Monday, July 4, 2022

How To Do Plastic Injection Molding

A Brief History Of Injection Molding Machines

How the Injection-Molding Process Helps Create Pretty Much Everything

The history of injection moulding machine for plastic parts is quite long. Plastic injection moulding machines have been around for a long time. First invented back in 1872 by Isaiah and John Hyatt . These first machines would have been used to mould buttons, combs and billiard balls. The initial molding machine was very basic. These machines were essentially a piston ram inside a heated barrel. However, the moulding machine evolved a lot during the 1930s and post WW2. The invention of the current screw & check ring by James Watson Hendry, allowed for greater control over the injection process.

The Injection molding cycle is quite straight forward. The moulding machines are used to perform the following steps within the molding cycle

  • Mold close
  • Ejection

Benefits Of Using Plastic Injection Molding

  • Cost Saving
  • Smooth finishing of plastic parts
  • Good Speed & Fast Production
  • Capable of using multiple plastic types

To be a top plastic producer it is significant to have knowledge of plastic manufacturing methods. The more you have a grip on technicalities and all courses of action the more you will be able to become a number one plastic manufacturer in the world.

Lets look at the process and see how plastic injection molding works

Where Has All The Pressure Gone

In Scientific Molding procedure, parts are filled 90% to 99% in the first injection stage: That is, we make a short shot. If the part is short we know the plastic pressure at the end of flow is zero. We also know the pressure in the nozzle from the reported or calculated plastic pressure. Nearly all machines report the pressure at transfer, or at the end of first-stage injection. This transfer pressure is not always peak pressure during injection.

Lets take an example: We are making a small 1-in.² insulator from PC with some thin walls in a 4-cavity mold. The machine reports a hydraulic pressure at transfer of 2040 psi , and the part is 96% full and short. To find the plastic pressure in the nozzle, you need to multiply 2040 psi × the intensification ratio of the machine, which in this case is 14.7:1. So the pressure in the nozzle is 29,988 psi. If we have a short shot, the pressure loss for filling the part to 96% full is the entire 29,988 psi.

Where did all this pressure go? Is there anything we can do to reduce the pressure loss? The process window will open up and quality will be higher if lower pressures are required.

The flow path is known: Plastic traveled through the nozzle, sprue, runner, gate, and finally filled the part. How much plastic pressure did each of these components eat up? To find out the pressure loss for each component of the flow path, we need to make a few short shots and maybe dig out a part or two.

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Tip #: Minimize The Part Volume By Reducing Wall Thickness

Reducing the wall thickness of your part is the best way to minimize the part volume. Not only does it mean less material is used, but also the injection molding cycle is greatly accelerated.

For example, reducing the wall thickness from 3 mm to 2 mm can reduce the cycle time by 50% to 75%.

Thinner walls mean that the mold can be filled quicker. More importantly, parts thinner parts cool and solidify much faster. Remember that about half the injection molding cycle is spent on the solidification of the part while the machine is kept idle.

Care must be taken through to not overly reduce the stiffness of the part which would downgrade its mechanical performance. Ribs in key locations can be used to increase stiffness.

Home Plastic Injection Molding With An Epoxy Mold

Injection Moulding

I made an epoxy mold of an existing plastic item. Then I was able to make copies of that item using a home plastic injection molding machine.This process should work for many different types of items or models. Note that the original item must have a parting line without undercuts. That means the model must be able to be separated into two halves from which the new molded part can be easily removed.Here are the steps:

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How Does Injection Molding Simulation Software Help

Molding simulation can help designers and engineers to understand risks early in the design process, allowing the ability to address these before becoming fully invested. Simulation software gives engineers, mold makers, and other molding professionals accurate digital prototyping solutions, and help bring better products to market faster.

Moldflow simulation can be paired with other simulation tools, including mechanical stress, vibration, motion, computational fluid dynamics , and Multiphysics, providing a fast, accurate, and innovative approach to solving the most challenging design problems.

Injection Molding: What It Is How It Works Who Is It For

Ronan Ye

Rapid Prototyping & Rapid Manufacturing Expert

Specialize in CNC machining, 3D printing, urethane casting, rapid tooling, injection molding, metal casting, sheet metal and extrusion

Injection molding is the most popular method for manufacturing plastic parts on the planet, so its not surprising that the global market for the process was valued at almost 260 million U.S. dollars, with projections for continued growth in the foreseeable future.

The technology is used in a wide variety of industries, including aerospace, medical equipment, and automotive, where advanced techniques such as overmolding and insert molding are used to create even the most complex parts with impeccable precision.

And when you consider just the wide range of injection molding benefits, understanding why millions of companies in some of the most demanding industries in the world are using it for bringing their part designs to life.

But why is injection molding machine manufacturing so effective? And how does it work?

