MOLD cost explained
The most significant factor in tooling cost for a plastic molded part is the complexity of the part. Injection molded parts with a lot of sharp corners, thin ribs or undercut features will generally be more expensive to make tooling for than parts without these type of features.
If the locations of the sharp corners prevent the use of a round cutter for machining the mold features, they will require a secondary machining process called EDM machining. Likewise, rib features that require machining deeper than 1/2 inch and thinner than 1/16 inch into the mold can also require secondary EDM machining.
Undercut features on plastic part designs will always increase the cost of the injection mold. Undercuts force costs into the mold design time and machining time. They can also sometimes require the molds overall size to become larger to accommodate the space needed for the "side actions" (moving parts) that form the undercut features.
There are many different injection mold construction methods for a lot of different part shapes, and many different reasons why they all have a place at the table in the injection molding industry. But, in basic overall injection mold construction we like to simplify them into two groups: "Insert molds" and "free standing molds". Our mold making page will give you a good understanding of each. Within each group there are hundreds of different styles, sizes, and complexities around the industry, but from a general cost point of view insert style molds will cost significantly less than free standing molds.
Within a molds construction there are many different styles of side actions, as mentioned above, that are used to release plastic parts with undercuts during the part ejection process. The lowest cost side action method, and used by many of our competitors, is frequently called a "hand load" in our industry. Hand loads are loose parts of the mold that get ejected from the mold with the part and need to be manually removed from the part then re-inserted into the mold every molding cycle by a person. Hand loads are a low cost tooling option to deal with undercut part features in the mold, but they increase part cost and can lead to inconsistent part quality. At MSI we prefer to use more "automatic" side actions in our molds because they allow for a faster and more consistent injection molding process which gives our customers better parts for a lower cost. Injection molds with automatic side actions do carry a higher cost to make than hand load molds, but for any production quantities higher than a few hundred pieces the payback in part cost savings will generally wash out the added mold construction cost.
Number of Part Cavities
The number of part cavities within the mold will directly affect the price of the mold. Parts that only require a low quantity of parts per customer order will be constructed as a 1 cavity mold to produce only 1 part per machine cycle, and parts with a high quantity of parts per customer order might be constructed as a 2, 4 or 8 cavity mold producing several parts per machine cycle. Multi-cavity molds will certainly cost more than single cavity molds, however, keep in mind that part pricing will have an inverse relationship to mold cost when considering mold cavitation and a projects total overall cost.
Perhaps the easiest tooling cost factor to understand is the relationship between part size and mold cost because the size of a plastic part is directly related to the mold size required to produce the part. It also follows that larger molds require larger amounts of building space, larger and more expensive CNC machinery and equipment to work on them, etc. However, there can be some crossover of the cost-to-size relationship in some cases. For example, a small injection mold with a very complex design can sometimes cost more than a larger injection mold with a very simple design.
INJECTION MOLDED PART cost EXPLAINED
Material Choice and Part Weight
The plastic material selection you make for your plastic part is not only critical to the part's function, it also plays a huge role its cost to manufacture. Raw plastic material cost can range in price from $1 per pound to $25 per pound. So care should be taken in plastic part design to minimize the weight of the part when possible. Choosing the correct plastic type can also help achieve faster cycle times or production rates of your parts to reduce overall costs. For small parts the cost of the material has a small effect on the piece price and the production rate has a large effect. On larger heavier parts the cost of the material has a large effect on the piece price along with the production rate.
$ Material Part Cost = $ Plastic Material Price x Part Weight lbs
Cycle Time and Mold Cavities
"Cycle time" is a term used in our industry for both injection molding and CNC machining. In simple terms it is the amount of time required for a machine to complete one production cycle. The total injection molding cycle time is made up of the following steps:
- mold closing time
- injection fill time
- injection pack/hold time
- cooling time
- mold opening time
- part ejection or take out time
- re-cycle time (which would only apply when parts must be ran in a semi-automatic mode with a machine operator)
So how does cycle time relate to part cost? Lets discuss the primary sources of the non-material related costs for injection molding.
Lets start with the machinery and equipment. Today's high-tech servo driven injection molding machinery is very expensive. The tonnage size rating of the machine and its surrounding support equipment will determine the overall capital investment required to run production. Small tonnage machine sizes are used for running small molds and small parts, and large tonnage machines are needed to run large molds with large parts. Even a small injection molding machine along with its support equipment will cost near $100,000 and very large machines will cost millions of dollars. It also follows that the machinery has a finite effective life cycle because it wears out and also loses it technological advantage to newer technology over time, just like everything these days. Knowing the cost being the capital ($) investment in machinery divided by its effective life cycle (time), its easy to understand that translates into an hourly operational cost of ownership of the equipment.
Secondary to the capital equipment costs dicussed above, would be a combination of fixed and variable costs like a factor for rent on manufacturing space, electrical power usage, etc. Through simple math these costs are also very easy to drill down into an hourly operational cost.
The sum of all these non-material related hourly costs combined and calculated out with a profit margin will become what is known as "machine rate" in our industry.
Finally, here is how the simple non-material related cost of an injection molded part is determined by cycle time:
The # of units produced per hour = (3600 sec / cycle time sec) x number of mold cavities
$ Non-Material Part Cost = $ Machine Rate per hour / # of units produced per hour
Since machine rates are all very competitive across the injection molding industry, cycle time and number of cavities in the mold are two significant determining factors in the total cost of a plastic molded part.
Packaging and Extras
Two more items that can really have a big pricing effect on a molded part are packaging and extra operations. Commonly requested items like the following can increase the cost of injection molded parts: