"Riddle me this…”
Sometimes I can be flexible, but often I’m not. I’m a part of your everyday life – in fact, you’d have a hard time without me – but you might not always recognize me. I am what I am today because I couldn’t take the heat. What am I?
OK, I acknowledge this a tough one. I’ll give you another clue.
I can be used to fill cavities.
When I’m all fired up and under a lot of pressure, I mold into my surroundings.
The answer, my friends, is plastic!!!
Actually, there are many different kinds of plastics, but the one property they all share is “plasticity” (duh), which is an inherent pliability that allows us to form the material into a variety of different products or parts. From water bottles to phone cases to car bumpers, you interact with plastic every single day in any number of different ways.
Here at Idea Planet, we use plastic to manufacture custom collectibles, life-size characters, retail point-of-sale displays, figurines, and more. That may sound pretty simple, but there’s actually quite a bit of science involved depending on what we’re creating. Let’s start with what plastic is, more precisely. I’m not going to go all Einstein here, but just know that plastics are mostly manmade materials, made from polymers, which are long molecules built around chains of carbon atoms. That’s plenty of chemistry for today…
Suffice it to say, there are many different kinds of plastic characterized by:
How the polymers are structured (polyesters, polyethenes, polyurethanes, etc.)
Whether they’re natural or synthetic (cellulose vs. nylon)
Whether they’re recyclable (can be mixed with other plastics without contamination)
Whether they can be reheated to a melting state – or not (thermoplastics vs. thermoset plastics)
Since each type of plastic behaves somewhat differently, the specific polymer used in manufacturing will depend on the product requirements, which in turn, also inform the most appropriate manufacturing process. It’s our job to assess the function and form of the item we’re making to recommend the right plastic and process to deliver the desired outcome.
So, let’s take a look at the six different types of plastic manufacturing methods and their advantages. I promise not to get too technical on you!
One of the most used plastic
manufacturing methods on the planet is injection molding. It’s my personal favorite due to all its inherent benefits, especially for custom figurines, weapon replicas, and intricate packaging décor. In this process, thermoplastic resin pellets are melted through the intense pressure of an auger moving the plastic forward to fill the 3D cavity of a custom, steel mold. The material then cools to a solid state in the shape of the intended part design. Wallah!
See the action animation here: https://www.youtube.com/watch?time_continue=143&v=b1U9W4iNDiQ
The design and creation of the mold is critical in injection molding – it requires a greater investment of time and costs but results in very low unit costs when quantities are high.
Other benefits? Hold on to your hat, because there are several:
Because injection molds are subjected to tremendous pressure, the melted plastic is pressed extremely hard against the inside walls of the mold. This excessive force makes it possible to incorporate more details into the design of the product or part. Complex, small, and intricate shapes are easily managed. Our product engineers work with the factory on mold design to ensure appropriate depth of detail and avoidance of undercuts that would interfere with extraction of the finished product.
And the icing on the cake is consistent accuracy and repeatability of this high level of detail. Injection molding is so precise that it’s virtually error-proof. In fact, accuracy is typically within 0.005 inches. This includes controlled wall thickness, which can be more challenging with other production methods.
Once the mold is made, actual mass production goes pretty quickly. Again, with the right mold, it’s simply a matter of cranking out the product. That means it’s perfect for large quantity production runs.
Since injection molding is a highly effective process, only as much plastic as is necessary is used. Any excess material, because it’s a thermoplastic, can be ground up and recycled for another use, thereby further reducing waste.
Another ingenious form of plastics manufacturing is blow molding. Perhaps you need a milk bottle or a gas can – an item that must be hollow and made as one piece. Blow molding is the answer. This technique propels air into a malleable, plastic, hollow tube, called a parison (no, not someone who lives in Paris…), which is placed inside the mold. Compressed air forces the plastic to inflate and expand against the walls of the mold, leaving the interior hollow – similar to blowing up a balloon inside a pre-formed cavity. The plastic hardens, and BAM! you have a bottle!
