Cold Runners Versus Hot Runners
Runners is the term used for the channels incorporated into a mold for the hot resin to travel through to reach each cavity.
Larger runners allow the resin to flow more easily and at lower pressures. However, large channels require more time to cool and create more scrap, both of which impact the part cost.
Smaller runners, on the other hand, minimize cooling time, scrap, and ultimately part cost. The downside to small runners is the higher pressure required to force the hot resin to flow through them.
A solution that facilitates the use of small runners while also minimizing the required pressure is to use what are known as hot runners.
Small heating elements are incorporated into the mold near the runners so as to keep the resin more molten allowing it to flow more easily at lower pressure.
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Nothing is ever free though, and the downside of hot runners is the additional mold complexity which always translates into additional costs.
In most cases, at least initially, you are best off using only runners without heating elements which are referred to as cold runners. Remember, always start with the simplest, lowest cost solution.
Fast Fabrication Of Short
3D printed injection molds in an aluminum frame with the finished injection molded part.
With affordable desktop 3D printers, temperature resistant 3D printing materials, and injection molding machines, it is possible to create 3D printed injection molds in-house to produce functional prototypes and small, functional parts in production plastics. For low-volume production , 3D printed injection molds save time and money compared to expensive metal molds. They also enable a more agile manufacturing approach, allowing engineers and designers to prototype injection molds and test mold configurations or to easily modify molds and continue to iterate on their designs with low lead times and cost.
SLA printing technology is a great choice for injection molding. It is characterized by a smooth surface finish and high precision that the mold will transfer to the final part and that also facilitates demolding. 3D prints produced by SLA are chemically bonded such that they are fully dense and isotropic, producing functional molds at a quality not possible with fused deposition modeling . Desktop SLA printers, like those offered by Formlabs, simplify workflow as they are easy to implement, operate, and maintain.
for the detailed case studies and to learn how to create 3D printed molds in-house for injection molding.
What Products Are Made From Injection Molding
Manufacturers all over the world use injection molding to mass-produce all kinds of goods. You can use injection molding machines to make anything from a large object like a garbage can to a small object like a mobile phone case.
The most widespread use of plastic injection molding is to make non-consumer parts used in situations where visual appeal doesnt matterlike mechanical partsbecause the injection molding process often leaves impurities behind on the plastic part. Why? Lets look at the process to find out.
Tire chains made from plastic injection molding
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What Is Injection Molding
Injection Molding is a commonly used technique in manufacturing. It is especially useful when one needs to manufacture large volumes of the same item. Injection molding allows the manufacturer to repeatedly create the same product with the same specifications, a good level of tolerance, and cost efficiently. All kinds of household items such as appliance housings, bottle caps, plastic parts, and auto parts are manufactured using the process of injection molding.
The process of injection molding involves pushing a melted plastic mixture under a certain pressure through a cavity of a tightly held mold. The mold is also under a certain clamping pressure depending upon the viscosity of the plastic fluid being injected. The molten plastic flows inside the mold and takes the shape of the mold. Then, the plastic is allowed to cool down and set. Finally, the mold is opened up and the solidified plastic part is pushed out using ejector pins.
Make The Second Half Of The Mold
Place the item back into the first frame. The first frame will already have the hardened epoxy in it. Now place the second frame onto the first one and align it correctly. The second frame will have a removable plate that will allow you to pour epoxy in the second frame. That way, the second half of your mold can begin to take shape.
You will then leave the second plates epoxy to harden overnight. The next day, you will have to pry open the two plates which are pushed up against each other. This separation of the frames should not be difficult because of the automotive car wax which you had applied on the first half of the mold.
What you should see is an almost mirror image of your item formed into the epoxy mold which would now be formed on the second frame.
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How To Make An Injection Mold
What is an Injection Mold?
An injection mold is a tool that when all the parts are assembled, fits into an injection molding machine to manufacture identical plastic parts. Injection molds start with a drawing by an engineer of the part required for an application. Next an engineer with knowledge of the injection mold making process makes a mold drawing.
How an Injection Mold is Made
Making an injection mold takes a great deal of skill and some very expensive machinery. They are generally made out of tool steel by CNC machines. They are the negatives of the actual part required. There is also a runner system that allows the hot plastic to fill all the mold cavities in addition to channels that help cool or heat the mold thus controlling its temperature. About 10% of a typical injection molding cycle is dedicated to actually injecting the molten plastic. The remaining percentage is dedicated to cooling and solidification, packing and hold.
The simplest injection mold is a straight pull injection mold consisting of two halves. The A side and the B side. The front side or A side faces the cavity and is referred to as the cavity. The B side faces the cavity and is known as the core mold. However, not all injection molds follow that simple pattern.
