How To Save Money On Your Future Injection Molding Projects

Injection molding is, in many ways, one of the most cost-effective manufacturing processes available — that fact is one of several reasons why it’s so commonly used. Among those other reasons: versatility, relative speed, and access and availability. In this piece, however, we’re going to focus on that cost-efficiency aspect, and how to maximize it by lowering your production costs in targeted areas. The value of injection molding lies in its ability to produce a broad range of part types and designs, at scale, with a relatively fast turnaround time. We’ll look through the lens of that value to help locate areas for even greater efficiency and lower costs.

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When assessing areas where you might want to reduce your injection molding costs, there are two big-picture areas to consider: what you can do as a product developer or designer, and what the manufacturer can do as your production partner. While injection molding may seem like a cut-and-dry process with little wiggle room, the opposite is true.

Injection molding, is to be sure, a rigorous process with a number of defined best practices. Though its versatility means that by nature, there are innumerable areas that can be tested and fine-tuned, on both sides: designer and manufacturer. Let’s take a look at a few areas for each.

What Can You Do As a Product Designer?

From small, one-person concepts to larger product development firms, the role of product design is, obviously, all-important in the final product — and has its share of bumps and snags. Difficulties and inefficiencies are not limited to those just starting — even entrenched design teams with longstanding processes are able to locate areas where overall efficiency can be improved. These include:

Design for manufacture: This concept has several names — one you’ll see most commonly is “manufacturability.” What it means, in a nutshell, is taking the practicalities of the manufacturing process into account when settling on a final product design. Note the use of the word “final” here — manufacturability is by no means meant to impede early brainstorming or concepting processes, which often work better with fewer limits placed on them. At some point, however, between the brainstorming process and the delivery of a design to the manufacturer, it’s helpful to take manufacturability into account.

From a cost-savings standpoint, you’ll see several benefits:

• • Fewer additional prototype iterations
• • Less input from the prototyper or manufacturer — more consultation time may mean increased costs
• • A product design that’s easier to implement, with fewer potential production modifications or workarounds
• • Design optimization with the luxury of time, rather than against potential production deadlines (when all potential optimizations may not be explored)
• • You’ll also see a finished product that’s closer to what you designed, which, while not necessarily a direct cost-saver, is a knock-on benefit

Familiarity with production processes: Similar, but not quite identical, to the above point — as a product designer, the more you know about available manufacturing processes, the better off you’ll be working with prototypers and production facilities. Even a baseline knowledge of concepts — such as optimal production runs for different processes, 3D printing and when it can be useful, the existence of different types of injection molding materials, and more — can help you ask the right questions to get the best price possible.

Research potential manufacturing partners: No two injection molding facilities are identical, no matter how much of a commodity it may seem that you’re producing. Price is, clearly, always an important factor in manufacturing, but it shouldn’t necessarily be the final word when sourcing a manufacturer. Try and find companies that, for example, have worked with similar industries and products to yours; that may focus more on production runs more suitable to you (whether 20,000 pieces, or 2 million); or have more experience involving potential cost-saving processes like 3D printing.

What Can a Manufacturing Partner Do to Lower Costs?

On the manufacturing side, there are several ways to offer lower costs to customers without cutting into your bottom line. As a manufacturer, these are worth looking into, and as a customer, they’re always worth exploring.

Make the most of available processes: Injection molding, and design for injection molding, shouldn’t live in an injection molding silo. Processes like 3D printing can lower costs and speed production everywhere from prototyping to mold creation, and can even be a viable solution for injection molded part components via over molding. Don’t ignore the widespread availability and ongoing advances in these areas.

Be a material expert: Understanding the properties and ideal applications of the full range of common materials used in injection molding can not only differentiate you in regards to customer service, it can also help optimize your molding processes — resulting in fewer part defects or rejects and thus higher margins for you. Developing a deep understanding of factors such as viscosity, flow rate, melt temperature and the molecular makeups of types of resins can pay dividends over time.

Take advantage of best practices: In the context of this conversation, communicate them to your customers. Many injection molding best practices are designed to optimize the process and reduce defects and errors: round corners, wall thickness parameters, draft and so on. These are cost-savers through the efficiency they introduce into the process. Other best practices, however, can have a more direct impact on production and material costs, such as using ribs in part design rather than solid walls wherever possible. Not only does this type of best practice reduce production time, it lowers the amount of raw material used overall in a production run, thus reducing material cost.

