As plastic technology advances, the limit to their applications continues to lessen. Plastics have advanced from the lightweight low-cost materials used in plastic bottles. They have now entered the realm of high-performance materials. Some plastics now outperform metals. Plastics like nylon 66 and polyphenylene sulfide can substitute metals in demanding applications. The use of long fiber composites has led to even higher performance. One of such very demanding applications is in gears. Plastic gears were once only used in toys or product models at best. Today they get used as functional parts in machinery. The developments in plastic gear owe a lot to injection molding. Having a process that can produce complex plastic parts to good precision. This goes a long way in the success of plastic gear manufacturing. For reproducible production to high precision injection molding is the go-to.
The injection molding process produces plastic gears with good precision. A mix of factors come to play in a successful production. The material used, the type of equipment, and the process conditions are key ones. You want to reduce the amount of post-processing required. This reduces time spent per product and this has a direct impact on revenue. Also, these factors affect product quality. This article looks at the case of plastic gear injection molding. In the process, we explore the factors that need special consideration for success. This involves an overview of the injection molding process first. Then and a focus on the aspects that are critical plastic gear molding. The article covers vast aspects of plastic injection molding. From the type of materials used to the processing of the final product.
Plastics Gearing the Modern Age
Plastic gears have been around for over half a century. Gears aid torque and motion in different devices and machines. You will find gears in cars, bicycles, washing machines, clocks, and sewing machines. Manufacturers seek to outperform the competition by developing better products. In the automotive industry for example. There seems to be a constant race towards lighter, stronger car parts. This reduces fuel consumption and improves performance. At the same time, this has to be while keeping cost realistic. Gears offer better system management over other options like cables. Making them lighter and stronger expands the range of applications. Today when one thinks of light and strong, polymers come to mind. A lot of these are high-performance plastics.
Image showing example of Injection Molded Plastic Gears
Gears have a characteristic shape. A core with several teeth at the edges. Quite simple. But achieving this to high precision is the challenge to gear manufacturers. The more precise the dimensions are the less the noise and errors in transmission. Taking the example of gear in a bicycle. As the gear rotates, each tooth must be a perfect fit into the gaps in the bike chain. It is this interaction between the gear and the chain that facilitates the movement of the bike. It translates the rider’s pedaling into the motion of the wheels. The level of noise you hear as a bike move says a lot about the gear system. In a poor-quality bike, there’s a mismatch between the teeth spacing and dimension and that of the chain. This means a good part of the rider’s energy gets wasted on wear and friction. This also impacts the life span of the gear. While regular oiling reduces friction, it doesn’t stop this. More complex gear designs This gives a good picture of the importance of well-made gear.
Plastic gears aim to address the limitations of their metal counterparts. The goals include reduced production costs and better performance. Gears get molded to somewhere between a few inches to around 18 inches in diameter. They can deliver over 10 horsepower.
10 reasons to use Injection Molded Plastic Gears.
In trying to manufacture good quality plastic gears. Processors must face some challenges. We’ll discuss these challenges and how to solve them. First, let us look at why even bother with plastic gears in the first place.
- High precision is as achievable in plastic as it is in metal
- Injection molding allows good replication to maintain manufacturing standards
- It is low-cost processing. Most plastics get processed at temperatures well below those needed for stainless steel. This reduces the energy and time consumed in heating and cooling.
- Plastics do better at resisting abrasion and wear than metals. This is even more so in high-performance plastics. This results in more durable parts.
- Plastics do not rust, unlike metals.
- Plastics achieve improved performance than metals. This is a result of the inherent properties of plastics.
- Plastics offer more design options. Complex designs are easier to achieve with plastics.
- No need for external lubrication. This is due to abrasion resistance
- Lightweight. This also means a lower inertia.
- Less noise compared to metal gears
Plastics Used in Injection Molded Gears
Generally, high-performance plastics get used in plastic gears. Thermoplastics work better for injection molding of gears. Thermosets may be more challenging to achieve precision with. The injection molding of high-performance plastics poses a different set of challenges. Some articles in this blog cover injection molding of high-performance plastics. Examples are polyethylene ether ketone and Nylon 66. So consider this when designing the process and tooling for plastic gear. Below are some of the plastics known for their use in plastic gear injection molding. The first three on the list are the most common choices.
