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Injection Molding Cycle And Reduction Techniques Of The Cycle Time

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If you’ve read my previous article on injection molding, you likely have a basic understanding of the injection molding process. However, the injection molding cycle is a critical aspect of the entire injection molding process, and it warrants further exploration.

Next, we’ll talk about the advanced aspects of the injection molding cycle. For those in the mold industry, mastering this knowledge is essential. Product designers need to understand the basic steps of the injection molding cycle.

Preparations Before The Injection Molding Cycle

1. Material Inspection

By examining the packaging, shape, size, and color of the injection molding materials, you can ensure they match the type and model required for the order, preventing the use of incorrect materials. Check for any packaging damage or signs of contamination, especially with transparent materials.

2. Coloring and Mixing of Plastic Materials

Plastic materials usually come in natural, white, off-white, light yellow, or transparent colors. To meet product color requirements, it’s necessary to add color masterbatch or powder before use. Typically, the product’s color is adjusted during the early trial production phase, with the ratio of color powder to masterbatch set and color limit samples created. During mass production, follow the material requirement sheet and work instructions closely for mixing.

Key Point for Mixing

Before mixing, clean the inside walls of the mixer. Mixers used for colored materials should be cleaned with mold cleaner or kerosene. Use the original packaging bags for materials when possible, or ensure substitute bags are clean and dust-free.

3. Material Drying

Excess moisture in materials can cause issues like splay marks, bubbles, and sink marks on the product surface, potentially leading to degradation and impacting appearance and quality. Thus, drying the plastic materials before molding is necessary.

Different types of plastic materials have varying levels of moisture absorption, so they are categorized as either moisture-absorbent (e.g., ABS, PA, PC, PMMA) or non-moisture-absorbent (e.g., PE, PP, PS, PVC, POM). Drying effectiveness depends on three factors: drying temperature, time, and material thickness. Materials can reabsorb moisture after drying, so if not used for a long time, they should be dried again under the same conditions.

4. Equipment Cleaning

Different molds, products, or orders may use different types of plastic materials or colors. These various materials and colors cannot fully mix when plasticized through the injection molding machine’s barrel, leading to potential quality issues, like easy breakage, lack of elasticity, significant color deviations, or the presence of black spots. Mold production might also become unstable, with some processes becoming unfeasible (e.g., nozzle clogging).

Therefore, when switching molds, it’s crucial to thoroughly clean the machine of any residual materials from previous molds or products.

5. Mold Preparation

Mold Cleaning

Before injection molding, clean the mold’s surface, cavities, gaps around inserts, nozzles, and runners of any rust preventive oil to prevent it from sticking to the product or clogging the mold’s vents, affecting molding stability.

For mirror-finished products, electroformed shells, or molds with strict post-processing appearance requirements, avoid using cotton swabs, rags, or old gloves for cleaning to prevent scratches on the mold surface. Generally, rinse with mold cleaner and blow dry with an air gun.

During the cleaning process, take care to avoid touching the mold surface with the air gun or other objects. When disassembling the mold for cleaning, place the disassembled inserts and mold cases in specialized plastic boxes and, if necessary, wrap them in foam or soft cloth for storage. Non-professionals should not perform mold disassembly and cleaning. It’s best to clean the mold before mounting it on the machine for easier cleaning, quality assurance, and time-saving during mold changeovers.

Water Connection

For the appearance and productivity of the product, connect the mold to water, mold temperature controllers, and chillers to maintain an ideal, relatively stable temperature with minimal external influence. After connecting the water and starting the mold temperature controller, it generally takes 15 to 30 minutes to reach the desired temperature.

Connecting the Hot Runner Power Supply:

For molds using hot runners, connect the hot runner power supply after mounting the mold and preheat for 15 to 30 minutes. Begin trial injection molding only after the display value on the hot runner control box reaches the set value.

Steps of Injection Molding Cycle

1. Clamping

Clamping in injection molding refers to securing and holding the mold closed before the injection and cooling stages. It involves applying a specific amount of force to the mold to counteract the high pressure created by the molten plastic material during the injection.

We call this specific amount of force a clamping force.

I’m sure many people have heard of injection molding machines with tonnages like 1000 tons or 500 tons. However, it’s important to note that these tonnages refer to the maximum clamping force exerted by the machine, not its weight. In the future, I will share a detailed introduction to clamping force and its significance in the injection molding process.

In a general sense, the clamping unit plays a vital role in securely fastening the core and cavity of a mold together. This unit comprises various essential components, such as the fixed platen, movable platen, tie bars, toggle mechanism (in toggle clamping systems), or hydraulic cylinders (in hydraulic clamping systems). Each element collaborates harmoniously to deliver the required force for firmly holding and sealing the mold.

