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Plastic Injection Molding Can Provide Inventors With Economical Manufacturing Costs.
Plastic Injection Molding - Cost of manufacturing information for inventors.
The wonderful thing about plastic injection molding is the low cost of the parts being produced when you need a large volume of parts.
The downside is the cost of injection molding tooling. The tooling, or the molds, range in price anywhere from $2000 for a simple one, to over a hundred thousand dollars for a big and complex injection mold.
The way the injection molders I've worked with figured out how much to charge per part was to weigh the part complete with runner to determine how much plastic resin is used for each part. The size of the part determines what size molding machine must be used, and how big the mold is etc.
The details of the design in a large part determines the cycle time of the machine. The cycle time is the amount of time it takes from the time a part is ejected from the machine to the time the next part ejects. Smaller parts require less "fill time", the time it takes for the machine to push the melted resin into the mold, and thinner part walls cool faster reducing the "hold time".
The customer is charged a set up fee which pays for installing the mold into the molding machine, and getting it ready to mold parts. Typically it runs a couple of hundred dollars and depends on things like how big the mold is and which accessories will be needed etc.
Since the set up fee is amortized over the whole run of plastic parts, the cost per piece is usually quite small. For example, if you run a batch of 10,000 widgets, and the set up fee was $500.00 you are paying only 5 cents per part for the set up. Of course, if you mold 100,000 per batch it drops to 1/2 cent per part.
Most plastic injection molding resins run a buck or two a pound, with specialized engineering resins costing over $5.00 per pound and more. Plastic resins are available with strengths approaching that of aluminum, and some able to withstand over 500 degrees Fahrenheit.
So if your part can be made from the $1.00 per pound resin, and lets say you get 10 parts per pound, your only paying 10 cents per part for the material.
Lets further assume that the cycle time for your part is 45 seconds, (80 parts per hour), and that your are being charged a rate of $60.00 per hour for the machine time. That comes to seventy five cents per part.
Now we can estimate the cost for each part for a run of say 10,000 plastic parts.
$500 set up fee..................................$.05
cost for material per part.....................$.10
cost for machine time per part.............$.75
Total cost..........................................$.90 per part.
As you can see, plastic injection molding cycle times have a large influence on part costs. Molding more than one part at a time can significantly reduce the cost. Putting 10 part cavities in the mold so the mold makes 10 parts per cycle would dramatically reduce the part per cost.
The more cavities the lower the manufacturing costs.
With injection molding, generally, the thinner the part, the shorter the cycle time since the part cools more quickly with thin sections.
The tooling is critically important. A poorly made tool can be very tricky to keep running, wear out quickly and or have lots of maintenance problems. That's in addition to delivering poor quality parts. The tooling needs to be properly maintained as well.
You can find plenty of injection molding shops by searching for them online. Most will be happy to work with you on your design. They are a good source to find out where you can have prototypes of your invention made as well. Many have machine shops and build prototypes as well as the tooling.
Sometimes they will be willing to amortize the cost of the tooling over a pre-determined number of parts. For large part runs the tooling costs can practically disappear leading to lower initial start up manufacturing costs.
If your parts need secondary operations such as drilling, gluing, painting, welding, etc. then you have to figure in those costs as well. Often some operations can be performed by the operator of the machine while he waits for the machine to cycle, and it becomes quite economical.
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