Precision casting with soft silicone at home, using 3D printing

Some people suffer from fetishism, but others enjoy it, like Orthodox farmers at Easter - a hangover. Why not? “I drank it in the morning - the day is free,” says the famous saying. So the author belongs to the second type, enjoying engineering fetishism in full. But in all we need measure and common sense.

This article is about how much success can be achieved with the use of "non-print" materials, on the old "Prusha", without any frills and special extruders. As a use case, the author moved on a multi-disk system and decided to maximize the installation density of the "spindles" while reducing vibrations. In the struggle for millimeters, a pair of dozens of absolutely identical small parts were needed with tolerances of 100 µm from very soft and "puffy" material. However, if a respected reader has already forgotten about the HDD, like a bad dream, generally abstract from the use case and pay attention to the casting nuances, it is also boring there.

According to the author, consumer vibration damping solutions have a psychological effect rather than a metrological one. Why? Probably because of the tight rubber that transmits most of the energy of the pulsations to the body. Therefore, it was very important for the author to measure the results of the technological process described here, albeit by amateur, but objective means. Miracles did not happen, but about a third managed to play.
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As for silicone, it is often associated with the material of the mold itself, and not with the molded product. Casting into molds in general is a rather tedious and entertaining business, but the author in any case dilutes the story with engineering trash for the public to entertain.

DISCLAIMER

This story is already a year old, I could fall behind the realities. If someone from dear readers mastered precision 3D printing with very soft material, you are welcome, share. The most interesting ideas can be embodied in the form of frames in the article, and if you are reading this article outside of Geektimes, check the authentic link in a couple of weeks, where all the updates and comments from the residents of the club will be available. If the link does not open, you probably shouldn't read more;)

Formulation of the problem

In order to achieve the maximum density of the spindles, the design of the slide must be guaranteed to keep the vibrating body of the disk on something quite soft, but strictly in millimeter from a solid obstacle. At the same time, the total gaps between the discs and the walls of the basket are only about 4 mm, and no one has canceled the slide itself, which must ride strictly along the guides, and not flow along them in the form of jelly.

Traditional approaches

In spite of the textbook nature of the human body, articles about soft materials that appeared at different times here and there are somehow too focused on its separate parts. If you take advertised in 2014. extruder Discov3ry , then, firstly, I do not have data on the softness of silicone (help). And secondly, the result of the “seal” causes me personally associations with a tube of toothpaste, which artistically squeezed onto a bar of soap. Yes, this accuracy should be enough to reproduce male dignity on a 1: 1 scale, but I have another task.

Problems: vibrations of hard drives

Once we started talking about men's virtues, let's move on to vibrations :) Well, when a single body rotates at a speed of 5..7 thousand revolutions per minute, then the vibration is even, imperceptible. But if there are several disks, their high revolutions slightly “walk” relative to each other, and low-frequency modulation appears with a period of one to two seconds . We add to this orchestra a fan in the basket and get a nasty floating hum, which our hearing aid immediately fixates. The faster the fan, the more aggressive the note.



If you add a dozen other disks, to the growth of acoustic noise of the “fan” group, you also add tangible parasitic vibration, which affects both the health of the disks themselves and the comfort of the people around them. Manufacturers of disks “for NAS” have already paid attention to this problem, struggling with both sources and the consequences of vibrations. At least, on paper, because, first of all, manufacturers care about volume, and to throw away precious millimeters of radius is still more expensive than investing in marketing. But even in the brochure we are talking about 5-disk systems.

The computer case works as a complex resonator, suppressing some frequencies and amplifying others. Therefore, the resulting noise spectrum of the system is rather unpredictable in its profile, but equally unpleasant, and the more disks, the worse for everyone.

So I decided that the problem should be stopped in the bud, creating a vibration-damping sled, but not solid rubber, but combining hard plastic with very soft, “stretchy” silicone inserts with a hardness of 25A in Shore. By the way, the softest thermoplastic elastomer for 3D printing has a Shore stiffness of about 85A, which is as far from silicone as a wrestling mat from a down pillow.

UPD: caution, recoil
However, do not get too carried away with a soft "suspension." The user MrRIP drew attention to the fact that the precision (in the full sense of the word) spindle drive mechanic does not like the hanging mode due to the recoil that occurs when positioning the head unit: try to jump from one boat to another. But, looking ahead, I will say: in the worst case, silicone inserts add about 2% to the head positioning time from edge to edge, with the next displacement, the effect is almost impossible to measure. Those. there is a return, but in the case of silicone dampers, it cannot be compared with a suspension on ropes, suspenders or elastic bands from underwear.

