Granules of a thermoplastic material are loaded into a hopper and fed into an injection molding machine. A heater melts the plastic granules into a molten liquid state. The molten plastic is forced into a mold of the desired shape using a screw-type feed or a ramming plunger. The mold is cooled by passing a coolant fluid (usually water) through the mold via hose pipes connected to holes drilled in the mold. The molten plastic is cooled in the die and solidifies, upon which the die is removed and opened to extract the part. Any excess plastic is trimmed off using machining tools.
The initial setup costs are high because of the price of an injection molding machine and the price of machining a high quality accurate mold. However, once the setup is done, the costs of the thermoplastic materials and manufacturing cost of the stock are very low indeed, which drops the cost of each piece manufactured. In fact, the price of an injection molded stock can be far less than the price of a wood stock.
Two rifles with Injection Molded plastic stocks made by Weatherby
Injection molded plastics are usually "foamed", because the weight of a solid plastic stock would be excessive. The finished stocks are typically lighter than comparable wooden or laminated stocks. Typical time to manufacture an injection molded plastic stock, including molding and packaging takes about 30 minutes. The stocks can be manufactured to the same accurate dimensions every time, which makes them suitable for mass production.
Virtually every major firearms manufacturer in the United States makes injection molded stocks and there's a lot of advertising dollars spent on them. These stocks are usually marketed as weatherproof, shock-resistant, accurate, durable etc. So how much of this is truth and how much is hype, the reader asks.
It is certainly true that injection plastics are not affected by moisture, so fog and rain do not affect them. However, they are no more waterproof than a properly constructed laminated wood stock. It is also true that injection plastics are resistant to typical scratches and dings. However, it must be remembered that the material of the stock is a thermoplastic. What this means is that it does get affected by temperature. For instance, if a person were to leave his weapon locked in his car in the hot sun, it could cause the plastic to melt and warp the stock. Conversely, if the person left his weapon locked in the car during winter time, the cold could make the plastic extremely brittle and it could fracture due to a slight impact.
As for accuracy, it is true that injection plastic stocks can be manufactured to very accurate tolerances indeed. However, the flip side is that injection plastics are not as rigid as wooden or metal stocks. This means that it does not really matter if the stock is manufactured to very accurate dimensions, since it flexes when the weapon is fired, thereby affecting the accuracy of the weapon.
As far as durability claims go, it is certainly true that most thermoplastics degrade very slowly indeed, when exposed to the sun and rain. However, most of them are also susceptible to severe deterioration when in contact with liquids like ethanol, acetone, kerosene, ethyl ketone etc. As it happens, these are the very same solvents that are used in practically every barrel cleaner available in the market today.
Despite all the hype, the strength of an injection molded stock is actually not much different than a walnut wooden stock and is definitely easier to break than a laminated stock.
They are also ugly looking, compared to the beauty of a wooden or a laminated stock.
However, these stocks are among the cheapest types of stock to manufacture and hence, they bring down the overall cost of the weapons (or increase the manufacturer's profit margin!!) This is why practically every firearms manufacturer in the United States offers some firearms with injection molded plastic stocks. They are also lighter than stocks made of other materials.