Well drilled and well placed vent holes could make the difference between a successful or a failed form
Surely most of you reading this article are already familiar with the vacuum forming process - a heated plastic sheet is pulled over a mold using a vacuum, which forces the sheet to take the shape of the mold.
Creating a strong and effective vacuum is something that Vaquform’s DT2 does exceptionally well. However, a strong vacuum is only part of the equation. In order for the vacuum to be effective, your Vaquform mold must allow air to freely escape from all its parts.
This is where vent holes come in.
What’s a Vent hole?
A vent hole is a small hole that you drill into your Vaquform molds. Vent holes can be many different sizes and be placed in different parts of the mold but they essentially have the same purpose. Vent holes allow air to escape from areas of your mold where they might otherwise get trapped during the forming.
Why do you need Vent holes?
Trapped air becomes a problem when molds have concave features relative to the forming bed and the sheet.
Here you can see the effects of trapped air on the formed sheet. The plastic is unable to fully envelope the mold because air cannot escape from in between the sheet and the mold, especially in the corners of the trapezoidal shapes.
When vent holes are placed in these concave features, the issue of trapped air gets resolved, and you end up with a clean and successful form - The sheet fully covers all the corners, bends, and other details of your mold.
How to drill vent holes
The concept of drilling a hole is simple enough, but when to take into consideration things such as the small size of the vent hole and the material that you're drilling into, it gets a little more complicated.
When drilling vent holes, your two biggest concerns are:
- The expulsion of material from the hole you're drilling.
- The dissipation of heat caused by friction between the drill bit and the mold material.
Drilling without giving thought to technique generally means simply pushing the drill through a material until a hole appears.
The problem with this method, as seen in the video, is that the material has difficulty escaping from the small hole and even smaller drill flutes, and that the heat from the friction may melt material with low melting temperatures such as 3D printed PLA. The vent holes you end up with leave a rough imprint on the surface, and will have diameters that are not consistent with the diameter of your drill bit.
Here are some drilling techniques you can use to help solve these problems:
This technique allows for adequate dissipation of heat, and allows for material to be expelled as the drill bores further into the hole.
In the example above, a 0.7mm drill bit is used to drill through the material. A conventional drilling technique results in the melting of the plastic material, which wraps around the drill bit when it's retracted, and then solidifies. This renders your drill bit ineffective at drilling clean holes. Furthermore, if you use the drill bit without removing the melted plastic, it will increase the effective diameter of your drill bit, and you will get much larger holes than the size of your bit.
Here we have a side by side comparison between the quality of vent holes you get with two different drilling techniques. The difference is clear. You get much better quality and more consistent holes with the peck drilling technique.
Lubricant & Water
Another way you can make your vent holes better is by using water and lubricant.
When it comes to materials such as PLA (used in 3D printing), the heat that comes from the friction between the drill bit and the material, is enough to cause melting.
Before you start drilling holes, we recommend spraying the surface of your mold with water, and dipping your drill bit in any lubricating agent you may have, such as spray lubricant, grease, oil, petroleum jelly, etc..
Misting the surface of the mold with water aids in keeping the surface of the mold cool, while the use of a lubricating agent allows easier passage of both the drill through the material, and the expulsion of material.
During the actual drilling process itself, drill the holes quickly; in and out, one quick motion*. This ensures that the chips don't have time to melt on the drill flutes.
*note: You ought to practice this movement a few times. Be sure to drill straight and true or you might bend or break your drill bit.
Drill Bit Options
You have many options when it comes to what kind of drill bit to use and what size is best for creating the most effective vent holes.
When it comes to what your drill bit is made off, most options will get the job done. However, there are some notable differences among the options you have:
- H.S.S. (High Speed Steel) is low cost and isn't brittle like the other materials. But as a softer material, its edge isn't as sharp which may increase friction, and therefore, heat build-up.
- Tungsten Carbide has very sharp edges which means it cuts quite cleanly. The trade-off is that it tends to be more brittle than other options, which may be a problem with smaller sized bits.
- Cobalt is a happy medium between the HSS and Tungsten Carbide. It's able to flex when a carbide bit would snap. It will also retain a sharp edge longer than regular HSS.
- When it comes to using drill bits made of Tungsten Carbide, the minimum diameter you use ought to be around 0.7mm. Any smaller and you're bound to break the bit as you're drilling the hole.
For creating general purpose vents, use 0.7 to 0.8mm drill bits. If you are using small forming tools, you can go as small as 0.4 to 0.5mm; there is rarely a need to go smaller than 0.5mm, since it is very difficult to detect the dimple imprinted by a 0.5mm diameter vent.
Generally speaking, several small vents scattered around are better than a single larger one located in a single spot. This makes for a more discrete hole while still being able to attain a high flow rate of air. This also makes for a more balanced stretch of the sheet
Where do you place vent holes?
The drilling vent holes for your Vaquform molds, is generally dictated by the presence of any concave or negative features*
*Any feature or shape on a mold that is inwardly curved such as hollow features, sharp inward corners, debossed details, and all other places where air might get trapped during the forming process.
In this example, the concave features of the mold are the trapezoidal cutouts, and the concave circle they surround (highlighted).
Ok but where EXACTLY do you place vent holes?
So far, we’ve established that we need to place vent holes in places where air might be trapped. But where to place vent holes exactly along those areas, could be the difference between a failed and a successful form.
- When placing vent holes, the idea is to place them along corners and edges, where the plastic will touch LAST as the plastic slowly envelops the mold during the forming process.
- The reason you want to go for these hardest to reach areas when strategizing where to put vent holes, is that you want to ensure that the holes do not get covered by the plastic BEFORE the plastic sheet has fully formed into or around the mold.
Let's take a look at this mold as an example. There are three circular cut outs, each around 3mm deep. We conducted a test pull with different vent hole configurations in each circle to see how it would affect the final form.
For the test pull, we:
- Did not drill any vent holes for cut out (A) in order to establish a baseline
- Drilled one hole in the center for cut out (B)
- Drilled several holes along the edges of cut-out (C)
Here is the result of the test pull:
- (A) For our baseline, we can see that the sheet barely takes the form of the circular cut out and that you are left with a purely convex curve.
- (B) The sheet was able to form until it reached the bottom surface of the cutout, but the sheet was still unable to form into the edges of the circle. This is because when the sheet was pulled over where the vent hole was drilled, the sheet covered the vent hole. This prevented any remaining air in the cut-out from escaping.
- (C) A successful form! Placing the vent holes along the edges meant that all the air was able to escape until the sheet fully formed into the cut-out.
The form above had a negative cut out with a FLAT bottom surface. If the bottom surface had been concavely curved, the result would have been different.
Here, we have a Vaquform mold made to from blister packaging for a designer toy. This mold requires some vent holes to be placed along the edges [y], and some in the center [x]
In this part of the mold, the vent hole ought to be placed in the center because of the convex shape of the bottom surface. The last area that the plastic will form over during the forming process is precisely where the vent hole is placed: the bottom/center*
*note: when drilling vent holes for larger round concave features, you can drill several smaller holes around the center for a more effective pull.
Whereas for this part of the mold, we place it along the edges, in accordance with how we know the sheet will form if the surface is flat. There are also vent holes along the convex features within this negative cut-out.
Some Final Tips
- Consider doing a test form to see where air gets trapped on a mold to avoid drilling unnecessarily.
- Best practice is to drill only as many holes as is necessary, and that you evenly space out the holes to ensure an even distribution of force throughout the plastic sheet.
Here's a bullet point summary if you’re here for a quick reference, don't have time to read through the whole article, or just have a short attention span just like most of us do.
- A vent hole is a hole that allows air to escape from the negative areas of your mold
- Without vent holes, air could get trapped in negative and/or concave areas of your mold, which will prevent the plastic sheet from properly forming over your mold.
- Look out for two things while drilling:
- Expulsion of material
- Heat building up in the drill bit
- Combat these by:
- Using the Peck drilling technique
- Spraying your mold surface with water
- Coating your drill bit in any lubricant
- Along corners and edges, where the plastic will touch LAST as the plastic slowly envelops the mold during the forming process
- Practice the aforementioned drilling techniques before you work on your molds.
- Use drill bits that are between the sizes of 0.5mm - 0.8mm.
- When drilling with tungsten-carbide bits which tend to be brittle, avoid using anything smaller than 0.7mm
- You can perform a test form on your mold to see where you’ll need to drill vent holes.
- Many small vent holes are better than one big one.
And there you have it!!
Hopefully these tips on drilling vent holes will help you improve the effectiveness of your Vaquform molds, and lead to making better forms with your vacuum former.
If you have questions, feel free to reach out through Facebook and Instagram, or by sending us a message at firstname.lastname@example.org