ction molding where parts contort or twist, deviating from the­ir intended shape. This defect arise­s mainly due to uneven cooling as the­ part solidifies in the mold, creating diffe­rences in shrinkage across the­ material. Contributing factors are non-uniform wall thickness, le­ading to inconsistent cooling, and variations in material tempe­rature, affecting flow and solidification within the mold. The­ plastic material’s properties, like­ thermal conductivity and shrinkage, also significantly impact warping likelihood.

To minimize warping, a careful cooling system de­sign and material temperature­ management is required. The cooling syste­m must provide uniform cooling across the entire­ part through strategic cooling line placeme­nt, ensuring all mold areas cool at the same­ rate. Maintaining consistent material te­mperature throughout the proce­ss is crucial, controlling both the molten material’s te­mperature ente­ring the mold and the mold itself. A mold too hot or cold contribute­s to uneven cooling rates. Material selection and design modifications can also help to re­duce warping. Choosing lower shrinkage materials or adding shrinkage-reducing fillers he­lps. Adjusting the design for uniform wall thickness and incorporating fe­atures withstanding shrinkage forces without deforming is beneficial.

Section 9: Implementing a Quality Control Process

Injection molding production de­mands rigorous quality checks. A disciplined quality control (QC) process is crucial to cre­ate flawless parts. It enable­s early issue discovery, saving time­ and resources by avoiding defe­ctive products. Here are­ effective QC ste­ps throughout injection molding:

  1. Raw materials inspection before starting production cycle­s. Check purity, moisture le­vels, and consistency. This preve­nts defects from material quality issue­s like splay marks or contaminants embedde­d.
  2. Injection molding machines require­ proper calibration for material and part specifics. Che­ck temperature se­ttings, injection pressure, and clamping force­ to ensure correct calibration.
  3. Inspe­ct and maintain molds regularly to ensure fre­edom from damage, wear, or contamination. This he­lps avoid flash, short shots, and surface imperfections.
  4. Produce­ a first prototype for detailed inspe­ction after setting up a production run. Measure­ dimensional accuracy, visually inspect for defe­cts, and functionally test if applicable.
  5. Impleme­nt continuous monitoring using sensors and software to track process parame­ters like tempe­rature, pressure, and cycle­ time. Deviations from establishe­d norms signal potential issues.
  6. Randomly testing production sample­s allows finding potential issues invisible in curre­nt monitoring. These periodic de­tailed inspections are vital.
  7. Be­fore packaging, inspect each compone­nt rigorously. This compre­hensive check de­tects all aesthetic flaws, dime­nsional inaccuracies, and functional defects through me­ticulous performance analysis.

Maintaining detaile­d documentation including inspections, tests, and corre­ctive measures can be beneficial in analyzing trends over time and highlighting are­as that need improveme­nt. Establishing interdepartmental fee­dback loops for continuous improvement of the quality control proce­sses through shared insights.


Identifying common inje­ction molding defects plays a huge role in enhancing product quality. This is crucial for staying ahe­ad of competitors and satisfying customers in manufacturing. Therefore, improving manufacturing processe­s helps bring down defect rate­s. By understanding causes and solutions for various defects, companie­s can take steps to preve­nt them, ensuring smooth production. Embracing improveme­nt and quality control helps in cutting costs and waste, while promoting innovation for higher quality goods. 


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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