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Category: News
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Why is Repeatability Crucial in Injection Molding Processes?
If you’re part of the plastic processing industry, you’ve likely encountered the term ‘repeatability’ or at least heard it in passing. Even after searching the internet, you might still be wondering, ‘What is repeatability?’, ‘Why is repeatability important in injection molding?’, or ‘Why is there such emphasis on repeatability?’ In this article, we will try to answer these questions with thorough and insightful explanations. We will also offer practical advice on how to achieve and sustain a consistent and repeatable process in injection molding.
Repeatability: A Gauge for Injection Molding Machines’ Capability
Repeatability is a key indicator of production stability; it reflects the ability to consistently produce parts with similar features across different production periods. Typically, products from a highly repeatable injection molding process exhibit very similar dimensions, color, gloss, and surface finish, regardless of their molding time. While many of these aspects depend on raw materials, additives, auxiliary equipment, and other factors beyond the injection molding machine, the machine’s repeatability is primarily evaluated based on dimensional features and product weight.
The easiest and most common method for assessing the repeatability of injection molding is to measure the shot weight or product weight across different molding cycles. Once you have the shot or product weight data, calculating variation and standard deviation becomes straightforward. In a highly repeatable process, variation usually remains below 0.1% of the mean shot weight. Conversely, significantly higher values often indicate a lower level of repeatability. Most machine manufacturers prefer this method as it provides a non-comparative scale, allowing buyers to quickly gauge the machine’s repeatability.
There is another method that assesses the repeatability of an injection molding machine through a machine capability study. This method is more popular among molders because it provides a comparative scale, allowing them to evaluate their machine in direct comparison with their other machines or even with the same machine across different production periods. The Machine Capability Index (Cm) is the primary indicator of repeatability in this method. It is derived from observing a specific feature in products from continuous molding cycles. To calculate Cm, at least 50 samples must be examined, and the formula used is as follows:
In a highly capable machine, the quality factor often equals 6, aligning the process with Six Sigma (6σ) standards. The quality factor is also linked to the number of out-of-tolerance products in the process; the higher the quality factor, the fewer out-of-tolerance products are produced.
The desired value of Cm is equal to or greater than 1.67, indicative of a very capable process or machine. Practically, it is not feasible to improve the Cm beyond 2.00.
For instance, in a Six Sigma process, if the Machine Capability Index (Cm) reaches 2.00, then only 0.002 products out of a million would be out of tolerance. This number is remarkably low, which is why there is no practical benefit to improving the Cm to values higher than 2.00.
The diagram below shows the diameter measurements of several samples produced by a hydraulic injection molding machine across different production cycles. The nominal diameter of the product is 50 mm, with a permissible tolerance of ±0.02 mm. In this example, the Machine Capability Index is 2.38, which is exceptionally high, indicating that the machine’s repeatability is outstanding.
Strategies for Achieving Repeatable Injection Molding Processes
Utilizing a capable injection molding machine with high repeatability is essential for achieving a consistent molding process. Servo-driven hydraulic injection molding machines, equipped with closed-loop control systems, offer inherent accuracy and stability. Full-electric and hybrid injection molding machines, known for even greater precision and stability, are ideally suited for producing high-precision products that demand the utmost in process repeatability.
The Yizumi Full-Electric Injection Molding Machine: High Repeatability for Precision Molding Highly capable injection molding machines guarantee repeatability over short periods; however, in the long term, several other factors must be considered. These include the rapid wear of injection unit parts, fluctuations in ambient temperature, unstable material properties, and more.
The one-way valve in the injection unit is susceptible to abrasion and corrosion. If it becomes worn out, the machine’s repeatability can be seriously compromised. In some cases, replacing the worn check valve alone can restore repeatability. However, more often, it is necessary to replace the entire screw and barrel set.
Factory temperature and humidity can vary at different times of the day or throughout the year. Many plastic processing factories do not practice ambient temperature and humidity control due to its high cost. To compensate for fluctuations in ambient conditions, some auxiliary equipment such as mold temperature controllers, chillers, dryers, and mold area dehumidifiers can be utilized.
Sometimes, material properties vary from batch to batch, leading to unstable product quality. Consequently, adjusting the machine settings becomes inevitable with each new material batch. One remedy for this issue is to use higher-quality, more stable materials, although this could undesirably increase production costs. Another solution is to exert greater control over the material by using auxiliary equipment, such as pellet dryers and gravimetric dosers.
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YIZUMI：Implement Lean Manufacturing Method, Realize Assembly-line Production
In the injection molding machine industry, product quality depends not only on materials, configuration, most importantly depends on whether the companies have the capacity to manage and control during development as well as the batch production process.
