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Troubleshooting Guide for Plastic Injection Molding

Troubleshooting guide for plastic injection molding, illustrating common defects like burn marks, weld lines, flash, jetting, sink marks, short shots, warping, splay, and flow lines with respective diagrams

Plastic injection molding is very important in the production of many products. The major defects are sink marks, short shots, flash, weld lines, and warping, depending on factors like uneven cooling, non-proper mold fill, and shrinkage of the material. Solutions include uniform part design, optimized injection parameters, and accurate material selection. An appropriate mold design, planned maintenance, and process stabilization are the keys to qualitative outcomes.

What will you learn?

This guide provides information on defect prevention, material preparation, and machine settings adjustment, and therefore, it is an essential tool to improve production efficiency and product quality.

Understanding Plastic Injection Molding Defects:

  • Sink Marks:
Red plastic part with a highlighted area showing sink marks, visible as depressions on the surface due to improper cooling during molding

Sink mark is the term used to refer to a small depression or hollow on the surface of plastic parts. They are produced when the plastic cools unevenly, thus making some parts shrink fast while others slowly. The thick sections of plastic take longer to cool, and sink marks are more common in that place. The materials containing overfillers also tend to result in this marking. Rapid cooling prevents them, and so ensures a more even result.


  • The wrong setting of the mold temperature can lead to sink marks.
  • Low injection pressure leaves molds undistributed.
  • Too hot plastic makes these marks more often.
  • An area with thick walls cools differently and causes dents.


  • Maintain uniformity of part thickness for better cooling to prevent sinking marks.
  • Modify the design of the part to eliminate thick sections and prevent uneven shrinkage.
  • Select plastics with fewer fillers or special cooling additives.
  • Enhance cooling in the mold by introducing additional cooling passages to improve heat control.
  • Employ post-molding methods such as annealing to relieve stress and avoid sags.
  • Short Shots:

Short shots can be caused by an incomplete mold filling by plastic. It’s a puzzle with missing parts. The part is left unfinished and does not suit the design. To prevent this, ensure that the mold gets filled. Adequate filling leads to the whole part. This ensures that each piece is as intended, without any gaps.


  • A defective mold design with weak venting halts the plastic flow.
  • Low injection pressure leads to the inability of the plastic to fill the mold.
  • Short injection time does not allow plastic to fill up all the corners.
  • Insufficient cooling time does not allow the plastic to be set properly.
  • The plastic is too thick and hard, and some mold parts are missing.


  • Increase injection pressure or rate for complete mold filling.
  • Increase the mold temperature for better plastic flow.
  • Modify the gating system for even mold filling.
  • Material and process knowledge enhances the success of filling.
  • Adjust parameters to emulate plastic response.
  • Flash:

The flashing occurs when additional plastic escapes from the edges of the mold. That overwhelmingly cools and solidifies, creating unwanted thin sharp corners or burrs around the part. It usually occurs on the parting line that connects the mold halves. It can also develop close to ejector pins and make the part surface not smooth. The flash should be removed to produce a neat and finished look.


  • Plastic leakage occurs in the gaps of mold, which results in flash.
  • The pressure in excess pushes the plastic out, forming waste bits.
  • High injection velocities also cause plastic flash.
  • Plastic shrinkage from mold surfaces contributes to flash.
  • Worn mold parts do not fit tight, thus making plastic ooze out.


  • Enhance mold construction for a good seal, preventing plastic leaks.
  • Decrease pressure and velocity to regulate material flow.
  • Use plastics that have less shrinkage to prevent extra edges.
  • Systematic checks identify and eliminate flash problems at the very stage.
  • Employ robots to have precise trim and smooth finishes.
  • Weld Lines:
Close-up of weld lines on a gray plastic vent, highlighting the slight imperfections where molten plastic has met and solidified

Weld lines are similar to sew marks on plastic surfaces. They form when streams of melted plastic do not meet precisely. Frequently, these lines are created around mold obstructions such as pins. When plastic flows together but does not fuse, these defects are formed. They may weaken the product in these areas. Detection and elimination of weld lines are vital for product quality. They have a two-fold impact on both appearance and structural strength.


  • Parts that have a poor thickness difference cause weld lines.
  • Fillers in plastic interrupt a smooth flow; hence, lines are created.
  • The incorrect temperature of molten material can create this kind of defect.
  • Inserts or holes in parts add complexity to the plastic flow paths.
  • Numerous gates in molds can lead to weld lines.


