Leafly Mould Blog

Injection molding is one of the most popular and effective manufacturing processes, because it is capable of producing high quality parts in large numbers, and generally very quickly. In a nutshell, injection molding is when a material (generally plastic) is heated until pliable, forced into a mold made for a specific purpose, allowed to cool and harden, and then ejected from the mold. Depending on how the mold is made (and what material is being injection molded), this process can be repeated over and over in order to create large numbers of a given product. Although popular and effective, injection molding is not a cheap process; projects typically cost between $10,000 and a few hundred thousand dollars to manufacture. Let’s look at some ways to ensure that cost effective injection molding is a reality for your project.

Tips for cost effective injection molding from the Creative Mechanisms team

Make sure you’re using the right material. Did you know that there are hundreds of plastics (let alone other materials) that can be injection molded? It’s important to consider what function you want a particular piece to accomplish, and which material is most appropriate to make that happen. Does a piece need to be pliable or rigid? Will it be exposed to heat or extreme temperature deviations? How does Factor of Safety affect the materials required for design? It’s a common mistake to assume that a state-of-the-art, top-of-the-line material is the right one to utilize, but if its good qualities aren’t pertinent to your project, then they are essentially useless – and may cost more money overall. For instance, why use a 40% glass filled nylon when polyethylene would do the trick just as well? The best material for injection molding is the one that best fits your requirements and is not simply the better material overall.

Identify where processes can be consolidated. There are a lot of secondary processes involved in producing a part from scratch. Such processes (like custom inserts, label printing, painting, etc.) can prove to be time consuming, as they require extensive setup – and in injection molding, time is money. All those extra costs – and the time that could have been saved with better production management – ultimately drive the part price up. The best practice is to try to combine all of these processes into one single robust process. Read more about the Creative Mechanisms process, and how we design for defect prevention in injection molding. 

Be selective when choosing who does the injection molding.Like most industries, the injection molding industry is full of small, mid-size, and large companies. One or the other may be more appropriate depending on your project. Smaller companies will generally offer more flexibility and lower costs, whereas prices may be driven up with large companies due to higher overhead, higher salaries, and sometimes more advanced technology. In general, it’s best to choose a company that has experience molding your type of product, as it will save time during the research and development part of the project. Remember that bigger and more expensive does not necessarily equal better quality. Creative Mechanisms is not an injection molding company, but over the years, we have built a reliable network of high quality, responsive manufacturing companies that have helped to turn ideas into reality.

Consider bulk production. Molding operations are rarely personal projects or projects that will produce small numbers. In the research and development phase alone, some projects produce hundreds or thousands of prototypes, as many benefits come from extensive testing and feedback. When the product moves into the production phase, it is even more important to be able to mold as many parts in one shot as possible. Molds for production should also have as many cavities as possible without compromising the quality of the parts produced. In a competitive market, a product must be the best it can be while also being affordable. That is why it is advisable to produce as many parts as possible at one time – because it spreads the setup cost out over more parts, thus leaving you with a lower price per piece. You can now sell your product in a competitive market. 

Is the mold design optimized for cost effective injection molding? In mold design, as in bulk production, it is beneficial if you can produce as many parts as possible in a single shot. For mold design, it is also very important to be able to eject the plastic product quickly and to be ready for the next shot without wasting movements. Rods, an air blast, or a plate are typically used for the ejection stage of injection molding. Every second in the injection molding process translates into money, so it is critical to minimize the mechanisms of molding to as few and as fast as possible. A design and engineering firm that is familiar with the nuances of injection molding will create parts that will lend themselves to optimized mold design.

Optimize product design and materials. You can save a considerable amount of money, especially in material consumption, with an optimized product design. Using ribs and gussets to reinforce a product, for example, will save on material consumption, as well as ensuring that the product has uniform wall thickness that is neither too thin nor too thick. Incorporating adequate draft is also essential, as it allows for quick ejection of the product from the mold, saving time and money. If there is a need for a mechanism in the product, there are quite a few to choose from. Many can be incorporated into the molding process without the need for secondary processes or machining. Some mechanisms, such as living hinges, take advantage of the properties of the material that was used to mold the plastic part. These mechanisms can be made directly from the molding process versus spending extra time and money on other processes, such as stamping.

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Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

What is Injection Molding:

Injection Molding is a manufacturing process for producing parts in large volume. It is most typically used in mass-production processes where the same part is being created thousands or even millions of times in succession.

Why Use Injection Molding:

The principal advantage of injection molding is the ability to scale production en masse. Once the initial costs have been paid the price per unit during injection molded manufacturing is extremely low. The price also tends to drop drastically as more parts are produced. Other advantages include the following:

• Injection Molding produces low scrap rates relative to traditional manufacturing processes like CNC machining which cut away substantial percentages of an original plastic block or sheet. This however can be a negative relative to additive manufacturing processes like 3D printing that have even lower scrap rates. Note: waste plastic from injection molding manufacturing typically comes consistently from four areas: the sprue, the runners, the gate locations, and any overflow material that leaks out of the part cavity itself (a condition called “flash”).

