How to Mold Tight Tolerance Parts

While cavity pressure measurement technology has been in use in the plastics industry for years, Mack has begun to use it much more frequently as we have transitioned into high tolerance, precision molding for orthopedic and medical device OEMs. To ensure that all dimensional specifications are precisely met, we have to control the process tighter than we can with standard injection molding machine features. And that’s where cavity pressure transducers come into play.

To explain, let me back up. The injection molding process occurs in three phases: filling, packing and holding. During the filling phase, the melt is injected into the cavity at an established velocity. When the cavity is volumetrically filled (packing), the machine switches over from injection pressure to holding pressure, which is much lower. During this phase, the lower holding pressure continues to fill the part incrementally to compensate for shrinkage as the part cools. At this point, the mold can be opened and the part removed.

In the standard molding process, the machine switches from fill to pack to holding pressure based on time, position or machine pressure. While adequate for many parts, these methods do not take into account variations in plastic melt viscosity, which can cause process and dimensional variation that can ultimately lead to flashing, underweight parts, sink marks and/or surface marks.

We have found that inserting a cavity pressure sensor into the mold (or into each individual cavity in multi-cavity tools) is the most accurate method for measuring volumetric filling. Rather than relying on standard machine variables, this approach makes it possible to control the pressure the mold needs to precisely fill and pack out the part.

Figure 1 graphically illustrates how the use of a cavity pressure transducer improved the stability of one dimension. In this case, it was the profile of a surface at a 45-degree angle. As you can clearly see, without a pressure transducer (left), this dimension varied outside the control limits. With a transducer, however, the dimension was steady.

With this type of data in hand, we are able to confidently make mold adjustments so that tight tolerances and high quality can be achieved and maintained throughout the life of the project. Most customers specify a minimum Cpk (process capability) of 1.33, which can yield 64 ppm defects. A Cpk of 2.00 yields .0198 ppm defects. With this technology, Mack can achieve a Cpk of over 10.00, theoretically producing no defects.

From a business perspective, this is critically important because of the confidence it instills in customers in our ongoing quality performance. Additionally, however, it can allow Mack ship-to-stock status, which translates into greatly reduced inspection and inventory costs.

Other benefits of using in-mold pressure transducers include:
• Reduced cycle times – the data can be interpreted to determine exactly when the part should be ejected, so there is no lag time.
• Consistent mold setup and repeatability – we are measuring tool output rather than machine input, which reduces the process variation caused by material and machine variations.

Finally, the most important benefit is customer satisfaction. This level of confidence ensures that we are providing a product that not only meets or exceeds product specifications, but customer expectations as well.

Contact: Tom Revane
tom.revane@mack.com