  • Revise section design for improved plastic flow.
  • Regulate temperature and speed to keep plastic melted longer.
  • Optimize gate placement for uniform flow.
  • Choose materials that will not make weld lines.
  • Comprehend molding and materials science for proactive measures.
  • Warping:

Warping is characterized as the plastic part changing the form significantly. It occurs when a part is de-molded. The part does not maintain its designed form or size. However, it deforms, rendering it unsuitable for its purpose. It makes the part look strange and out of place.


  • Warping occurs due to uneven cooling where the parts distort.
  • Parts come out in sections of different thicknesses and thus cool at different rates, hence warping.
  • Various plastics shrink in different ways, which influences the form.
  • Other causes of warping are the mold design and injection settings.
  • Semicrystalline materials warp more, but so do amorphous plastics.


  • Guarantee accurate control of the molding process.
  • Rib the part to avoid warping.
  • Set mold temperatures to equalize cooling rates.
  • Change water temperatures flowing through molds for uniform cooling.
  • Adjust the temperature of each mold half as required.

The Role of Material Selection and Preparation:

The selection and proper preparation of the appropriate plastic are extremely important for molding success. It guarantees the right operation, and the final parts fulfill the requirements.

Material Properties:

water with low viscosity and weak intermolecular bonds, olive oil with medium viscosity and medium strength intermolecular bonds, and honey with high viscosity and strong intermolecular bonds

Viscosity is a parameter of plastic that depends on the thickness or fluidity, which influences the mold filling. The melting point of plastic is also critical; it must be the same as the mold design. Humidity is also involved since water affects plastic behavior. These characteristics ensure the mold is filled to the right degree and the end product is of the required quality. Appropriate material selection is paramount in achieving effective results in injection molding.

Material Preparation:

Proper preparation of materials is the most important factor for defect-free molding. Drying plastics before application eliminates water, which leads to defects. Thorough mixing of materials guarantees that every component is of the same quality. High material quality reduces the risk of problems such as warping or weak areas. Adherence to drying and mixing specifications allows plastics to behave consistently in the mold. Such careful preparation results in fewer defects and, consequently, a higher quality of the final products.

Correct material choice and processing minimize defects, improving product quality. This strategy forms the basis of superior plastic injection molding.

Machine Settings and Process Parameters:

Setting and processing parameter adjustment is a skill of perfection in injection molding. It guarantees the manufacturing of perfect, high-quality plastic parts.

Injection Speed and Pressure:

Precisely setting the speed and pressure of the injection is of the essence for the perfect molding. Proper settings guarantee the complete filling of molds and minimize the typical defects. High speed or pressure results in parts defects. If the mold is too low, filling might be difficult. Exact settings guarantee quality parts. Having control over these settings improves the production of non-defective plastic parts. This equilibrium is very important for the best molding results.

Temperature Control:

The right temperature, 120 to 180°C, is extremely important in successful injection molding. Each type of plastic requires a particular temperature to melt and solidify properly. Proper temperature control eliminates defects, making the material flow and cool the way it’s supposed to. This precision is a critical factor in the production of high-quality parts. Materials determine the best temperature settings; thus, knowledge is essential for defect-free manufacturing.

Process Optimization Techniques:

The application of scientific principles significantly improves the molding processes. Fine-tuning settings according to accurate data maximizes the yield. This approach determines the optimum conditions for each material. Hence, it eliminates defects by a great proportion. Quality parts are consistently produced. This method is essential for high productivity and quality in injection molding.

Mold Design and Maintenance Strategies:

Proper mold design and servicing are important in obtaining the best injection molding solutions. They make sure the molds work well and produce high-quality plastic parts.

Mold Design Considerations:

Proper mold design highly improves the production of the part. Angled surfaces make the part that is removed easily lift out. Uniform wall thickness ensures the absence of defects, such as shrinkage. Well-thought curves and corners help maintain uniformity. Optimum rib dimension reduces shrinkage and speeds up mold filling. The innovative designs prevent weld lines and cracks. Vented and valve-gated molds minimize scrap and enhance quality and efficiency.

Regular Maintenance:

Regular maintenance of molds keeps the last and ensures quality parts. Cleaning and checking molds eliminate defects due to the accumulation of material or wear. Lubrication keeps them in the proper working order, preventing damage. Early maintenance will keep mold performance, which will help avoid expensive problems. Comprehensive maintenance history and trained crews provide productive preventive care.

Mold Modifications:

At times, adjustments are needed for molds to fix some issues. This could involve changing shapes or placing extra components within the mold. This can enhance mold fill performance or part cool rate. It is an attentive procedure that needs to know the areas which have to be improved. These adjustments serve to produce parts of the high quality on a consistent basis.

Implementing Design of Experiments (DOE) for Troubleshooting:

Introducing DOE as a troubleshooting tool simplifies the process of problem-solving in production. It improves productivity, assures quality, and qualifies compliance that sustains innovation and customer satisfaction. Below are some steps of DOE for troubleshooting.

