Plastic gear injection molding process and materials

Table of Contents

Plastic gears are moving toward larger sizes, more complex geometry, and higher strength, with high-performance resins and long glass fiber-filled composites playing an important role. In the past 50 years, plastic gear has undergone a change from a new material to an important industrial material.

Today they are embedded in many different applications, such as cars, watches, sewing machines, structural control devices, and missiles, to transfer torque and form of motion. In addition to existing applications, new and more difficult gear applications will continue to emerge, and this trend is still in progress.

The automotive industry has become one of the fastest-growing areas of plastic gear, and this successful change is encouraging. Car manufacturers are trying to find auxiliary systems for all kinds of car drives.

They need motors and gears rather than power, hydraulics, or cables. This change has led to the penetration of plastic power gears into a wide range of applications, from lift doors, seats, and tracking headlights to brake actuators, electric throttle segments, turbine regulators, and more.

The application of plastic power gear is further expanded. Precision plastic gears are often used to replace machined metal gears in applications where large sizes are required, such as washing machine drives using plastic, which changes the size limit of gears.

Plastic gears are also used in many other applications, such as vibration-damping drives in ventilation and air conditioning systems (HVAC), valve drives in flow facilities, automatic scrubbers in common rooms, power screwers for surface stabilization in small aircraft, and screw and weight meters and control devices in military applications.

A large-size, high-strength plastic gear

Because of the advantages of plastic gear molding which can be molded larger, with high precision and high strength characteristics, is an important reason for the development of plastic gear.

How to design a gear configuration that maximizes transmission power while minimizing transmission errors and noise is a challenge. This requires high machining accuracy for the concentricity, tooth shape, and other characteristics of the gear.

Some helical gears may require complex shaping actions to produce the final product, while others require core teeth in thicker parts to reduce shrinkage.

While many injection molding specialists have achieved the ability to produce the new generation of plastic gears using the latest polymeric materials, equipment, and processing techniques, a real challenge for all processors will be how to fit this entire high-precision product.

The difficulty of control

The tolerances allowed for high-precision gears are generally difficult to describe as “good” as stated by the American Plastics Industry Association (SPI).

But today most molding experts USE THE LATEST injection molding machines equipped WITH MACHINING CONTROL UNITS THAT CONTROL the precision OF molding temperature, injection pressure, and other variables in a complex window to shape precise gears.

Some gear-forming specialists use a more advanced approach, placing temperature and pressure sensors in the cavities to improve consistency and repeatability.

Plastic gear manufacturers inspection also need to use specialized inspection equipment such as double-toothed side rolling detectors to control gear quality, and computer-controlled detectors to assess gear tooth surfaces and other features. But having the right equipment is just the start.

Those trying to enter the precision gear industry must also adjust their injection molding environment to ensure that they produce gears that are as consistent as possible in every injection and every cavity.

Since the behavior of mechanics IS often the decisive FACTOR in THE production of mold precision gears, they MUST focus on the training of their employees and the control of the operation process.

Because the size of the gear is susceptible to seasonal temperature changes, and even the temperature fluctuations caused by opening the door and letting a forklift pass can affect the dimensional accuracy of the gear, the injection molding manufacturer needs to strictly control the environmental conditions in the molding area.

Other factors to consider include a stable power supply, suitable drying equipment that controls polymer temperature and humidity, and a cooling unit with constant airflow.

In some cases, automatic technology is used to remove the gear from the forming position and place it on the transfer unit in a single repetitive action to achieve the same cooling method.

Important molding cooling steps

The machining of high-precision parts is compared with the requirements of general molding processing, which needs to pay more attention to the details and the measurement techniques required to achieve the precise measurement level.

This tool must ensure that the injection molding temperature and cooling rate in the cavity are the same for each molding. The most common problem in precision gear machining is how to deal with the symmetry cooling of gear and the consistency of each mold cavity.

Precision gear dies generally do not exceed 4 cavities. Since the first generation of molds produced only one gear with few specific instructions, tooth inserts were often used to reduce the cost of secondary cutting.

Precision gears shall be injected from a gate at the center of the gear. Multiple gates are easy to form fusion lines, change the pressure distribution and shrinkage, and affect the gear tolerance.

For fiberglass reinforced materials, because the fiber is arranged radially along the welding line, it is easy to cause eccentric radius “collision” when using multiple gates.

A molding expert can control the deformation of the tooth groove, and obtain a controllable, consistent, and uniform shrinkage ability of the product based on good equipment, molding design, used material stretching ability, and processing conditions as the premise.

When forming, it is required to accurately control the temperature of the forming surface, injection pressure, and cooling process.

Other important factors include wall thickness, gate size and location, packing type, amount and direction, flow rate, and forming internal stress.

The most common plastic gears are straight teeth, cylindrical worm gears, and helical gears. Almost all gears made of metal can be made of plastic.

Gears are usually shaped by a split die cavity. When machining helical gear, because the gear or gear ring forming the teeth must be rotated during injection, attention should be paid to its details.

