Using the fishbone diagram method to find the cause of the defect
Using the fishbone diagram method (also known as a cause-and-effect diagram or Ishikawa diagram) to identify the causes of injection molding defects is an intuitive and systematic quality analysis method. By categorizing various factors affecting product quality, forming a branching structure similar to a fishbone, the relationship between defects and causes is clearly demonstrated. This method emphasizes multi-dimensional analysis, breaking down complex problems into specific, traceable factors. This facilitates collaborative root cause investigation and avoids blind trial and error. In injection molding production, common product defects such as sink marks, flash, and weld marks often have causes that involve multiple factors, including raw materials, molds, equipment, processes, and personnel. The fishbone diagram method can systematize these dispersed factors, providing a basis for developing targeted improvement measures.
Determining the analysis object and drawing the main bones are the first steps in the fishbone diagram method. It’s necessary to identify the specific defect type and focus on the core issue. First, the defect must be precisely defined. For example, for “sink marks on the plastic part surface,” the location, size, frequency of occurrence, and other characteristics must be described to avoid deviations in the analysis direction due to vague definitions. The main bones are usually represented by arrows pointing to the defect name, symbolizing the core of the problem. When conducting a team analysis, it is necessary to ensure that participants include operators, machine adjusters, mold engineers, quality inspectors, and others, providing information from different perspectives to ensure a comprehensive analysis. For example, for a weld mark defect on a certain ABS plastic part, the main bone can be defined as “obvious weld marks on the ABS plastic part.” Branch analysis can then be carried out around this main bone, laying the foundation for subsequent factor classification.
Classifying the major factors is a key step in the fishbone diagram method. Influencing factors must be categorized by nature. Common categories include people, machine, material, method, and environment, covering the entire injection molding process. “People” factors involve the operator’s skill level, sense of responsibility, and training, such as whether work instructions are strictly followed and whether process parameters are adjusted properly. “Machine” factors include the performance of the injection molding machine, mold, and other equipment, such as the machine’s pressure stability and mold venting. “Material” factors focus on raw material characteristics, such as the resin’s melt flow rate, moisture content, and additive content. “Method” factors cover process parameter settings and operating methods, such as injection speed, temperature profile, and holding time. “Environment” factors involve the production environment, such as temperature, humidity, and cleanliness. This categorization helps to organize scattered causes into clear categories, making the analysis more organized.
Expanding from the major symptom to the intermediate and minor symptom is a process of delving deeper into specific causes, requiring progressively more detailed analysis until a verifiable root cause is identified. For example, under the major symptom of “material,” the intermediate symptom could include “poor raw material flowability” and “high raw material impurities.” Within this intermediate symptom, the minor symptom could be further refined to include “low melt flow rate,” “insufficient raw material drying,” and “excessive recycled material.” Within the major symptom of “method,” the intermediate symptom could include “inappropriate injection speed” and “low mold temperature.” The minor symptom could be further refined to include “slow injection speed at the weld” and “mold temperature at least 5°C below process requirements.” This process requires a combination of production data and empirical judgment. For example, by comparing weld marks from different batches of raw materials, one can determine whether “raw material batch variation” is a contributing factor. By adjusting the mold temperature and observing changes in the weld mark, one can verify the validity of the “low mold temperature” symptom.
The ultimate goal of the fishbone diagram method is to verify the root cause and develop corrective measures. Experimentation or statistical analysis is required to identify the key cause, avoiding prioritizing minor factors for improvement. Potential causes listed in the fishbone diagram can be verified one by one using a process of elimination. For example, for weld marks caused by “poor mold venting,” adding venting slots at the weld can be used to conduct trial molds to observe whether the defect improves. For “improper process parameters,” orthogonal testing can be used to adjust parameters such as injection speed and temperature to find the optimal combination. During the verification process, detailed data, such as defect incidence and part dimensional changes, should be recorded to quantitatively demonstrate the authenticity of the cause. Once the root cause is identified, actionable corrective measures should be developed, such as “testing the melt flow rate after each batch of raw materials arrives” and “cleaning the mold venting slots weekly,” with clearly defined individuals and deadlines for completion. Finally, by continuously tracking the improvement results, a closed-loop management system is established to ensure effective defect control.
The application of the fishbone diagram method should avoid common pitfalls, such as an incomplete listing of factors, failure to refine causes to a verifiable level, and neglecting teamwork. During the analysis process, all participants should be encouraged to speak freely, avoiding premature intervention based on subjective judgment. Furthermore, the analysis should be grounded in practical on-site conditions, avoiding theoretical explanations that are divorced from production conditions. For example, instead of simply stating “outdated equipment,” the cause should be more specific, such as “wear on the equipment’s check ring leading to a drop in holding pressure.” Furthermore, the fishbone diagram should be dynamically updated, with analysis content promptly supplemented as production conditions change and new issues arise, ensuring continuous and effective quality improvement efforts. By correctly applying the fishbone diagram method, companies can establish a systematic defect analysis mechanism, improve problem-solving efficiency, reduce quality costs, and ultimately achieve stable and controllable injection molding production.