2. Injection

As mentioned earlier, before the injection molding cycle, the plastic material undergoes a heating process known as plasticization, transforming it into a molten state. During the injection step, the injection molding machine’s screw or plunger moves forward, propelling the molten plastic material through the nozzle and into the mold cavity.

By orchestrating this regulated motion, a precise and uniform flow of the molten plastic into the mold is ensured. By exerting pressure, the screw or plunger pushes the molten plastic through the nozzle and into the mold cavity, filling it with the desired shape and pattern.

This crucial step requires careful injection speed, pressure, and timing control to achieve optimal part quality. The molten plastic material takes on the form and dimensions of the mold cavity as it flows into it. Maintaining a proper balance between the injection speed, pressure, and cooling time is important to achieve high-quality, defect-free molded parts.

The injection stage is often referred to as the filling process as well.

3. Dwelling or cooling

During the dwelling or cooling step, the mold is kept closed to maintain the pressure on the solidifying plastic. This helps prevent any shrinkage or warping if the part is prematurely removed from the mold. The duration of the cooling time can fluctuate depending on several factors, including the material employed, part geometry, and additional considerations.

Proper cooling is crucial for achieving high-quality molded parts. It allows the plastic to fully solidify, ensuring uniformity and strength throughout the part. The cooling process can be enhanced through various methods, such as using cooling channels or applying external cooling techniques like air or water cooling.

4. Mold Opening

In this step, the clamping unit of the injection molding machine releases the clamping force to separate the core and cavity.

The mold opening process starts with the movement of the movable platen, which is connected to the moving side of the mold. The clamping mechanism, such as hydraulic cylinders or toggle mechanisms, is actuated to retract the movable platen and separate it from the fixed platen. This action creates a gap between the mold halves, allowing for the removal of the molded part.

5. Ejection

Ejection refers to the process of removing the molded part from the mold cavity once it has been released during the mold opening phase. Here is a special mention: With automation’s popularity, most Chinese injection molding companies are using automatic robots to get the molded parts.

The ejection process typically involves using an ejector system consisting of ejector pins or plates strategically placed within the mold. The ejector pins or plates actuate to forcefully push the parts out of the mold cavity. The design of the mold includes features such as ejector pin holes or ejector plates to facilitate the ejection process.

Once the mold has been opened, the ejector system is engaged, and the ejector pins or plates extend into the mold cavities. The pins or plates make contact with the molded part, applying sufficient force to push it out of the mold cavity. The ejected part is then guided and collected for further processing or inspection.

Ejection must be carefully controlled to ensure that the part is safely and effectively ejected without causing any damage or deformation. The ejection force and speed are determined based on the specific characteristics of the part, such as its size, shape, and material properties. Proper ejection can help prevent part sticking, distortion, or other defects.

In some cases, additional auxiliary ejection mechanisms may be incorporated into the mold design, such as air blasts, stripper plates, or robotic systems, to aid the ejection process, especially for complex or delicate parts.

6. Mold Closing

During this phase, the clamping unit of the injection molding machine exerts the required force to bring the core and cavity of the mold back together again. This step ensures the proper alignment and closure of the mold, setting the stage for the next cycle of the injection molding process.

Factors Affecting Injection Molding Cycle Time

In the injection molding industry, for any project, injection molding factories must quote the machining parts before manufacturing the mold or upon receiving an existing mold from a client. This requires the factory to accurately estimate the product’s cycle time for the project.

Even with flow analysis, cycle times remain estimates. While flow analysis can predict fill and cooling times with considerable accuracy, it cannot foresee the conditions of the mold, machine, and often the operator’s performance. Nor can it predict the process conditions on any given machine, especially older ones. Hence, estimating cycle time accurately is crucial when quoting for injection molding parts.

For projects with low annual production, underestimating cycle time won’t cause significant losses. However, for projects with very high annual production, the accuracy of cycle time estimates becomes crucial. In the injection molding industry, a profitable factory’s mantra might well be “cycle, time, cycle, time, cycle, time.”

Factors Affecting Cycle Time

The total cycle time of a mold is the combined time of the following 11 steps. When attempting to reduce cycle time, it’s necessary to optimize these steps individually and consider their interplay.

  1. Mold closing and locking time.
  2. Filling time.
  3. Packing and holding time.
  4. Screw delay time.
  5. Screw plasticizing time.
  6. Screw retract time after rotation.
  7. Cooling time.
  8. Cooling delay or idle time before mold opening.
  9. Mold opening time.
  10. Part ejection (and robot capture) time.
  11. Ejection mechanism (and robot) return time.

