Posts Tagged ‘uhmw’

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.

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UHMWPE (Ultra-High Molecular Weight Polyethylene) often abbreviated as “UHMW” is one of the most popular and well known industrial plastics. UHMW is seen as a “Jack of All Trades” in many applications while being more available and less expensive than other plastic options. While this is true to a degree, UHMW has limitations that means it will not solve all problems in all situations. In fact, it has some properties which make it deficient and the “DIY” community should keep this in mind.

When to use UHMW:

UHMW excels at taking impact and it is virtually unbreakable, it’s also very slick and the colder the operating temperature is, the better it performs. This is why UHMW is so popular in applications such as toboggans or as rails on sleds: impact + cold + slickness are all important to the application, and it does really well in this application. UHMW is also easy to work with as it drills, saws, and lathes well. It functions as wear pads, sliding blocks, fenders, and in other impact absorbing applications.

When NOT to use UHMW:

The main application we here at Redwood Plastics see UHMW incorrectly used in is bearing applications. UHMW is a poor choice as a bearing for two reasons:

1.) it is difficult to machine to tight tolerances (+-) 0.02″ are the best you can usually hope for and

2.) UHMW has low load bearing capabilities (500-800PSI). In a DIY bearing application nylon or Tuffkast will be superior.

The issue with tolerances is important to hit home: UHMW is not a dimensionally stable material. Not just compared to metals but even compared to many other plastics. We regularly receive drawings and requests for UHMW parts where we cannot quote based off the requested tolerances (or have to get a written waiver for acceptable tolerances). This lack of dimensional stability extends to thermal expansion and contraction which is why it is so hard to guarantee tolerances: if the part is manufactured in 30 degree weather but used in 90 degree heat the dimensions of the UHMW will drastically change!

We hope this points you in the right direction if UHMW is or isn’t right for your project. But if you have questions, or might be wondering what an alternative material could be, please contact us and we’d be glad to help!

First things first, we here at Redwood Plastics (the administrators of the Plastichowto blog) are happy to answer your application related questions. That said, we might be able to save some time with your future DIY projects by helping you narrow down what plastics you might need for your application. If you’re still hesitant or have additional questions just ask us. Without further adieu, a list of plastics and where they might fit in to various applications:

UHMW polyethylene:

An incredibly versatile plastic, UHMWPE is the “Swiss army knife” of plastics. It’s good at a lot of things, great at a few things and substandard for a few applications. If you need an impact-zone UHMW works as it’s virtually unbreakable and it also works as synthetic ice (though unlike HDPE, it will dull skate blades). It functions well as light-load bushings (under 500 PSI) but cannot hold tight tolerances (+-) 0.05″ is about the best you’re reasonably going to get. We’ve seen UHMW used as sheaths for bladed objects and cutting boards as well. UHMW has benefits of being inexpensive compared to other industrial grade plastics and is also widely stocked.

Nylon:

Nylon’s main application in DIY applications is as sheaves and bushings. Nylon machines well and can handle 4000PSI in operation, which is why it works so well under load. You do need to be aware if your application is exposed to the sun (and therefore need UV-stabilizer) and if your application is marine. For “wet” applications acetal is recommended as a substitute because nylon will swell. In addition, nylon gets brittle below -10 degrees C.

Acetal:

An excellent all-around plastic, acetal can be substituted for nylon in most applications as it handles a similar load for applications like bushings and sheaves. In addition, acetal is the best engineering plastic to machine. It holds tight tolerances and is excellent for small, complex parts. A popular DIY application for acetal is replacing metal parts in paintball guns.

Tuffkast:

A premium nylon-like material, Tuffkast was designed to overcome the drawbacks of nylon. Tuffkast is better in the cold and absorbs much less moisture than nylon. It can be substituted in most applications the only potential issue is that it is softer than nylon, which may mean reduced wear properties in some applications. Otherwise, it is extremely versatile and would delight most DIY’ers if they only knew it better.

