Plastic lumber is one of the most intriguing materials for the DIY community because it’s so easy to wrap your head around how one might use it. Since it comes in profiles that are the same dimensions as wood and bears the same load as wood (if not better) substituting it for wood makes a lot of sense. Plastic lumber has very low maintenance requirements: most of it is UV stable and rot proof as well as resistant to splitting and insect attack. A concern you need to have is that a fiberglass reinforced HDPE product should be used when bearing weight, such as a deck without a reinforced backing. Also, despite UV resistance, the color will fade somewhat over time so you need to have fair expectations.

Another expectation is price. People are usually exposed to cheap single plastics or otherwise “commodity” plastic found in consumer products. Unfortunately for members of the public, the inexpensive cost of these materials does not reflect the cost of higher grade engineered plastics. When members of the DIY community pursue plastics the first time around their “sticker shock” often leads to the exclamation “but it’s just plastic!” We need to change our way of thinking: besides the cheap commodity plastics, plastics are a premium material that offers significant advantages over metal and wood at a premium price. However, you really do get what you pay for!

Plastic lumber will look great for years and function well in many applications but just know that you’re getting into an investment.

The application we found today highlights the means, not the end. We found a short Youtube video that shows how one “DIYer” manufactured UHMW hex nuts in his own shop. There isn’t necessarily a prevailing application for these nuts though. They will not have the sheer strength or rigidity of metal; however, as one commentator noted on Youtube – the ideas shown in this video could be applied to functional applications such as spacers or high-quality washers.

Perhaps more than anything, this application highlights how UHMW polyethylene is a “Jack of All Trades” material. UHMW performs good service in a variety of applications due to balanced properties. It does have deficiencies and should not be used as an automatic substitution everywhere but particularly for a spacer/washer application, it’s ideal where tolerances are not critical.

Hopefully the video inspires you to a successful application!

 

HDPE vs. UHMWPE

Posted: September 30, 2019 in Education, HDPE, UHMW
Tags: , , , , , ,

Hobbyists are often unsure of the differences between closely related plastics. When it comes to two commonly available plastics in the polyethylene family: “High Density Polyethylene” (HDPE) or “Ultra-High Molecular Weight Polyethylene” (UHMWPE or simply UHMW) – what is the difference? Are there any times it’s critical to use one verses the other?

The answer is in many cases the differences are not pronounced but there are a couple things to watch out for.

Where the products are more or less equivalent is any application where they’re being used as, for example, a spacer, a lining, a surface, or other “static” applications. Both products will appear to be the same color and will be the same weight. It’s when mechanical properties start getting involved that you should be concerned. Firstly, UHMW takes significantly more impact than HDPE does, so if you have an application where a part is being banged around – UHMW should be your choice. UHMW is also significantly slicker and an application involving “sliding” UHMW is the ideal choice. An example would be as sled rails on a snowmobile or toboggan: UHMW will traverse the snow much better! You can in fact find some good Youtube videos that make a comparison between the two materials on this application.

One note about the static applications listed above is that HDPE is superior as a cutting board. While both products are food grade and work great in the application, UHMW will dull blades over time whereas HDPE will not. With both products being highly available, HDPE would be the better choice. This is why HDPE is preferred for synthetic ice as well.

Speaking of other properties the plastics are pretty comparable with their UV resistance (poor), chemical resistance, availability, machinability, and heat tolerance. As always, if you’re unclear what to use for your application just contact Redwood Plastics and Rubber for assistance.

A lot of what we offer here on the “DIY” blog is material information; however, as the hobbyist, you actually have to cut, drill, and machine the plastic yourself. It’s beneficial to educate yourself on practical fabrication matters. Most hobbyists grow up working with metal and wood; however, industrial plastic is only usually encountered when one knows to look for them – usually when one is a teenage “DIYer”.

The prevalence of CNC machines in recent years makes machining knowledge even more critical. In the video linked below, Instagrammer Tom Zelickman educates on issues he’s had with machining plastics and how to overcome various challenges including his stressing about one “aggressively” machining plastics and cleaning them.

You can view the whole video on YouTube here:

Ultra-High-Molecular-Weight Polyethylene (or “UHMW” as it’s commonly known) is one of the most popular industrial plastics. Unfortunately, due to its well-known reputation, people overestimate the properties of the material and their expectations of it are, frankly, too high. The plastic hold poor tolerances due to thermal expansion. This means that the tolerances of a part when machined may be much different then when the part is actually installed.

In addition, the part’s geometry will change due to thermal expansion once installed. This has created an odd paradox – customers try to impress on us that somehow due to wishful thinking these poor tolerance holding properties of UHMW will somehow ‘not exist’ if we just hope they won’t. That statement may not make sense but it’s actually what we often encounter at Redwood Plastics and Rubber with customers. They dislike the wide tolerances of UHMW and ask us if there are ways to account for the thermal expansion issues in machining. We can mitigate some effects but expecting UHMW to hold good tolerances simply doesn’t work.

This short YouTube video highlights some issues with UHMW and serves as a good primer of the challenges you will face with the material.

 

For some reason PVC plastic, DIY, and summer seem to be a magic trifecta! The plastic is strong, commonly available, and easy to work with. It has a disproportionate number of applications in the DIY community due to it being such a commonly available pipe. What are not so common are videos as good as this one we’ve found! Unlike many DIY videos, this one states exactly how the project was made even down to specific measurements for the drill holes for the spray nozzles. The video also includes a life test and troubleshooting of what would appear to be a common problem. Once the project is said and done it appears to work marvellously!

Don’t take our word for it, see for yourself:

Redwood Plastics and Rubber, the sponsor of this blog, offers many resources on their website to help their customers work with plastic. Some of these charts and information are ‘overkill’ for what the DIY community really needs, however. In this case, more basic information is best. The good news is their website actually has a page that links to several sub-pages that give general information on fabrication and care for plastics. These guides include some of the most common plastics used by the DIY community: UHMW, acrylic, polycarbonate, and acetal.

You can find the guide and the link to the sub-pages here: https://www.redwoodplastics.com/tools-data/fabrication-machining-guidlines/