Working With Styrene

Working With Styrene

Posted with permission By: Jim Banner http://members.shaw.ca/sask.rail/
Saskatoon Railroad Modellers Group http://members.shaw.ca/sask.rail/con.../workstyr.html



First of all, what IS Styrene? Styrene is a solid plastic made by polymerization of styrene gas. It is commonly sold in three forms:

1.Expanded styrene usually referred to as Styrofoam and used in the building industry as insulation.

2.Sheet styrene, with or without designs molded or pressed into one or both faces.

3.Injection molded mechanical or decorative parts, and model kits.

All three forms of styrene are useful in modeling.
A wide variety of plain and decorative styrene sheets can be purchased at hobby shops.



CUTTING STYRENE

On the list of things that make working with styrene a pleasure, ease of cutting must be close to the top. Unlike wood, styrene has no grain to worry about. And unlike metal, only two inexpensive tools are required for cutting styrene. The tools are a sharp knife, preferably with replaceable or snap-off blades.

1. Deciding where to cut. Cutting lines can be marked on styrene with a pencil. Straight cutting lines need to be marked only at the ends, but curves should be marked for the full length. Curves can be laid out free hand or with compasses or by tracing around something with the right diameter, depending on the required result.

2. Scoring the plastic. For straight cuts, position the edge of the ruler EXACTLY over the cutting line. This is easy if you put the tip of the knife exactly on one of the pencil marks and slide the ruler up to it. Then put the tip of the knife on the other pencil mark and rotate the ruler until it again touches the knife. Recheck the first end, then draw the knife along the ruler edge to score the surface of the styrene. If the knife is sharp, only a light pressure is required. Ideally, the ruler should be over top of the piece you want, so that if the knife wanders at all, it wanders into the scrap. For curved lines, it is often necessary to follow the line by eye. Try to keep the knife in contact with the styrene at all times, so that the score line is continuous. Often it is easier to turn the material than it is to turn the knife, so that you never have to cut at an awkward angle.

3. Breaking the plastic at the score. With the score towards you, bend the ends away from yourself. For straight cuts, this is a snap (pun intended). To convince yourself just how easy it is, try it a few times on a scrap piece of styrene. Also try snapping a piece that has not been scored, just to get a feel for how tough this stuff really is. Curves are a little harder to snap. Usually it helps to work in stages, first gently bending the plastic at the score line, working along from one end to the other. Then work back, bending a little more. Keep working back and forth, bending a bit more each time until the score line penetrates right through and the plastic separates.

4.Cutting holes in the center of a piece by this "score and snap" method is somewhat harder. Round holes, particularly larger ones, are not too bad if you keep working gently around them until the center breaks free. Rectangular holes, for example holes for windows, are more difficult. Often cracks will develop at the corners no matter how carefully you work. One way to avoid this is to drill a small to tiny hole at each corner. ( If you don't have a drill, use a heated pin to make the holes - just don't breath the smoke. Then connect the holes with knife scores. On the front of the sheet, use the ruler as a guide and connect the holes in the desired rectangle. On the back, connect the corners in a X. Press on the center of the rectangle from the front of the sheet to snap all the scores and pop out the scraps.

5. If required, is to remove the burr left from scoring. The fatter the knife, the duller the knife and the more pressure used, then the more material that will be pushed up on each side of the score line. See figure at left, below. This ridge can be felt by dragging a finger nail across the cut edge of the sheet (DO NOT drag your fingers along the edge, a cut edge can be sharp enough to cut you back!) In many applications, this ridge is of no consequence but in others, it does matter. It can cause enough separation between pieces that solvent welding won't work properly. The drawing in the figure below shows a vertical knife blade creating burrs as it scribes a horizontal piece of styrene. The right hand drawing shows two pieces of styrene being held apart by a burr on one of them.

SCRIBING STYRENE

Scribing styrene, that is, making shallow grooves in its surface, is a useful technique when it comes to replicating walls, blocks, tiles etc., or for simulating cracks or joints. Sometimes a knife cut made with a hobby knife is enough - perhaps for a crack that has formed but has not yet opened up. But often a good healthy groove is needed to make a strong statement that can be seen many feet away. How do we make the grooves i.e. how do we scribe styrene?

The answer is to use a scribing tool. If you are fortunate enough to own a Richards Formica knife, the kind that looks like a carpet knife with a little triangle of carbide set in the tip, you are in business and can skip the rest of this section. If not, you can make a servicable tool from an old half round file, or even from a cheap new one, using a bench grinder.

In the diagram above, such a file is shown in the insert in the upper right corner. You are looking at the flat side of the file, and the tang would be out of the picture to the right.

The file is converted into a scribing tool in three steps. Firstly, the teeth are ground off the flat side of the file for about the last inch. This is to make it slide easily along your straight edge and getting it smooth is worthwhile. Next, a hook about 1/4" wide and protruding 1/8" or a little more is formed on the end of the file. It is formed by grinding away the bottom edge of the rest of the file, leaving the hook at the end.

Then turn the hook into a sort of blunt knife. This is shown in the left side of the diagram above. The bottom of the knife must slope up toward the end, by at least 15 degrees. This relief angle is to allow the tool to be tilted up enough to let you wrap your fingers around the handle, and still allow the tool to cut. If relief angle is more than about 20 degrees, the tool will tend to dig in and tear the work.

