Injection Molded Plastics
This week, we’re going to cover injection molded plastics! This is the most common type of process used for manufacturing plastic parts. This is typically how most plastics in the consumer electronics, automotive and medical industry are made. The biggest reason to select high pressure injection molding is because the piece price can significantly drop when you get to higher and higher volumes. Typically, if you intend on making more than 1000 parts, your best bet is to go with injection molding.
High pressure injection molding has numerous benefits over other types of plastic manufacturing methods:
1. It’s very repeatable – once the process is dialed in, you can produce thousands of parts in a very predictable and repeatable way.
2. It can be very low cost per part
3. The tooling is capable of making up 1 million parts or more depending on material.
4. High flexibility in material choice – numerous polymers and colors that can be selected
5. There are numerous advanced “in mold” processes that can further reduce the cost of decorative parts
6. Low waste – depending on how your tool is made there is no waste, or there is a small amount of waste that can be recycled
7. You can get production representative prototypes.
So why don’t we do injection molding for everything? Well there are a few disadvantages:
1. High up front investment – tooling can be expensive and is often paid for up front and not amortized into the piece price
2. It can be longer lead times to manufacture the tool and industrialize it (tool bring up and process tuning)
3. There are design and size limitations.
This video below is a great animation of how the injection molding process works.
https://www.youtube.com/watch?v=b1U9W4iNDiQ
That video represents what we would typically call an open/close tool. The part is a very simple design with no under cuts. The next video shows a demonstration of how a slider works. This is how you achieve undercuts. You can also do collapsing cores like in the casting video I showed. There are some limitations to how many undercuts can be done and how many sliders are involved, but for some very complex tools you can add as many as 20 actions.
https://www.youtube.com/watch?v=1XUt7q-wkTw
Let’s quickly go over the disadvantages, and then we can go through the advantages one by one.
High Up Front Investment
Some of the really big injection molding tools can cost up to $1mil to make. But for the most part, the smaller parts will all be under $100k. Tools are made by CNC machining the cavity and core from solid block aluminum or steel. Then you need to add all the fittings, hoses, mechanical actions and runner systems. So, an injection molding tool can be quite an investment, but a worthwhile one as we’ll see later on. It is also common for suppliers to ask you to pay for the tooling up front. Sometimes you can negotiate to have them amortize the cost of tooling into the cost of the parts, but you typically have to be a big well known company to get them to agree to that. At the very least, don’t ever agree to pay 100% of the tool cost upfront. It is quite normal to pay for say 50% up front, and then 50% once you’ve approved the part manufactured by the tool. Or sometimes you will have 30% up front, 30% when tool is completed and 40% when you approve the parts.
Longer lead times to manufacture tools
Lead times for tooling can be anywhere from 4 weeks for small tools to 40 weeks for really big tools. It takes quite a bit of advance planning to ensure you kick off your tool at the right time and have it complete when you need to manufacture parts. Also, don’t forget to include anywhere from 2-4 weeks for industrialization. This is where they test out the tool to ensure it is operating correctly and tune the process for producing parts. It’s pretty typical that something is found in the tool and it needs to go back to the tool maker for fine tuning or adjusting.
There are design size and limitations
Injection molding machines are categorized by their tonnage, which is their clamping force. The pressure of injection is so high, that a too low tonnage machine will actually come apart and spurt plastic everywhere out the sides of the tool. This is why there is a limitation to how big of parts can be manufactured. Some of the largest machines in the world (which are very rare) are 6000T. These can do parts as big as a 3 yard dumpster. Small parts can be made on 50T machines (think your thumb nail), medium parts on 500 – 1000T machines (the size of your hand), and larger parts on 1500T – 2000T machines (size of your chair). In terms of design limitations you have to add draft to all the walls of your parts, they can’t be perfectly upright and straight because otherwise they won’t come off the tool. There are also limitations to the thickness of ribs called wall thickness ratio. You will get sink marks in the surface is your ribs are too thick. These are just some things to think about when trying to design more structure into the parts. Now, let’s walk through the advantages one by one:
Repeatability
No matter what process is chosen for manufacturing, there are always variabilities in the parts. This is why all parts must be designed with tolerances – the amount of variability you can accept. The tighter the tolerances the more expensive your parts will be. Plastic injection molding tends to be very repeatable because the molding machines are highly controlled and mold flow analysis software is quite sophisticated. So you can very well predict before your part is ever made how it will turn out. That being said, it takes an experienced engineer to review the design and run the mold flow analysis to ensure the results come out right. Due to the high pressure nature, there are inherent stresses in the parts, and if the injection points and pressures aren’t correctly done you run the risk of a flat part turning into a Pringles potato chip once it fully cools!
