The majority of the cost for plastic injection molding will be the machining of the injection mold. Injection molding costs are calculated based on several factors. The first and most important factor is the size, followed by the complexity of the mold. Large, complex molds will cost more than smaller, simple molds. The type of mold will also help determine price. Prototype injection molds will cost less than molds used for production runs. Molds built specifically for long production runs will be more expensive. Molds made for extended production will need to be made of high-strength steel that can stand up to repeated use.
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The last factor to consider is the number of injection cavities machined into the mold. High-production molds can have multiple, identical injection cavities. These are typically referred to as muti-cavity Molds. Family molds are molds that have two or more similar cavities that aren&#;t necessarily identical. An example would be the right and left side of part that snaps together. These molds will have multiple cavities that will produce the same part or related parts in just one pressing, reducing the number of times the mold needs to be opened and closed while increasing production efficiency.
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.
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 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.
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:
The overall cycle time can be estimated as:
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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.
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.
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.
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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