Keep machinery updated: Just like fuel efficiency in automobiles, energy efficiency with production machinery is continually improving. While it may not be feasible to purchase new machinery every few years, it’s a worthy idea to keep in mind as, at scale, increased energy efficiency can materially lower your overhead costs. If a new machine isn’t on the table right now, look at updating or retrofitting components, or even overhauling your maintenance routine and schedule. Any of these can help you get more out of what you already have.

Injection Molding Tooling Cost

Injection molding tooling for a mid-level order (around - small parts), can cost up to $10,000. For more complex geometries and large orders, the cost can go up to $100,000.

Tooling costs depend on how complex or large your parts will be. In other words, if your part is complex (for example, if it has intricate geometry or dense walls), the manufacturer may need to use a special (i.e. more expensive) machine to complete your order.

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Injection molding tooling cost increases exponentially if we are talking about a large custom-designed part. From the operator's perspective, it is preferable to use small (desktop) injection molding machines (and even 3D printers) in order to fulfill low-volume orders.

Large, industrial plastic injection molding machines can cost over $200,000 and have additional costs that are related to skilled labor training, maintenance, monitoring, and even industry regulations. These types of machines are reserved for high-volume orders.

Injection Molded Parts

Injection molded parts cost varies based on mold upkeep, the time to form a part, and factors around the plastic. These include the part rate, the estimated amount of scrap (usually 3-5%), the plastic weight, and sometimes a mold maintenance fee, which covers the cost of injection mold warranties. The quoted price should also include any setup costs for the molds.

1. Injection Molding Cycle Time

Injection molding cycle time takes up about 60% of final part cost. The part rate comes down partially to cycle time and partially to cavitation, which is covered in the next section. Generally, manufacturers will have set hourly rates they charge for their machines.

The molding cycle can be divided into two parts:

• Injection time. This represents the time it takes for the polymer (material) to be injected into the mold. It is usually a few seconds and depends on the machine's power and shot size.
• Cooling time. This represents approximately 80% of the entire cycle time. It heavily affects the quality of the end part, as it is the time it takes the material to cool and shrink.

The overall cycle time can be estimated as:

In this equation α is the thermal diffusivity coefficient and h max is the maximum wall thickness in millimeters. While the first is a material-specific constant, the latter is a design choice or requirement. As such, depending on the part's complexity, the cycle time can take up to two minutes.

As a manufacturer, it is preferable to maintain the cycle time to as low as a number as possible, since that translates into the production of more parts in a given time.

2. Mold Cavitation

Mold cavitation cost, another factor in price per part rate, increases with the number of cavities. Cavitation is the amount of identical cavities that produce parts per mold. The injection mold itself can cost thousands of dollars.

Mold cavitation is related to part size and design. If your part is relatively simple, single cavity molds are usually more affordable. However, if the part's geometry allows it, it is preferable to go for multiple-cavity molds to speed up the production process (making production less expensive over time). Larger parts may, however, not allow for this.

You can either buy a ready-made injection mold or have one made for you. Ready-made mold use is a common practice and the less expensive option, especially in the electronics industry since standardization is quite prevalent. You can also have one produced for you, which is better for unique designs.

Another aspect to keep in mind is that for custom injection molds, the geometry has to be compatible with the machine used by the manufacturer. This constraint may affect your project (and possibly design), so it is best to make sure the company you are interested in can fulfill the order.

3. Plastic Weight

Plastic weight cost includes both the plastic formed into parts and the hardened plastic left over in the mold. You'll also have to cover the cost of the plastic weight, not only in the parts but in the runner, sprue, and gate as well (the runner and sprue are channels molten plastic flows through to get into the mold, while the gate prevents plastic from flowing out once it's inside). For example, if you end up with 4lbs of parts, and 1lb of leftover plastic that hardens in the sprue and runner, you'll pay for 5 lbs.

Set scrap rates will also go into the final cost of injection molding. These are for purging the plastic injection molding machines of previous plastics from the barrel and screws as well as reloading them with your plastic.

4. Injection Molding Setup Costs

Mold setup costs include optimizing the process as well as physically setting up the materials and machinery to create your parts. These are part of the final price per part. The process will be optimized, and there may be some setup time that goes into preparations, which most companies will charge as its own flat fee. This may encompass processes like drying the resin, hanging the molds, arranging water lines, external troubles, and any type of sensors in the mold. It may also include any type of special cooling required or specific gating scheme that acts as a hot manifold system (which keeps runners hot enough to keep the plastic in them liquid).

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