Image From Ferris.edu

A sprue is simply the channel that guides molten plastic from the nozzle of the injection molding machine to the entry point for the entire injection mold tool. It is a separate part from the mold tool itself. A runner is a system of channels that meet up with the sprue, typically within or as part of the mold tool, that guides the molten plastic into the part cavities within the mold tool. There are two principal categories of runners (hot and cold) which you can read about here. Lastly, the gate is the part of the channel after the runner that leads directly into the part cavity. After an injection mold cycle (typically only seconds long) the entirety of the molten plastic will cool leaving solid plastic in the sprue, runners, gates, part cavities themselves, as well as a little bit of overflow potentially on the edges of the parts (if the seal isn’t 100% right).

Thermoset material, such as an epoxy resin that cures once exposed to air, is a material that cures and would burn after curing if one attempt is made to melt it. Thermoplastic material by contrast, is a plastic material that can be melted, cool and solidify, and then be melted again without burning. With thermoplastic materials the material can be recycled are used again. Sometimes this happens right on the factory floor. They grind up the sprues/runners and any reject parts. Then they add that material back into the raw material that goes into the injection molding press. This material is referred to as “re-grind”. Typically, quality control departments will limit the amount of regrind that is allowed to be placed back into the press. (Some performance properties of the plastic can degrade as it is molded over and over). Or, if they have a lot of it, a factory can sell this re-grind to some other factory who can use it. Typically regrind material is used for low-quality parts that don’t need high performance properties.

• Injection Molding is very repeatable. That is, the second part you produce is going to be practically identical to the first one etc. This is a wonderful characteristic when trying to produce brand consistency and part reliability in high volume production.

What Is The Downside To Injection Molding:

Up front costs tend to be very high due to design, testing, and tooling requirements. If you are going to produce parts in high volumes you want to make sure you get the design right the first time. That is more complicated than you might think. Getting the design right includes:

• Designing and then prototyping the part itself to specification
  • Initial prototype development is typically completed on a 3D printer and often in a different material (such as ABS plastic) than the final part will be constructed in
• Designing an injection mold tool for an initial production round
  • Typically generating 300-1000 injection molded prototypes in the production material requires the development of an injection mold tool.
• Refining any and all details in the injection mold tool prior to mass-production in an injection mold manufacturing plant.

Potentially negative aspects of injection molding include the following:

• Two of the major disadvantages to injection molding are the high tooling costs and large required lead times. Tooling is almost a project in and of itself and only one phase of the entire injection molding process. Before you can produce an injection molded part you first have to design and prototype a part (probably via CNC or 3D printing), then you have to design and prototype a mold tool that can produce replicas of the part in volume. Lastly, and typically after extensive testing in both of the aforementioned stages, you get to injection mold a part. As you can imagine, all of the iteration required to get the tool correct prior to mass production requires both time and money. It is rare that you would prototype an injection molding tool. It does happen though, especially for parts that will be made in a multi-cavity tool. For example, let’s say we were going to injection mold a new shampoo bottle cap. That cap would likely have threads to attach it to the bottle, a living hinge, a snap closure, and potentially some overmolding too. A company may choose to make a single cavity tool of that part to make sure all of the features will mold as desired. Upon approval, they will make a new tool, that is capable of molding, for example, 16 caps at a time. They do the single cavity tool first so if there are any issues, they don’t have to pay and wait for it to be fixed 16 times for each cavity.

• Because tools are typically made out of steel (a very hard material) or aluminum it can be difficult to make changes. If you want to add plastic to the part you can always make the tool cavity larger by cutting away steel or aluminum. But if you are trying to take away plastic you need to decrease the size of the tool cavity by adding aluminum or metal to it. This is extremely difficult and in many cases might mean needing to scrap the tool (or part of the tool) entirely and start over. In other cases you might be able to weld metal into the cavity that is undesired.

• Injection molding necessitates uniform wall thickness. If you were to cut a cross-section of the Panasonic mold above you would notice that the wall thickness is approximately 2-3mm thick throughout. Keeping walls from being too thick is important to prevent inconsistencies in the cooling process resulting in defects like sink marks. A good rule of thumb is to keep walls less than or equal to 4mm thick. The thicker the walls the more material you will use, the longer the cycle time will be and the higher your cost per part will be. Conversely, if wall thickness is any thinner than 1mm or so you might experience trouble filling the mold tool (resulting in gaps or short shots). Designers can compensate for this potentiality by using a material with a higher melt flow index like Nylon which is often suitable for walls as thin as 0.5mm. Different manufacturing techniques like CNC don’t require uniform wall thickness at all.