Reduction Techniques Of Injection Molding Cycle Time

For injection molding factories, shortening the injection molding cycle is an indirect way to boost profits. For those commissioning product production, reducing the injection cycle can speed up the efficiency of getting products to market. However, this involves ensuring product quality. Thus, finding a balance point where the injection cycle is shortened without compromising product quality is the direction every injection molding practitioner should strive towards.

Mold Design Optimization

A well-designed mold can contribute to shorter cycle times. This includes considerations like proper gate placement, optimized runner and cooling channel design, and efficient part ejection mechanisms. Mold flow analysis software can be utilized to identify potential design improvements and optimize mold geometry.

Clamping and Opening Time Optimization

The injection molding cycle begins and ends with clamping. Clamping usually occurs in four stages: fast clamping, slow clamping, low-pressure protection, and high-pressure locking. Opening typically involves three phases: slow-fast-slow opening. Optimizing the speed and position of clamping and opening can reduce their time. New injection molding machines feature regenerative clamping hydraulics (differential clamping) for higher speeds.

Injection Time Optimization

Injection starts after high-pressure locking and can employ multi-stage injection. High injection speeds are usable during the injection phase as long as they don’t cause defects like bubbles or burning.

Holding Time Optimization

Holding starts after injection, usually at a lower pressure than injection pressure. Its primary function is to compensate for shrinkage, filling in any depressions as the molten material cools and contracts, ensuring the finished product is full (without dents) when ejected. Once the runner solidifies, holding pressure becomes ineffective and can end. Holding pressure can vary across multiple stages (generally decreasing gradually), divided by time. The overall holding time can be determined by weighing the product or ensuring it has no dents. Start with a short holding time and increase slightly with each injection until the product weight no longer increases or the level of dents is acceptable.

Cooling Time Optimization

The “cooling time” set on an injection molding machine is the period from the end of holding pressure to the start of mold opening. The goal of “cooling time” is to continue cooling and setting the product so it doesn’t deform upon ejection. Start with a longer cooling time and reduce gradually with each injection until the product just stops deforming upon ejection, then holding time need not be decreased further.

Charging Time Optimization

Charging begins at the start of the “cooling time”. If charging time is longer than “cooling time”, it indicates insufficient plasticizing capacity, affecting the production cycle. Enhancing plasticizing capacity can shorten the cycle time, achievable by: A. Barrier screws can increase plasticizing capacity. B. Large diameter screws can increase plasticizing capacity. C. Increasing screw channel depth can increase plasticizing capacity. D. Increasing screw speed can increase plasticizing capacity (not for shear-sensitive plastics like PVC, PET). E. Reducing back pressure can increase plasticizing speed. F. Using hydraulic shut-off nozzles allows plasticizing during clamping and opening. G. Using equipment that can plasticize throughout the cycle except during injection and holding.

Barrel Temperature Optimization

Using the lowest barrel temperature that still ensures smooth injection filling can shorten the “cooling time”.

Clamping Force Optimization

Use the lowest possible clamping force that doesn’t produce flash, shortening the time needed for high-pressure locking and extending the life of the mold, injection molding machine’s tie-bars, elbows, and platen.

Cooling Efficiency Optimization

Optimizing mold water channel design can improve heat exchange efficiency and product cooling uniformity, shortening cooling time. Using ice water cooling can shorten the “cooling time” if it meets product quality requirements.

Ejection Time Optimization

On small injection molding machines with low ejection force, pneumatic ejection can be faster than hydraulic ejection. Independent hydraulic, pneumatic, or electrical control can achieve simultaneous mold opening and ejection. For multiple ejections, using the injection machine’s vibration ejection means ejector pins don’t have to fully retract each time, shortening the time for multiple ejections.

FirstMold would like to reiterate to all our peers: Shortening the injection molding cycle should be optimized while ensuring the product’s quality, dimensions, appearance, functionality, and materials remain unaffected. Otherwise, any effort to optimize will be meaningless.

Key Parameters in the Injection Molding Cycle

ParameterParameterParameterParameter
Injection PressureInjection SpeedHolding PressureMold Temperature
Screw Rotation SpeedBack PressureMelt TemperatureMold Venting
Clamping ForceMaterial Moisture ContentShot SizeScrew Back Position
Cavity PressureCooling Water TemperatureInjection RatePart Ejection Mechanism
Runner SystemScrew L/D RatioSprue SizeCooling Line Design

The professional terminology in this table is quite important, with some terms even requiring the buyer’s product designers to master. We will select a few key terms for detailed introduction in the future.

Conclusion

Understanding and calculating the injection molding production cycle benefits injection molding factories in managing every aspect of customer product production in a compliant manner. I’m Lee Young, and I highly recommend this article to peers in the industry or newcomers to injection molding. If you have any questions, feel free to share them in the comments section of this article.

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