Injection Molding 101 – Everything You Need To Know - LEADRP

Injection molding (or injection moulding) is a widely used manufacturing process in many different industries such as aerospace, automotive, medical, and even consumer products. Here you’ll find the answers to the most common questions we get asked about injection molding

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What is injection molding?

Injection molding is a method of producing parts by injecting material into a mold. Metals (for which the process is known as die-casting), glasses, elastomers, confections, and, most commonly, thermoplastic and thermosetting polymers can all be used in injection molding. The part’s material is fed into a heated barrel, mixed, and forced into a mold cavity, where it cools and hardens to the cavity’s configuration. After a product is designed, usually by an industrial designer or engineer, molds are made from metal, usually steel or aluminum, and precision-machined to form the desired part’s features. 3D printing materials like photopolymers which do not melt during the injection molding of some lower temperature thermoplastics can be used for some simple injection molds. Injection molding is widely used for producing a wide range of parts, from very small to very large. The ability to produce parts with varying geometrical shapes and sizes is determined by the type of machine used in the operation.

The history of injection molding

Injection molding equipment originates from metal die casting processes. For example, John Wesley Hyatt is credited with inventing a patent for the first plastic injection molding machine. This patent was granted in August . However, the origins of injection molding date back to when Brothers Francis and John Downing invented and patented the bottle-making machine that produced embossed bottles from a continuous sheet of glass. This technology made it possible to create seamless bottles that could not be created with traditional blow-molding techniques.

How does plastic injection molding work?

Injection Molding takes plastic resin, heats it up, and forces it into a mold. The mold is usually made of steel or aluminum. The mold is made in two halves, called the A-side, and B-side. The A-side is attached to the injection unit on the injection machine. When the molten plastic comes out of the barrel of the machine, it fills the mold and then hardens.

Once the plastic has hardened in the mold, both sides are opened and the part is ejected. The process starts again when you close the mold back up and pressurize it to push more plastic in.

The injection molding process requires the use of an injection molding machine, raw plastic material, and a mold. The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part.

What are the benefits of plastic injection molding?

#1 Very high production rates

One of the most appealing characteristics of plastic injection molding as opposed to other processes is its very high production rates. Depending on the complexity of the design, the size of the part being molded, and other factors, individual molds can produce hundreds or even thousands of finished parts per hour. This allows manufacturers to keep costs low, while still benefiting from a fast turnaround time for their products.

#2 High tolerance precision

With the use of this process, you can produce high tolerance precision parts. A mold is used to create the shape of your part. The molds are held to very close tolerances by design. For example, if you need to make a thousand identical parts, you can use the same mold and they will all be exactly the same size and shape.

#3 Low labor costs

Compared to other manufacturing processes, plastic injection molding is relatively labor-free once a mold has been manufactured. Many companies that offer injection molding services are able to produce large quantities at a very low price point because they do not need to hire much labor for this type of work. This allows them to remain competitive in today’s market and keep prices low for consumers.

#4 Environmentally friendly

When compared to other manufacturing processes like CNC machining, which creates a lot of waste by cutting away at raw material, plastic injection molding is a much more eco-friendly solution.

#5 Very durable

Another great benefit of plastic injection molding is the ability to create very durable products that do not scratch or break easily. You can also choose from different types of plastics based on your product’s needs, like an impact-resistant or heat-resistant plastic.

#6 Variety of plastics available for choices

There are a variety of plastic resin material options to choose from for use in the plastic injection molding process. Each material has its own unique properties; therefore, understanding the differences between them is crucial to ensure selecting the most suitable material for your intended application.

What are the typical materials for plastic injection molding?

Injection-molded parts can be made from a variety of thermoplastic materials including ABS, nylon, polycarbonate (PC), polyoxymethylene (POM), polypropylene (PP), polystyrene (PS), polyurethane (PUR), thermoplastic elastomers (TPE) and TPU.

#1 Acrylonitrile Butadiene Styrene(ABS)

ABS is a commonly used plastic injection molding material with three main ingredients: acrylonitrile, butadiene, and styrene. Each of these monomers imparts specific properties and provides ABS terpolymer with a robust combination of features. ABS offers high strength, toughness, and resistance to impact and temperature. It is easily molded and gives a high-quality glossy surface finish. This plastic polymer does not have a specific melting point.