Parts of an Injection Mold
The engineer designing the mold will decide if it requires an insert tool or free standing mold.
The Runner System
Injection Molding Machine
Molds For Plastic Injection
Receiving high-quality plastic parts starts with buying a quality built mold. Well work with you every step of the way to ensure youre familiar with the different types of molds for plastic injection that are available, and well make sure youre completely informed before you buy. Below we provide you with a valuable understanding of basic injection mold construction types and the benefits of each.
Insert molds have a cavity side and core side insert that fit inside of a custom mold base here at MSI. They are a great way for us to offer customers economical tooling at very rapid lead times. The insert style design is great for small and medium size parts that require low quantity part orders or fast delivery of parts.
Even though insert molds are low cost in basic design, they are made with the same high quality materials and components as free standing plastic injection molds and provide the same high quality defect free parts.
- Low cost
- Average lead times from 5 to 15 days
- Good for small parts
- Good for 1 cavity molds and small order quantities
Free standing molds, also known as stand-alone molds, provide an all-in-one design. They are built to include the mold base, inserts, and all components needed to be fully functional alone.
A summary of free-standing molds for plastic injection includes:
- Higher cost
- Average lead times of 3 to 8 weeks
- Best method for parts that will not fit into insert molds
- Best choice for multi-cavity molds to reduce part cost
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How To Build A Vacuum Forming Mold To Make Your Own Plastic Parts
Building custom parts often means learning new skills or getting out of your comfort zone, especially if you want to create something that is really cool. While most hot rodders can fabricate brackets and panels from sheet metal and tubing, building parts from plastic is not as common. How these kinds of parts are made carries a perception of being out of reach for the casual enthusiast. 3D printing is helping bring plastic fabrication into the shop, but even 3D printing comes with a steep learning curve that might put it out of reach. There is, however, a production method for building one-off plastic components that is easily within reach of any hot rodder: vacuum forming.
Unlike other plastic forming processes, such as rotational molding and injection molding, vacuum forming uses heat and vacuum to form plastic over simple pattern molds. This economical method of producing plastic can be found in the things that you see and handle every day. Nearly every business sign you see is vacuum formed plastic. Those clamshell packages holding the parts you buy? They are mass-produced on a vacuum forming machine. While the mass production machines cost well over $100k, you can build your own simple former for less than a $150.
Injection Molding Wall Thickness Constraints
If you take apart any of the plastic appliances around your home youll notice that the walls for most parts are about 1 mm to 4 mm thick , and uniform for the entire piece. Why? Two reasons.
First of all, thinner walls cool faster, shortening the cycle time of the mold, the amount of time it takes to make each part. If a plastic part can cool faster after the mold is filled, then it can safely be ejected sooner without warping, and because time on the injection machine costs money, the part is less expensive to produce.
The second reason is uniformity: In the cooling cycle, the outer surface of a plastic part cools first. Cooling causes contraction if the part is of uniform thickness, then the entire part will shrink away from the mold uniformly as it cools, and the part comes out smoothly.
However, if the part has thick and thin sections next to each other, then the molten center of the thicker area will continue to cool and contract after the thin areas and surfaces have already solidified. As this thick area continues cooling, it keeps contracting, and it can only pull material from the surface. The result is a little dimple on the surface of the part called a sink mark.
Sink marks merely show poor engineering on hidden areas, but on cosmetic surfaces, they can require thousands in retooling costs. How do you know whether or not your part has these thick wall problems with injection molding? Luckily, Fictiv has a tool for that.
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Injection Molds From Your 3d Printer
Last time we checked in with he had bought a benchtop injection molding machine. This time, he shows off how to 3D print molds. If you have ever had to spend to make tooling for injection molding, youll appreciate being able to make molds relatively inexpensively.
To test his workflow, created a little 3D figurine and brought it into TinkerCad. From there he created a mold and used Lychee Slicer to print it using resin.
There was some manual finishing needed and that poses a problem since the two halves of the mold need to line up exactly. He mentioned he should have put alignment pins in, and we agree. We might also have put a way to bolt the halves together so you could, for example, sand the two pieces as a single unit.
The molds wound up in a mold frame and the results were impressive. We cant help but think that the machine itself shouldnt be that hard to create. It looks like little more than an arbor press, a heat chamber, and a piston.
Design For Injection Molding
There are several factors that may affect the quality of the final product and the repeatability of the process. To yield the full benefits of the process, the designer must follow certain design guidelines.In this section, we outline common defects of injection molding and basic and advanced guidelines to follow when designing parts, including recommendations for keeping the costs to a minimum.