To find out, lets dig into the subject and answer all of the important questions, including what it is, the process behind it, its main advantages, and how to choose the right service provider.

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The Injection Molding Process

Injection molding is widely used today for both consumer products and engineering applications. Almost every plastic item around you was manufactured using injection molding. This is because the technology can produce identical parts at very high volumes at a very low cost per part .

But compared to other technologies, the start-up costs of injection molding are relatively high, mainly because custom tooling is needed. A mold can cost anywhere between $3,000 and $100,000+, depending on its complexity, material and accuracy .

All thermoplastic materials can be injection molded. Some types of silicone and other thermoset resins are also compatible with the injection molding process. The most commonly used materials in injection molding are:

  • Polypropylene : ~38% of global production
  • ABS: ~27% of global production
  • Polyethylene : ~15% of global production
  • Polystyrene : ~8% of global production

Even if we take into account all other possible manufacturing technologies, injection molding with these four materials alone accounts for more than 40% of all plastic parts produced globally every year!

How Much Does Injection Molding Really Cost

How It’s Done: Injection Molding with QLP

Out of all the costs for injection molding, tooling makes up the majority. Tooling makes or breaks the plastic injection molding process because proper tooling is key to having a successful production run. Tooling pricing depends on the complexity of the part, the length of the production cycle, and thermoplastic resin material choice.

Costs also depend on the type of resin and the amount of resin needed to create each part. The size of the part correlates directly to the size of the injection molding machines which are rated by tonnage, or the amount of force needed to hold the mold closed. Injection molding machine cycle time can increase depending on the complexity of the part. These factors all play a part in the costs associated with the plastic injection molding process.

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Get Frames To Make The Mold

The first step in our injection molding setup is to create a mold. We will use epoxy to create our mold. Epoxy is actually a two-part liquid. It is initially in liquid form, but it can then be poured into a mold or frame. It then solidifies and hardens.

You need to get a frame in which you can pour the epoxy to make your molds. You will need two frames for two molds. Moth these molds will clamp against each other. Each mold will make one section of your injection molded part. You can buy a frame for making your epoxy mold here.

You can always make your own frame, but that will add a significant amount of time to the whole process. If you really want to make your own frame, then make sure that the frames have a cavity for the epoxy to pour itself in and that they also have alignment pins so that the two frames line up correctly. The link we posted above is for a set of frames which have the alignment pins and the cavity.

Scientific Versus Traditional Moulding

Traditionally, the injection portion of the moulding process was done at one constant pressure to fill and pack the cavity. This method, however, allowed for a large variation in dimensions from cycle-to-cycle. More commonly used now is scientific or decoupled moulding, a method pioneered by RJG Inc. In this the injection of the plastic is “decoupled” into stages to allow better control of part dimensions and more cycle-to-cycle consistency. First the cavity is filled to approximately 98% full using velocity control. Although the pressure should be sufficient to allow for the desired speed, pressure limitations during this stage are undesirable. Once the cavity is 98% full, the machine switches from velocity control to pressure control, where the cavity is “packed out” at a constant pressure, where sufficient velocity to reach desired pressures is required. This lets workers control part dimensions to within thousandths of an inch or better.

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Start Small And Prototype Fast

Before you commit to any expensive injection molding tooling, first create and test a functional prototype of your design.

This step is essential for launching a successful product. This way design errors can be identified early, while the cost of change is still low.

There are 3 solutions for prototyping:

  • 3D printing
  • CNC machining in plastic
  • Low-run injection molding with 3D printed moldsThese processes can create realistic prototypes for form and function that closely resemble the appearance of the final injection molding product.
  • Use the information below as a quick comparison guide to decide which solution is best for your application.

    Prototyping with 3D printing

    Prototyping with low-run injection molding

    Min. quantity: 10 – 100 partsTypical cost: $1000 – $4000 totalLead time: 5 – 10 days

    Make Your Own Injection Molding Machine

    Plastic Injection Molding Explained, and how Billiards and ...

    Tried, true, and totally doable. Start cranking out solid thermoplastic parts.

    Sometimes theres a tool you want to buy, but you cant justify the cost. In that case, why not make it? For me, a plastic injection molding machine fell into that category and it turns out theyre not hard to make.

    I wanted to make solid plastic parts for some of my amateur science experiments. There are a number of ways to make things out of plastic, each with its advantages and disadvantages. Often just cutting raw material to the desired shape works best. Some plastics can be cast by pouring a liquid resin with hardener into a mold . Vacuum forming works well for making things out of thin sheets of plastic . I considered making a 3D printer, but for the few plastic parts I envisioned needing, it wasnt worth the time and money.

    Plastic injection molding has been around since thermoplastics were invented. Its a great way to make many copies of a part quickly, and what I like best is that its easy to reuse old plastic objects to make new ones.

