Injection Moulding Basics for Model Makers

Here's some introductory information on injection moulding tech from the point of view of model makers. (ie: not just how it works, but how the technology affects someone assembling a commercial model kit)

Hope it's useful!

What is injection moulding?

Injection moulding (or molding) is an industrial manufacturing technique for producing plastic parts. A special mould, usually made from solid steel, is constructed. The mould contains a bunch of recesses representing the negative of the shape that's desired.

Hot molten plastic is pumped under pressure into the mould, which is also called a die or a tool.

The plastic cools down and solidifies, the two halves of the mould are pulled apart, and the finished piece is extracted.

The vast majority of commercial model kits are made using this basic technology, since it’s capable of accurately reproducing really fine details and textures in the plastic. Injection moulding machines are expensive pieces of industrial equipment, and mould production is also quite costly. But once that investment is made, it’s doesn’t cost much to churn out identical parts, making it ideal for mass production on a large scale. (ie: it’s not a manufacturing technology accessible to garage kit makers, the way resin casting is: see below)

However, the simple fact that the moulds have to be made in two halves introduces certain limitations in terms of what can be produced.

Overhangs

Injection moulding means you can’t have areas of a part that overhang or undercut. The reason is simple - you wouldn’t be able to pull the finished piece out of the mould if you did! The part has to be able to be pushed easily out of the mould.

Let's say you wanted a sphere with a simple projecting rectangular block.

And here's the problem! The sphere can't be pulled downwards from the upper half of mould, because the projecting overhang gets stuck.

This fundamental fact limits the potential range of shapes that can be made using the technology. If a kit designer wants a part that hangs over another part they generally need to make it two separate pieces that the model maker has to put together. (or have complex multipart moulds - see the bit on slides below)

Faking overhangs/undercuts

Alternatively the designer can fake it and design a part that approximates the appearance of the desired shape. You might, for example, have a shelf or strut underneath an overhanging part. This sort of compromise is commonly seen in model kit parts, because faking an overhang, rather than making two separate parts, is commonly done for price-sensitive products.

Let's say you wanted a part with angled fins like this. The angles make it impossible to pull the part out of the mould.

One fix would be to angle the whole part so that the fins were vertical and the base part angled instead. That would work in this case, though the sides of the base have to go at a funny angle, which could be a problem.

But often a manufacturer will just fake it by adding material to one side of the angled bits, so that the part pulls neatly out of the mould.

Slides

So how does Bandai do it? Some of their models have fine side details that clearly would be an undercut in a mould!

The large arch is a giveaway on a Bandai kit that a slide was used. Notice how the two detailed struts have fine details on both the top face and the side. This would be impossible with a normal two-part mould. So in this case a side-sliding section was built into the mould to make these details possible.

They pull off this stunt by employing special sub-sections of the mould, sometimes called slides or side actions, that move sideways before the main halves of the mould are separated. This is a complex and expensive technology, and one used by few other model kit manufacturers.

Here's a three-part mould for our ball with projecting bit. The top half of the mould has been split into two, with one piece sliding sideways and the other bit going up as normal.

Draft

If the sides of the finished part are perfectly parallel then it can be difficult for the manufacturer to remove the component from the mould. There's a bunch of friction, especially with longer or taller parts.

This part would be difficult to extract from the mould.

So injection moulded parts typically have slightly angled sides rather than parallel sides. This tapering is known as draft, and is usually a degree or two. It doesn’t sound like much, but draft is why you don’t usually see perfect boxes made using injection moulding.

This is a slightly exaggerated example of what draft looks like.

Power adapters are a really common example. Most affordable AC adapters have enclosures which look like this. In other words they’re almost a hexagon in cross-section.

A few companies, Apple being one example, employ “zero draft” technology which yields almost straight sides, even for simple things like power adapters. (the term "zero draft" can be a bit of marketing hyperbole on the part of injection mould manufacturers...  you'll notice that the adapter box below has nearly parallel sides, but not quite!) Of course, since this technology raises the cost it’s not commonly used for consumer products.

The same problem can afflict model kits. Consider this two-part R2-D2 foot by De Agostini/Fanhome. The top and bottom of this thing should be perfectly parallel to the floor, but they're at noticeable angles because of draft. It's only a 2-3° draft, but you can really see it.

This product was clearly optimized for cheapness rather than accuracy. Making the foot from 4 or 5 separate components would have improved its appearance dramatically (no draft; that awful visible seam down the middle would be gone), but would have cost more to manufacture.

The photo on the left shows the two actual parts that make up the De Ago R2 foot.

The photo on the right has been Photoshopped. I altered the angles to show what the foot should actually look like, if it didn't have draft issues.

A part with pronounced draft is difficult for the model maker to fix. Sometimes you can sand the entire surface of the offending area and make it flat. But often you end up with a sort of subtle curvature to the surface rather than a truly flat one, which isn’t great either.