Acrylic/Polycarbonate:

Acrylic and polycarbonate are used where you need a clear plastic. Acrylic is not as strong as polycarbonate but it has better scratch resistance. Both plastics are much stronger than glass. In addition, acrylic resin is naturally UV-stabilized where only special grades for polycarbonate are. Safety glass, windows, canopies and tabletops are DIY applications for these plastics.

Phenolic/Industrial Laminates:

Industrial laminates are a very large family of plastics where different resin systems, substrates, and additives mean there isn’t a “one size fits all” phenolic. For the DIY community phenolics are fairly expensive and difficult to work with as you require dust exhausting systems and masks. Some are glass-filled which means special tooling to work with them. Phenolics can handle a large amount of load, often in excess of 20,000PSI and are primarily used in mechanical applications by the DIY community. So anywhere the plastic’s job is to provide strength, rigidity and form. It’s hard to narrow down specific applications for this – we’ve seen almost everything! But what comes to mind is, for example, replacing the metal collars on a sailboat (which hold the sail masts firm and steady) with phenolic. Remember, unlike metal, phenolics won’t corrode due to salt and water.

There are several other plastics we could talk about but their use by the DIY community would be limited. If you’re interested in discussing your application feel free to contact us.

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UHMW polyethylene has countless applications for the DIY community but as we head into winter there is one simple application that we have to highlight on this blog. That is the UHMW plow blade for snow plows. Let’s not over-complicate things: this is just a UHMW strip with some holes drilled in. But in order to get what you need, you need to know what to ask for! First of all know that the material you likely want is the reprocessed-black UHMW grade. While being less expensive than the natural grade, the cross-linking in its production actually improves wear properties. A nice bonus when you’re also saving some money.

You also need to know what thickness of blade you want. In general, the blades are almost always 1″, 1.5″ or 2″ thick with 1.5″ being the most common. If you don’t know what you need, pick 1.5″. Many plastic companies will pre-drill holes for you if you need them for a fee (usually about $80.00) but we would recommend you do not. Firstly to save money, secondly you need to remember that UHMW is not a dimensionally stable plastic. Changes in temperature may expand the blade from when the holes were drilled meaning by the time you get it – the holes don’t line up! To avoid disappointment we recommend you do the drilling if at all possible.

If you have questions about UHMW snow plow blades please contact us.

SAMSUNG

SAMSUNG

Over the years we over at Redwood Plastics (the company that sponsors this blog) get inquiries all the time asking for synthetic ice. Most customers have seen it, but they do not know what it is, how much it costs, or how to install it. This can cause disappointment in people who dream of skating on an all-weather, all-year rink.

The first “fact” to discuss is what is synthetic ice in the first place? It could be one of two types of plastic. Companies that specialized in synthetic ice, of offering pieces that are perhaps 24″ x 48″ or 48″ x 48″ with interlocking segments, that synthetic is usually high-density polyethylene (HDPE) material in 3/8″-1/2″ thick. However, when asking for synthetic ice from a general plastics distributor you will usually be offered Ultra-High Molecular Weight Polyethylene (UHMWPE or shortened, UHMW) this slicker, stronger, and yes – more expensive – polyethylene variant is widely stocked. Almost no plastics distributor will stock purpose-made interlocking sheets of synthetic ice: the market doesn’t warrant the inventory space.

The next question that comes up is cost and here is where a lot of the disappointment occurs. All synthetic ice is “expensive” but UHMWPE, as an engineering-grade industrial plastic, is more so. Prices will vary but a ballpark price for 3/8″ thick material would be $400.00/sheet and $600.00/sheet for 1/2″ thick in a 48″ x 120″ sheet. For a power skater who is also procuring a harness and wants to “skate on the spot” for training, this price can be OK, because only one sheet will be needed. But many potential customers want to build an entire rink out of the material and that becomes cost-prohibitive for many upon quoting.

