An Overview of Injection Moulding Process Optimisation / Scientific Injection Moulding
Injection Moulding Process Optimisation is a scientific method of developing injection moulding processes. It offers many advantages over non-scientific trial and error methods.
Enables the engineer to make fact based decisions.
Typically offers a 50% reduction in mould validation time.
Step-by-step management of optimum part performance, quality and output.
Fully documented process development.
Systematic management of part development, mould and equipment performance and processing.
It has applications across a range of moulding technologies.
It is flexible to suit the manufacturer’s needs.
It enables establishment of both a cosmetic process window and dimensional process window.
Offers easier identification of tooling or product design issues.
Provides documented evidence of the process of parameter selection.
It is a repeatable method of parameter selection for new moulds.
It can often improve cycle times.
Delivers a more repeatable moulding process.
Generates a reduced or zero scrap rate of parts.
Offers energy savings for the manufacturing plant.
There are many studies that can be completed and they are not all required all of the time. Below is a breakdown of the studies available for a new tool qualification as Moulding Optimisation Ltd would recommend.
Dry Cycle Study
This would be typically completed at the mould makers during the FAT stage but it is often wise to repeat this once the tool is delivered. Typically the mould would be dry-cycled for around 4 hours at the correct, or at least anticipated mould and water temperature. After the run the mould is stripped down and inspected for wear.
Selection of Process Temperatures
This is often just a section of the workbook recording the selected process temperatures based on both experience and the material manufacturers’ recommendation.
Water flow rates
This is often completed at the FAT stage and repeated once installed at the moulding company. A record of the flow rates per cooling zone and a diagram of the mould tool and water pipes is created for future reference. Any future change in the piping up of the mould or the water flow can be detrimental and can be considered a deviation from the original validated process.
Initial Process Setup
This stage is to set up the machine with a basic process. Using the recorded process temperatures, recommended metering parameters and a mid-range injection speed the shot is built up to define an initial shot size and then a token hold pressure is introduced, usually around 40-70% of the VP position pressure. This gives a basic valuation of the mould and it is during this stage that can discover issues early on in the optimisation process.
Injection Time Study / Rheology Study
As polymer melt is non-Newtonian, as injection rates increase, more shear forces are applied and the melt temperature increases and the viscosity reduces. In simple terms, a faster injection speed will yield better repeatability but there is a point at which too much injection speed and/or shear is just wasting energy and can cause other issues. An injection speed that is too slow can introduce variability, especially between material batches.
A study is completed that records the peak injection pressure and injection time at several injection speeds and generates a relative viscosity value that is then plotted on a graph. An optimum injection time is selected from the graph output where the variability is low, usually where the line starts to flatten out, and where further speed increases yield no further viscosity reduction.
Balance of Fill analysis / Cavity Balance (and correction)
After the injection speed study it is standard to measure the imbalance between the cavities. Some imbalance between hot runner tips/drops can be as much as 20%. While it is down to the tool manufacturer to provide a mould tool with a balance to a perceived acceptance level (typically an imbalance of <10% is accepted) this is not always possible without modification of the tip temperatures.
The imbalance must be measured and recorded. This can be completed by filling the mould and cavities to around 80% of the expected final product weight and determining the range between the most filled cavity and the least filled cavity.
The hot runner manifold temperatures and tip temperatures can then be manipulated to offset the effect of the imbalance. With effort, the cavity imbalance value can be reduced, typically from 16% to <2% during this optimisation stage.
Reducing the imbalance of multi-cavity moulds will increase the available process window. It is imperative that the minimum imbalance possible is achieved to ensure the largest possible process window. This can take time but is a worthwhile investment of that time.
Pressure Drop Study
A study can be carried out to see how much pressure is being lost through the hot runner or runner system. This method requires a puck or false spru bush to be put between the machine nozzle and the spru bush and the machine cycle started. The specific pressure is noted and compared with that of injection into the mould.
Hold Time Study / Gate freeze Study / Hold pressure study / Pressure vs Weight Study / Hold Time vs Weight Study
The exact studies required depend on the type of runner in use and goes beyond the scope of this page. In most cases, a study to determine the best hold time or the range of hold times possible is undertaken.
A study to determine the best hold pressure from a process point of view and/or dimensional view would often be carried out. A DoE (design of experiments) can be carried out post optimisation to further refine the hold parameters to dimensional requirements and develop anticipated process ranges.
Repeatable metering is fundamental to a robust moulding process. In order to select the best metering parameters, experiments can be carried out on the metering parameters. Similar products with the same base parameters could likely utilise the outputs from earlier studies as long as the temperatures, screw speed, barrel size and shot weight are similar.
Screw speed optimisation
The screw speed is usually set to the recommended screw surface speed value. The remaining studies then follow that value. A change in screw speed, barrel size, barrel temperatures or shot size would usually require the following metering studies to be repeated.
Screw Back Pressure Study
With decompression set to a mid-value (typically 6mm) parts filled to around 80-90%, a range of screw back pressures are set and variability assessed across 10 or more (recommended 15-20) shots. A graph is plotted and the back pressure with the least variability becomes the set point.
Decompression Distance Study
Using the chosen screw speed and the selected back pressure and parts still 80-90% full, a range of decompression distances are set across 10 or more shots (recommended 15-20 shots) variability is assessed and a graph is plotted. The decompression setting with the least variability is selected.
Some newer electric moulding machines such as Fanuc have a function that can reset the check ring, giving the option to exclude this study.
Decompression Speed Optimisation
Using much the same method as the decompression distance study and the screw back pressure study, the decompression speed can be optimised and a value chosen. If time constraints are present then this section can often be omitted.
Recent experimiments carried carried out by Moulding Optimisation Ltd have concluded a faster method for determining the correct decompression speed. Contact us for more information.
Cooling Time Study
This can be completed several ways.
The cooling time can be used as a factor in a DoE. When chasing dimensions it can be selected from the response optimiser, or outside of a DoE the value can simply be chosen where the dimensions and quality are as required from a range of cooling time settings.
Another method is to measure with an infrared thermometer or thermal imaging camera, the post ejected components at a range of cooling times. The cooling time is determined when the ejection temperature meets the HDT (heat distortion temperature) value + 10%. The HDT value can usually be found in the material data.
Clamp Force Study
There are clamp force calculations that can predict the clamp force required based on the projected area of cavitation but it is essential to physically test the situation once you have the actual tool, in the actual press with the actual process.
A study is completed whereas the clamp force is reduced from maximum (within reason) in increments until the part weight increases or defects are seen. Data is collated and a plotted on a graph and the least clamp force setting that is without defects is chosen (+10%). This study can save a lot of energy by ensuring that the clamp force applied is not significantly more than necessary.