Setting up 5-stage DOE:

  • PLAN: At the planning stage, we determine the goal of our experiment and the key factors. We choose exact settings for each factor to see how they determine the outcome. The response that we intend to measure is selected. This crucial step set the stage for the experiment, guiding our attention to certain details.
  • SELECT OA: Select an orthogonal array (OA) and specify replications for each of the experiments. This option guarantees testing in all conditions, making data collection complete. The correct OA is crucial in an efficient kind of study that causes precise and trustworthy results. It helps us to achieve critical, operational, and actionable insights in the quickest manner.
  • CONDUCT: Before starting, it is essential to write a comprehensive test plan. The appointment of a team member to observe trials guarantees the consistency of this strategy. The monitoring of any unexpected changes ensures the integrity of the experiment. The recording of deviations is crucial for a clear interpretation of results, preserving the focus and direction of the experiment. This systematic process ensures reliable and valid findings.
  • ANALYZE: In this phase, we assess the results and analyze the data. We apply graphs, charts, and software analysis to our results and expose the statistical significance of our data. This conversion of raw data into intelligence guides us in making informed decisions. It is an essential step that ensures we have correct and valid findings that will lead to what we should do next.
  • CONFIRM: Finally, we forecast the optimal conditions for each parameter and verify them. This validates if our results work in practice. Checking our optimized setup guarantees constant part quality. It is essential to demonstrate that our findings bring the process to the expected level.

Benefits of DOE in troubleshooting:

Troubleshooting DOE has many advantages. It enhances quality, lowers costs, and shortens time to market. It also guarantees customer delight and regulatory adherence.

  • Optimization of Process Parameters: DOE carefully studies the variables such as temperature, pressure, and material flow in molding. It determines the optimal settings by considering the interactions of these factors. This method considerably enhances the quality of the product and manufacturing efficiency. Fine-tuned modifications of the parameters lead to excellent results.
  • Cost Reduction: It simplifies the injection molding process, resulting in a substantial amount of waste reduction. Improved productivity leads to again of quality products. As a result, material costs are reduced, and production becomes more cost-efficient. This way eliminates operational costs, indirectly making the manufacturing process cheaper.
  • Quality Improvement: It reveals how differences influence the output product. Manufacturers can further refine the processes for improved accuracy and aesthetics. Consequently, products acquire better resistance and structural integrity—the careful methodology results in more constant high-quality outcomes.
  • Faster Time-to-Market: It greatly reduces the process development and troubleshooting time. Rapid development drives products rapidly from the drawing board to the market. This fast advancement brings a significant competitive edge. It allows companies to be a front-runner in innovation by swiftly changing new ideas into consumer-ready products. A shorter lead time increases the company’s positioning in the market and profits.
  • Scalability and Reproducibility: It makes the processes strong, scalable, and repeatable. This has significant implications for mass production. It maintains the quality of all batches. Consistency is the key to mass production.
  • Risk Management: DOE focuses on the critical factors that impact quality, thereby acting as an effective approach toward risk management in manufacturing. Detection of these factors leads to the proactive resolution of issues, and, therefore, defects are reduced. Such an approach eliminates production risks and provides uniform, quality output. Early involvement simplifies the processes, increasing product reliability.
  • Innovation and Experimentation: It encourages a systematic approach to testing new ideas, thus leading to innovation. It allows us to experiment with various settings to find better ways to mold. This results in better products and processes. Studying different parameters reveals approaches that are more effective.
  • Customer Satisfaction: DOE’s continuous quality and uniformity of the products lead to customer satisfaction. Reliability and compliance with specifications guarantee the trust of the customer. Loyal customers are key to the success of an organization, which results in feedback and repeat business. This link is very essential in staying ahead in the market.
  • Regulatory Compliance: It guarantees that medical and automotive industry parts follow very stringent rules. It gives an understandable way to demonstrate processes and the quality of products. This ensures that products comply with the required standards. Documentation of every stage aids in achieving these standards.


In conclusion, achieving perfection in the plastic injection molding process demands extreme knowledge of the entire process, defects recognition and their elimination, and material selection to machine settings. This manual has provided some important approaches that will help in improving product quality, cutting costs, and achieving an effective production process. Adopting these practices will substantially benefit manufacturers, engineers, and quality control personnel in the plastic injection molding industry.
HiTop Industrial, equipped with state-of-the-art technology and mastery in mold design and maintenance, is an integral player. Utilizing HiTop’s complete solutions, businesses will benefit from the high-quality molding combining innovation, effectiveness, and customer satisfaction. Contact us today, and let’s begin your work.

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