By Brad Harvey, Lab Technician at RJG
Instrumenting an injection molding machine with sensors can be a daunting task. It is a huge investment, after all, so how do you ensure that it will pay off in the end? The key is knowing the right questions to ask before you even begin. Here are the 5 most important questions to ask in chronological order.
1. What do you want the sensor to do for you?
This is the most important question to ask yourself before installing sensors in your mold. But most people stall right here because they’re not sure what functions a sensor can actually perform. A lot of people just want to instrument a tool because they’ve heard they should run DECOUPLED MOLDING® processes or they want to try out process control software. Others are curious about the ability to collect data, track a part to a molding cycle, resolve ongoing challenges, or create a template to maintain or transfer processes. Whatever the answer, this single question needs one before proceeding so you can make decisions to work toward your goal moving forward.
Once that question has been answered, we can begin to explore the many different functions sensors can serve if used correctly. Some examples are: monitoring pressure at a specific area in the cavity, transferring the injection portion of the cycle to hold via pressure or temperature, detecting mold defection, sequencing valves gates, sorting, and the list goes on. Once you know what you want the sensor to achieve, you have some important decisions to make.
2. Where Do You Want to Install the Sensor?
Now to install it. Conventional installation techniques to consider include post gate, mid-cavity, and end of fill, but there are more less common locations in addition to these. You may need one or a combination of these techniques depending on your particular project. In some cases, it’s not necessary to sensor every cavity.
It’s also important to note how hot the steel is where the sensors will be mounted. The electronics blank out after 140 degrees Fahrenheit, so that’s something to keep in mind.
The Benefits of Each Sensor Location
Post gate sensors allow you to know the moment plastic enters the cavity and at what pressure. With this information, you can perform various studies, including pressure loss (from nozzle to gate), pack rate, and gate seal (pressure lost when the hold pressure is released). In addition, post gate sensors are often the location of choice when transferring off cavity pressure, but other locations can work as well depending on your machine and percentage of barrel usage.
Mid-cavity sensors can also aid in performing calculations like determining pressure lost through the cavity and cavity deflection. In addition, they can help with timing the firing pattern of valve gates on larger parts, like car panels, where manifolds are being implemented. Used in this application, sensors can help with moving knit lines or reducing the flow length ratio. Mid-cavity sensors are also a good option for very small parts, as they can typically represent an average pressure in the cavity.
End-of-fill sensors can also be used in calculations but add an additional beneficial attribute: monitoring the pressure required to fill your part. Short shots have always been an issue in the injection molding industry. If used in conjunction with a post gate sensor, you can see the pressure lost through the cavity and observe the cooling behavior of the plastic, which is critical in semi-crystalline polymers.
Temperature sensors can be used to detect the time at which the flow front passes over a particular area in the cavity and the surface temperature of the plastic or steel. You can then use this telemetry to fire valve pins or adjust accordingly.
Knowing some of the different functions of instrumenting a tool will greatly aid in deciding what sensor type and location you’ll need to achieve your goal.
3. What Type of Sensor Technology Do You Need?
Once you determine what you want to achieve and where you need to place your sensors, then the next step is to determine what type of sensor technology to use. There are several factors to take into consideration, including available real-estate in the tool and cost. The two pressure sensor options we offer are Strain Gage and Piezoelectric. Strain Gage sensors generally are more cost effective, require less maintenance, and have less noise but require more space.
4. How Should You Install the Sensor?
The most common installation styles for pressure sensors are direct and in-direct. Direct is when the sensor comes in contact with the plastic, sometimes called flush mount. In-direct is when the sensor is mounted away from the area of interest, and the pressure is transferred to the sensor via transfer pin, static pin, or ejector sleeve.
After decades of sensor installs, customer feedback, and testing, we strongly believe that mounting the sensor in the clamping plate and utilizing a transfer pin to transfer the pressure is the most robust configuration. Some of the benefits include:
- Mitigating the chances of damaging the sensor due to the tool’s action or while it is serviced on the bench
- Ease of accessibility
- Minimizing heat exposure to the sensor
- Improving the sensor’s life by removing it from the movement and shock of the ejector plate
5. What Load Capacity Do You Need?
Next, we move towards determining the load capacity of the sensor. The load capacity is determined by the predicted force that will be exerted on the sensor. You can make this determination in several ways, the preferred method being a flow simulation with predictions of the pressure in your chosen area. If this is not available, you can resort to the material datasheet which often times gives a pressure or tonnage factor. This gives you the recommended pressure per square inch required within the cavity to mold that particular resin properly.
If the tool under consideration for sensoring is already built and simulations are not a luxury you have, you can perform a Short Shot Study. To do this, you fill the part gradually at your preferred linear speed or volumetric flow rate and observe the filling pattern. Determining Post Gate (PG) is fairly straight forward, however determining Mid-Cavity and End of Fill (EOF) can be a little trickier depending on the material, flow rate, and geometry. The key is to not make too many assumptions without supporting your decisions with actual data.
A Real-World Example
Let’s imagine for a minute that you want a cavity pressure sensor for your tool. Let’s run through each question to create an implementation plan.
- What do you want the sensor to do for you? The goal is to ensure you hit a particular cavity pressure on each cycle, eliminate part variation, and sort out suspect parts.
- Where do you want to install the sensor? Your parts are very small in size, so a mid-cavity sensor would be sufficient in representing the average cavity pressure.
- What type of sensor technology do you need? Because there is limited real estate, you opt for the piezo option.
- How should you install the sensor? You choose to go with a flush-mount installation because the simplicity and minimal hardware requirements offer a small footprint in your limited space.
- What load capacity do you need? The plastic flow simulation software provided data showing where the plastic would flow and that the recommended sensor capacity in your mold is 3585 ppsi.
The Key Takeaway
Putting a sensor anywhere in a tool can give you some form of data, but it may not be value-added. The key is knowing upfront what you want the sensor to do for you.