#2 Polycarbonate (PC)

A polycarbonate is a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonate has a high degree of stiffness and thermal resistance due to its molecular structure and a reasonably high viscosity when processed. Even so, polycarbonates can be molded and thermoformed with great ease, making them a popular choice for a wide range of products.

#3 Polyoxymethylene (POM)

Polyoxymethylene also known as polyacetal/acetal/polyformaldehyde/Delrin, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic polymers, it is produced by different chemical firms with slightly different formulas. POM is characterized by its high strength, hardness, and rigidity to − 40°C.

#4 Polypropylene (PP)

Polypropylene also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and is partially crystalline and non-polar. PP is inexpensive and easy to access, and due to flexible consistency, PP is widely used for manufacturing storage containers, such as bottles and plastic boxes. Polypropylene has a strong resistance to fatigue and chemical corrosion, making it useful for most types of plastic storage containers, kitchenware, water bottles, and even insulation and piping systems.

#5 Nylon Plastic (PA)

Nylon plastic (PA) is a synthetic thermoplastic polymer commonly used in injection molding applications. It’s a versatile, durable, flexible material often used as a more affordable alternative to other materials like silk, rubber, and latex.

#6 Acrylic (PMMA)

Acrylics are a group of polymers prepared from acrylate monomers. These plastics are noted for their transparency, resistance to breakage, and elasticity. They are also commonly known as acrylate polymers or polyacrylates. Acrylate polymer is commonly used in cosmetics, such as nail polish, as an adhesive. The most common acrylic plastic is polymethyl methacrylate (PMMA).

#7 Polyethylene (PE)

Polyethylene thermoplastic materials are generally divided into multiple groups, based on density. These include low-density polyethylene (or LDPE), medium density polyethylene (MDPE), high-density polyethylene (HDPE), and ultra-high molecular weight polyethylene (UHMWPE or UHMW). In general, the higher the density, the higher the tensile and flexural strength, chemical and abrasion resistance, and surface hardness.

#8 High Impact Polystyrene (HIPS)

This cost-efficient material offers excellent machinability, dimensional stability, impact resistance, and aesthetic properties. It is highly customizable and can be glued, printed, bonded, and decorated with ease.

Design tips for injection molding parts

Designing parts for injection molding can be simple, but there are a few basic rules you should follow to avoid common pitfalls and make sure your part is made in the best way possible.

To design parts for injection molding, try to keep these rules in mind:

#1 Uniform wall thickness

Contact us to discuss your requirements of automotive injection molding. Our experienced sales team can help you identify the options that best suit your needs.

Wall thickness should be no less than 4mm (0.16″) and no greater than 8mm (0.31″). Part walls thinner than 4mm (0.16″) will be difficult to inject, leading to flow lines, surface imperfections, and short shots. Walls thicker than 8mm (0.31″) will require more material and longer cooling times, which can lead to warpage or increased cycle times.

A wall thickness between 1.2 mm and 3 mm is a safe value for most materials.

Below are the recommended wall thicknesses for some of the most commonly used injection molding materials.

MaterialRecommended wall thickness [mm]Recommended wall thickness [inches]Polypropylene (PP)0.8 – 3.8 mm0.03” – 0.15”ABS1.2 – 3.5 mm0.045” – 0.14”Polyethylene (PE)0.8 – 3.0 mm0.03” – 0.12”Polystyrene (PS)1.0 – 4.0 mm0.04” – 0.155”Polyurethane (PUR)2.0 – 20.0 mm0.08” – 0.785”Nylon (PA 6)0.8 – 3.0 mm0.03” – 0.12”Polycarbonate (PC)1.0 – 4.0 mm0.04” – 0.16”PC/ABS1.2 – 3.5 mm0.045” – 0.14”POM (Delrin)0.8 – 3.0 mm0.03” – 0.12”PEEK1.0 – 3.0 mm0.04” – 0.12”Silicone1.0 – 10.0 mm0.04” – 0.40”

#2 Round all edges and corners

The uniform wall thickness limitation also applies to edges and corners: the transition must be as smooth as possible to ensure good material flow. For interior edges, use a radius of at least 0.5 x the wall thickness. For exterior edges, add a radius equal to the interior radius plus the wall thickness. This way you ensure that the thickness of the walls is constant everywhere (even at the corners). Wherever possible, round all corners to increase both the looks and the durability.

