A Guide to Plastic Fabrication

1) Plastics and their Properties

2) Manufacturing Processes

1) Plastics and their Properties

Plastic is the name given to a state of matter between solid and liquid with properties of both. Commonly occuring materials which exhibit this property include candlewax and sugar.

Most of the materials referred to as 'plastics' are in fact solid at normal temperature and some undergo a chemical change when they are moulded and cannot be made plastic again (eg bakelite,tufnol,fibreglass)

It is probably more correct to refer to them as polymers as their structure consists of simple molecules which are linked to form chains thousands or millions long. It is this structure which gives rise to their properties -

Lightweight & strong but not particularly heat resistant

The materials we use to fabricate fall into two general types -

Glazing Plastics - naturally clear plastics such as acrylic, polycarbonate, styrene, clear PVC. These materials are normally used for where appearance or transparancy is important such as point-of-sale displays or light diffusers.

Engineering Plastics - naturally translucent white materials such as polypropylene, polythene, nylon, or PVC. These materials are generally much stronger and less prone to notch sensitivity. They are used for industrial applications such as tanks, bearings, shock pads etc. In these applications, appearance is less important and they are not normally supplied with a protective masking.

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Glazing Plastics

a) Acrylic
This is the material with which we mainly work. It was developed in the 1930's - mainly for use on aircraft cockpits. Trade names include Perspex, Plexiglas, Altuglas, Transpalite, Senosan, Evolite, Vedril, Repsol.

As well as clear, it can be obtained in white opalescent, dense white, opaque,translucent and transparent colours, patterned, mirrored and high- impact.

Acrylic is 'notch sensitive'. This means that if any part of the material fails under stress, then the weakness will run through the body of the material in the form of a crack. This means that all cut or machined edges must be clean and chip free and that internal corners must be avoided as far as possible.

As acrylic melts at about 120 degC, great care should be taken to avoid any build up of heat.Acrylic also has an extremely high rate of expansion when heated - up to 4mm per metre over a 20 deg rise in temperature. When heated to forming temperature, approx. 4% shrinkage will occur. This can cause problems in line-bending and also in final use.

b) Styrene
Styrene comes in two forms -

Clear Styrene - A clear plastic similar to acrylic but about 1/3 of the strength. As it has a very low softening point ( about 700) it is much more difficult to machine,bend and buff.

High Impact Styrene (HIPS) - Only available in solid colours. This is a tough, flexible plastic which is relatively cheap. It is often used in fairly thin sheets of about 1 - 2mm thickness.

c) Polycarbonate
Polycarbonate is similar in appearance to acrylic but about 100 times greater in impact strength and approx. three times the price. It is often referred to by the trade names - Lexan or Makrolon. Due to the high strength, high softening point and tendency to absorb atmospheric moisture - it is a difficult material to fabricate.

d) PVC and PVC Foam
These materials are available in a variety of forms -

Clear PVC - similar to acrylic with a pronounced blueish tinge. It falls between acrylic and polycarbonate in terms of strength and price but is easy to fabricate.

Industrial grey PVC - A relatively cheap plastic for industrial use.

Coloured PVC and PVC Foam - for decorative uses. PVC Foam is one of the cheapest coloured plastics for display uses.

Flexible PVC - A rubber-like material normally only used in industrial applications such as strip curtains or flexible doors.

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Engineering Plastics

All engineering plastics are similar in nature and it can become confusing due to the range of materials available. Across the range of properties, each one has a particular feature which may make it better in a certain application - but any of the others can usually be used for the same application.

a) Nylon,Acetal and PTFE
These materials are quite expensive and so are normally only used for machining applications.

b) Polypropylene
This is the cheapest of this type of plastic and so is most commonly used for fabrication. It can be hot-air welded quite easily.

c) Polythene
Slightly more expensive than polyprop. but particularly suited for applications involving food contact. Easy to weld. It comes in a confusing variety of forms - Low density (LD) High Density (HD) High Molecular Weight (HMW) and Ultra High Molecular Weight (uHMW). They are all similar but because of price, we normally fabricate in LD and HD. HMW and uHMW are mostly used for machining.

d) PVC
In its cheaper forms or in flexible grades, PVC can be used in industrial applications and it is easily welded. Unlike polyprop and polythene, PVC can also be solvent bonded.

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2) Manufacturing Processes

Sawing

Plastics can be sawn using a TCT circular saw or fine-tooth Jigsaw.

The objective is to obtain a clean cut with as little chipping as possible.

Sawing leaves toothmarks on the cut face. These normally have to be machined or scraped smooth before an acceptable finish is achieved. Remember to allow 2/3mm for this process when setting the size for cutting.

Difficulty can be experienced with very thick sheet due to cracking and thick sheet due to heat build-up causing the swarf to melt in the cut or on the riving knife. To allieviate this, the sawblade should be set as low as possible to create a scraping rather than biting action - and the sheet should be as fully supported as possible underneath to minimise chipping. For extremely thin sheet, a cover plate on top can also help.

Thick sheet should be cut using multiple cuts of progressive depth.

On multiple cuts, it is more accurate to register at right angles to the cut which gives a constant error - rather than parallel which leads to an accumulating error. If this is not possible, the strips should be turned every other cut to balance the errors.

When sawing against a fence and stop, ensure that a gap is left to prevent swarf interfering with the accuracy of position.

SAFETY -

- The blade height should always be as low as practical

- Never place the hand or fingers along the line of the cut or over the edge of the panel

- Never allow the hand or fingers within 2 inches of the sawblade - if neccessary, use a stick to guide the material

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Drilling

Plastics can be drilled with modified HSS drills,"bullet" drills, flat bits or hole saws.
HSS drills require a zero or negative rake and 60 deg tip angle.

As with sawing, a scraping action is required and the speed and feed must be controlled to achieve this. Again, the work must be as fully supported as possible to eliminate chipping at the back of the hole.

On deep holes, or when using a hole saw, "pinch" drilling is needed to remove swarf and eliminate heat build-up. If neccessary, air or water can be used to cool the drill bit.

When drilling against a fence and stop, ensure that a gap is left to prevent swarf interfering with the accuracy of position.

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Machining

Against Fence

The work should always be pushed rather than pulled - against the rotation of the cutter.
As the cutter rotation is tending to pull the material away from the fence, pressure should be exerted to counteract this as the panel is moved through. (fig 1)

Except for internal cuts, it is preferable that the amount of material removed is less than half the diameter of the cutter.

Narrow panels should be supported against a block to avoid twisting. Ensure that the block used is square at the corner.(fig 2)

When a lot of panels are to be fenced,a back-stop should be fitted to guide the panels as they clear the cutter. To avoid twisting, two clamps should always be fitted. (fig 3)
Consideration should be given to fitting a back-stop if a large, expensive panel is to be machined.

When making cut-outs, the fence should be set so that if the panel moves avay from the fence, it will take the cutter into the scrap - except when the scrap is to be cut free, in which case the set-up must ensure that the waste material is not trapped between the cutter and fence or stop. (fig 4)

When machining against a fence and stop, ensure that a gap is left to prevent swarf interfering with the accuracy of position.

SAFETY -

- The cutter height should always be as low as practical

- Never place the hand or fingers along the line of the cut or over the edge of the panel

- Never allow the hand or fingers within 2 inches of the cutter - if neccessary, use a stick to guide the material

Off Template

Plastic panels can be clamped to a template by mechanical stops, double- sided tape or with vacuum.

The frequency charger must always be switched on before the router is started. The router head must never be allowed to drop freely and the cutter must always be lowered into the cut at the start of a cut.

Loose masking should be removed or cut back before machining.

Always blow away swarf from the machined panel before removing to avoid clogging the filter of the pump or contaminating the double sided tape.

For greatest accuracy, when possible machine against the same part of the pin all the time.

For thick or very tough material, machine through in stages or fly-cut the material with a jig-saw.

By Hand

A plunge router can be used by hand against a fence, using the template follower or the base of the router.

When using the flat base of the router, always use the curved part of the base against the template or fence.

SAFETY -

Always use the depth stop. Release the clamp before laying down the router, so that the cutter is not projecting from the base.

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Polishing

When polishing, the finished surface will reflect the quality of the edge so that saw-marks or cutter marks will still be visible.

All sawn panels should be machined before polishing. If even better quality is required, the edge can be scraped with a stanley blade and\or finished with finishing paper.

When scraping, always hold the blade in both hands and take long strokes. Avoid any cuts on the surface. Finishing paper should be wrapped round a block or taped to a flat board. A circular motion should be used.

Buff Polishing

If the plastic is to be glued or printed within 1" of the polished area after polishing, it must always be buffed.

Always keep polish on the buff and use only light pressure.

Always buff off a corner, not onto it. (fig. 5)

When buffing out scratches, polish across the scratch over a wide area. (fig 6)

Flame Polishing

When flame polishing, do not allow the torch to stop on the material at any time.
Ensure that the atomiser is serviced regularly and always carry out a leak test after refilling.

