This article explains the best way to remove kit parts from their sprues. The vast majority of scale model kits will consist mainly of either injection molded plastic, or cast polyurethane resin. In either case all except the very largest parts will be attached to the sprues on which they were molded/cast.
This is best for both the manufacturer and the modeller. From the manufacturer’s point of view they do not have to take the time to remove all the parts and take the risk of losing them. From the modeller’s point of view, having the parts attached to sprues makes it easier to check that they are all present. There are also other benefits covered later.
When To Remove Parts
It is normally best to leave parts on their sprues for as long as possible for several reasons:
- While small parts remain on their sprues they are easy to find. Normally, parts are grouped on their sprues in some sort of logical order. For example, on a tank model there may be one sprue with all the parts for the turrret, another for the suspension etc. Therefore, when building the turret it is easy to find all the turret parts if they are still on their sprue.
- Most manufacturers will stamp part numbers on the parts. However, some parts are too small to do this and with other parts there is no place to stamp a number that will not be visible on the finished model. In these cases, the part number may be stamped on the sprue next to the part. Clearly, as soon as the part is removed it is no longer possible to tell what number it is. With a model containing several hundred parts, many of which may be very similar, this can cause difficulties.
- As soon as you remove small parts from the sprue there is the risk that they will be mislaid.
- Some small parts may be easier to paint when on the sprue. It depends on how and where the part is held, but if you have to paint a small part separately from the main model then the sprue may make a convenient holder. However, remember to do any clean-up of the part before cutting it from the sprue if the intention is to paint it on the sprue.
Follow the maxim – ‘Leave on the sprue until ready to glue’ .
How To Remove Parts
Never be tempted to break or twist the part away with fingers. Doing so runs the risk of damaging the part. The best way is to use purpose designed side cutters or flush cutters. These remove the parts easily leaving hardly any of the sprue still attached to the part. The flat side of the flush cutter should normally be put against the part to be removed. It is very important to avoid the part flying off when it is cut, so gently hold it and the sprue in one hand whilst operating the cutter with the other hand.
It is advisable to keep a small container to hand to put small parts in when they are removed from the spue. It is often convenient to remove several similar parts together such as wheels so getting into the habit of placing them together as soon as they are cut away minimises the chance of losing one of them.
If a flush cutter is not available, then a sharp craft knife will do fine. In this situation, put the sprue on a surface and apply light pressure with the knife using a rocking motion. Make sure the cutting surface is protected. Try to cut the part at the thinnest part of the sprue, but do not cut right against the part as the action of the knife may cause distortion. As with the flush cutter, care should be taken to avoid the part flying away and this should not happen if the cutting is gentle.
For very delicate parts consider supporting them from below with a ball of Blu-Tak.
It is very easy to damage clear parts and even small scratches are difficult to repair. The plastic used for clear parts also tends to be more brittle than normal styrene and the part may break off when pressure is applied damaging it beyond repair.
Once again, side/flush cutters are best, but do not cut right up to the part as the pressure put on the plastic may create stress marks on the part where it joined the sprue. If side cutters are not available then either be very very careful with a knife or use a razor saw if one is available to gently saw through the sprue. In very difficult situations, it may help to heat the knife so that it melts through the sprue rather than cuts it.
Polyurethane Resin parts are usually encountered when converting or detailing a model kit and usually come as an after market accessory, perhaps with brass photo-etched frets.
However, it is possible to buy entire kits made primarily of resin. These will normally be of specialist subjects and be manufactured by small companies. For example, Accurate Armour make a range of British military vehicles that are not covered by the really big kit makers such as Tamiya, Dragon or Hasagawa.
Resin parts are made by pouring a liquid into a rubber mold and then splitting the mould when the resin has hardened. Many resin parts may be supplied separately, but some smaller parts do come on sprues. Resin is harder and more brittle that styrene so although the same principles apply to removing them from the sprue, extra care needs to be taken. Another problem with resin is that often there is a large ‘stub’ of resin left on the part from where it was poured into the mold and this can be difficult to remove.
If a part is attached to a sprue with a narrow connection or has a small ‘mold stub’ then it can be removed with a knife or flush cutter as for a styrene part. However, if the sprue attachment or ‘mold stub’ is large, there is no alternative than sawing it off with a razor saw. There is no short cut to this and patience is required. It may be tempting to snap the piece off when it is partly sawn through but this often will just leave an irregular lump behind that will then need to be removed and you may damage the part.
|Do not inhale the dust produced when sawing or sanding resin parts. Either use a dust mask or ‘Wet and Dry’ abrasive paper with water.|
Removing Large Delicate Parts
Occasionally, large delicate parts need to be removed and it is difficult to see how it can be done without risking damage to the part. For example, in the photograph on the right two sides of storage baskets need to be removed. These are in 1/72nd scale and are about as small as it is possible to make injection molded parts.
Each of the parts are attached to the sprue in six places and the attachment points are quite thick. Any attempt to cut through the attachments with either a knife or side cutters risks bending the plastic parts.
The solution is to remove a section of sprue with the parts attached and then gradually work inwards cutting away the sprue. The first stage in this case is to cut through the sprue in four places as indicated by the red circles. This will remove a section of sprue with the parts attached as shown in the photo on the right.
It is now possible to cut off the sprue from the parts without putting any pressure on the parts.
Even if a flush cutter is used there will probably be a small amount of the sprue left attached to the part. The part is also likely to have a seam line where the two halves of the mould were joined and may have other defects. Therefore, immediately after separating the parts examine each one and clean it up with a knife, abrasives and files as necessary. The part is now ready to attach to the model.