#3 Add draft angles
To make the ejection of the part from the mold easier, a draft angle must be added to all vertical walls. Walls without a draft angle will have drag marks on their surface, due to the high friction with the mold during ejection. A minimum draft angle of 2° is recommended. Larger draft angles (up to 5 °) should be used on taller features.

#4 Take care of the ribs

Ribs serve as structural features that help maintain the part’s overall stability. They are thin wall protrusions that extend perpendicularly from a wall or plane. Adding ribs rather than thicker walls will offer greater structural support.

#5 Avoid undercuts if possible

Injection molds are made up of two halves, which means they have parting lines and a “blindside” that cannot be reached by the injection nozzle or runner system.

#6 Transition from thick to thin

Parts will form better if plastic flows through features moving from greater to lesser wall thickness starting from the gate(s) (where the plastic first flows in to fill the part).

Besides, injection molding part design is not just creating an injection molded part that functions in its environment, but one that will delight your customer. It’s a given that you need to understand what it is you’re trying to design for manufacturability — but taking the step further, here are some suggestions on how to make your part stand out:

1. Think in terms of functional design

2. Make it easy to assemble

3. Make it easy to manufacture

4. Minimize the cost of manufacturing

5. Follow the design for manufacturability and assembly (DFMA) principles

Start Injection molding with LEADRP

Nothing compares to injection molding as a mass-production process. It is an efficient way to produce large quantities of parts ranging from a few grams to several kilograms. The injection molding process requires an injection molding machine, raw plastic material, and a machined mold. As a manufacturing process, injection molding is one of the most versatile ways to produce plastic parts. It allows you to create complex shapes at low costs; however, it can be challenging to get your product off the ground with an injection molder without first investing in tooling.

I’m looking forward to making better molds and better parts. Thank you to FormLabs people for your help making this seem possible!

My resources:
I have a table-top molding machine from LNS (https://www.techkits.com/products/model-150a/) and a couple small mud-frames The press is great but it is very low-tech. I am running Solidworks - but I don’t have the plastic mold-analysis feature. Oh, and I have a Form2 printer (w/ high temp, grey pro & some others). For molding, I have some virgin PP that molds nicely with the sample (metal) mold LNS supplied.

My issues:
(1) What I think I could use help with is how to design a mold that helps me utilize these tools. What I mean is that the mold press doesn’t let me know how much material I’m pressing into the mold so I get a lot of short-shots and blow-outs. I should probably make some strategically-placed big-ish exits (1mm diameter) so that material can escape and then when I see this, I’ll know it’s filled. (This might mean I don’t get as much packing pressure, but maybe that’s a trade-off)
(2) Another thing I want to improve is the main gate I was over-complicating things and put in a funnel-shaped washer on my first runs & that was really hard to clean. I think a flat washer will be much better.
(3) The LNS press comes with a standard clamp and an extra quick-release clamp. I want to design my molds so that I can get the most out of the clamping (to hopefully get less blow-outs). I did one hybrid mold where the ballast was made from my FDM printer (much less $ to run) and it filled out the mud frame & only the important parts when on the Form2. I would do this again.
(4) Lastly - I wonder what is a good first-part to test with this setup? Anybody want to suggest a simple part to make - and share an STL of the mold? Mold making and 3d printing are lots of fun but a little overwhelming - so many possibilities…!

Thanks,
Juliana

The MicroMolder is not out yet. They currently claim the early bird shipments (10 in US and 10 internationally) will start shipping at the end of June, but could be delayed again due to shortage of components.

Including this one, I know of 3 injection molding machines launched on Kickstarter over the past 7 years. I know one was called the AllForge, can’t remember the other one. All have been fully funded, but none have delivered yet. They all get into the testing stage and then you don’t hear anything about them again.

I’d be leary about pre-ordering until they actually start shipping. The people who pre-ordered the AllForge after their Kickstarter closed lost all of their money. By the time the company folded it was too late to get their money refunded by their credit card companies. It was in a similar price ranges the MicroMolder.

BabyPlast injection molding machines out of Italy are shipping, but I have no idea of cost, since you have to contact a sales person for pricing.

I felt that the 3D printed molds were for the small injection molding machines that are being discussed here, not for machines run by injection molding companies. I have clients that have their own small machines (most are the manual kind). They just need someone to make the molds.

