The part examined is shown in the following picture, which also shows a rough layout of the cooling channels in the mold. The mold is designed to produce two different parts that are then joined to form a mirror bracket for an industrial vehicle. The two parts are called upper bracket and lower bracket, according to their position on the vehicle.
Gate definition (type, number and position)
Several gating solutions were considered and then simulated to evaluate their advantages and disadvantages. These solutions are described below, together with the considerations behind the final choice.
In this phase the two parts making up the mirror bracket were studied separately. The main simulation results are listed below, from which the conclusions described later were drawn.
Upper bracket – direct gating with hot nozzles:
Two solutions of this type were simulated, both with the nozzles on the upper surface of the part but with a different number of gates.
The next picture (2 hot nozzles) shows the filling time of the part on a colour scale: blue marks the areas that fill first and red the last areas to fill. The same result can be displayed as iso-filling lines: simply put, a continuous line means that the material reached the area it bounds at the same instant. This view is very useful to show where two flow fronts meet and create weld lines (marked by the pink arrows).
Below, several filling stages before the flow fronts meet can be seen, to understand how the weld lines are formed.
The picture shows the same result for the solution with three hot nozzles (marked by the yellow cones). It simply highlights that the number of weld lines naturally increases (see the pink arrows).
After checking the position of the weld lines, their severity must be evaluated, using the result that shows the material temperature at the moment it reaches the different areas of the part (temperature at flow front).
These results show that the weld lines form at temperatures not much lower than the melt temperature in the barrel, a sign that they are probably not severe. In both cases.
Upper bracket – side gating with submarine gates:
Here too two solutions were tried, with 3 and with 4 submarine gates.
Another important aspect for predicting the deflection of the part, and therefore directly related to the choice of the most suitable gating, is the evaluation of fibre orientation in the material.
The previous pictures show some results for the solution with three submarine gates. Several weld lines are formed which, given the flow front temperature result for this solution, should not be severe.
The following pictures show the same results for the solution with 4 submarine gates. As expected, the number of weld lines increases, without any significant improvement in their formation temperature.
Upper bracket – direct front gating, cold runners:
Two solutions were evaluated: one with a single gate and one with 2 opposite gates.
The following pictures highlight the weld lines for the solution with 2 opposite gates: there are two weld lines, one of which (the one closer to the centre of the part) is of medium severity in terms of both formation temperature and position.
The next pictures evaluate the solution with a single fan gate. There is only one weld line, formed at low temperature but in a position considered less relevant from an aesthetic point of view.
Upper bracket - conclusions:
The hot runner direct gating solution, although the best in terms of temperature drop during filling (also because there would be no cold sprue to lose further heat), was rejected for several reasons:
- high manufacturing cost;
- gate marks, although small, on the most visible surface of the part;
- a high number of weld lines, although of low severity, in very visible positions.
The submarine gate solution, although it guaranteed low production costs (no secondary operations), was rejected for the following reasons:
- gate marks on surfaces that are hidden but used when assembling the two brackets;
- a high number of weld lines, of medium severity, in very visible positions;
- difficulty in making a submarine gate in one area of the bracket;
- a fibre orientation likely to cause unwanted deflection of the parts.
NOTE: when choosing the gate type, the problems of both parts were actually evaluated, since the mold as a whole had to be balanced.
The gating chosen is therefore a single direct front gate, which has the following advantages:
- easy manufacturing and low mold cost;
- essentially uniform fibre orientation, indicating acceptable deflection;
- compared with other solutions, the possibility of reducing shrinkage in the thick areas, since they were chosen as gate locations for both parts;
- a low number of weld lines.
It also has the following disadvantages, considered less relevant than those of the other solutions:
- a high temperature drop during filling, with consequent low-quality weld lines, which however form in areas hidden after assembly;
- degating is needed to remove the gates;
- a higher clamp force is required than with other solutions.
The solution with two direct gates was rejected because it produced weld lines of medium severity on surfaces visible even after assembly, and it required double degating; the second gate would also have been difficult to make, and the fibre orientation was worse.
Lower bracket – gate choice:
As anticipated in the previous conclusions, a direct gate was chosen for the lower part as well, for the reasons already listed.
Some results of the various solutions are briefly shown below; not all the simulations are detailed as for the upper part, since the considerations are similar in many respects.
As the picture shows, with 3 hot gates this part too has many weld lines in visible areas.
The pictures below show that submarine gates, whether 3 or 4, give a non-uniform fibre orientation on the two side walls of the part. This may cause unwanted and hardly controllable deflection.
The following picture shows that, with two opposite direct gates, the weld line (of medium severity) forms in a very visible area on the lower bracket too.
Moreover, compared with the orientation obtained with the solution shown below (single direct gate), the fibre orientation is much less uniform, as shown in the picture.
The filling time of the solution adopted shows that there are no weld lines. In addition, the last area to fill is hidden after the parts are assembled.
In the chosen solution the fibre orientation is also much more uniform, with a fibre direction favouring a limited deflection.
Filling balance and runner sizing
Simulation made it possible to size the runners so that the cavities fill in a balanced way. The different tests carried out are briefly listed below, with the balance results obtained.
The picture shows the symbols used to define the runner dimensions. A trapezoidal runner section was chosen, as shown in the picture.
For the runner feeding the upper bracket (shown in pink in the picture) the following symbols were used:
- h = height; b1 = major base; b2 = minor base.
For the runner feeding the lower bracket (shown in blue in the picture):
- H = height; B1 = major base; B2 = minor base.
The simulations run with the different dimensions are listed below, with the related filling time results to compare the balance.
- Simulation with dimensions: h=7 mm; b1=10 mm; b2=7.5 mm - H=9 mm; B1=12 mm; B2=9 mm.
