Plastic Injection Molding Cycle Time: Calculation and Reduction Techniques

Injection molding cycle time is a reflection of production efficiency. A comprehensive understanding of this cycle is crucial, as it not only dictates the quality of the final product but also impacts the overall production timeline.

Estimating the Injection Molding Cycle Time: Key Components and Calculations

Injection molding is a cyclical process, and understanding its various stages is essential for optimizing production efficiency. The injection molding cycle time, often synonymous with terms like “injection molding process” or “plastic molding cycle,” comprises several crucial components:

Cycle Components

Material Storage Time & Cooling:

These two processes occur simultaneously. When calculating the molding cycle, the longer of the two is considered. Typically, the cooling time includes the material storage duration.

Injection Pressure Holding Time:

This duration is determined by the polymer’s properties, the product’s shape, and quality requirements such as appearance and dimensions. It’s influenced by factors like injection pressure, injection rate, screw rotation number, back pressure, and temperature. To ensure quality, it’s vital to seek the shortest possible time for this stage.

Injection Process:

Generally divided into three phases – slow, fast, and slow. The formula for estimation is:

T = W/20~50%V + t

T: Total injection time

W: Total injection volume (product weight x number of pieces + sprue weight)

V: Maximum injection speed of the molding machine

t: Time constant required for screw start and stop

Depending on the machine’s tonnage, the time constants vary: 80T~200T takes 1~2s, 200T~500T takes 2~3s, and 500T~1000T takes 3~4s

Pressure Holding Time:

This begins once the mold cavity is filled and lasts until the pressure holding ends. The duration is typically chosen based on the product’s appearance, shrinkage, dimensions, and deformation requirements.

Cooling Time:

This refers to the time from the end of pressure holding to the mold opening. Factors influencing this duration include product shape, wall thickness, mold cooling water design, mold temperature, and melt properties. To ensure optimal product quality, it’s essential to minimize the cooling time. The mold’s cooling water design plays a pivotal role in determining the cooling duration.

Mold Opening & Closing Time:

Influenced by machine size and mold structure. Mold core structures, mold rack transmission mechanisms, and three-plate mold mechanisms can all impact this time. Typically, 80T~200T takes 4~8s, 200T~500T takes 6~10s, and 500T~1000T takes 8~15s.

Product Ejection Time:

Determined by ejection speed, ejection stroke, and ejection method (automatic, manual, or robotic). Automatic ejection is generally used for products with lower appearance requirements. Ejection times typically range from 0.5~2s. When using robotic arms, the mold can start closing once the product is out of its range, taking about 3~8s. Manual ejection usually takes 1~3s longer than robotic ejection.

By understanding these components and their interplay, manufacturers can make informed decisions, optimizing the “injection molding cycle time” and ensuring efficient production.

Optimizing the Injection Molding Cycle Time: Strategies for Efficiency

Reducing the injection molding cycle time, also known as the plastic molding cycle, is a pivotal strategy for enhancing production efficiency and cutting down production costs. It’s an aspect of injection molding that deserves significant attention. Here, we’ll explore various methods to optimize this cycle time.

Mold Opening and Closing Time Optimization

The injection molding cycle can be calculated from the start of one mold closure to the beginning of the next. Mold closure typically has four phases: rapid closure, slow closure, low-pressure protection, and high-pressure lock. Mold opening usually consists of three stages: slow-fast-slow opening. By optimizing the speed and position of mold opening and closing, we can reduce this time. Newly designed injection molding machines come equipped with regenerative mold hydraulic circuits, allowing for faster mold closure speeds.

Injection Time Optimization

Injection begins after the high-pressure lock completes and can be done in multiple stages. To avoid defects like bubbles or burns in the product, it’s advisable to use a high injection speed.

Pressure Holding Time Optimization

Pressure holding starts after injection and is generally lower than the injection pressure. Its primary purpose is to compensate for shrinkage, filling the depressions formed by the cooling melt and ensuring the product is full and free from indentations when de-molded. Once the runner solidifies, further pressure holding becomes redundant. The overall pressure holding time can be determined by weighing the product or ensuring there are no indentations. It’s best to start with a shorter pressure holding time and incrementally increase it until the product weight stabilizes or the indentation level is acceptable.

Cooling Time Optimization

The cooling time parameter on the injection molding machine is the duration from the end of pressure holding to the start of mold opening. The objective of this cooling time is to allow the product to continue cooling and solidifying, ensuring it doesn’t deform when ejected. This time is typically determined through experimentation. It’s advisable to start with a longer cooling time and gradually reduce it until the product remains undistorted upon ejection.

Material Storage Time Optimization

Material storage begins with the cooling time. If the material storage time exceeds the cooling time, it indicates insufficient plasticizing capacity of the screw, affecting the production cycle. Several methods can enhance plasticizing capacity, such as:

• Using barrier screws.
• Employing larger diameter screws.
• Increasing the groove depth of the screw.
• Raising the screw’s rotation speed (not suitable for shear-sensitive plastics like PVC or PET).
• Reducing back pressure to increase plasticizing speed.
• Using hydraulic nozzles to allow plasticizing during mold opening and closing.
• Utilizing equipment that can plasticize throughout the cycle, excluding injection and pressure holding times.

Locking Force Optimization

To prevent flash or excess material around the product, it’s essential to use the lowest possible locking force. This not only reduces the time required for high-pressure locking but also extends the lifespan of the mold, the injection molding machine’s tie bars, toggle joints, and platens.

Barrel Temperature Optimization

Ensuring smooth injection filling while using the lowest possible barrel temperature can reduce the cooling time.

Cooling Efficiency Optimization

Optimizing the mold’s water channel design can enhance heat exchange efficiency and product cooling uniformity, shortening the cooling time. Using chilled water for cooling, while ensuring product quality, can further reduce the cooling time.

Ejection Time Optimization

For smaller injection molding machines with minimal ejection force, pneumatic ejection, which is faster than hydraulic ejection, can be used. Independent hydraulic, pneumatic, or electrical controls can enable simultaneous mold opening and ejection. For multiple ejections, the machine’s vibration ejection can be used, eliminating the need for the ejector pins to fully retract every time, thus reducing the ejection time.

By implementing these strategies, manufacturers can significantly optimize the injection molding cycle time, ensuring efficient and cost-effective production.

The Injection Molding Cycle Time Control Specialist

A good injection molding company must know how to calculate and reduce injection molding cycle times. Mastery of this aspect is not just about efficiency; it’s about delivering consistent quality, reducing costs, and ensuring timely production. At Prototool, we pride ourselves on our deep understanding of the injection molding process. Our expertise in calculating and optimizing the injection molding cycle ensures that we not only meet production demands but exceed them. Prototool consistently seeks ways to improve and innovate in the injection molding industry. Our commitment showcases the vital role of knowledge, experience, and dedication in this field. Choose Prototool, and you’re choosing a partner committed to excellence in every facet of the injection molding process.