Fortunately many model kits are of planes or cars or whatever, and such vehicles are mostly curved anyway. It’s only things with boxlike shapes, such as the R2-D2 here, where this is a real problem.

Flash and parting lines

The point where the two halves of the mould join always result in a slight raised line on the surface of the final product. This is known as a parting line or informally a seam, and can be fairly subtle.

However, molten plastic sometimes oozes into the parting gap, resulting in a thin sheet of material running around the part, like a kind of papery halo. This is known as flash, and is typically caused by moulds which aren't properly clamped shut.

Here's flash which has appeared around an ejector pin hole.

Older model kits had massive amounts of flash, needing cutting and filing off. Modern kits require less, but even the best kits usually benefit from a little touch-up with a fine file or whatever. Subtle parting lines are common on all injection moulded kit parts, though better kit designers will try to conceal the lines by placing them on the edges, rather than the faces, of objects.

Sink

If you look at a flat or relatively smooth surface, you sometimes see sunken areas in the material. These “sinks” or “sink marks” normally correspond to where there are thicker areas on the opposite side, such as where struts or beams support a flat wall.

The depressions occur because the plastic cools more slowly where it’s thicker, resulting in differential cooling and thus different shrinking rates. Even relatively high quality moulding can sometimes have sinks. They’re a pain to get rid of because they’re noticeable and are a hallmark of cheap crappy toys, yet are subtle and shallow enough to be tricky to putty and fill convincingly.

Sometimes a kit manufacturer will deliberately mould a part in two pieces, especially if it's thin at one end and thick at the other. That reduces the risk of sink. The drawback is that you then might have a gap or seam between the two pieces that needs filling.

Mould misalignment

One of the worst problems to deal with, and a common one with kits of decades gone by, is the problem of the two halves of the mould being misaligned to each other. This is generally caused by poor tolerances, or worn-out equipment being used, in the manufacturing process. The result is a part which sticks out on one side but not the other.

The reason this really sucks is because fixing the problem inevitably involves reshaping the part - maybe cutting back on one side and puttying on the other. Details can also be easily lost during such repairs. Poorly made human figures can be ruined by this problem.

Fortunately, quality kits these days rarely have serious misalignment.

Weld, flow, or knit lines

Sometimes you can see slight traces of wavy lines in the plastic, revealing the path that the molten plastic took when it entered the mould. These can appear around obstructions in the mould - where the plastic has to flow around them like islands in a river. If the part is to be painted such flow lines aren’t usually a problem as they often disappear with a layer or two of paint. Test first - maybe a prime and sand is needed first.

However if it’s a part that’s bare plastic - maybe a clear window for a vehicle - then these lines can be quite objectionable.

Plastics

There are two basic types of plastic - thermoset and thermoplastic. The former is a plastic that, once it cools down from its hot liquid form (or cures following the mixing of resin and hardener), is permanently solid and cannot be remelted. Polyester and epoxy resins, used in stuff like printed circuit boards and fibreglass, are typical thermoset plastics. Bakelite, the early plastic used for making things like desk phones decades ago, is also thermoset. Thermoplastics, however, can be reheated to make it a viscous liquid again.

Injection moulded models are usually made from polystyrene (PS) or ABS (acrylonitrile butadiene styrene) polymers; both thermoplastics. PS is the more common type for glue-together or snap-together kits. Incidentally it is, in fact, basically the same polystyrene used to make foam coffee cups. It's just that the PS in cups is "expanded polystyrene" which as been foamed with gas. ABS is a sturdier plastic and generally used for larger structural pieces, or strong products like Lego bricks. Different additives are put into the plastic to affect its properties - colour dyes, or substances to make the plastic stiffer or more flexible.

One way in which model makers can use thermoplastic properties is sprue stretching. This is a way of obtaining thin rods or threads of plastic. Just heat a piece of sprue – hold it near a soldering iron or high over a candle flame – and slowly stretch it as the plastic softens.

Ejection pin marks

Metal pins are traditionally used to push the cooling part out of the mould. Most injection moulded pieces, therefore, will have small circular marks, sometimes called ejector pin witnesses, across one surface.

A well-designed kit will try to put all the pin marks on the back of a part, or somewhere inconspicuous like a runner. But sometimes you have to waste a bunch of time sanding and puttying the stupid marks away. Since the marks are raised around the edge but often slightly sunken on the inside, like a crater, they require both cutting and filing down, and filling.

Sprues, runners, and gates

The typical model kit is made up of a bunch of parts joined by thin plastic rods. Known as sprues and runners, these rods conveniently allow for parts to be shipped in fairly flat bags, with parts neatly separated to avoid damage. Parts are joined to the runners by thinner, flatter, segments called gates.

Technically the sprue is the rod at the point where the plastic enters the mould, runners are the disposable internal rods joining up any parts, and gates are the thin points where the runners touch the parts. But most model makers use the terms sprue and runner interchangeably.

