The first shot of a new mold determines the fate of the rest of production. In most workshops this stage is passed by trial and error: a parameter is changed, the part is looked at, another parameter is changed. In the end a setting that works is found, but why it works is not known. Months later, when the material batch changes or the shift warms up, the same setting no longer holds and nobody remembers on what basis each value was chosen.
Systematic mold proving is the alternative to this: a procedure whose steps and order are set in advance, with what is proven at each step written down. The flow below is used not only for new molds; it also serves as a framework that can be applied piece by piece when the process has gone wrong on an existing mold.
What needs to be on the table before going to the machine
The biggest waste of time in a trial is looking for missing information at the machine. Before starting, the following must be ready:
| Document / information | What it is for |
|---|---|
| Purpose of the trial | Exactly what is to be proven in this trial (dimensions, surface, cycle, cavity balance) |
| Mold mounting dimensions | Whether it fits the machine, centering and ejector connection |
| Ejection function diagram | Which movement in which order, with which safety |
| Core puller and needle valve nozzle flow diagram | Signal sequence and interlocks |
| Hot runner zone layout | Which zone belongs to which nozzle, sensor types |
| Temperature control plan | Which circuit feeds which area of the mold |
| Processing data for the material | Temperature range, drying, peripheral speed limit |
| Technical drawing of the part | Dimensional tolerances and critical surfaces |
| Flow check | Verifying that each cooling circuit is actually flowing |
The temperature control plan and the flow check are the ones most often skipped. A single circuit that is not connected or is blocked makes all subsequent measurements meaningless; details are in our what is mold temperature control article.
Setup order
The procedure proceeds in five blocks, and the order is not accidental — each block sets up the precondition for the next.
1) Machine setup. The hydraulic, electrical and pneumatic connections are made. The temperature control medium is connected, set and its flow measured. The hot runner temperatures are entered. The clamping unit is set.
2) Mold. The mold is checked, the plasticizing temperatures are set, the mold is mounted and a function check is carried out. Then the clamping force, mold protection force and mold movements are set.
3) Plasticizing conditions. The unit movements are set, followed by a realistic cooling time. Then screw speed, back pressure, decompression, screw retraction speed and metering stroke are entered. The block closes with the selection of the switchover point and the metering stroke.
4) Injection conditions. The injection speed and pressure limit are set. Partial fills are made until the part is about 98% full. A suitable holding pressure is used as a starting point, the time is entered, the profile is optimized and the cushion is checked.
5) Close-out. If the required quality has been achieved, the monitoring times are entered, the dwells are optimized, the remaining cooling time is set and the part quality is verified one last time.
Partial filling: the backbone of the trial
Partial filling is the only way to see how the mold fills. There are several conditions for doing it correctly:
- No melt must enter the cavity during the holding phase. For this the holding pressure is chosen very low; otherwise the switchover point is overshot and the partial fill is no longer partial.
- The set switchover point must also be verified by eye — the value on the screen and the actual fill ratio of the part are not the same thing.
- A long enough plasticizing delay is given so that no melt escapes into the cavity during metering.
- Scaling up is done in small steps; large jumps hide where the flow front slows down.
- Each partial fill is labeled with the switchover point or metering stroke it belongs to. In this way the front position can be traced back.
- Sufficient pre-fill must be ensured for the ejector positions to work properly.
A partial fill series shows at the same time where a short shot begins, where air is trapped and where weld lines fall. The findings are matched with the solutions on the short shot and weld line pages.
Flow front speed and the injection profile
For the surface quality to be consistent, the flow front speed must stay constant. But while the screw advances at a constant speed, the volumetric flow is constant, not the width of the front: near the gate and at the end of the flow path the front is narrower, so the speed rises there.
That is why the injection profile is typically set up as slow-fast-slow. Once the front widths in the different regions of the part have been determined, the screw advance speed is adapted to keep the front speed roughly constant.
Slowing the profile again toward the end of filling is also a safeguard: as the front narrows, a constant flow speeds the front up, which leads to a pressure rise and flash.
Switchover point: the 98% rule and when to look again
The switchover from injection to holding should take place when about 98% of the part volume has been filled. Switching over early means a short shot, switching over late means overpacking; the latter carries a risk of mold damage as well as flash.
The critical point is this: the switchover point is not a value to be set once and forgotten. It must be checked again whenever any parameter that affects the metered melt volume changes:
- Back pressure — changes the compressibility of the melt
- Decompression — changes the plastic volume in front of the screw
- The first injection stage — changes the closing behavior of the non-return valve
If one of these three changes while the switchover point is left at its old value, the part weight quietly drifts and the cause cannot be found for weeks.
