First, read Scientific injection moulding – part 1.
In part 1 of this article we described the three initial steps of the Scientific Injection Moulding methodology, i.e. the basics of how to fill the cavities with the polymer melt. In this part we will see how to establish a process window with regards to melt temperature and packing pressure and what time we need for the packing and cooling phases in order to obtain a baseline for a stable process producing good quality parts.
4. Process window
When the phase of the process where the cavity is filled with plastic material is defined, it is now time to set the packing or holding pressure to make the plastic part resemble the tool steel geometry while the material solidifies and shrinks. The link between process and critical dimensions or other properties is now starting to become clear; known as the moulding area diagram, or MAD window. There will always be a difference between the part and the steel due to a linear shrinkage factor, but any other warpage or displacement can be considered as a defect and should be reduced to a minimum.
One initial target for the packing pressure is to aim for the theoretical weight of the part. However, as the density data from the plastic data sheet is only roughly stated, the final process window must be fine-tuned around verifiable data.
The process window for packing pressure and melt temperature can be derived from the conditions where sink marks in thick-walled sections, respectively flash at split lines, start to form.
Figure 6. An example of a process window with respect to melt temperature and packing pressure.
5. Holding pressure time – gate seal study
Besides the actual level of packing pressure, the holding phase is also defined by its length in time. The practice to find this needed time is often referred to as a gate seal study. The holding phase of the moulding process should, in theory, compensate for the shrinkage of the material in the cavity as the part goes from liquid to solid. This compensation can be done up to the point where the gate is frozen and no more material can be pressed into the cavity. Note that in combination with a hot runner the gate never completely seals. The essence of this important step is however to establish the proper holding time until no more material is required.
The standard procedure for this step is to measure the weight of the parts moulded at a constant holding pressure, with increasing holding time and to plot the results in a graph. Where the graph is starting to flatten out, the gate, including most of the part, can be regarded as solid and the optimum hold pressure time can be estimated. For demanding applications it can be recommended to add a margin to the holding pressure time.
Figure 7. Picture showing still melted plastic (blue) 4 seconds into the packing/holding phase in an injection moulding simulation study.
Figure 8. Graph showing a gate seal study to find a suitable packing/holding time.
6. Fine-tuning cooling – Cooling study
The cooling phase of the process cycle has until now been set to a good-enough time based on a moulding simulation study or from experience. During the previous five steps in the setup process, the goal of each individual step has been to optimize the process for its ability to produce consistent quality over time, without focusing too much on the quality of the part itself. The logic behind this approach is of course that the process has to be fairly optimized before any decisions are made to adjust tool dimensions or any other adjustment with impact on the part that may be very costly to adjust.
During the final step in the 6-step process setup the cooling time will be optimized according to actual part dimensions. Preferably two large dimensions are evaluated at different cooling times and when both dimensions show stability at acceptable values, in regards to nominal value and tolerances, the cooling time can be defined.
Figure 9. Example of a cooling study showing dimension dependence of cooling time.
Summary
We have in two articles described a well-established and accepted procedure to develop and document the setup of a moulding process according to scientific principles.
Although Scientific Injection Moulding may not always solve each process problem, the process derived from this methodology gives you a well-defined starting point for further problem solving. Good luck!