Once again about silicone

Silicone is an amazing material in terms of physico-chemical properties. For example, its two-component version with a catalyst on tin or platinum prior to vulcanisation of polymerization (solidification) both in viscosity and in tackiness is reminiscent of fresh honey. But after solidification, the silicone is very inert and has almost no adhesion with anything. Products made of "platinum" silicone can be safely used in the kitchen, and certainly in medicine as well. This limitless chemical inertness of silicone is sometimes annoying: in order to glue the silicone part, it must first be coated with a special primer, the usual glue does not take silicones.

Used two-component silicone is dosed in a ratio of 10: 1 with a catalyst by weight. The densities of both liquids are close to unity, so it is possible to dose in the right ratio and by volume. Pulling the “honey” with an ordinary syringe is inconvenient, but with the use of a dripless dispenser from the antipyretic drug Nouwonahren - just right. The solidification takes several hours, so you can mix the silicone with the catalyst, slowly, in a small cup, preferably with a round bottom, but you can and so.

If the instruction allows contact with the skin, it is most convenient to mix not with a stick, but with your own little finger: soft tissues collect material well from all nooks of dishes; for healthy skin, silicone is completely harmless UPD: but work with gloves, out of harm's way. Before solidification, the material is dissolved with white spirit , after that it is easily removed mechanically, but only from smooth surfaces (do not wipe yourself, stock up with napkins beforehand). To evaluate the properties of the material, try to separate the thinnest layer remaining on the cup after solidification, it stretches, but almost does not tear.
Silicone freezing with platinum catalyst can inhibit (break, retard) various materials, including sulfurous, latex, some rubbers and even wood. But the main thing is that ABS and PLA plastics do not belong to hardening inhibiting materials , which means that you can print a mold and get a product from a very soft and inert material with a previously unattainable tolerance . Just like that: we take and cast everything at home. Or not everything?

Mold for small parts casting

Usually, they take, roughly speaking, a bucket of silicone and mix it with a mixer with a bottle of catalyst, then pour it into forms and start degassing , i.e. remove air bubbles. But in order to better understand the process of pouring silicone into a “group” mold for small parts, the reader is asked to imagine loading honey into the honeycomb. Yes, I am so drawn to hexagonal shapes ...

The usual form may consist of two halves, printed by ABS and compressed with five screws with press washers and wing nuts. If the external symmetry of the form differs from the internal, it is recommended to make a “key” in the form of a “sawed” angle, as on a processor socket. Otherwise, you can confuse the orientation of the halves and cast something unexpected, spending a lot of time in vain. It is recommended to bring the adjoining edges after printing to smoothness by polishing, but do not soar them in acetone. In the morning, we open the form and pick up fresh dampers, for some we have the thinnest, easily removable "collar", arising due to poor contact and degassing features, which are described below.

Case of bubbles

"Magic" bubbles can be considered as those in drinks, whirlpool baths or, at worst, in washing machines. When casting, the harmfulness of bubbles is inversely proportional to the dimensions of the part, i.e. degassing is needed the stronger, the smaller the detail. Otherwise, the bubble can easily destroy a key element of the type of carrying "trunk" damper with a thickness of a couple of millimeters and a length of five. That is why, by the way, when you stir with your little finger, even, excuse me, a three-day nail creates bubbles like a paddle in water. Tested in practice, it is better to shave to zero to work with gloves.

Silicone sellers to my question about the method of degassing, without batting an eye, recommended using a vacuum chamber . But here's where to get it at home, cut it out of the microwave? So, the pressure will have to be used at normal atmospheric pressure, but then how to remove the bubbles with a clear lack of buoyancy force? It, by the way, is proportional to gravity, however, it does not fly to Jupiter. And here I recollected one story about prototyping metallurgists, cleaning bubbles from another liquid alloy in ... a centrifuge. Since even the metal was obeyed in the workshop, we can do anything with silicone even at home.

I assembled the simplest centrifuge from a chain and ballast, which during rotation orients the plane of the form strictly perpendicular to the radius, creating a unidirectional multiple dominance of the centrifugal force over all others. If you rotate a shape similar to a brush on a meter chain in a vertical plane, to overcome gravity you will have to do about 30 rpm, which will create a “pulsating” acceleration from zero to 2g. This is about how to fly from Pluto to Jupiter and back. The centrifugal force is directly proportional to the length of the chain and the square of the rotation frequency, therefore, by increasing the chain to one and a half meters and pushing up to 60 rpm, you can get a sixfold gain, i.e. approximately 1g..7g.

Enough for a start, but there is a nuance. When the bubble pushes out, how to pour silicone into its place without disassembling the form? This is easy to do if you provide fill holes in the upper part, adding to them the top of the technological sink with a supply of material. The bubble will float, the contents of the sink will flow down. But in order not to splash silicone with such vigorous rotation of the form, I added a third element to it: the cover.

