Mould venting
Mould venting
Here is an example of a brief presentation on mold venting for the Plastics conference Formy a Plasty Brno 2025
1,Hello.
I have prepared a short presentation about mold venting.
and it occurred to me to start with a quote from a book by Mr. Lubomír Zeman.
2,this quote is about the diesel effect,
it is shown here in the picture.
In reality, the most common way the diesel effect is dealt with in practice
is actually...
ordinary cleaning of the parting line on the mold!!!
Everyone is familiar with this.
It starts causing trouble, some paper strips get stuck on it in production, it gets a bit better for a while, when quality threatens to reject it, a toolmaker is sent in, he "adds venting" there (exactly in the way Mr. Zeman mentioned) and it works... for half a shift... And this keeps going in circles, only the intervals when the mold is actually producing keep getting shorter, and when it is standing still more often than it is running, the mold is taken out and sent for disassembly and complete cleaning in maintenance.
However, if the mold is left running with a strong diesel effect, cavitation occurs and a burnt hole appears in the mold, and that is a serious problem.
3, So, everyone definitely knows this defect, right?
May I ask?
who is here
1. Tool shop, mold manufacturing
2. Mold maintenance,
3. Setters and molding shop, production
4. Technologists, project engineers, process engineers, white‑collar staff, managers...
What is being presented here is part of practical courses for toolmakers and mold maintenance staff, which are held in Liberec at Libeos.
Main points to take away:
• (A) "Phantom imprint" – how to tell from the venting on the mold whether it really works
• (B) How individual parameters and dimensions affect the efficiency and durability of venting
• (C) An overview of mold settling and how to vent molds that settle a lot (unfortunately, there are more and more of these because of, well, China…) or because of saving money in the wrong place.
• (D) And anyone who makes it to the end without falling asleep or disappearing for a beer will hear with their own ears the self‑cleaning effect of a perfectly executed venting groove.
4. There are certainly many problems that have their root cause in poor mold venting.
Burn marks, trapped air, sometimes streaks, surface appearance defects, short shots, and sometimes, paradoxically, parts with flash...
A large share of these problems would not need to be solved if the "venting" on the molds worked as it should.
Cleaning and maintenance costs could be cut in half, and scrap rates even more.
Poorly vented molds become contaminated quickly, which requires shorter intervals between regular maintenance.
This is logical, because contamination and burnt condensate act as a perfect seal.
Cleaning is a type of work that can be easily assigned and checked, and does not require highly qualified personnel.
But how to really recognize that a mold has insufficient venting?
It can be seen… on the compression marks, (A)phantom.
Look at the photo in the middle: opposite the functioning venting there is no pressure mark. (This will be discussed further.)
Do not sidestep venting problems by simply dumping them on the process engineer.
When it is done correctly on the mold, there will be peace for a longer time.
The time saved can then be invested, for example, in real preventive and predictive maintenance, which can never be done in a rush or as a simple task assignment.
5. In order to inject a quality part, it is necessary to ensure
that the air in front of the melt front can escape, will not obstruct the flow, will not become trapped and enclosed in the moulding, will not be compressed and therefore will not overheat and burn.
Try to imagine the mould as an empty garden hose when the end is pinched and the water is turned on.
If the hose is squeezed hard, the water will move forward, compress the air in front of it and will not reach the end.
If the grip is released, the hissing air can be heard and then the water shoots out at full pressure.
If the end is not pinched at all, the air cannot be heard and the water flows out calmly and without a pressure surge… (the switching point from filling pressure to slower holding pressure).
Compared to water, plastic has a much higher viscosity (the opposite of fluidity) and will not flow into a certain gap even at full holding pressure.
These values for a specific plastic can be found in the tables (B).
Unlike water or metals, plastic flows through the mould cavity in what is known as a "fountain flow": it flows through the centre of the wall as a hot liquid core and, when it comes into contact with the cooler surface of the mould, it solidifies and forms a frozen layer. As this layer cools, it becomes thicker and stronger. In the areas where the plastic arrives last, venting is most effective and also most needed.