YIZUMI introduced the lean manufacturing method in 2018 to achieve high-quality and rapid delivery. The improvement of the logistics system and the building of the assembly line ensure delicacy management through the whole processing chain, from components arrival to product delivery. At present, YIZUMI injection molding machine below 320 tons has been put into flow-line production, which is able to produce a unit of injection molding machine in 20 minutes. The cycle time is significantly shortened while the product failure rate is obviously reduced.
Better quality, faster delivery, lower cost
With multiple components, complicated procedures, large floor space, high-quality requirements, and great dependence on personnel’s skill levels, injection molding machine assembly is a systematic process, and it generates enormous challenges for enterprise operation when facing large orders.
YIZUMI has introduced the “continuous flow” of lean manufacturing method to the assembly procedure and built up clamping unit assembly line, electric box assembly line, injection unit assembly line as well as general assembly line. The entire production process has shortened the delivery time, effectively improved the product quality and output per unit.
Relying on the assembly line production, the injection molding machine capacity below 320T of YIZUMI has grown by about 60% year on year in 2020 when the hardware resources and personnel did not change a lot. The transformation of assembly-line production mode has played a huge role in improving the overall operational efficiency of YIZUMI.
Above all, YIZUMI constantly pursuits better qualified rate of products in terms of quality management. In the past few years, YIZUMI strictly controls the qualified rate of components, thus the failure rate of the finished product keeps dropping.
Standardized operation, high-level skills of the employees
- Assembly line production leads to more specialized, specific working procedures, the skills of every assembly specialist are gradually standardized.
- YIZUMI promotes staff training and examination, paying equal attention to theories and practices. Employees’ operating skills and assembly quality are both significantly increased based on grading and certification.
Traceability of the entire process
- There are thousands of parts in a unit of injection molding machine. Any mistake caused by anyone or any process will be finally reflected in the machine. A form will record relevant information on every procedure to prevent errors and trace the problems.
- Establish on-site management system, specify the responsible person to deal with the problems after they occur. If the problems are not solved, a red sign will be presented to alarm and the next procedure could not be carried out either. This could help to supervise the responsible person to work in a strict manner.
Visible management
Visible Kanban management including material management Kanban, scheduling Kanban, takt time is adopted to clarify production schedule, efficiency, and quality data, ensuring real-time tracing, precise management of schedule, and quality.
20 minutes for one unit of injection molding machine
- Set up general assembly lines of injection molding machines below 320T, suitable for mixed line production of injection molding machine ranging from 60T to 350T;
- The average takt time is 20 minutes, cycle time has been sharply reduced;
- Achieve fixed position of duties, fixed post for the materials, the on–site operating status can be improved fundamentally.
* The Data above were acquired by testing in the YIZUMI factory, only for your reference, please refer to the actual equipment.
Source: https://www.yizumi.com/en/news/t9/i2111013
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Tonnage Upgrade! FF300T Brings Spray-free Air Outlet
YIZUMI electric injection molding machine not only performs incredibly in the field of producing small-sized plastic parts with high precision and high cleanliness but also provides cost-effective solutions for medium and large plastic parts. YIZUMI FF electric injection molding machine and HE series electric-hydraulic hybrid injection molding machine with clamping forces of over 300T have been successfully applied in the market this year. For example, the FF460, UN480HE are supporting stable production of 3C components in customer’s workshop.
YIZUMI FF300 electric injection molding machine will be shown at DMP 2020 during November 24th -27th to display the injection molding solution for spray-free material. The solution combines an electric injection molding machine featuring high injection response with spray-free materials to cut down the spraying process during production. Products with high gloss, metallic effect, and various colors that meet market demand can be produced in a more environmental and cost-saving way.
Value Advantages
SmartClamp intelligent system, the precision two-stage mold opening and closing technology combined with low-pressure injection function, SIC technology and spray-free technology;
Precision：All-dimensions upgraded FF series electric injection molding machine， product repeatability reaches 0.3%；
Part: Decorative Panel of Auto Air Outlet
Number of cavities：1
Material：Spray-free ABS
Part size (L×W)：310×227 mm
Part weight: 17.7g
Wall thickness: 2mm
Cycle time: 30s
Partner: Zhaoxing KEYAN Mold, Shenzhen JIANCAI New Material
Create more possibilities for customers is our aim! Based on IPD process, YIZUMI has developed ranges of electric injection molding machines such as FE series electric injection molding machine, FF series electric injection molding machine, HE series electric-hydraulic hybrid injection molding machine and has mastered core technologies including electric injection, precise plasticizing control, precise control of mold opening and closing, and injection compression molding. Our flexible product configuration solutions can apply to different-tonnage machines, of which clamping force ranges from 60T to 460T. We provide customers with solutions in various industries. So far, YIZUMI electric injection technologies and products are widely used in sophisticated electronics, medical treatment, packaging, automobile parts and other industries.
