Mad About Science: Injection molding

By Brenden Bobby
Reader Columnist

Injection molding touches all of our lives, whether we realize it or not, yet it’s a process that not many people have ever been exposed to. Unless you’ve worked in a milling shop or factory that manufactures plastic goods, chances are you’ve never even heard of the process, which has been used to create virtually everything in your daily life — from plastic cookware to your office chair and even your phone case.

It all begins with plastic pellets. Plastic pellets are refined from crude oil after a number of involved steps we’ll talk about another day. Often, a factory will purchase a huge amount of plastic pellets from a manufacturer or a recycler, depending on their need. There are a wide range of plastics out there, and not all of them can be recycled to the specifications required for production.

Today, we’re going to talk about a form of production near and dear to my heart: the production of miniature figurines. Companies like Games Workshop, which develops figures for Warhammer Fantasy and Warhammer 40,000, are the most recognizable, though there are many other companies out there that produce minis for things like D&D or Pathfinder. 

High impact polystyrene, or HIPS, is used in the production of minis. This is a durable and impact-resistant form of inexpensive plastic that can be dissolved in acetone and limonene, a substance from citrus peels that gives them their fresh aroma, and is also used in flavoring some foods.

The HIPS pellets are poured into the hopper of the injection molding machine and heated up to around 220 degrees Celsius, or 428 degrees Fahrenheit. Then, the liquefied plastic is injected into an aluminum or steel mold to form its final shape.

This mold is specially designed by engineers and accounts for a number of factors. Channels in the mold direct the molten plastic to the spaces it needs to fill to form the item. As the liquid will always follow the shortest path and the one of least resistance, engineers have to be careful to balance the mold properly or it may end up with gaps and defects that could require a new mold. 

Additionally, engineers need to account for the air inside of the mold as it’s heated up and pushed out by the molten plastic. As the air heats up, it expands, which can cause an explosive force inside of the mold. Over time, this will deal irreversible damage to the mold, though a well designed mold should be able to withstand hundreds of thousands of injections.

After the mold has been filled, it is cooled, often using water in a closed circuit, somewhat similar to the radiator of your car or the liquid cooling of a high-end computer. The water is cycled through pipes and hoses, drawing the heat from the mold and the plastic before it’s flushed to a cooling tower to disperse the heat and start the whole process again. After about 30 seconds, the plastic cools, the mold separates and ejection pins push out the plastic sprue, where it will fall into a collection bin or a conveyor to be sorted later.

If your finished product is defective, you can often throw it into a grinder to break it down into pellets to be reused. While this is true of thermoplastics like HIPS, PLA or PET, there are other plastics called thermosets that will not melt after being subjected to heat. Epoxy, silicone and polyurethane are examples of thermosets and are much more expensive to manufacture if there’s a defect in the process.

The truly impressive part of this process is the creation of the mold. This usually begins with a computer assisted design (CAD) program. Engineers design the mold in the computer and then send that design to a computer numerical control (CNC) machine equipped with a milling tool that will bore out a perfect replica of the computer design. 

When it comes to minis, this mold is usually carved from an aluminum block due to its malleability in comparison to steel. The drawback is that aluminum isn’t as strong as steel and will often need to be replaced sooner. You can almost think of this process as 3-D printing in reverse — rather than adding material, the computer is carving it away to create a void for the plastic to mold to. This is the stage where engineers need to factor in efficient design that won’t lead to failures or the destruction of the mold from explosive gasses. Depending on the size of the mold, these aluminum blocks can cost anywhere from a couple of hundred dollars to a couple of thousand. This means that even minor failures in the creation of the mold can be very costly.

This also means that your first sprue is very expensive, but each subsequent product becomes cheaper and cheaper.

If you’re like me and learn more from watching a process than reading about it, consider checking out Goobertown Hobbies’ video “How Models are Made: Injection Molding Hard Plastic Sprues — HIPS” on Youtube. While this process isn’t completely universal for all products, it’s close enough to get an understanding of how manufacturers create things like phone cases, the plastics on staplers and even plastic furniture. This particular application is interesting because of the detail involved with the process, which is often skipped for the other products in our day-to-day lives.

Stay curious, 7B.