Around 32kg out of every 100kg of plastics processed globally uses injection molding. This makes injection molding an important manufacturing process. The success of injection molding lies in the efficient management of time. Failure in time management affects profit and can lead to losses. This can be a financial loss due to low production output. It can also be a loss of materials and tools due to damage. Timing is very important in injection molding. Time translates to money in manufacturing and injection molding epitomizes this. It is even more so in more complex injection molding systems. Where even a fraction of a section can result in a defective product. The injection molding process is a series of events. From the introduction of pellets to the final release of products. These events must be well-orchestrated cycle after cycle for optimal output. The goal of every processor is to keep cycle types low and output high. In this article, we look at how you can minimize cycle time in injection molding. In doing so, we review the injection molding process. We then go on to look at how each stage contributes to cycle time and how this gets minimized.
An overview of Injection molding
To optimize the cycle time, the whole process needs to get evaluated. The injection process begins with the plastic pellets in the solid form. These go into the hopper and get fed further down the barrel. The screw rotates and moves backward. In doing so it shears and melts the plastic. The melted plastic moves to the melting chamber in front of the screw. Back Pressure applied to the screw ensures melt compaction. It also controls the backward movement of the screw. Once the melt gets filled to the set shot size, this halts the backward motion of the screw. The screw then performs its final role in the cycle. This is to push the melt into the mold via the sprue, runners, and the gate. The melt fills the mold and gets held under pressure as it cools. Cooling occurs bypassing cooling fluid through cooling channels passing through the mold. Once cooled, the mold opens and the hardened part gets ejected. For simple ejection systems, ejector pins push out the molded part in one direction. In more complex product releases this can involve the use of slides and guide pins. All these events that make up the injection molding cycle are time-dependent.
Total cycle time = Melting time + injection time + Cooling time + Ejection time
The ejection happens at an instance. It is often negligible in conventional injection molding. The cooling time makes up a large proportion of the cycle time. Cycle times range between 10 and 30 seconds. Could be longer depending on the size of the product and thickness. To reduce the cycle time, the time for each of these stages should get evaluated.
8 ways to shave off a few seconds from your cycle time in injection molding
The cycle time for any given product depends on a variety of factors. For example the size and thickness of the product. The material used for the part is also important. Some plastics have a higher melting point than others and thus need more time to take in more heat. They also need more time to release this heat. The machine itself also determines the cycle time. Parameters such as the rotation speed and the length to diameter ratio affect cycle time. The following sections summarize measures you can take to minimize the cycle time.
1. Conformal cooling channels
Cooling is a major part of injection molding. This is because every event in the cycle culminates in the mold where the product forms. The rate of cooling and how it occurs matters a great deal. Inefficient cooling of a part can result in problems such as warpage and flow marks. It can also cost an unnecessary amount of time. The conventional mold system has straight channels drilled into the mold walls. These channels pass cooling fluid through the mold. The mold exchanges heat with the fluid stream and cool. These straight channels tend to be at varying distances from the mold cavity/product. This results in uneven cooling. Today the introduction of direct metal laser sintering (DMLS) provides better cooling options. Conformal cooling channels offer more efficient cooling. This is because they run through the mold while conforming to the shape of the cavity. This allows good contact between the cooling line and product. It also aids even temperature distribution preventing hot spots and warpage. In doing so, it saves cooling time. The cooling time difference can get as high as 55%. This can lead to significant time savings using conformal cooling. You can go from 90 seconds down to 40 seconds cycle. The image below compares the conventional straight channel with the conformal cooling channels.
Image of (a) Conventional cooling channel and (b) conformal cooling channel
2. Manage back pressure
The back pressure controls how fast the screw moves backward. It ensures that the screw does not move back too fast. But this should not be to the extent that it slows down the process. If the back pressure is too low, the screw moves back too fast. If the backpressure is too high the movement is too slow. Other than the time it takes the screw to reach the check valve it also affects the pressure exerted on the melt. This in turn affects product quality. Optimized backing pressure ensures good melt compaction without compromising on injection molding pace.
3. Maintain optimal viscosity
In the injection molding process, the viscosity of the melt is important. This affects how the melt passes through the barrel. It also affects how it travels through the runner and the gate to fill the mold. A lower viscosity means it is less resistant to flow. The viscosity relates to the temperature of the melt. Different plastics have their melt viscosity.
4. Use Optimal Product Design
The general rule here is to make your walls as thin as possible. Use the wall thickness that you need for product functionality only. Any thicker will only consume more material and increase the cooling time. This is why all stakeholders need to get involved in the early stage of product design. This way a lot of cost gets saved by avoiding design features that add little value but increase the cost.
5. Invest in experience
If you have an operator that has worked with a machine for a long time, foster that experience. Over time an operator gets used to a machine and knows the ins and outs of it. Although the theoretical injection molding process is well established. Every piece of equipment has its peculiarity. Considering that the injection molding machines can last for decades. In that time it can run up to a million cycles. Some operators can spend their entire career with a machine. Familiarity with equipment can improve operation and hence save cycle time. So rather than alternating operators on different machines. You might want to consider allowing the operators to master equipment.
Another way to invest in experience is to outsource. Where you don’t have the long-term experience and expertise within your company outsourcing works as well. In areas such as having the right mold design, the right tooling, and operations you can consult. Companies such as Cavitymold.com have several years of experience in injection molding. You can contact such to discuss your process, design, or troubleshooting.
6. Close monitoring of parameters
Having the right functioning sensors can save a lot of time. This helps you keep an eye on process conditions and ensure they do not deviate from optimal values. You should also have alarms and indicators in place to give early warning signs. This way problems get detected before they get a chance to occur. This prevents the need for downtime or halting between cycles. For example, maintaining the temperature keeps the melt at the right viscosity. If the temperature falls below set values this affects flow rate and mold filling. By setting a temperature range and alarm this gets prevented. Equipment downtime is one of the ways time gets consumed. When the machine is down for repair or a difficulty with a part stuck in the mold, no production is happening. This means no money is getting made during this time. Money is being spent on repair.
7. Pre-cooled Mold and cold cooling fluid
The colder the mold, the higher the temperature difference between the melt and the mold. This means the mold can take away a lot of heat from the melt upon entry. This temperature difference makes for a stronger heat transfer driving force. Likewise, the cooler the cooling fluid is, the more heat it takes away from the mold. So having your cooling water at a very low temperature can shave off some seconds from the cooling time. A well-designed mold can save you a significant amount of time in injection molding. Cavitymold.com offers professional mold making service which includes mold design and manufacturing. We’ll work with you to get the best mold design and material that best suits your product.
8. Insulation
This is in particular important for hot runner systems. Ensure that the heat from the hot runner gets kept away from the mold. Otherwise, this will add to the cooling time. It will also mean you are using up cooling water to quench the heat from the runner. Insulating the mold section from the runner helps minimize cooling time.
Conclusion
Time management is an important part of operating injection molding machines. Efficient time management makes for a successful injection molding process. The total cycle time comprises four process times. These are, the melting time, the injection time, the cooling time, and the ejection time. By proper monitoring of the operation conditions, cycle time gets minimized. The cycle time relates to other factors like temperature and pressure. So in optimizing the cycle time you also are optimizing product quality.
Contact www.cavitymold.com to discuss your injection molding process and products. We’ll help you get the best mold design that saves you time and money without compromising quality.