Thing is, sprues, runners, and gates are really a side-effect of the injection process, in a way. They actually represent the narrow tunnels into which the molten plastic flows. They must therefore be carefully placed by the manufacturer - both to allow plastic to flow correctly to all parts of the product, but also to serve as useful trees and runners for the finished kit.

Sprue cutters

Sprue cutters or nippers are an essential tool for the model maker. They’re simple plier-like hand tools with two sharp blades that allow the builder to separate the parts easily and cleanly from the runners.

They come in two basic types - cutters with a pair of blades that meet in the middle, and shear-action side-swiping blades. Both can work well if they’re thin and sharp enough.

The side-shear blades work pretty well with soft styrene plastic used in most kits. But I find side-shears are a bad choice for parts printed using brittle 3D-printing acrylic resin. The side-shear blades tend to crack the resin, whereas centre-meeting blades are less likely to. Check out this page for more information on cutting 3D printed acrylic resin.

I recommend buying a sharp, high quality sprue cutter, and never using it for anything other than plastic. Cutting metal wire, for example, can notch the blades.

Undergated sprues

A traditional aspect of making a plastic model kit is carefully cutting parts off their runners. The problem is that sometimes, despite your best efforts and sharpest sprue cutter tools, a bit of plastic remains and results in a small nub that must be trimmed. Trimming the projecting part can sometimes result in damage to fine details.

One approach taken by Bandai and a few other makers is to undergate parts. This means carefully placing runner contact points below important areas, thus minimizing the area of exposed - and easily damaged - detail plastic. It seems like an obvious thing to do, but many kit makers don’t do this.

Multiple colours

Bandai in particular employ a sophisticated moulding process that lets them have multiple colours of plastic on the same tree of parts. They do this by putting special valves into the moulds.

These gates let plastic flow or prevent its motion. Through doing this they can close a gate, inject plastic of one colour, open the gate, then inject plastic of a different colour or material or transparency. The final product - a multicoloured tree of parts - is then removed from the mould.

This is how Bandai kits can have, say, black, red, and transparent parts on the same physical tree. Parts can also be the same colour but almost touching, such as the incredible ball and socket joints built into their figure models.

Similar technology is used for expensive “double shot” ABS keycaps on traditional mechanical keyboards, where the key legends are actually moulded into the part rather than being simply printed on the surface.

Glues or cements

There are different ways to glue plastic pieces together. The most common are solvent-based glues or cements. These actually dissolve the plastic upon contact. So if you run a small amount of the solvent between two pieces and press them together you're effectively welding the two parts. Technically that means they aren't "glues" as such. Glues harden to form a material that sticks permanently to both surfaces, whereas the solvent melts the plastic and basically vanishes.

The most effective solvent cements are liquids. They can also be applied precisely with small paintbrushes, and can run through joining surfaces through capillary action. The thick gel-like tube cement sold to kids is much messier and hard to apply neatly.

Different solvent formulations exist for different plastics. Weaker solvents, like many consumer-oriented model glues, only work with the same plastics. 

Stronger solvents can often weld different but similar plastics. But other times you need to use a surface adhesive, such as epoxy, rubber contact cement, or ACC/superglue. These glues are also necessary for fastening completely dissimilar materials, such as brass and styrene.

Mould release agent

Sometimes you get kits with blobs of dark grease - typically brown or black - sticking to parts. These are caused by the greasy chemical agents used to make it easier to pull the parts from the mould. High quality kits generally don’t have the messy blobs, though cleaning any parts before painting is important. The slight film of oily grease may be nearly invisible, but can still interfere with paint adherence. Generally the greasy crap comes off with a little wash in warm water and dishwashing detergent, perhaps with a soft brush. But sometimes plastic parts are permanently stained by the stuff.

These release agent stains are particularly noticeable around the ejector pins on this Moebius model kit.

Resin casting

How does resin casting differ from injection moulding? Well, there are a number of similarities but many differences.

With resin casting you start with a model of the thing you want to copy. This master is then placed into a container, and silicone rubber is poured around it. The silicone is cut away once it's hardened, and you end up with a perfect mould of the master.

Two-part polyurethane (PU) resin is then mixed and poured into the mould. Unlike injection moulding, it's not done under pressure or at high temperatures - pouring liquid at room temperature is fine. One of the parts of the resin is a catalyst, which hardens the plastic up – a thermoset polymer. No specialized injection machinery is needed.

All these factors make resin casting perfect for home and garage operations. You don't need to invest tons of money on an injection mould, and pay a factory tons of money to use it!

Good resin castings can be reasonably detailed, but typically aren't as good as high-end injection moulded parts. Also, the silicone mould wears out quickly and a relatively small number of copies can be produced. As the mould wears, the quality of the castings also goes down. Finally, since the silicone is flexible, it's easy to cast a resin replica that's not perfectly flat if the mould flexes.

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