    So I built an injection molder based on the plans in Vincent R. Gingerys book Secrets of Building a Plastic Injection Molding Machine. David Gingery could be considered a forerunner of the Maker Movement he and his son Vincent have written a whole series of books on building tools for the machine shop.

    My Build

    Temperature Control Upgrade

    Using the Injection Molder

    Some Tips

    Making Molds

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    Examples Of Products Made With Injection Molding

    If you look around you right now, you’ll see at least a few products that were manufactured with injection molding. You’re probably looking at one right now actually: the casing of the device you are using to read this guide.

    To recognize them, look out for these 3 things: a parting line, witness marks on the hidden side and a relatively uniform wall thickness throughout the part.

    We’ve collected some examples of products commonly manufacturing with injection molding to help get a better understanding of what can be achieved with this manufacturing process.

    Lego bricks

    Lego bricks are one of the most recognizable examples of injection molded parts. They’re manufactured using molds, like the one in the picture, which produced 120 million lego bricks before it was taken out of commission.The material used for Lego bricks is ABS because of its high impact resistance and excellent moldabillity. Every single brick has been designed to perfection, achieving tolerances down to 10 micro meters .This is partly achieved by using the best design practices, which we’ll examine in the next section .

    Bottle caps

    Model airplanes

    Model airplanes are another common example of injection molded parts. The material used here is mostly Polystyrene , for its low cost and ease of molding.What’s interesting with model airplane kits is that they come with the runner system still attached. So, you can see the path the melted plastic followed to fill the empty mold.

    Car parts

    Consumer electronics

    What Are Some Of The Considerations For Injection Molding:

    Before you endeavor to produce a part via injection molding consider a few of the following things:

  • Financial Considerations
  • Entry Cost: Preparing a product for injection molded manufacturing requires a large initial investment. Make sure you understand this crucial point up front.
  • Production Quantity
  • Determine the number of parts produced at which injection molding becomes the most cost effective method of manufacturing
  • Determine the number of parts produced at which you expect to break even on your investment . Build in a conservative margin.
  • Design Considerations
  • Part Design: You want to design the part from day one with injection molding in mind. Simplifying geometry and minimizing the number of parts early on will pay dividends down the road.
  • Tool Design: Make sure to design the mold tool to prevent defects during production. For a list of 10 common injection molding defects and how to fix or prevent them read here. Consider gate locations and run simulations using moldflow software like Solidworks Plastics.
  • Production Considerations
  • Cycle Time: Minimize cycle time in as much as it is possible. Using machines with hot runner technology will help as will well-thought-out tooling. Small changes can make a big difference and cutting a few seconds from your cycle time can translate into big savings when youre producing millions of parts.
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    How Does Plastic Injection Molding Work

    The manufacturing process for molded parts is complex. Before your parts are even molded, we’re there to help you with comprehensive manufacturability feedback and consultative design services, so you can avoid mold design pitfalls before the resin hits the mold. From there, our production process is documented and optimized using scientific molding so you get high-quality parts consistently and reliably with every production run. Critical-to-quality dimensions you call out on models let us know what areas are most crucial to the success of your parts. Once the parts start molding, the process and samples are qualified using automated inline CMM technology, generating a detailed FAI and process capability study with no additional lead time.

    • Shipped within days
    • pilot runs
    • functional prototyping

    Our thermoplastic injection molding is a standard process involving an aluminum mold with no heating or cooling lines running through it, which means cycle times are a bit longer. As part of the scientific molding process, our molders independently optimize and monitor fill speed, hold pressure, and hold time, and periodically inspect part quality and cosmetics. During the molding process, we use an automated inline CMM to ensure dimensional accuracy, as well as generate a detailed process capability study and FAI report.

    Plan For Dimensional Deviations

    MoldFlowXpress demo in SolidWorks aiding injection Molding

    Speaking of warping, you need to accept that its a natural part of subjecting materials to extremely high temperatures and cooling them down. If you need an extremely high degree of dimensional accuracy for your injection molded parts, this warping is something you will need to anticipate. Every resin behaves differently, so you may need to print your mold multiple times before you can get this right.

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    The Detail Of The Process

    During the molding process, the plastic pellets will be first delivered into the high-temperature injection barrel through the hopper, where they are heated, melted and plasticized into a sticky molten flow, which will then be injected into a lower temperature closed mold through the injection nozzle at a high speed under the great pushing pressure exerted by the plunger or the screw.

    Under the great pressure, the molten plastic will fill the entire cavity and will also be compacted. After that, the plunger or the screw will return after a period of pressure holding. At this point, it is possible to flow back from the cavity into the sprue and runner system.

    When the mold opens after cooling and forming, the product will be released from the mold cavity.

    • Position.

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