The free flame should always be directed away from the work. (fig 7)

When the lit torch is in its holder the flame must always point away. Ensure that nothing lies within 30 inches of the path of the flame.

The edge to be polished should be clean and free of swarf. Any blackening or blemishes should be buffed out or scraped and re-polished.

Meths must not be used after polishing. However meths should be used on white/opal acrylic before polishing to remove grease.

Acrylic is the only material that can be flame polished. Polycarbonate and PVC can be buff polished. Polycarbonate can also be solvent polished with Methylene Chloride. Always ensure that the edge to be polished is vertical so that excess solvent does not run into the face of the sheet.

Whitening caused by cloroform runs can be removed by flame polishing.

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Cementing

Material Cement Set Time Cure Time Strength
         
1) Acrylic Chloroform 4/5 mins 30 mins 30%
  Tensol 12 (ICI) 20 mins 3/4 hours 60%
  Tensol 70 (ICI) 4 hours 24 hours 90%
         
2) PVC PVC Cement 5 hours 1 hour 70%
         
3) Styrene Polysyrene 5 hours 30 mins 70%
         
4) Polycarb Methylene 1 min 10 mins 50%
         
Polypropylene None      
Polyphene None      

 

(1) - 4 can also be bonded with superglue, silicone, epoxy resin and fibreglass.

Except for Tensol 70, the other cements are solvents and will not work unless the edges are in contact.

Important - When using solvent cements on acrylic - ensure that the solvents do not come into contact with any stressed areas ( heat bends or polished edges)

Capilliary action is used to draw the cement into the joint (except for PVC cement which must be brushed onto the surface). Unfortunately, this effect can cause blemishes if any supporting tape or blocks are in contact with the joint surface so that care must be taken to avoid this. Also care must be taken to ensure excess solvent cannot run onto the bench.

To avoid excess solvent "pooling" it is often wise to set the bench at a slight angle. This also make long joints easier to glue as the solvent can be run down by gravity.

If a joint will experience flexing in use, it should be built up using "fillets". As well as increasing the bonded surfaces, this also reduces the "notching" effect. (fig 8)

For display joints, edges may be bevelled and masked before glueing with Tensol 70. The joints are overfilled and the excess glue removed just after it has set, but before it is fully hardened When masking bevelled joints for display, the tape should be cut with the bevel.

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Hot-Air Welding

Welding is used for polypropylene, polythene, PVC and polycarbonate. The welding rod must be of the same material.

Ensure that the speed welding nozzle is clean with sharpcutting edges.

The parts to be welded must be supported in place and blocked to ensure air cannot escape through the seam.

The welding nozzle should be held so that the air slot is parallel with the seam to be welded. At the end of the weld, the rod is cut with the cutting edge by means of a twist of the wrist. The shoe at the end of the nozzle should force the softened welding rod into place. (fig 9)

To weld slowly, set the heat to a lower setting.

To finish off an awkward or unsightly weld, the excess material can be cut away with a stanley knife, ground away with a file or tooled using the gun and a cold screwdriver or chisel.

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Hot Line Bending

Material Approx Bending Temperature
   
Styrene 70oC
PVC 70oC
Polyprop 90oC
Polythene 100oC
Acrylic 130oC
Polycarb 180oC

 

Polycarbonate can absorb moisture from the atmosphere which can cause bubbling when heated. This problem is also occasionally experienced with extruded acrylic sheet. For this reason, these materials should not be left unmasked for any length of time before bending.

If "bubbling" is experienced, this is caused by moisture in the sheet. It is internal and cannot be removed by polishing. "Burning" - caused by the sheet coming into contact with the hot wire can sometimes be scraped out.

When Acrylic sheet is heated, shrinkage takes place which causes stress in the sheet. This can result in "bowing" if there is insufficient material on each side of the bend to counteract this effect. Bowing can be reduced by using an off-set jig or by "half -checking" the material where it is to be bent. This reduces the thickness of the bent area and consequently the stress caused by shrinage is less. However, the strength of the part is considerably reduced when it is "half-checked".

Generally, the edge used to register the bend should also be used in the bending jig.In this way, any variance in panel size is in the free part of the material and there is less chance of bend varying. (fig 10)

Any dirt or roughness in the jig will cause "mark - off" in the bend and should be avoided.

Generally, bending against a previous bend should be avoided to reduce error. (fig 11)

Bending introduces a lot of stress into the material. No solvents or glues should be used near the bent area, even after it has cooled.

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