UPD:
The casting workers and vacuum workers came here, gave me a few ideas on the vacuum chamber, at the same time writing out both “precision” and “silicone vulcanization”, and “mold”, and many more for that, but gave some tips .

Shrinkage

Some materials shrink after solidification, but the silicone I use is non-shrink, as it is written in the instructions.

Degassing

Equipment for degassing (removal of non-magical bubbles) is a vacuum chamber with a pump, pressure gauge and other fittings. The user hungry_ewok suggested in the comments a whole monument of plastic sewage parts with a section of 300mm, a variant from dlinyj - an old pressure cooker. At the same time, hungry_ewok and kellakilla recommend a Komovsky condo pump (from a physics classroom, can be driven by an electric motor), and wormball , MotttoR and dlinyj suggest using a compressor from an old refrigerator.
Pumps like care and oil, do not like abrasives and aggressive fumes, can give off light oil air suspension. The fittings fit plumbing. The required vacuum depth depends on the viscosity of the material; vacuumization can be done in two stages: first, the material itself in the bucket until complete boiling away, then the filled form.
For small forms, a syringe for 50 “cubes” is enough: the bubbles can be removed directly in the syringe, holding the “nose” to it. But it will not affect the pouring of the form itself, it will only remove the effect of “soda” - Igor_omsk , BigBeaver , Wandy .
ATTENTION: Depending on its cosmic depth, the vacuum can be traumatic. At a minimum, safety glasses are required. Doubt - buy a camera for decontamination in the store, there is a safety instruction. It will be with whom to ask.

The form

It is clear that during serial production it is very important to optimize the casting mold so that it does not have to be twisted in a centrifuge or shaken on a vibrating table. This is the real, professional engineering. However, I have an emphasis on the amateur approach, for a single prototype it is possible to equip a centrifuge, it will simply remove some form errors by brute force. And, by the way, “for centrifuging it makes sense to make a round multi-bed shape with sprues from the center”. Thanks to vbifkol , rfvnhy , BigBeaver .

Clearly, the acceleration in 7g seemed to me a little, so version 2.0 of the centrifuge uses little mechanization in the form of a drill clamped in a rack with a rocker arm in a cartridge. At 400 rpm, the shoulders with a form and a weight deflect perpendicular to the axis of rotation, describing a radius of 10 cm and giving an almost “sunny” 18g, expelling bubbles even from very thick honey.

If you need to cast a lot of parts, it is good to have two forms of the same mass, and it will not be so harmful for a drill. But I took my one single form and again visited the fastener shop, this time weighed a white piece of plastic with screws and "sheep" in front of puzzled sellers. As a result, even managed to use weights that are compatible with version 1.0. But the reader can do better by going to the grocery store and choosing any hard fruit or fruit of a suitable mass, for example, a turnip or a sour apple. It is inconvenient to grip carrots with a bolt, but nothing is impossible. However, see below about safety, grab in a supermarket and form with fasteners, it is easier and more legitimate than to drag the scales home from the store.

Safety engineering

If you print and mount a cover for a form that is too lazy, think about splashing the environment with harmless, but sticky, drops. If the form is small, you can use the salad bowl of the appropriate size, dropping the form into it right on the rack, then you only have to let off the salad bowl, but it is better after solidification. But still the lid is somehow more technological. I tried and so and commercials.

UPD:

Carefully read the safety documentation issued by the industry regulator.
I give the found excerpts ToolDecor 15A for professional use.

Brief verbal description
Low hazardous effect on the body slow-burning mixture.
8.2.1 Limiting and controlling exposure at the workplace
General protective and hygienic measures:
Do not eat, drink or smoke when working. Wash hands after work and before meals.
Individual means of chemical protection:
Respiratory protection
Not required
Hand protection
Recommendation: Protective gloves made of butyl rubber, protective gloves with a neoprene layer, protective gloves made of PVC. Gloves are suitable for use up to 60 minutes. The choice of suitable gloves is determined not only by the material, but also by other qualitative attributes, which differ significantly from different manufacturers. When selecting gloves, take into account the permeability and break times specified by the manufacturer.
Eye protection
Protective glasses

Do not forget about the physical safety of loved ones and surrounding objects during the rotation of cargo, its head and other parts of the body are invaluable. When using chain 1.0, it is better to wear a helmet on your head, retiring to a room without windows and mirrors. Surrounding can be warned, but the sight of a person in a helmet with a brush scares and so. When using a mechanized centrifuge 2.0 you should never hold a drill with an unbalanced accessory in your hands without having the grip of a Terminator. In general, the imbalance is not useful for the drill, so it is necessary to balance, at least approximately. I firmly fastened the drill in the rack, the rack fastened to the table with clamps, and during launches I hid behind the screen from a 16-mm chipboard, holding my own loins closer to ... the vice in which this plate was clamped. And not what some readers thought: not to break through the grip, behind them is the safest place.

