How sensors enable shot-to-shot data control for high-volume injection molding

Injection molding sensors that go beyond a machine’s standard equipment improve product quality and production efficiency.  

By Ben Harp, Polymer Medical

For medical devices used in and on sensitive areas of the body, the part quality must be high to guarantee the patient’s comfort. Small adjustments can prevent defects like sharp edges on a parting line when doing a long run.

So much time and energy goes into designing and developing products — even single-use disposables — that tracking shot-to-shot data ensures that investment pays off with the maximum number of quality pieces per run.

Injection molding heats polymers to a molten state and forces them into a mold where the polymer solidifies to form Class I, II, and III components and devices. Controlling temperature, position, and pressure are among the factors that help meet the particularly high requirements for manufacturing medical devices. Advanced sensors like thermocouples are some of the measurement tools. With a combination of direct and indirect sensors (whether in direct contact with the melt or positioned behind an ejector), manufacturers can detect deviations from an ideal molded part.

As the injection molding process and part develops, facility managers can tap into sensors to pull data into a central monitor. This allows shot-to-shot visualization of the device-making process. While molding and assembly firms can measure an array of variables, the primary ones they monitor are temperature, position, and pressure.

Gathering shot-to-shot data requires placing independent sensors on a molding machine as well as ancillary production machines and the diagnostic components in a cell. With independent sensors (those beyond what the machine calls for) placed across the manufacturing process, a firm can control the parameters within an upper and lower specification limit. If a limit goes out of specification, the machine will reject the part and notify a robot to remove that part for that condition. This ensures the customer and patient receive a product made to specification vis-à-vis the initial validation protocol.

To gather this data, contract manufacturers and their partners can install sensors on resin hoppers, dryers, and inside and outside molds as well as on other locations like barrels. Sensors such as resistance temperature detectors (RTDs) and thermocouples monitor the temperature of water, dryers and the melted plastic throughout the inject and hold phase.

Manufacturers use linear transducers and encoders to monitor the important positions of the molding machine’s screw as it travels during injection. Monitoring the “shot size” is a critical way to prevent injecting too little or too much material, which can cause defects. Pressure sensors track the pressure of the plastic melt as it makes its way from the barrel into the cavity. The goal is not filling the mold, but stopping the injection velocity and applying pressure to pack out the cavity geometry while adjusting the pressure to meet the dimensions.

While the manufacturers of injection molding machines already install sensors that provide a look into the injection molded process, auxiliary equipment sensors are critical to monitoring other key properties of the plastic resin. For example, if you have a hygroscopic material that readily absorbs moisture, sensors will monitor the drying process at different temperatures and times along with how much humidity the material pulls from the air drying the part. There is also a temperature control unit for the water flowing through a mold to manage the shrink rate so the part is solid when ejected.

Sensors for better maintenance and better products

Sensors also shine a light on how a contract manufacturer’s equipment is performing. The data that advanced sensors gather can help a contract manufacturing and assembly firm determine which machines consistently deliver the best part.

In fact, process monitoring techniques available via Industry 4.0 solutions enable contract manufacturers to pull in machine information and monitor it from shot to shot. With sensors, engineering teams can establish tight bands, or tolerances, for, say 10 to 15 characteristics critical to the part they’re producing. They can then monitor production to ensure they stay within those bands.

With sensor data, operators and owners can see the machine changing its ability to mold parts over time. With that data and an eye on trends, engineers can plan preventive maintenance and even know when to begin the process of buying a new machine — or a different brand.

Independent sensors: The checks and balances for a superior part

A contract manufacturer can rely on up to 40 or 50 independent sensors connected to their injection molding machines to collect shot-to-shot data. And to properly manage any process, there should be about 10 to 15 sensors in place. Managers, engineers, and operators collaborate to decide where an independent sensor is critical for making a part and archiving data about the piece. Contract manufacturers can also pull in signals and log data for other elements to measure and store the DNA of an injection molded part.

Since sensors track every shot’s temperature, pressure and position, a firm may choose to chart and catalog this over time for all the parts they make. If a run of parts for a splitter or clamp doesn’t look identical to the last set, a manager can look at sensor data with a time stamp and see why there’s an anomaly. While the parts may not have a rejectable anomaly, the sensor data can pinpoint this variance and allow a manager to correct the process before there is a problem.

Installing independent sensors on injection molding equipment also ensures the machine’s sensors are accurate. Independent sensors supply the trend data over time to tell you if a machine component might be out of calibration. By contrast, relying solely on the machine’s built-in sensors and an annual calibration means potentially producing inconsistent parts. Independent sensors on auxiliary equipment can report if a machine requires calibration more frequently. Independent sensors also help monitor temperature and pressure separately for tighter tolerances.

Final thoughts

As the adage goes, “Whatever is worth doing at all, is worth doing well; and nothing can be done well without attention.” By paying attention to each step of manufacturing medical devices, firms can achieve consistency through tighter tolerances. Machine and auxiliary sensors are a tag-team approach. Tightly monitoring and controlling the process benefits OEMs and delivers a safe, quality product for patients.

Ben Harp, president and co-founder of Polymer Medical, has 30 years of materials science and process engineering experience and is president of the board of directors of Healthy Manufacturers of New York State (HMNYS) and the Manufacturers Association of Plastic Partners (MAPP). He earned his MBA and bachelor’s degree in mathematics from Canisius University.

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The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Design & Outsourcing or its employees.