Finding the effective holding time by weighing
Instead of setting the holding time by guesswork, it can be measured, and the method is simple: the holding time is increased step by step and the part is weighed at each step. The moment the weight stops increasing significantly, the effective holding time has been reached. Continuing to hold beyond this time only lengthens the cycle.
There are two conditions: the holding pressure must be kept constant throughout the experiment and the weighings must be done under the same conditions. The method also prevents melt from flowing back out of the part as the gate freezes — if backflow occurs, the weight and dimensions deviate in an undefined way.
With soft TPE compounds this method is not reliable: because the material is highly compressible, the weight curve does not give a clear plateau.
Cooling time and the cost of mold temperature
For economic reasons the cooling time is kept as short as possible; you can start from calculation programs or empirical values. Three things are decisive: wall thickness, mold wall temperature and ejection temperature.
There is a rule of thumb to keep in mind here: the price of heating the mold wall by 10 °C is an extension of about one fifth in cooling time. Raising the mold temperature has benefits for the surface and weld lines, but its price is paid directly in cycle time. For a detailed calculation see the how to shorten cycle time article and our calculators.
The mold wall temperature also affects shrinkage and therefore dimensions. That is why dimensional corrections to the mold must only be made with parts produced in the relevant cycle — grinding a mold with a measurement taken in a different cycle is an expensive mistake.
The process window
The quality of a part can only be guaranteed within a certain range of settings. This range is called the process window. The critical point is that the settings are in the middle of the window, not at its edge.
The reason is simple: the material, the machine and the auxiliary equipment have their own tolerances. If the setting is in the middle of the window, these tolerances do not change the result; if it is at the edge, a change of batch or a few degrees in ambient temperature drops the part into scrap. The window also defines the permitted tolerance of change in the machine settings.
When a quality problem arises in production, the first job is to verify that all components are working perfectly: processing temperatures must be measured, drying must be checked, the set values must be compared with the actual values. Only after that can the machine settings be optimized systematically.
Two rules when changing parameters
One parameter at a time. If more than one variable is changed at once, it is impossible to tell which one worked; moreover, parameters can mask each other.
No part is assessed before thermal equilibrium is reached. Especially if the melt or mold wall temperature has been changed, the parts are only meaningful once the system has reached thermal equilibrium. The first parts immediately after a change are misleading.
If the parameters are tangled: a trial plan
Sometimes the combined effect of several parameters cannot be resolved by trying them one by one. In that case the parameters that determine quality are combined systematically in a trial plan. The number of trials is 2ⁿ; 8 trials for three parameters:
| # | Injection time (te) | Holding pressure (tn) | Mold wall temperature (Tw) |
|---|---|---|---|
| 1 | + | + | + |
| 2 | + | + | − |
| 3 | + | − | − |
| 4 | − | − | − |
| 5 | − | − | + |
| 6 | − | + | + |
| 7 | − | + | − |
| 8 | + | − | + |
The `+` and `−` signs show the high and low value of the parameter: te+ fast injection, te− slow; tn+ low holding pressure, tn− high; Tw+ high mold wall temperature, Tw− low.
A practical tip: the trials are not carried out in the order written in the table but grouped by temperature. For the plan above the efficient order is 2, 3, 4, 7, 1, 5, 6, 8 — first all the low mold temperature trials, then the high ones. Because mold temperature is the parameter that takes longest to reach equilibrium, this ordering markedly shortens the total trial time.
Commissioning is a record-keeping job
The real gain of the procedure is not that a working setting is found at the end; it is that what each value was chosen on the basis of remains in writing. Partial fill photos, the weighing curve, the volume at which the switchover point was found, the limits of the process window — these are information that will be used throughout the life of the mold.
If cavity pressure measurement is possible, the most valuable part of the commissioning record is the pressure curve: it is the reference against which comparison is made in the following months when the process drifts. The method is in our cavity pressure measurement article, and the equipment side on the cavity pressure measurement systems page.
Once commissioning is complete, the mold needs to enter the maintenance plan; the periodic inspection items are in the mold maintenance and periodic inspection plan article. For surface and dimensional defects encountered during commissioning you can use our molding defects and solutions guide, and for your mold's temperature control needs see our temperature control systems page.
If you need support in commissioning a new mold, you can contact our technical support team.