If a dear reader ever changed the cartridge on the drill, then surely paid attention to the bolt with the left thread. Fortunately, I remembered this feature in time, and the spontaneous separation of the rotating rocker arm was prevented. By that moment, the right-hand bolt from the “straight” stroke of the drill is already weak enough enough for the centrifuge to start suspiciously ignoring the increase in speed commands. The screen was not useful, but if something had happened, it would be very appropriate. If you clamp a right-hand screw in the chuck and fix something on it with a nut, the reverse mode of the drill is obligatory, otherwise the nut will turn away, the load will break and fly away somewhere. Take care of the bolts , dear friends;)

Mat

The savvy engineer, of course, will take a professional CAD system and, by the rigidity of the material, will model the shape that is optimal for a given “sagging”. A real ninja will just remember the mat and count everything on a piece of paper. However, part of the experimental conditions was the use of free software, I don’t have practical support for material. Therefore, a specific type of dampers had to be chosen experimentally, having printed a trial form of dampers of various shapes. Then they were alternately fastened on a sled (there was no glue at this phase, the hexagons proved to be excellent). A skid disk held by the fingers was carefully “worn” on a slide clamped in a vice with dampers upwards, after which the reaction to the static mass was studied. In other words, I decided to solve the problem of raw materials by the method of direct physical modeling , sorry for ignorance, dear readers.


Experimentally, those dampers were chosen that, after sagging, they left one millimeter of stock to hard plastic. Then the form was corrected and reprinted. The ebb, finally, the first batch of dampers, I fixed them in a printed slide, using ... "instant" cyanoacrylate glue. Of course, it is poorly suited for flexible silicone and doesn’t really like heating, but there wasn’t a special primer on hand, it holds at least somehow.

Performance measurement

DISCLAIMER: The author is aware that he measured not the basket vibrations, but the vibrations of the smartphone somehow fixed to it, with accelerometer limitations and partial loss of the pulsation energy. During the experiment, not a single smartphone was damaged: you wear the device for a couple of years without any problems, but you should give it “for temporary use”, the screen is broken for a week guaranteed ...

So, the smartphone was chosen by weight: not new with a shovel screen, but a light old Samsung S3 Mini, so as not to dampen vibrations. The lid was removed so as not to spring. There was a lot of fuss about fixing the device on a 5-disc basket using aluminum strips and a stack of gum for banknotes. From the applications I stopped at the VibSensor : the layout along the axes, the adjustable averaging period, the graphic spectrum, the preservation of the results - all that is needed for such experiments. Working with the screen, sandwiched by the plate is a separate art.

But a metrological problem arose: even the averaged results of the vibrometer in a few minutes were walking simply indecently for unknown reasons. How do you like this series: 36, 35, 31, 28; or, for example, 51, 42, 68, 61. Another would have spat and counted on some average, but I remembered the presence of thermal calibration in the disks, which occurs at the most unexpected moment, sounding like intensive head work. And when with the help of smartctl it was possible to force all five participants in the experiment to simultaneously pass the endless surface test, 25, 25, 26, 25, 28, 27 or 18, 18, 17, 17, 19, 17, 19 turned out to be enough. it was possible to even talk about something.

I will not give the tables, only the dimensionless results for a fairly variegated company from 5-year, 10-year-old models and contemporaries. Let us assume that the disks do not relax lying flat, but work in a rack “on the side”, with the axis of rotation of the spindle parallel to the horizon. Each such disk at work jumps up and down and fidgets back and forth from the rotation of pancakes, and also dangles left and right from moving the head unit inside. When using vibration-damping dampers, a fixed smartphone weighing 110g receives two times less energy from bouncing compared to hard slides, and from fidgeting and rattling it gets even less by about 20%. Why? Because gravity creates a more durable channel of pulsation energy transfer between bodies. In other words, other things being equal, the gain from the floor mat is greater than from soft walls.

Is it a lot or a little? Tactilely, on one basket only, this is not understood, but it is worth installing the basket into the case, subjectively, the difference is already felt even on 5 disks. However, I want to remind you that my main goal is a demonstration of the technological process, and I was not going to be limited to 5 disks;)

findings

• "Print" the unprintable - excellent gymnastics for engineering ingenuity.
• The effectiveness of vibration-damping decisions should be confirmed by objective measurements, and not be based only on subjective “worse / better” assessments.
• For degassing, instead of a vacuum chamber, you can use a centrifuge, “shedding” small and complex elements of parts.
• The described technological process is very accessible and applicable not only to the combination of silicone and ABS plastic.
• The limiting factor, in addition to the complexity, is the size of the print.
• The combination of silicone inertness and 3D printer accuracy opens up new niches in areas such as telemedicine and prototyping complex products.