Low Cost
As long as you are making a few hundred parts at a time, injection molding comes out very low cost. This is mostly due to the cycle time, which for very small parts can be as low as 15s or for very large parts can be up to 80s. But it’s safe to say that on average a cycle time is between 35 and 45s. So in 1 hour, your tool can produce 80 parts at a 45s cycle time. If you have a $50 per hour machine rate, that means it only costs you $0.62 per part of machine time to manufacture. This isn’t the whole story around cost of course, you need to account for set up costs, overhead costs, labor costs, raw material and profit. But compared to the same part made by CNC machining or printing - which could easily run you $80+ in machine time – it’s a bargain! When it comes to much higher volumes, you can achieve ridiculously low prices because those other costs are spread out over more parts. For example, many of the parts in your cellphone are run on machines 24 hours a day, 7 days a week and the tool only ever stops once a month for a few hours for routine maintenance! So setup costs, overhead costs and labor are all spread over a high qty of parts which is why it can be so low.
High volume tooling
As in the example above these tools can produce well over 1 million parts. So again the upfront investment in tooling can be spread over so many parts it’s almost negligible. For example, a tool that can make a plastic part the size of your hand will be approximately $30,000. If you make 500,000 parts off that tool you are paying $0.06 per part in tooling costs. Which is again quite a bargain compared to other manufacturing methods.
Flexibility in Materials and colors
There are several thousand different types of plastic materials that can be used in injection molding. But the most typically used ones fall into these general classifications: Polypropylene (PP), Acrylonitrile Butadiene Syrene (ABS), Polyamide (Nylon), High Density Polyethylene (HDPE), Polycarbonate (PC) and ABS & PC blend. There are many brands and specific formulas of these plastics and if your volumes are high enough you can get custom colors made too.
Decoration
The most common plastic appearance method that can be done in mold is called graining. This is adding a texture to the cavity of the mold that the part will take on. Almost all plastics have grain to them, otherwise they would just be shiny and slick. Grain can also be very fancy design or can imitate other textures. For instance, plastic parts in cars often have grain to imitate leather or fabric. There are some pretty incredible advances in the world of injection molding where you can do In Mold Decorating (IMD) This is where you can add in molding painting or high gloss protective coatings. You can insert films into the mold that cover the surface of the plastic after injection and provide a specialized color or printed pattern. Or you can even insert piece of fabric and back injection mold it and receive a finished part out of the mold with a fabric covering!
Low Waste
If you chose to manufacture your tool with a hot runner system, there is actually almost no waste when manufacturing. The only waste comes from the startup and cleaning of the machine when finished. This is because the plastic stays hot at the injection point inside the tool. Then the gate closes, and cuts of the plastic flow and there are no runners or legs left on the part. In a cold runner system, as soon as the plastic transfers from the machine to the tool it begins to cool and the cut off point is higher up the chain. So when the part is ejected from the tool you have to cut off the runners or legs of the part. This plastic can easily be re-ground and go on to manufacture more parts.
Prototyping
It’s important to make a distinction between prototype and production injection molding. Prototype injection molding can be a lower tool cost up front when the tools are made of aluminum. Aluminum as a raw material is more expensive than steel, but it is must softer and therefore it cuts faster on a CNC mill, so the tool require much less machine time and is therefore significantly less cost. Prototype tools can get you anywhere from 5000 to 25000 parts depending on the grade of aluminum used. You can add sliders and other mechanical actions to the tools or you can go ultra low cost and use hand slides. These are placed in the tool by hand, then when the part is ejected they are removed by hand, and then placed back in the tool again. This makes your upfront cost very low, but then your cycle time is higher so your piece price is higher.
That concludes a high level overview of injection molding. It can get far more detailed and complicated than this, but hopefully this provides enough information to help you understand if injection molding is right for you. Next week we’ll explore more plastic manufacturing methods.