Archive for the ‘Education’ Category

Admittedly plastic welding is a fabrication technique that is going to be attempted by few “Do-it-yourselfers”. Plastic welding will require the purchase of special tools and filaments which are not always easy to come by.¬† In addition, since most plastic welding is done by specialists, it is difficult to get fabrication tips from plastics distributors or manufacturers. You’re likely going to have to search the internet for ideas and while we cannot vouch for the tips, YouTube seems to have several good videos to get you started off right.

In particular the company “Techspan” which manufacturers plastic welding equipment has, what looks to us, as a good 4-1/2 minute video on the basics of plastic welding. In particular we like that it talks about preparing the plastic for fabrication/welding (something often overlooked in these types of videos) and it goes into topics such as “tacking” which is welding two pieces on a 45 degree angle to each other. We’d prefer to let the video speak for itself and we’ve linked it below:

 

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Sheave Design: Advanced

Posted: August 9, 2017 in Education
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On our previous post we taught you the bare minimum that is required to design a simple sheave. Now, we’re going to discuss some ways to add advanced features to your sheave. Firstly, is how to calculate webbing. “Webbing” in regards to a sheave is removing material outside of the hub and outer diameter to lower the overall weight of the sheave. A webbed sheave would look like this:

 

 

 

 

But how do you know how much material you can machine off? The math is actually quite simple: W = 1.2r where W is the “minimum web thickness” and “r” is the radius of the rope or cable. So for example, based off a 1″ diameter rope the minimum web thickness is 1.2″. This naturally segways us into a discussion about the hub. As you can see in the picture of the webbed sheave, the hub has to be wider than the webbing. But by how much? Again, the math is simple: H = 1.5b where H is the “hub width” and b is the bore size. Lets assume the bore is 1″ again, you would then require a hub no thinner than 1.5″. Typically the hub is at the very least as wide or wider than the rim, so always run off that rule of thumb.

Finally, the last calculation is to figure out a press fit if you’re going to push a bearing into your sheave bore. You need to know how to precisely bore but leave just enough room that the bearing won’t slide around. That is done as follows:

 

This will give you the bore diameter you require to fit your bearing. You now have all the tools to make not only a sheave but a fairly complex one if you’re so inclined. If you want to download the whole Redwood Plastics sheave design manual, where this information was taken, you may do so here.

If you need a quotation on some sheaves or sheave materials please contact Redwood Plastics.

 

 

 

 

 

We found a great little video on YouTube by user “Make:” which in just three minutes (before credits) gives all sorts of great tips for the DIYer using acrylic in their applications. It starts off with a good point that not everyone knows: acrylic comes in both cast and extruded forms. Yes, they have differences that are important in an application. Yes, there is a difference in cost. Other important tips include how to thermoform, glue, and most importantly – drill, the plastic. Drilling is especially important as the plastic can crack easily, so using lubrication and a soft touch is essential to prevent a wasted part. It will help you answer some questions you didn’t even know to ask prior to diving in: do you have all the materials you need? How are you planning to smooth the edges of the acrylic?

We’ll let the video do the rest of the talking:

Are you a “DIYer” who makes plastic bearings at home or perhaps looking into doing some? Not sure how to calculate a press fit or a running clearance for your bearings? Fortunately there is a useful tool available from Redwood Plastics available¬† on their website: the machinist chart for plastic bearings found here.

This chart provides valuable information for bearing manufacturing using Redco 750 or Redco nylon bearing materials. While many in the DIY crowd like to use UHMW polyethylene for everything, including bearings, this is not a good bearing material and has large and variable tolerances. The bearing chart is not intended to be used for UHMW bearings. Acetal is similar to nylon and therefore nylon’s values can be substituted directly.

Choosing Your Plastic

Posted: March 8, 2016 in Education
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The question comes up a lot in the DIY community: “I am planning to make _______ so what plastic do I need?” The good news is selecting a plastic for your application is very similar to how heavy industry would select a plastic for their projects. You need to select a plastic based on certain properties it gives you, not some sort of bias to what you “heard” worked – and especially not based on color!

So when thinking about your what you need for your application consider the properties of these plastics:

 

UHMW Polyethylene:

-Slick, low-coefficient of friction

-Good wear properties

-Good abrasion properties

-Thermoformable (at home)

 

Polyurethane:

-Elastomer: will regain its shape after depression (excellent as mallet heads)

-Excellent cut & tear resistance

 

Nylon:

-Bearings (takes up to 4000PSI)

-Wear plates

 

Tuffkast:

-Replaces nylon in cold operating environments or where impact is a concern

 

Acetal:

-Small parts, replacing metal

-Where machinability is key, can hold tight tolerances

-Replacing nylon in “wet” applications

 

PVC:

-Anywhere a “frame” is needed.

 

Industrial laminates/Phenolic/Micarta:

-Mechanical or electrical applications

-High load bearing

 

We wanted to bring up something amateur plastic project enthusiasts and “Do-It-Yourselfers” often don’t consider until something goes wrong: the weathering of plastic. This was inspired by the a recent visit from a “diy’er” Jason, a hunter. Jason wanted to show us a table he built for processing his game. It was a homemade outdoor table with a white plastic surface likely natural or virgin-white HDPE or UHMW polyethylene. The surface was discolored and had numerous small cracks where Jason (rightfully) was concerned about bacteria growing in the cracks but even more so, he was curious on what caused the plastic to degrade.

The answer for Jason and the culprit with many plastics is UV (ultra violet light or sun) exposure. The chemistry would take to long to explain but suffice to say the sun has damaging effects on plastic. Many plastics become more brittle and crack while others discolor, usually by becoming yellow. If you’ve ever been to an aircraft museum and seen the yellow tinted plastic (polycarbonate) canopies on aircraft? That’s from weathering and UV-exposure. If you look, very closely, at those canopies you’ll see subtle cracks that create a type of haze obscuring vision, as in the picture below.

In most plastic applications for home machinists and project enthusiasts the UV exposure is more of an annoyance than anything. We recently had a sailboat owner frustrated with the discoloration of his polycarbonate hatches he made: the same issue with those aircraft canopies. The most important thing for your project is to figure out of there would be a safety issue caused by a part weathering. In the case of Jason the hunter the cracks on his table might harbor bacteria but since the meats would eventually be cooked (and raw meat has lots of bacteria anyways) this probably isn’t a critical issue. However, in another application – such as the increasingly common homemade roller coasters – the failure of a part, such as a wheel, could be very dangerous.

If you want UV protection there is some good news. Most plastics can be procured in UV-stabilized versions. The problem is these versions are more expensive and sometimes prohibitively more expensive if the material needs to be shipped in on a special order. In the competitive world of plastic distribution there simply isn’t margin or warehouse space to stock the UV-stabilized version of every plastic. The other thing you need to realize is that UV-stabilization in plastics isn’t permanent, it just buys you time – usually no visible degradation over 10 years – but the plastic will eventually lose its UV-stabilizing properties and degrade.

The point of this article is to get you thinking and remember to consider on your next project if weathering is a concern. And for those of you with weathered plastic in your applications? Well now you know why!

For more information, contact Redwood Plastics.

weathered_Canopy