- Polyoxymethylene (POM)
- Polybutylene Terephthalate (PBT)
- Polyamide (Nylon) (PA)
- Polyphenylene sulfide (PPS)
- Liquid Crystal Polymers (LCP)
- Thermoplastic elastomers (TPE)
- Polyethylene ether ketone (PEEK)
These plastics offer properties such as abrasion resistance, surface lubrication, crystallinity, and toughness. Also, they provide better dimensional stability. Materials such as these get used to fabricate gear teeth that are less than the thickness of human hair. Other plastics such as polypropylene do get used. But these are in far less demanding applications. They could also be in applications where the above-listed polymers can’t get used. Or are unnecessary and less expensive plastics would do. For example in the design of gears for toys.
Special Consideration When Injection Molding Gears
The injection molding process for plastic gear manufacturing follows the conventional procedure. Gears form very crucial parts of appliances and machines. So they get made to high precision. The difference between the injection molding of gears is precision. The following are some of the factors to consider in achieving such precisions.
Precision Mold Engineering
Good precision is a very important in-gear performance. Precision can get lost if injection molding is not well designed. The processor and mold maker need to be very scientific. Shrinkage can result in loss of precision if this is not considered in the design. The level of shrinkage varies for different types of plastic. The direction of shrinkage also varies. Some plastics shrinkage are isotropic and while others are anisotropic. This could range from a fraction of a percentage to a few percentages. One approach in plastic mold fabrication is to test run with the core. The mold is partly fabricated without the teeth. A test run is then done with this mold. This gives the shrinkage % of the specific material. The rest of the mold is then fabricated based on this shrinkage. It is easier to adjust the dimensions of the core than that of several teeth.
The mold maker needs to measure parts to high precision. This calls for specialized tools for precision measurement in mold fabrication. The tools for machining and fabrication of the mold also have to be of high precision. When finding a mold maker for plastic gear manufacturing lookout for these factors.
Number of Cavities
Keeping the number of cavities to only a few reduces the chances of variation. Generally, about 4 cavities get used. This also reduces the distance between the injection section and the molding section. The shorter the distance of the runners the less likelihood of uneven filling or cooling. If at all there is any manual activity in the mold opening and release. Having fewer molds reduces variation. Filling the molds from a single gate also improves precision. Having many gates risk the development of weld lines. The gate gets located at the center of the gear to achieve the best filling. Other factors such as gate design and runner types are also important. These do not have fixed values for plastic gear injection molding. The goal is to ensure consistency.
Image showing a mold next to the molded plastic gear. The attached runner shows the cavity filling occurs from the center.
The precision of the process is also important. Even with a mold made to good precision the process also needs to be precise. Inconsistencies in process parameters like time, injection speed, pressure, and shot size. These will show in the final product. For example the time from when the product gets released to its journey across the conveyor belt. This affects the rate of cooling of the product, hence shrinkage. This could affect precision. Uniform cooling occurs within each mold and across all mold. There are different ways of achieving this. These include accurate control of flow through cooling lines, automated removal, and transfer.
When molding gears with complex shapes their removal from the mold might not be a simple push. In the simple design, the ejector pins can push out the product in one forward action. Where you have complex gear like a worm or helical design this needs more thought. The motion to release is in more than one direction. The mold release can be manual to allow for human judgment. Or it can involve the programmed motion of the ejectors. The movement in the x, y, and z-direction can be pre-programmed for effective automation. Mold release must occur without damage to the product.
Overmolding, 2 Shot and Insert Molding
Rather than fabricating a mold with the cavities for teeth, inserts are an option. One way is to fabricate the mold with only the core of the gear. The teeth cavities are then gotten as inserts. This method reduces the need for the refabrication of teeth. This might be in case of errors in previous fabrication or damage. Inserts are also used to achieve different core designs. For example, having gear with a hole at the center.
Reducing the amount of noise and wear makes for better gear performance. This is possible using elastomers. An elastomer gets molded on the edges of the teeth using 2 shot molding. This elastomer absorbs the force between the two bodies on impact hence the sound. This is one way of using insert molding in gears. It also makes for longer-lasting gears by reducing wear. Similarly, overmolding gets used to reduce the cost of manufacturing the gear. The core gets made of a strong enough material. This part does not experience as much wear as the teeth. Its main role is strength and load-bearing. But the teeth need more resistance to wear. So a different polymer can get overmolded unto the core. This meets the type of resistance needed by the teeth. For example, teeth made of polymers with inherent lubrication.
Plastic gear molding demands high precision injection molding. Several factors determine the level of precision achieved. This begins with the type of plastic used as raw materials. Having tight control over process parameters also plays an important role. So does the design of the mold. Successful plastic gear injection molding is a combination of several factors.