There are many cost drivers in injection molding, in order to reduce the final cost of your projects, we analyze the main factors in injection molding, and summarize some actionable design tips in parts designing process.
The main cost in injection molding are
- Mold cost: this is determined by mold designing and machining cost in mold manufacturing.
- Material cost: the materials’volume usage in parts manufacturing and price per kilogram determine the material cost.
- Production cost: this is the cost of injection molding machine operation, it normally is determined by process time.
Mold cost is constant at start-up, it will not be influenced by total number of final manufactured parts, while the material cost and production cost are dependent on production volume. For low volume production in the range of 1,000 to 10,000 units, mold cost has greatly impact on the overall cost at approximately 50-70%. So it is worthwhile to alter your parts design, in order to simply mold manufacturing process and reduce overall cost. For high volume production in the range of 10,000 to 100,000 units, the material and production cost will take the main contribution of overall cost. In this case, your design efforts should minimize both volume part and molding cycle time.
In Runsom, we collect design tips to minimize the overall cost in injection molding as following:
- Straight-pull mold
Any side-action cores and in-mold mechanism will increase the mold cost of 15% to 30%. This means there will be a minimum additional cost of $1,000 to $3,000 for molds tooling. So we recommend that avoid using side-action cores or other mechanisms unless necessary, in order to keep your production on budget.
- Avoid undercuts
There is no doubt that undercuts will add mold cost and machining complexity, a clever redesign should always eliminate undercuts.
- Smaller molded parts
Smaller parts will be molded faster with less cycle time, this will increase production output and decrease parts unit cost. In addition, it will also have less material cost and low mold manufacturing price.
- Multiple parts fit
Fitting multiple parts in the same mold is a common practice in the injection molding process. Normally, 6 to 8 small parts can be fit in one mold, this will reduce the total production cycle time by 80%. Parts with different geometries also can be fit in the same mold, this will reduce overall cost of assembly.
- Avoid small details
Small details in molds will need more machining and finishing time. Especially for small text, sometime it will need special machining techniques like electrical discharge machining, this will result to higher mold tolling cost.
- Lower grade finishes
Finishes in mold are normally required manual operations, this is an expensive process, especially for high grade finishes. We recommend don’t apply high grade finishes, once your parts are not for cosmetic applications.
- Minimize parts volume
Reducing wall thickness is the best way to minimize parts volume. This will not only reduce material application, but also accelerate the injection molding cycle. Such as, reduce wall thickness form 3 mm to 2 mm will decrease cycle time by 50% to 70%. Thinner wall means molds can be filled quicker, and parts will cool and solidify faster. As we know, half of injection injection cycle time is spent on the solidification process once molding machine is kept idle.
We also need to consider that thinner wall will reduce parts stiffness, which will downgrade its mechanical performance. So ribs should add in key locations to increase stiffness.
- Secondary operations
In lower volume production less than 1,000 parts, secondary operation is a cost-effective way to complete injection molded parts. Such as, you can drill a hole after molding rather than using side-action cores in molds.
Start Injection Molding
Once your design is optimized for injection molding, we will start your project manufacturing with injection molding.
- Start prototyping
Before you start any expensive injection molds manufacturing, you should create and test a functional prototype of your design. This step is essential for starting a successful product. It will identify design errors early in low cost change.
There are 2 solution for prototyping:
These process will create realistic prototypes for plastic form and function, and resemble final injection molded parts appearance.
- Pilot run
As design finalized, we can start injection molding process with a small pilot run. Our minimum order volume for injection molding is 500 units. For this quality, we usually manufacture the molds form aluminum, aluminum molds are easy to manufacture with low cost, and can finish at least 5,000 injection cycles.
At this step, the unit cost varies between $1 and $5, this is depend on parts geometry and materials. Our typical leading time is 6 to 8 weeks. Once you just require thousands parts, this will be the final production step. Aluminum molds have similar physical properties and accuracy identity as tool steel molds.
- Scale production
For massive quantities parts production form 10,000 to 100,000 units, special molds is required. These molds are normally CNC machined for tool steel, and can withstand millions of injection molding cycles. In addition, they are also equipped with hot-tip gate and intricate cooling channels to maximize production volume. In reason of molds complexity of designing and manufacturing, the typical unit cost varies from cents to $ 1, and the lead time is 4 to 6 months.