The next barrier faced in a synthetic ice application is that customers usually do not know how to “make” a rink defer that responsibility to the plastic distributor. Unfortunately, most distributors just sell plastic and do not operate like a hardware store where representatives are well-versed in most potential applications you would use a particular plastic for. They can tell you how slick it is and how it will react to sun exposure all day long – but they won’t be able to tell you how to make a rink. What they can also tell you, which often comes as a surprise, is that the UHMW cannot be “glued” to a floor. You must figure out another way to keep the sheets stationary. Most often this is done by the customer designing a frame which is just big enough to contain their rink: the frame and pressure of the sheets pushing against each other keeps the rink stable.

One of the other common questions we get asked is “how long does the rink last?” Impossible to answer with accuracy since there’s no baseline in how much people use it. But perhaps 2 years is typical before the surface is so worn the sheets need replacement. Half-inch thick plastic will slightly last longer as there’s more area to get chewed up.

Synthetic ice remains an excellent DIY project for the skating enthusiast; however, you need to understand the facts about what it’s going to involved to design and purchase your rink.

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UHMW polyethylene is probably the most popular engineering plastic for use in DIY projects. But how do you work with it? People often get overly concerned with fabrication, especially in regards to the “tolerances” needed for most home projects, but regardless here are some tips to get the most out of your UHMW application.

Sawing:

UHMW can be cut with either a circular or band saw. A band saw is best as it will vent heat away from the plastic and allow for faster cutting speeds. The band saw blades may be 10-30mm wide and about 1-2mm thick with the circular pitch between 3 and 10mm. To prevent the saw blades from becoming jammed, the teeth must be set at approximately 0.5mm. When using circular saws, saw blades with a minimum setting 0.5mm are also preferred. Normal cutting speed for band saws is 1,000-2,000m/min and for circular saws, 3,000-4,000 m/min.

Drilling:

Lower RPM drilling is recommended unless compressed air, water, or cooling oils are used – UHMW melts easily. Twist drills are most commonly used but pointed drills and circular cutters can be used for higher diameter holes.

Welding

Because of its high melt viscosity, friction and butt welding are the only practical methods for joining Redco UHMW by welding.

Machining

Machining is the principal method used to fabricate finished parts from UHMW. UHMW can be sawed, turned, planed, milled drilled, stamped and welded easily on woodworking or metalworking machines. The following general directions should be observed in these operations: To obtain surfaces of high quality, tools should always be sharp. For the most purposes, normal tool steel is satisfactory through many fabricators use special steels.

The optimum cutting speed is between 250 and 1,000 m/min. At lower cutting speeds cooling is not required, but at higher cutting speed range, water cooling or the use of soluble cutting oil is essential. In all cases, care must be taken to avoid heat build-up in the machining operation, so that the work piece does not smear the cutting edges. In milling and turning, the feed should not be too fast and the depth of the cut should be greater than 0.3mm.

Bonding

Questions about solvent bonding are common but unfortunately this is not recommended with UHMW. It resists most solvents and, at best, will create a below-average to poor bond. Mechanical fixation is recommended wherever possible.

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Jet Boat UHMW

Posted: June 13, 2016 in UHMW
Tags: , , , ,

Late spring has hit the northern hemisphere and in many places it’s warming up consistently now. For many parts of Canada and the U.S. “jet boating” is a seasonal pastime. Jet boats are wide, somewhat flat-bottomed boats designed to shoot across shallow creeks, even sometimes jetting from creek across land, to a creek on the other side. Needless to say, the hulls of these boats take an absolute beating!

That’s where industrial plastic comes in. UHMW polyethylene is used on the bottom of these boats to protect against impact and abrasion. UHMW tests as unbreakable according to ASTM D256 and is also very slick, helping the boat pass any obstacle. For the most part, the boat bottoms are flat perhaps with a slight curvature. The material used is typically 3/8″ or 1/2″ thick UHMW either black-reprocessed or black virgin. The virgin material is more expensive but has about 20% better properties in most respects. If you watch jet boating videos on the internet you almost always will see a black plastic on the bottom of the boat – that plastic is UHMW.

Our recommendation is to mechanically affix the UHMW using bolts or weld washers. UHMW is available from certain vendors with “adhesive” but we have only heard of poor results and this is not recommended!

If you’re looking for a quote on UHMW for your jet boat you can contact Redwood Plastics.

Jet_Boat