It sounds hard, but it can be done rather easily if you take the time to draw a 15 degree angle on a piece of paper using a protractor or compasses. Then you can align the bottom of the file with one leg of your angle and slide the file along that leg until the edge of the knife meets the other leg of your angle. It is then easy to see how close your relief angle is to 15 degrees. Of course, if you happen to own a machinist's protractor, just set it at 175 degrees and checking the angle is dead easy.

Once you have established the relief angle, you can sharpen the knife. The knife should be sharpened to a rather blunt angle, somewhere between 45 and 60 degrees. This is harder to measure than the relief angle, but it is not so critical. Just draw an angles of 45 and 60 degrees on a piece of paper and check by eye that the knife edge is between these two limits.

The knife should be sharpened its full length - not hard, it is after all only 1/4" long, and it should be sharpened to the same angle either side of vertical. If you are good with a grinder, it helps to sharpen only a little on the side toward the flat side of the file and a lot more on the side toward the round side of the file.

However, this is more than a necessity and getting the knife sharpened from end to end without destroying the relief angle is far more important. Finish off the scribing tool, if you want, by wrapping some duct tape around the file a comfortable distance up from the knife.

To scribe styrene with the formica knife or the scribing tool, first make pencil marks near the tops and bottoms of the lines to be scribed.

Next place the cutting tip of the knife or tool on the top mark of the first line to be scribed. Note that the cutting tip of the scribing tool is the end of the knife nearest to you. Slide the ruler over until it touches the side of the knife. Swing the ruler until it is the same distance away from the bottom mark, judging by eye. Lastly, draw the knife or tool toward you to make the groove.


Tip 1 - if the near edge of the sheet being scribed is aligned with the near edge of your work table, the knife or tool will fall into space at the end of the groove rather than gouging your table.

Tip 2 - if the sheet being scribed is just a bit taller than the finished product, you can start scribing just a bit down from the top, which is much easier. If the sheet is already finished size, you can still start a little bit down from the top, then after all the lines are scribed, turn the sheet around and finish off the last bits scribing towards yourself.



JOINING STYRENE

If the pieces of styrene fit tight together without gaps then solvent welding is the strongest way of joining the pieces. Solvent welding is done with a liquid that will slightly dissolve the two surfaces to be joined, allowing them to flow together and become one piece. The handiest liquid is any of the plastic cements sold for the job, for example Plastic Cement. Note that these solvents should be used only with adequate ventilation.

A cheaper alternative which should ONLY be used outdoors or possibly in a properly constructed and ventilated spray booth, is lacquer thinner. While it does not produce as good a joint due to its quick evaporation, lacquer thinner is an attractive alternative when building large scale structures and large amounts of solvent are required.

When the pieces to be joined do not fit tightly together, a solvent with some filler in it is required. This is where tube glue comes into play. The idea is that the filler fills up the space. This does happen initially, but over a period of time, as the solvent slowly evaporates through the bonded pieces of styrene, the filler tends to shrink, either weakening the joint or possibly distorting it.

Sometimes pieces of styrene need to be joined so that they can occasionally be taken apart and reassembled. This can be done using machine screws either with nuts or with tapped holes. Styrene taps beautifully using ordinary taps without lubrication. It does not do as well using self tapping screws. Styrene can also be joined with pop rivets, but backing washers must be used behind the work, and should also be used under the head of the rivet as well.

Sometimes styrene needs to be glued or bonded to other materials. Many things work here, including some mixed solvent cements that will bond dissimilar plastics (make sure styrene and the other plastic are both listed on the label). For indoor applications, cyanoacrylates work well, but do not seem to last in outdoor work.

For small jobs, Walthers Goo will stick styrene to just about anything, but in larger jobs may cause distortion of the styrene if the evaporation of the solvents is delayed, for example when bonding styrene to metal.

For large jobs, e.g. laminating styrene onto plywood, contact cement works well. Automotive weatherstrip cement works well for small laminating jobs, and for bonding to metal, but can be messy. One product that does not work all that well is silicone seal, but it still has it uses, for example bonding glass windows to styrene (sticks well to the glass, not so well to the styrene).



PAINTING STYRENE

Most paints will stick to styrene well enough to allow them to be used indoors in locations that will not require a lot of handling. For brush applications, oil based paints such as Testors flat colors, and various brands of water based acrylics work particularly well. Household latexes also work, but the heavy coats needed for color coverage tend to obscure fine details.

Where a lot of handling is required, for example rolling stock, or where the project will be exposed to the weather, lacquer applied with an air brush is the best choice. When applied with an air brush in light coats, it attacks the surface of the styrene just enough to make a really strong bond without causing crazing.

A bit of practice with some styrene scraps goes a long way toward finding out just how light or heavy a coat to use. NEVER use spray cans of lacquer directly on styrene, the flood of material that they produce is guaranteed to cause problems.

Water based paints applied with an air brush; their advantages seem obvious; easy cleanup with water and no harmful solvents that require using a spray both and/or a mask to absorb the vapors. However, plain water may not completely clean the air brush, and a good mask to absorb the mist of fine droplets of paint is still necessary.

Often the durability of paint can be improved by applying a clear top coat over it.

wow thanks alot for this it will help me out alot!