January 1, 1970

The difference between injection molded plastic gears and machined plastic gears

The Difference between Injection Molded Plastic Gears and Machined Plastic Gears

In the past, OEMss used plastic gears in only a handful of applications. This was because of the uncertainty around their reliability or operational life owing to thier response to temperature, chemicals, and moisture, among other application specific variables. This was largely complicated by the fact that the Lewis equation commonly used to calculated traditional metal gears does not apply to plastics. This lack of polymer specific calculations and material data sheets that don't explain material properties in operating conditions caused many engineers and designers to avoid plastics.

Nowadays, more manufacturers are opting for plastic gears for industrial equipment, automobiles, medical equipment, automation,and electrical appliances. This is because plastic gears, unlike metal ones, run lubrication free are lightweight, low cost, chemical resistant, and corrosion-resistant. Moreover, they have good vibration absorption, quiet operation, and optimal shock absorption.

There are two main processes used for the manufacture of plastic gears; CNC machining and injection molding. CNC machining entails removing layers from a plastic rods using computer software to produce your desired gear shape. On the other hand, injection molding entails melting pellets of resin and forcing that melted liquid to mold under high pressures  into your desired shape. The flash cooled material is then ejected from its mold as a finished part. Plastics typically shrink while cooling something we will discuss later as a major hurdle to precision and reliability of injection molded parts.

Picking the right process for producing your plastic gears is more than simply a tradeoff between the elements that differentiate CNC machining and injection molding. Below is a consideration of the leading aspects that differentiate the two processes and the common pitfalls and advantages of both machined plastic gears and injection molded plastic gears to help ease your choice.
 

Cost

When initially designing or choosing plastic gears for your application your sensitivity to cost should be among the first elements you examine. Comparing the two methods of producing plastic gears, the cost is the primary consideration for most people. The cost of the production process you choose is affected by your production volume, component size, equipment price, and material savings. The equipment cost for injection molding is typically thousands of dollars charged upfront.

In general, injection molding is the cheaper choice if you have a high lifetime volume meaning you intend to use your machine for a long time. If you want to produce less than 5000 -10000 plastic gears, CNC machining is cheaper.

In terms of material savings, injection molding is also cost-effective. CNC machining involves subtractive manufacturing. This means you make plastic gears by cutting, removing, and drilling away excess materials to get the desired shape. This naturally generates waste that increases the manufacturing cost because you cannot reuse it. Conversely, injection molding is a formative manufacturing process that produces little waste, beyond mis-formed rejected parts, thus cutting back on material costs.

Manufacturing time

The fastest option for producing a low volume of plastic gears is CNC machining. For instance, if you want to manufacture ten parts in two weeks, this is your ideal choice. On the other hand, if you need 50000 parts in four months, the best option is typically injection molding, assuming the molds already exist. If you need to produce new molds, which you need to do every 100,000 parts that process could take months to complete before you can start forming gears. This is because the process takes time to make its mold and ensure the products meet your specifications. Making a mold takes weeks to months to complete.

Quantity

In general, the high upfront cost of injection molding pays off for large production volumes. CNC machining is cheap if you want to manufacture a few thousand items.Creating molds in injection molding might have a high upfront cost, but this is spread over thousands of manufactured parts. At high volumes, machined gears come down to 2 components material cost, run time, and scrap. Material costs and runtime will naturally be higher in the machining process but when factoring in scrap rates and the high tooling costs inherent in the injection molded process the costs may converge.  Not all materials can be machined or injection molded that is why it is so important to consider how you will manufacture your gears before you begin the material selection process. Even at high volumes, if the application falls at the edges of what plastics can handle or needs to be reliable for its calculated life machined specialty polymers might be your only option.

Quality

Injection molding might produce parts with slight cosmetic imperfections and because of their forming time will have internal stresses and a degree of dimensional instability. If you want the highest quality levels, CNC machining is the ideal choice. Injection molded gears are commonly made in the middle of the AGMA quality range AGMA 7 or AGMA 8 are the norm however if you are willing to invest in expensive molds or hob an injected blank you can achieve up to AGMA 10. Machined plastic gears are normally AGMA 9 but can reliably be made to AGMA 11+ for the medical and semi-conductor industry. As in Injection molding with the higher quality rating there is a higher cost. 

Precision

If you want your plastic gears to hold precise design specifications, the ideal choice will be CNC machining. Through injection molding has fair initial accuracy within .005 inches of your design specification, machining gives you lasting precision within .001 of your design specifications. These stresses introduced to the parts during the high speed injection process will cause the material to lose its peak material properties and the parts can suffer over time. Machined gears, when properly machined, can be cut to relieve stress and allow for a material to maintain its mechanical and dimensional properties through out the life of an application.  Superior precision is essential in the medical equipment, aerospace, semiconductor, automation, and telecommunications industries where machining is most commonly used. Other sectors in which slight inconsistencies make little to no difference often settle for injection molding.

Top material options                                                             

CNC machining allows a diverse range of plastic materials for the creation of gears. This is beneficial if you need a specific plastic or high-performance plastic for your product. The plastic selection for injection molding is limited. Even so, rubber and some flexible materials can only be injection molded.

From the above tidbits, you can correctly conclude that machining is the best choice for prototyping, precision tolerances, manufacturing components with large volumes, complicated geometries and for quantity runs under 5,000 to 10,000 units. On the other hand, injection molding works for large quantity runs for parts that won't be exposed to varying environmental conditional and applications low torque and limited shock loads, but where smooth surface finish is important and cost is the main driver.

When considering injection molded gears the most common materials include

Nylon

Acetal

Polyketone

Machined Gears are most commonly made from

Cast Nylon 6

Non Hygroscopic Materials

Gravity Cast Power Core

Acetal

PEEK 

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