- Simulation with dimensions: h=5 mm; b1=8 mm; b2=6 mm - H=7 mm; B1=10 mm; B2=7.5 mm.
- Simulation with dimensions: h=5 mm; b1=8 mm; b2=6 mm - H=6 mm; B1=9 mm; B2=7 mm.
- Simulation with dimensions: h=6 mm; b1=9 mm; b2=7 mm - H=7 mm; B1=10 mm; B2=7.5 mm.
As the pictures show, the dimensions used give a filling that is not perfectly balanced in all cases except the last one.
These results showed that the most suitable dimensions for a good balance are those of the last test, where the lower bracket has only a slight advantage and therefore fills just before the upper one.
To define even more precise runner dimensions, more complete simulations were run, varying the values obtained in this phase.
Filling balance, gate dimensions
Fine-tuning the runner dimensions:
The symbols used to describe the runners are those given above.
With the solution chosen previously (first picture in the figure above) the lower bracket fills first, if only slightly. By then adjusting the thickness of the smaller runner, an almost perfect balance was achieved (with the parameters used in simulation), as shown in the various pictures of the previous figure.
Gate sizing:
Two identical gates were chosen, even though the parts have different sizes and volumes, simply because the smaller part (which could have had a smaller gate) has a very thick area right at the gate; a larger gate was therefore chosen, equal to the one used on the lower bracket.
The first simulation used gates with a 5 mm x 2 mm section at the part, the second 5 mm x 3 mm.
At first fan gates were used, because filling was slightly better, but after evaluating all the results it was decided to use rectangular gates (5 mm x 3.5 mm) because, although filling was slightly less efficient, the other results improved.
Two important results must be evaluated for this gate. First, it must be checked that it is large enough not to reach a velocity that would cause the material to degrade during filling. In the upper picture (5 x 2 gate) the shear rate limits are reached and exceeded (red areas), because the gate is too small. In the lower picture the material does not reach those values during filling: it is within the limits recommended for the chosen material.
A second important result for choosing the dimensions is the temperature of the material in the gate area during the packing phase. If the gate is too small it "freezes" quickly and does not allow good packing of the parts, especially in very thick areas.
The picture shows that, at the same instant, after about 16 seconds in total, the gate of the upper bracket in the case shown in the figure above (with smaller dimensions) is already frozen and detached from the part. As a result packing is not sufficient to counter the surface sink marks caused by the solidification of the adjacent thick area.
The picture shows in green all the material still below the transition temperature, and therefore still fluid; where there is no colour the material is already above that temperature and can be considered "frozen".
Cooling circuit evaluation (layout and temperatures)
The cooling layout used is the one proposed in the design phase.
The temperatures used are:
- 90 °C for all circuits except the two central cooling circuits (those running through the middle of the parts) on the moving half;
- 80 °C for the two central circuits on the moving half.
These temperatures result from a cooling optimisation carried out in simulation, while still taking into account the temperatures recommended by the material supplier.
The picture, showing the surface temperature of the parts before ejection, shows a fairly uniform temperature overall, except in the central part of the lower bracket.
We therefore recommend improving the cooling of that area too, for example with bubblers or local baffles.
Complete cycle and process parameters
Holding pressure profile:
A holding pressure of 80 MPa (800 bar) was used, taken from a range of recommended values for filled nylons.
NOTE: the value chosen is the lower one, so it is recommended not to use lower values, but rather higher ones.
To define the holding time, the "temperature" result is used again, looking for the instant when the gates "freeze" (below the transition temperature).
The recommended pressure profile starts, as said, at 800 bar for about 16 seconds and then drops to 500-600 bar for another 7-9 seconds.
This is because after about 16 seconds the end sections of the parts begin to solidify, so less holding pressure is needed. In addition, after about 25 seconds the gates are already frozen and there is no point in continuing the packing phase.
Cycle time:
The overall cycle time is strongly affected by the solidification of the parts and by the condition in which they are to be ejected. They do not need to be completely solid at ejection: it is enough that the material is frozen where the ejectors act, together with a good overall percentage of solidification.
Several cycle times were tried in the simulations, arriving at 75 seconds, divided as follows:
- filling time = about 2 seconds
- mold open-close time = assumed 5 seconds
- holding time = about 23 seconds
- cooling time without flow = about 45 seconds
The situation at the end of the 75 seconds set is shown below, in terms of the areas of the part still fluid and not completely solidified.
The recommended ejection temperature for the material used in simulation is 170 °C.
Required clamp force:
The recommended pressure profile requires a certain machine clamp force, given by the following graph.
The simulation shows that just over 300 tonnes are needed to run the process as recommended, specifically in the initial part of the holding phase.
General summary of the recommended specifications and parameters:
The data below should be taken as approximate values, and in any case closely related to one another: changing one of them could cause even considerable changes in the others and, above all, in the results presented above.
The filling balance was also obtained using the following geometric and process values.
- Material: Radilon S RV 300 100 NAT - Radici Novacips.
- Geometry: as shown in the picture:
- Cycle time: 75 sec. NOTE: this cycle time depends on the size of the thick areas of the parts, especially on the upper bracket.
- Total injection time: about 2.3 seconds (from entry into the sprue to complete filling of the part).
- Melt temperature: 275 °C (temperature at the sprue inlet).
- Mold temperature: 105 °C (used in simulation as average mold temperature).
- Coolant temperature: 90 °C for all circuits except the two central cooling circuits (those running through the middle of the parts) on the moving half; 80 °C for the two central circuits on the moving half.
- Packing profile: 800 bar for about 16 seconds, then down to 500-600 bar for another 7-9 seconds. NOTE: specific values, not machine values.
- Velocity/pressure switch-over: at 99% of the part volume.