But it is precisely there, "at the end of filling", that the specified tabulated value of the venting gap must be followed exactly.
To know everywhere that the safe value of the gap must be maintained (and where venting is most needed at the end of filling), it is necessary to have in hand a moulding injected with 95% of the shot volume without holding pressure.
Every process engineer knows this when it is necessary to tune the switching point on the machine. It helps a lot when the process engineer works closely with the toolmaker and they do not just toss the problem back and forth like a ping-pong ball.
6,Functional venting in the parting line of the mold should have the shape of a narrow primary groove close to the contour of the molding. The gap must connect the cavity and the primary groove. (B)
Sharpened venting must not remove too much of the supporting surface in the parting line, otherwise the mold will settle and the venting will soon be crushed.
The better the mold is vented, the less it will become clogged.
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7,This is quite important to understand: Try blowing as hard as you can through a straw in a soft drink, and then shorten the straw to 2 cm. You will immediately understand why the primary venting groove should not be far from the shape…
Not only does such venting have up to 20x!!!! worse efficiency. A much worse effect is that it quickly clogs with condensate. (B + D)
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8, here on the left is the classic venting from old textbooks, and on the right is an option for how to significantly improve it.
Here it is explained to toolmakers that they can calculate those grooves as the sum of the cross-sections of those vents.
9, This slide describes one of the methods a toolmaker uses to grind a general shape of a reduction with hand tools, achieving an accuracy of one hundredth of a millimeter. Naturally, if it is possible to machine it using equipment such as CNC EDM or a magnetic grinder, then it is of course done, or arranged to be done, on a machine.
10. In this picture, note that correctly executed venting appears as a "phantom" imprint on the other half of the mold, visible as the absence of bruising.
It is always recommended to pay close attention to these details.
Even without being a toolmaker, it is clearly possible to see bruising, impressions, pressure rust on the mold, and in most cases it is absolutely obvious where the venting is functional and where the gaps are closed during injection, i.e. not connected to the contour.
11,In this picture, there is heavy bruising of the parting line. The original machined surface protrudes above the bruised current level of the parting line by 0.04 mm. It is clear that the former 0.02 mm gap has long since disappeared (if it was ever actually ground in at all).
When pressure rust and bruising appear where there should be a vent groove and gap, there can be no talk of functional venting.
The smaller the load-bearing surface of the mould, the faster it will bruise, settle, and develop venting problems.
Pressure pads in the mould frame do not help much.
12. Is there an idea of how large the clamping force needs to be for a given molding?
When starting an apprenticeship a long time ago at Plastimat, there is a memory of how a colleague (Mr. Syrový) in the mold repair shop used to say: imagine that at a certain moment, the plastic is pressing on every square centimeter in the mold with a force of one ton!!!
One ton per square centimeter is a pressure of 1000 bar! Such an enormous pressure, in fact even twice as much, is needed to get the melt from the barrel all the way to the end of the part.
This simple shop-floor rule of thumb is useful when checking the correct clamping force of the machine, or when figuring out why the mold opens during injection, "breathes", bends, or why the slides move back during injection or holding pressure.
The classic injection molding process (if there are no ultra-modern electric machines with top-notch setters) works so that the machine first closes the mold with full clamping force...
(and that means tens of tons – for a part about 2 cm like this; hundreds – for a part about 15 cm like this; and thousands!!! of tons for, for example, an automotive part or a crate for fruit)
With this force plus a bit more, the mold has to stay closed during injection; otherwise the melt will open it slightly and there will be flash. At the same time, the mold must be able to withstand this force over hundreds of thousands of cycles without creating burrs on the parting line or crushing the venting.
13,There are various methods of measuring and checking in mould making: special modelling clay, Plastic Gauge, spotting ink, feeler gauges, glass needles, shims, Brinell magnifier...
Spotting ink and Plastic Gauge are methods that can show how the mould behaves at full clamping force, when it is necessary to take into account the elastic deformation of the mould according to Hooke's law, below the yield point...
