Plastic Injection Molding
Plastics can be found everywhere, from homes to offices, they have become a prevalent part of society and have grown alongside us into modernity. Over time and with technology they have become more complex and can be found in everything from Lego bricks to synthetic human body parts - all thanks to the advent of plastic injection molding.
Injection molding was developed to resolve an obstacle around the billiard ball. In the 19th century, billiard balls were comprised of ivory, which was harvested from the tusks of African elephants. This ravaged the elephant population, then a billiards manufacturer came along and began offering a $10,000 (roughly 3 million today) prize to anyone who could develop an alternative. This provoked John Wesley Hyatt to develop one of the first plastic synthetic celluloid and patented a device that could mold the celluloid material into a billiard ball.
Let’s take a look at plastic injection molding: what it is, how it works, its advantages and cost investment to leave you better prepared to tackle your next plastics project.
What is Plastic Injection Molding?
While plastic injection molding may not be rocket science, it follows an intricate and complex manufacturing process. The principles have remained largely the same since its inception - small plastic beads, melted down and injected into a mold cavity, cooled and solidified to be left with a piece of molded plastic.
The entire injection molding process consists of four steps, clamping, injection, cooling, and ejection. These four steps define the quality of the end product, all of which results in an exact and continuous process.
The Injection Molding Process Cycle
The process cycle for plastic injection molding is very quick and typically lasts around 20 seconds, but can be anywhere from 5 seconds to a couple of minutes. The four stages of the plastic injection molding process are:
Clamping: Before any material flows from the out of the nozzle, the two halves of the mold need to be closed. For the molds, one half is allowed to slide in and out, while the other half is fixed. The half that can slide is pushed into the fixed mold with enough force so that no injected material can come out during the injection cycle.
Injection: The plastic pellets (resin) are dropped from the hopper into the heating system where the raw material is melted. Once molten, the plastic is injected into the mold which was pre-clamped in the first cycle. The amount of material injected into the cavity is known as “the shot.” The shot is calculated by taking the amount of material needed for the actual plastic part plus any scrap, also known as “flash.” The injection cycle is the primary driver for costs.
Cooling: The hot molten plastic is inside the mold also needs to cool. As the molten cools it solidifies into the desired shape. While cooling, there will be some physical changes to the plastic, such as shrinkage. Therefore, it is important to understand the characteristics of the plastic part to properly design the tool to take into account shrinkage which will get you the desired shape of the product. Not allowing for the proper amount of cooling time will cause defects in your plastic part.
Ejection: After the cooling time is reached, you can take the part out of the mold. Usually, the new part will be stuck into the mold so there will be an automatic feature built into the mold to force it out. If your machine doesn’t have that, then you will need to take it out manually, which can get expensive due to the added time. Sometimes, a spray can be used, so the plastic doesn’t stick as much to the mold. After the part is ejected, then the mold can be re-clamped and the cycle starts over at step 1.
After the ejection cycle is completed, there is usually some secondary process that occurs. If the part has a few different cavities, then there will be something known as flash, which is the excess material that is not intended to be on the final part. Flash is removed with cutters or scissors. Depending on the resin, the scrap can be reused.
This repeatable process makes Injection molding ideal for producing high volumes of plastic parts since numerous parts can be produced in each sequence by using multi-cavity molds. One can create the same part thousands or even millions of times in a single course, which is a noteworthy advantage considering this is all done with minimum scrap loss, high tolerance precision, repeatability, low labor cost, large material selection, and minimal need to finish parts after demolding.
ADVANTAGES OF PLASTIC INJECTION MOLDING
Low Cost and Scalability
The primary benefit of injection molding is its versatility and capacity to scale production en masse. The price per unit during injection molded manufacturing is extremely low, once the initial mold costs have been paid. Unit costs also tend to drop significantly as more parts are produced.
Unlike traditional CNC machining which cuts a substantial percentage of an original plastic sheet or block, injection molding has a significantly lower scrap rate.
Consistent and Repeatable
The second part you produce will practically be identical to the first one, which makes injection molding extremely consistent. This is an excellent characteristic when producing plastic pieces requiring complex shapes and precise dimensions, which is excellent if you are looking to build a high-quality brand around consistency and reliability in high volume production.
Quick and Efficient
Injection molding can generate a large volume of parts per hour. Though this depends on the number of cavities in your tool, you can expect anywhere between 15 - 30 seconds for each cycle. A quick injection molding cycle also means high production efficiency and automation.
Once you have a tool produced, you can easily modify the color of the part that you are producing. During the molding process, pre-colored resin pellets are added. This is the easiest and most economical method to color plastics.
DISADVANTAGES OF PLASTIC INJECTION MOLDING
High Initial Tooling Cost
Tooling is essentially a project in and of itself and one part of the entire injection molding process. There is a high cost of entry for the tooling due to the cost of design, testing, and tooling demands. When producing parts in high volume you want to ensure a high-quality design that is done right the first time.
The process of injection molding comes with some restrictions that come mostly with a plastic part design, which can make the structure more complicated. A mold is made from two massive steel halves that pull apart to release your part from the tool. While this may sound simple, this massive tool does this continually which can create stress if not designed properly. Poor tool design can have heavy implications down the line in terms of poor quality, re-tooling costs and waste. Upcoming EPower Corp blog articles will go into more detail on how to properly design around these limitations.
Not Ideal for Thin-Walled Parts
Injection molding requires consistent wall thickness. Keeping wall thickness consistent is important to prevent deviations during the cooling process resulting in defects.
Investments & Costs
One of the advantages of plastic injection molding is that the mass production cost is quite low. However, the disadvantage of this same process is that the tooling or molding cost is quite high compared to other processes.
The mass production price of a plastic injection molded part takes into account a few factors. These factors are material costs & production costs. Together, these two costs make up around 75% of the final cost.
Material Cost - The material cost is made up of the plastic pellets that are melted and shot into the mold. The price for the material differs depending on the type of plastic that you are using. The overall material is mostly made up of the weight of the product plus any scrap.
Production Cost - The production cost takes into account for the entire process cycle and also for the labor. From the previous section, we learned that the process cycle consisted of clamping, injection, cooling, and ejection. The costs of these four steps are also taken into account for the production cost. The other cost factor for production is the cost of the direct operator.
Other Costs - Other costs that make up the rest of the costing for plastic injection molding are the indirect factors that go into the part as well. These can include overhead, electricity, tooling changes, quality and more.
The price of the tooling is dependent on two main factors, and those are the mold base and the machining of the cavities. The mold base takes into account the size of the mold that will be needed to build your part. Therefore, the larger the part, the larger the mold base, thus, the higher the investment required to make that mold. Also, if many complex cavities need to be cut, then the price of the mold will also be higher. Other factors that affect the cost of the tool is the quality of the steel. If you have a part that will be produced in high volume, then higher quality steel is required compared to the steel needed for a lower volume part.
Plastic injection molding is one of the most preferred methods for manufacturing parts because if its many advantages over other manufacturing methods. Not only is injection molding simpler and more efficient, it is also extremely reliable.
Now that you’re familiar with the basics of plastic injection molding, you can now take it into consideration as a viable production method for your product.
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