You’ll also find that most molding companies won’t accept aluminum or steel molds made by other injection molding companies. They will only use the mold they make with their equipment. There are two main reasons -

1. Mold making is a HUGE moneymaker for those companies. The clients try to shave pennies off the cost per part, but the cost to make the mold doesn’t ever have any wiggle room except to change the material the mold is made from. They charge thousands for the simplest mold, which is why everyone is trying to find an alternative ways to make molds.

2. A badly made mold provided by a customer could damage their equipment and take weeks to fix if they have to order parts or bring a technician out. Would you let a client provide their own resin that comes in an unlabeled bottle for your Form 3L when you print their job? You’d tell them no, because it could damage your 3L or create a huge mess that you’ll spend a half day cleaning up.

It could also be argued that experimentation for a client cuts into your profit margin and slows down the flow of work through your business. When I get an odd material request for one of my FDM printers, I know I’ll have to spend time to dial in the settings and might have 2 or 3 restarts on a print job to get the print as perfect as can be. This means what should be a 60 hour print job could become a 180 hour print job (or longer), so I have to give the client an estimate for 180 hours (which will be crazy expensive) or an estimate for 60 hours and eat the cost of wasted filament, electricity, wear and tear, and the money lost from other jobs being delayed. It also adds a lot of unknowns to the production schedule for clients in the queue behind the oddball request. If I tell a client that their print job will be done sometime between 2.5 days to 7.5 days, they will look for someone else to print their project.

I’m in the process of setting up an injection molding shop right now, and I just thought I’d drop in and explain some of the non-financial reasons companies are hesitant to use molds designed by clients. As someone else already mentioned, there are of course financial incentives, but there actually are some functional reasons as well.

Mold design is very technical. Depending on the resin used, the geometry of the part, and the gate location, something as minor as a 0.5mm difference in gate size, or even just the shape of the gate, could as much as double the injection pressure needed to fill a part. Beyond that, it might be impossible to fill a part with any amount of pressure, or you might get poor quality fills, or ugly artifacts, if the gate is in the wrong location. For all these reasons, it is very common that a mold will have to be reworked and tweaked several times before you get a good part fill. If this is being done by a customer, who doesn’t know your process, equipment, or the capabilities of your machine, getting all these factors right, at the very least would take an insanely long amount of time to keep trading molds back and forth, if it wouldn’t be flat out impossible.

For all these same reasons, quite often shops use very expensive simulation software to get an idea how a particular resin will work in a particular mold, and this software gives them a good idea where to start on their settings, to get a good fill. If you are just handed a mold with no simulation, not reports, and no starting numbers, then there is a lot of trial and error, trying to guestimate the right values, chewing up both machine time and manpower. This could very quickly end up with the “cheap” customer provided mold, actually being a more expensive job than if they had just designed the mold themselves.

There’s also the fact that you have literally no way to know how good a mold designer your customer is. It’s easy to design a mold that is just never going to produce quality parts. Sink marks, weld lines, burning, poor surface quality, these are all artifacts that can happen no matter what you do, if the mold is poorly designed. Customers don’t have good things to say about your business if the parts you make for them look like garbage, and sometimes even refuse to pay if you can’t deliver the quality they expect. If they are providing the mold, you have no control over the final quality of the part, and that presents not just a huge reputational risk, but a very high chance of a bad customer interaction, that will leave both parties wishing they had never even tried this. No one actively wants to go seeking hard feelings.

The last thing is just straight up production process issues. There is basically no decent way to cool a 3D printed plastic mold insert. In a normal aluminum or steel mold, you’re going to have plumbing running through the mold to a temperature control unit, which will keep the mold in a certain temperature range. This way, you have some control of plastic flow, crystallization, shrinkage, and demold times. With a plastic insert, it’s probably going to be too cold when you first inject plastic, giving you a short shot that doesn’t fill the part, and then after some number of shots it’s going to get too hot, and either damage the insert, or causing warping on demold, because the part hasn’t cooled enough. This means it is both time and material consuming to trial and error how many shots you have before you get a good one, and then how many good ones you get before they go bad again. That’s just not something a production job shop wants to deal with for a small order.

I hope that cleared up some of the issues, and explained some of why it can be so hard to find a company willing to deal with it.

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