Of course, different mold shapes are used for a variety of containers. The air pressure and amount of plastic determines the thickness of the walls, and the level of detail in the form is much lower. You should know there are a couple different types of blow molding based on how the parison or preform is made, but let’s move on to the overall advantages of blow molding:
Tooling costs are typically lower
Compared to stainless steel injection molding tools, the cast aluminum molds for this technique are less costly and faster to make, adding to the overall efficiency of the project.
Hooray! No need to fit parts together and deal with seam lines that may not be watertight.
High production capacity allows manufacture of high quantities in a shorter time period.
Finished bottles and containers can be recycled and the plastic reused instead of adding to our landfills or sadly, sinking to our ocean floors.
Also known as roto-molding, this technique offers a second option for manufacturing hollow objects, like a rubber ducky or doll head, for instance.
In this case, liquified plastic “slurry” (generally PVC or vinyl) flows into the mold which then enters a furnace. The mold is spun on two perpendicular axes at a slow, even speed, causing the slurry to disperse and coat the entire interior of the cavity. At this point, I can’t help but think of that crazy ride at the fair that uses centrifugal force to flatten you to the outside walls. But I digress…
After the “spin & cook” cycle is done, the mold is dipped into water to cool. Small needle-nose pliers are used to extract the pieces out of the mold cavities. Interestingly, at this stage, the pieces will appear somewhat deformed or misshapen. They are then re-heated in a secondary process where they regain their original, intended shape. Crazy, huh? Hard to believe that roto-molding has been around since 1855 and is still going strong today!
Similar to blow molding, the aluminum (or copper) molds for this technique are less costly, which also means a lower, upfront commitment for new product development. Time for tooling is also less than that for injection molding.
Uniform wall thickness and stress-free parts (created without pressure in the mold) enhance the hollow product’s structural integrity, function, and durability.
Think as small as a miniature doll head and as large as a 21,000-gallon tank. Wow.
Since there are no sprues, runners, off-cuts, or pinch-off scrap, little material is wasted. What is left over, through scrap or failed part testing, can usually be recycled.
Extrusion molding is perfect for products that are formed in a continuous profile (kind of like a long strip of licorice!). Plastic is melted and pressed through a die (instead of a mold) to create a fixed shape which cools and hardens and then is cut to the desired length. If you need a visual, think back to the cool, spaghetti-making Play-doh machines we had as kids – basically an extrusion toy!
This saves time and costs (how often do you hear that?)
Extrusion molding allows high volume production at high speed.
The best way to explain this form of manufacturing is to compare it to making waffles (don’t you love my analogies? J) – the batter goes into a hot waffle iron, the lid is pressed down, and out pops a finished culinary delight. The same principle applies to compression molding – the heat and intense compression force the plastic into all areas of the mold. Lift the top plate, and you’re done – without the sticky mess of maple syrup.
A great example of a compression molded product is the melamine dishware you might use outside on your patio. While the amount of heat and pressure can vary based on the product you make, imagine a compression force as high as 2,000psi – that’s like ten baby elephants sitting on your chest!
Typically, this is because the molds are less complicated & can be made from less expensive metals.
Longer fibers result in stronger and stiffer materials.
Compression molded plastic parts generally are stronger, stiffer, lighter, and more resistant to corrosion than metal parts. They are also highly resistant to heat and therefore are microwavable.
Because the overall cost is lower (you don’t want to invest in an expensive tool for a low quantity product). The production cycle times can also be faster than with injection molding.
Last, but not least, is plastic thermoforming – in this process, a heated, pre-softened plastic sheet is stretched over and onto a molded form either using pressure or a vacuum. This ensures the sheet completely conforms to the desired three-dimensional mold shape. Depending on the product, either thin-gauge or thick-gauge thermoplastic sheets are used. For high volume production run, the sheets are fed through on rolls. Check out this cool video on manufacturing hard-shell suitcases! https://www.youtube.com/watch?v=pVPAHrlKGko
We are fans of thin-gauge thermoforming for creating the blisters that hold our products securely in their retail boxes.
Again, based on simpler design and lower metal costs.
Textures and small detail can be added to the mold, but the overall, structural shape must be more organic (without sharp corners).
Do you feel like a rocket scientist now? That’s quite a bit of information I just spewed. As you can see, some serious brain power needs to go into each and every custom plastic product you manufacture to get the best possible result.
And that’s no guessing game!