14,So this is my method; this is how I have modified probably hundreds of molds.
Venting in slides and ejectors has the advantage that it is not affected by clamping force and is not influenced by mold settling.
It is clear that with classic venting in the parting line, if the clamping force is increased above a certain limit, the venting closes, gets pressed shut and sealed as the mold settles. After a certain number of cycles it then settles further due to material fatigue.
Ejectors and slides are flush in a plane perpendicular to the clamping force, and that has no effect on the venting.
This is worth using wherever possible, and especially on "cut‑corner" molds with a small supporting surface.
15,anything that doesn't move will quickly get clogged and needs to be taken apart and cleaned.
Anything that moves gets clogged more slowly.
Ideally, it moves, is accessible, and has the right groove shape that does not get clogged by condensate.
16, a standard, highly effective method of venting, which should, however, be applied already at the stage when the mould is being designed and fine-tuned by the designer, is the use of ejector pins at the locations where air is trapped.
The graph shows the measured values at an overpressure of 1 bar for a given ejector diameter with a fit ød g6/H7 – L4 x ød.
The graph also includes values measured for ejectors lubricated with a thin film or a thick layer of lubricant.
17,This slide shows the tolerance zones in the ISO system and illustrates how significantly individual clearances can differ from each other.
This is why specific ejector positions in molds should be marked, even if they have identical nominal dimensions.
It is much easier to trace the cause of a defect when it appears consistently in the same place, rather than when it moves around depending on how the components are assembled each time.
add an image of an L-shaped hole 4–6×d and 1–2×d
18. To give an idea of what happens in a non‑vented mould, Aleš Ausperger at Libeos created an animation for one of our test moulds that we use for trials. It is a simulation for a mould where venting is missing or has become clogged.
The air pressure of 260 bar in the animation is an extreme that probably does not occur in reality, but notice what happens when plastic flows into a clogged mould up to about ¾ of its volume.
A part that is ¾ full = gas pressure in front of the melt of 10 bar and a temperature of +300 °C, which is already a value right at the limit or above the processing temperatures of common plastics, and it can be expected that the plastic will behave differently than usual.
At higher temperature the viscosity drops, the melt suddenly accelerates, the air can no longer keep up at all and overheats much more, until a burn mark occurs, a diesel effect, eroding the surface of the mould and the parting line (cavitation and flash). Another effect is that hot compressed gases and combustion products are forced into the parting line and into a partially clogged gap. After passing through the gap they lose speed, lose pressure, are hot and then cool down… (the triple point does not work only at the edge of clouds) they condense and gradually seal the gap completely, and the whole process gains dynamics, accelerates and ends in a clear, heavy diesel effect with erosion. At this stage, mould damage then progresses very quickly. This is called an exponential curve… nothing happens for a long time and everything seems fine, it grows slowly, then speeds up and shoots up steeply…
And again there are two main effects and phenomena that mutually potentiate and accelerate the degradation of the plastic at the end of flow up to the diesel effect, when the combustion products and the plastic itself, plus the surrounding steel surface, become carbonised, ignite and ultimately erode the surface of the mould and the parting line (cavitation and flash). Another effect is that hot compressed gases and combustion products are forced into the parting line into an insufficient and partially clogged gap. After passing through the gap they lose speed, lose pressure, are hot and then cool down… (the triple point does not work only at the edge of clouds) they condense and gradually seal the gap completely, and the whole process gains dynamics, accelerates and ends in a clear, heavy diesel effect with erosion. At this stage, mould damage then progresses very quickly. This is called an exponential curve… nothing happens for a long time and everything seems fine, it grows slowly, then speeds up and shoots up steeply…
19,here is an example comparing venting with a regular standard ejector and a very high‑performance venting ejector. And in the last video, when it is played, the self‑cleaning effect can be heard. Listen to the sound when the air flows through the correctly shaped groove. It is not rocket science; when the air flows at a sufficiently high speed, the slot, which works like a nozzle, does not clog because the gas does not have time to condense in it.
20, Thank you for your attention. Martin Marek HelixPin