• Oftentimes large parts cannot be produced via injection molding as a single piece. This is due to the size limitations of injection mold machines and the mold tools themselves. For example of a large injection molded part consider the shopping carts at Target. Although the machinery exists to mold very large pieces (e.g. 1000 ton presses roughly the size of a train’s caboose), using it is very expensive. For this reason, objects that are larger than a typical injection molding machine’s capability are most often created in multiple pieces. CNC machines have similar limitations regarding product size while 3D printing has even more limitations. CNC is limited to the travel and size of the bed in the milling machine while large 3D printed parts often need to be printed in multiple pieces and then bonded together.

• Large undercuts require experienced design to avoid and can often add costs to the project.

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:

1. Financial Considerations
   1. Entry Cost: Preparing a product for injection molded manufacturing requires a large initial investment. Make sure you understand this crucial point up front.
   2. Production Quantity
      1. Determine the number of parts produced at which injection molding becomes the most cost effective method of manufacturing
      2. Determine the number of parts produced at which you expect to break even on your investment (consider the costs of design, testing, production, assembly, marketing, and distribution as well as the expected price point for sales). Build in a conservative margin.

1. Design Considerations
   1. 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.
   2. 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.

1. Production Considerations
   1. 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 you’re producing millions of parts.
   2. Assembly: Design your part to minimize assembly. Much of the reason injection molding is done in southeast Asia is the cost of assembling simple parts during an injection molding run. To the extent that you can design assembly out of the process you will save significant money on the cost of labor.

An Example (Designing For Injection Molding)

Designing a part that’s suitable for injection molding versus one that’s suitable for machining, thermal forming, or 3D printing means taking into consideration some of the differences between the various fabrication techniques and recognizing when your project is better suited to one or the other. Typical parts you might want to injection mold include joints, brackets, or housings. For example, most consumer electronic tools are made with a plastic shell (housing) that’s injection molded and used for the body of the tool.

Consider the housing for an electric drill produced by Panasonic (see below):

Picture courtesy of Panasonic

 

One of the most obvious advantages to injection molding is that the housing serves multiple purposes. First, it serves as a handle for the end user to interact with. It also acts as a receptacle for the battery and motor as well the location of various screw bosses that will be used to fasten the device together once the internal parts are assembled. In other words, injection molding is extremely effective when you need to organize a lot of internal parts within a housing. As a consequence, it’s a fantastic way to reduce the number of total parts (“piece count”). Of note, this part is also an overmolded part. For more on this process read here.

Some of the other reasons that injection molding is a good fit for this example include the fact that the drill is being produced in large volume. That is, Panasonic is creating a large number of copies of the same drill handle. Injection molding is wonderful for this kind of high volume production because the high initial costs pay the manufacturer back over time with low per unit costs. For this same reason injection molding can be a poor choice for low volume production. Additionally of note, there are some design constraints if using injection molding. For example, the part has nearly uniform wall thickness (which is important in order to avoid defects), and the part is made with a thermoplastic material (allowing for solid plastic stock to be repeatedly melted for the procedure). If you were designing a part with a thermoset material then injection molding would be more nuanced. You can injection mold a thermoset material but you can only do it once. Trying to melt a thermoset plastic a second time will result in burning the material. Similarly, a part with varied wall thickness would require more attention in the mold tool design to ensure uniform cooling and to avoid defects during production.

Conclusion

Injection molding is a great technology for finished production on a massive scale. It is also useful for finalized prototypes that are used for consumer and/or product testing. Prior to this late stage in production, however, 3D printing is much more affordable and flexible for products in the early stages of design.

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Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

At least to some degree, asking what plastic injection molds cost is a lot like asking “How much is a car?”. Will your mold be a Pinto or a Porsche?

The determining factors in the cost of your mold are a combination of the part size, complexity, material, and anticipated quantities. For instance, if you wanted 1,000 washers per year, we would recommend a single cavity mold, meaning it makes one washer per machine cycle. In that case the mold would probably be $1,000-2,000.

On the other hand, if you are going to need 100,000 xbox controllers every month, we would build a 12 cavity hardened “family” mold which made four Fronts, four Backs, and four Button Trees every cycle, and you’d better have $60-$80,000 or more to invest.

Of course these examples are extreme, but it illustrates the range of costs to anticipate. Your particular part will most likely fall somewhere in between that, and the molds that Rex Plastics builds average around $12,000.

Material selection is another consideration in determining what plastic injection molds cost. If your product requires a fiberglass filled material for instance, it will most likely need a mold made from hardened tool steel due to the wear those materials cause when being injected under high pressure.

If you have a target price established for your parts, it will be helpful to let your mold builder know that, because they can design the mold with that in mind, and plan for the right number of cavities to achieve that price. The more cavities (or parts made per cycle), the less expensive the parts will be, also allowing for higher output.

While you don’t need to know every detail about the types of molds available, it’s always a good idea to come prepared. Supplying the basics to your mold builder will help them build the best tool for your project. It’s also recommended that you find a mold maker who also has production capabilities. The company running the production won’t want to build an inadequate mold because they know they have to use it!

Whatever your project requires, one thing Rex Plastics won’t do is suggest you invest in a Ferrari when all you need is a Fiat!

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Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold