Different materials, characteristics and solutions.

Different materials, characteristics and solutions.

Most of the time, our customers have a pretty good idea as to what sort of material they want to use for their next job. This is often based on experience; we used it last time and it worked well. But sometimes, it’s not that clear as there can be a bit of a crossover between the attributes of each material. So, here’s a brief overview of some of the materials we commonly use, what makes them appropriate for some applications as well as some of the limitations to their use.

When it comes to custom wood, we commonly use Uniform Medium Density Fibreboard (UMDF). UMDF is variant of Medium Density Fibreboard (MDF), and is unique in that UMDF maintains the same physical properties (i.e. density) from the outside and through the centre. This means that UMDF will incur the same levels of expansion/shrinkage throughout an entire job, meaning that the external surface remains consistent and fair. MDF on the other hand is denser towards the surface than it is in the centre, meaning that levels of expansion & contraction will be inconsistent across an entire job. This means that the external surface of the job becomes uneven (as per the image below, which shows the surface variation evident in multiple layers of MDF). 

When one of the reasons for using CNC cutting is accuracy, it could be perceived as a bit of a contradiction in objectives if MDF is used, with its dimensional instability. This is because the job, when it comes off the machine is accurate to within 0.05mm (for example), but depending on atmospheric conditions (humidity) this tolerance could quickly be eroded and become variable. But looking at it from another angle, MDF is a more economical solution than UDF. What this means is that the choice of MDF or UMDF for a job depends on what the job itself is to be used for. For example, simple sheet cutting lends itself to MDF, whilst making a larger, more complex mould that uses multiple layers of material is going to be more suited to UDF because of the consistency of shrinkage across the job (in other words you don’t get lumps and bumps all over the job, and then need to hand finish it, and hence introduce the ‘human’ factor).

Other ‘wood’ materials that can be used include Plywood, which has excellent strength and aesthetic properties (i.e. it looks good when cut, and does not necessarily require painting), or a wide range of natural timbers including pine, birch or rimu (to name a few). Choice of material depends a lot on a combination of the required structural properties (i.e. how strong it needs to be), and the desired finished ‘look’. When the two objectives contrast, an option is to combine the strength of two materials (i.e. pine or MDF) with a natural timber, such as rimu. In this case, the pine gives the required strength (and cost-effectiveness!) whilst the rimu offers the necessary visual look.

Polystyrene is a material we’ve had heaps of experience with, as this is where Styrotech started by making polystyrene core windsurfing boards. Styrotech CNC is setup in terms of machines, tooling and extraction systems, so is well placed to tackle virtually any polystyrene job you can imagine.

By nature of its properties, polystyrene is not as accurate as custom wood (it’s not as dense, and is made of lots of little ‘beads’ joined together), but it is a relatively light and cost-effective material. Basically, the higher the density, the stronger the material and the finer the surface finish, but the down side is that it gets heavier.  The lower the density, the lighter the material is, however the downside will be that the quality of surface finish declines (meaning you need to spend longer working on it to get the material to a ‘final’ shape). Polystyrene is available in a wide range of densities from 12kg/m3 through to 28kg/m3. Typically, we’re using 24kg/m3 foam as this is an excellent balance between strength, quality of finish and overall weight. 24kg/m3 polystyrene is ideal for making the structure of a mould, for example a yachts mast or boom (above left), or superstructure, a canopy mould for a ute, or basic shape features of a campervan. For applications where the finished weight is important (e.g. windsurf or surfboards), a lighter density can be used (typically 12kg/m3) which gives the overall shape and structure, whilst maintaining as light a weight as possible. The image above right shows an SUP blank, made from 12kg foam.

As well as polystyrene, there are a range of different foams available to use including urethane, PVC and EVA.

Urethane foam is quite dense (ranging from 40kg/m3 through to 100kg/m3), and once machined gives a high quality of finish. Its fine (dense), holds a shape very well and is relatively easy to work with; however, it’s not the cheapest material and, without taking the appropriate precautions and safety measures, is quite toxic. Urethane foams were until quite recently the material of choice for the surfboard industry (that is until 2005, when Clark Foam, the largest supplier of surfboard blanks in the world, closed down), however, due to its environmental footprint, is now typically replaced with polystyrene.

Closed-cell PVC foamboard is a light-weight material used primarily in the manufacture of signs and displays. It is considered robust for outdoor use, being immune to rain and resistant to wind and sunlight. However, it cannot be cut using any sort of heat based process as this will result in the material giving off really toxic chemicals.

EVA foam, which has numerous applications, some of which include deck pads on surfboards, handle grips, flotation devices, bumper guards (i.e. to stop abrasion/impact with other materials or your head - see images below left and right) and sports equipment. EVA foam is available in a wide range of densities ranging from approximately 30kg/m3 through to 400kg/m3. Similar to polystyrene, the higher the density, the heavier it is, but the final surface finish will be better.

Plastics, including Polyethylene, Polypropylene and acrylic are materials we cut often for a wide range of customers and applications.

If the objective is to make a model or mould of an object that contains a high level of detail that needs to be maintained, for example an engineering component, visual model or topographic map, Ebalta (tooling board) is the ideal solution This is due to it being able to retain a high level of detail and is very dense, so both paint or a surface finish (such as Duratec) will adhere very well to it. The images below demonstrate a topographic map being machined, from Ebalta, and shows the level of fine detail that can be acheived using Ebalta (tooling board). 

 

 

 

 

 

Choosing a material can be a very straightforward process, or one that involves a great deal of consideration. This short article is designed to give you a bit more background on the different materials we often use. Of course, if you have further questions please feel free to contact us to discuss what you are thinking or what your question might be. 

About the Author

Jonathan Squires With a background in business and product management both in New Zealand and internationally, Jon is currently working with Styrotech CNC as Sales and Marketing manager.

Having toyed with product design for some years I entered the Formica Formations competition to test my blue sky design process, and was delighted to have won and had Beehive fully realised.

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Graham Roebeck - Structural Integrity


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