Gas station without pumps

2013 June 30

SMD tools and custom plastic parts

Filed under: Uncategorized — gasstationwithoutpumps @ 02:02
Tags: , , ,

I did a little printed-circuit board design over the last couple of years, designing some motor controllers for Arduinos, some sensor boards, and some boards for my Applied Circuits class.  Almost all my designs have been for through-hole parts, so that I could assemble them with a soldering iron.  (That was an essential constraint for the boards for the course, as the students had never done any soldering before.)  Some of the sensor boards had surface-mount parts, but relatively easy ones to hand solder (0.05″ gull-wing leads).

Almost all electronic parts are cheaper and smaller in surface-mount packages, though, so I’ve been wondering whether I should get a hot-air rework station to be able to work more easily with surface mount components.  Sparkfun has a hot-air rework station for about $100 and Amazon has several, including a Kendal 898D 2-in-1 with a soldering iron for about $82.  I’m curious about how good these tools are and whether they are worth the price (my soldering iron, which works fine for through-hole parts, is only $15 from Parts Express).

One reason I’m considering getting a hot-air rework station is that my son is getting interested in hardware.  He’s been interesting in programming for years, and last year he did some fairly low-level programming involving interrupts for the data logger, but he’s not been particularly interested in the hardware. This summer he’s been teaching himself to use Eagle to do a surface-mount printed-circuit board design for a product design he and a friend of his came up with.  I don’t know whether they will ever finish the design and actually manufacture it, but their goal is to get a completed design by the end of summer and do a Kickstarter project to get enough funding to do a small run of 50.

My son has been learning a lot about product design, computer engineering, small-scale manufacturing, product pricing, design for upgrade, wear leveling in flash memory, power supply components, placement, … .  This project is easily the equivalent of many senior engineering capstone projects I’ve seen.

I’ve been learning some stuff along with him, as he asks me for advice and we have to search the web for information.  For example, we found that there are now small-run production services like Seeedstudio that could manufacture, populate, and test 100 small PC boards for about $14 a board (plus the price of the components).  One gotcha is that they need an engineering prototype, so you have to make one by hand first, hence the usefulness of a hot-air rework station.  I even looked into getting a reflow oven (which would allow doing more than one-at-a-time prototypes), but I don’t think I want to spend $390 (for the cheapest reflow oven I could find) when it is not clear that either of us will ever get beyond doing one-at-a-time prototyping.

I think that they may be able to make 50 of their devices for about $3500 for the assembled electronics and they want to sell at around $100–120 retail (direct sales).  Their biggest problems will probably be in the mechanical design: making the custom cases and stuff like that.  Although they have access to a 3D printer, I don’t think it has the precision and strength of materials that they need—they could probably make a prototype, but not 50 units that way.  Small scale manufacturing for mechanical parts seems to be harder to find and more expensive than for electronics.  Companies like Shapeways can do higher quality 3D printing, but the cost would be about $50 for the product—way too high for their $100–120 price point.

Short-run injection molding companies  (like DragonJewel) also exist, but the tooling is expensive: probably about $3000–5000 for what they want, which is too much for a run of only 50–100 units.  (The same tooling could be used for a run of 1000 or even 100,000 units, at which point it becomes cheap, but they’re not thinking of taking their first design to that scale.)  I mentioned DragonJewel by name only because it was one of the few sites we found that had any sort of estimate of costs.  Most of the sites are “quote only”.  Of course, real prices will require a quote, but it is good to know whether you are talking $300, $3000, or $30000 before you even start thinking about a service. Protomold has an automated quoting system, where you just upload a CAD model and they quote the costs, but they do have a minimum of about $1500.

Resin casting seems like a more feasible technique for very small runs.  Companies like Specialty Resin and Chemical claim prices like $50–100 for a mold and $4–8 a part, which is in the right ballpark for the cases for their product, if cast resin is suitable for the cases.  (Again, I picked this company out of many found by Google, because they gave a rough estimate of prices, and not just a “call for a quote” number.) The mold has to be made from a sample, but 3D printing by Shapeways might be a suitable way to make the sample. For that matter, there are companies (like Scicontech) that make molds from 3D printed models in house and do the casting, so there may be ways to go from CAD file to molded part quickly within one company—I’m actually surprised that Shapeways doesn’t provide that service. Perhaps the constraints on what is moldable are too complex to communicate easily.

 

 

2 Comments »

  1. You could get the case/enclosure made with a 3D printer, and then use it to create a silicon mold. You then use the silicon mold to cast urethane parts which may have acceptable mechanical properties, depending on the design and details required. First you make a box out of aluminum or plastic, and then fill it with the silicon mold material and your 3D model, let it set, and remove the master. Now you can cast urethane parts. You’ll also want/need a vacuum pump for de-gassing the silicon and urethane. It’s all reasonably doable and doesn’t cost much. Time and patience may be the biggest factors.

    You need to plan your box/mold carefully so that you can remove your parts, but the flexible silicon helps with this. You may also want to vacuum bag your mold while casting the urethane parts.

    And for all I know, nowadays there may be better materials/resins to use than urethane.

    Comment by Doug Smith (@bcphysics) — 2013 June 30 @ 22:42 | Reply

    • Silicone molds for urethane resins do seem to be the common approach. I don’t know whether either of the boys is good enough with tools or interested enough to make their own molds for the cases, and the cost of having the mold making and casting done professionally (assuming that they design the case with the limitations of silicone molds in mind) is not prohibitive (unlike the mold making for injection molding, which would have to be amortized over more than 1000 copies).

      I think that outsourcing the case manufacturing would make sense (given that I don’t want urethane resin all over my living room). Even selecting the right resin for the case could be difficult for a beginner, but trivial for a professional.

      Comment by gasstationwithoutpumps — 2013 June 30 @ 23:39 | Reply


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