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Hire Experienced Designers For Injection Molding
Given the many design considerations for injection molding, it takes special skills, training, and knowledge to implement all of them and get the part right. This is something only experts are capable of, and you will find plenty of them through Cad Crowd.
As a source of experienced engineers and product designers, we can provide you with an individual or a team of professionals who can help you develop injection molded parts from scratch. We will match you with pre-vetted and highly qualified design professionals.
We also offer contract manufacturing services. Well connect you with leading U.S-based manufacturers that will get you all set up when youre ready to start production.
Common Injection Molding Defects
Most defects in injection molding are related to either the flow of the melted material or its non-uniform cooling rate during solidification.
Here is a list of defects to keep in mind while designing a part for injection molding. In the next section, we’ll see how you can avoid each of them by following good design practices.
When certain sections cool faster than others, then the part can permanently bend due to internal stresses.Parts with non-constant wall thickness are most prone to warping.
When the interior of a part solidifies before its surface, a small recess in an otherwise flat surface may appear, called a sink mark.Parts with thick walls or poorly designed ribs are most prone to sinking.
As the plastic shrinks, it applies pressure on the mold. During ejection, the walls of the part will slide and scrape against the mold, which can result to drag marks.Parts with vertical walls are most prone to drag marks.
When 2 flows meet, small hair-like discolorations may develop. These knit lines affect the parts aesthetics, but also they generally decrease the strength of the part.Parts with abrupt geometry changes or holes are more prone to knit lines.
Trapped air in the mold can inhibit the flow of the material during injection, resulting in an incomplete part. Good design can improve the flowability of the melted plastic.Parts with very thin walls or poorly designed ribs are more prone to short shots.
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Set Up Your Epoxy Molds On An Injection Molding Machine
Now that you have your epoxy molds ready, you can finally set them up on an injection molding machine before pouring in molten plastic. There are quite a few injection molding machines available online which are fit for a DIY or domestic work environment. We suggest the Model 150A from LNS Technologies.
The Model 150A is a relatively cheaper alternative to expensive injection mold tooling machines and injection molding machines. The Model 150A has clamps which will hold your epoxy molds together and pressed up against one another with some degree of pressure. The clamp frames have alignment pins which will ensure that the two halves of your epoxy mold are correctly oriented.
The Model 150A also has a small chamber in which you can put your plastic pellets or tiny cut-up pieces of plastic soda bottles. A temperature controller allows you to set the temperature that you want your molten plastic mixture to be. The molten plastic then gets injected into your mold cavity through the machines injector.
Injection Molding Machines: How Do They Work
An injection molding machine consists of 3 main parts: the injection unit, the mold – the heart of the whole process – and the clamping/ejector unit.
In this section, we examine the purpose of each of these systems and how their basic operation mechanics affect the end-result of the Injection molding process.
Watch a large injection molding machine in action while producing 72 bottle caps every 3 seconds in the video here:
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Cost Drivers In Injection Molding
The biggest costs in injection molding are:
- Tooling costs determined by the total cost of designing and machining the mold
- Material costs determined by the volume of the material used and its price per kilogram
- Production costs determined by the total time the Injection molding machine is used
Tooling costs are constant . This cost is independent of the total number of manufactured parts, while the material and production costs are dependent on the production volume.
For smaller productions , the cost of tooling has the greatest impact on the overall cost . So, it’s worthwhile altering your design accordingly to simplify the process of manufacturing of the mold .
For larger volumes to full-scale production , the contribution of the tooling costs to the overall cost is overshadowed by the material and production costs. So, your main design efforts should focus on minimizing both the volume part and the time of the molding cycle.
Here we collected some tips to help you minimize the cost of your Injection molded project.
Benefits And Limitations Of Injection Molding
Injection molding is an established manufacturing technology with a long history, but it’s constantly being refined and improved with new technological advancements.
Below is a quick rundown of the key advantages and disadvantages of injection molding to help you understand whether it’s the right solution for your application.
Benefits of injection molding
Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost.
The recommended minimum production volume for injection molding is 500 units. At this point economies of scale start to kick-in and the relatively high initial costs of tooling have a less prominent effect on the unit price.
Almost every thermoplastic material can be injection molded. This gives a very wide range of available materials with diverse physical properties to design with.
Parts produced with injection molding have very good physical properties. Their properties can be tailored by using additives or by mixing together different pellets to achieve the desired level of strength, stiffness or impact resistance.
This means that hundreds of identical parts can be produced every single hour.
Limitations of injection molding
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