The worm gear produces less noise than straight teeth when running, and after forming, it is removed by rotating out the cavity or using multiple sliding mechanisms. If a sliding mechanism is used, it must be operated with high precision to avoid obvious split stitches on the gear.

New process and new resin

More advanced plastic gear-forming methods are being developed. For example, the second injection molding method, through the design of an elastic body between the wheel shaft and the teeth, makes the gear run more quietly, when the gear suddenly stops running, can better absorb the vibration, and avoid damage to the teeth.

The axles can be remolded to other materials, with the choice of more flexible or more valuable, self-lubricating composites.

At the same time, gas assisted method and injection compression molding method were studied as a method to improve the quality of gear teeth, and the overall precision of gear and reduce internal stress.

In addition to the gear itself, the molding personnel also need to pay attention to the design structure of the gear.

The position of the gear shafts in the structure must be aligned linearly to ensure that the gears run in a straight line, even when the load and temperature change, so the dimensional stability and accuracy of the structure are very important.

To take this into account, gear structures with certain rigidity should be made of materials such as glass fiber-reinforced materials or mineral-filled polymers.

Now, in the field of precision gear manufacturing, the emergence of a range of engineered thermoplastics offers more options than ever before.

The MOST COMMONLY USED MATERIALS, SUCH AS acetAL, PBT, AND POLYAMIDE, CAN produce gear equipment with excellent FATIGUE resistance, WEAR resistance, smoothness, high TANGential stress strength, and ability to withstand vibration loads caused by the operation of reciprocating motors.

For crystalline polymer must be formed at a high enough temperature to ensure the full crystallization of the material, otherwise in use due to the temperature rise above the molding temperature, the material occurs secondary crystallization and lead to injection molded gears size change.

Acetal, as an important injection molded plastic gear manufacturing material, has been widely used in automobiles, appliances, office equipment, and other fields for more than 40 years.

Its size stability and high fatigue and chemical resistance can withstand temperatures up to 90 ° C and above. Compared with metal and other plastic materials, it has excellent lubrication properties.

PBT polyester can produce a very smooth surface, the maximum operating temperature can reach 150℃ without filling modification, and the glass fiber reinforced product operating temperature can reach 170℃. It works well in comparison with acetal, other types of plastics, and metal materials and is often USED IN GEAR structures.

Polyamide materials, compared with other plastics and metal materials, have the properties of good toughness and durability and are often used in turbine transmission design and gear frame applications.

The operating temperature of polyamide gear is up to 150℃ when it is not filled, and the operating temperature of glass fiber reinforced product is up to 175℃.

However, polyamides have the characteristics of hygroscopicity or lubricant resulting in size variation, making them unsuitable for use in the precision gear field.

Polyphenylene sulfide (PPS) has high hardness, dimensional stability, fatigue resistance, and chemical resistance up to 200 ° C. Its application is penetrating the application field of demanding working conditions, the auto industry, and other end uses.

Producing precision gears made of liquid crystal polymer (LCP) has good dimensional stability. It can tolerate temperatures up to 220 ° C and has high chemical resistance and low forming shrinkage changes. The plastic material has been used to make gear with teeth about 0.066mm thick, equivalent to 2/3 of the diameter of human hair.

Thermoplastic elasticity allows the gear to run more quietly, making the gear more flexible and able to absorb impact loads well. For example, a low-power, high-sallowpeed gear made of copolyester

thermoplastic elastomers allow for some deviations while maintaining adequate dimensional stability and hardness, while reducing operating noise. An example of such an application is the gear used in curtain actuators.

Materials such as polyethylene, polypropylene, and ultra-high molecular weight polyethylene have also been used for gear production in relatively low temperature, corrosive chemical, or high-wear environments. Other polymeric materials have also been considered but are subject to many harsh restrictions in gear applications;

Polycarbonate has poor lubrication, chemical resistance, and fatigue resistance. ABS and LDPE materials usually can not meet the lubrication, fatigue resistance, dimensional stability, heat resistance, creep resistance, and other performance requirements of precision gear. Most of these polymers are used in conventional, low-load, or low-speed gear applications.

The advantage of using plastic gear

Compared with plastic gears of the same size, metal-machined plastic gears operate well and have good dimensional stability when temperature and humidity change. But compared with metal materials, plastics have many advantages in cost, design, processing, and performance.

The inherent design freedom of plastic molding ensures more efficient gear manufacturing than metal molding.

Inner gears, gear sets, worm gears, and other products can be injection molded from plastic, which is difficult to mold from metal materials at a reasonable price. Plastic gears are used in a wider range of applications than metal gears, so they push gears toward bearing higher loads and delivering more power.

Plastic gear is also an important material to meet the requirements of low and quiet operation, which requires high precision, a new tooth shape, and excellent lubricity or flexibility of materials.

Gears MADE OF PLASTIC GENERALLY DO not REQUIRE SECONDARY processing, SO compared with stamped and MACHINE-made metal GEARS, THE cost is guaranteed to be reduced by 50% to 90%.

Plastic gears ARE lighter and more inert than metal spur gears and can be used in environments where unlike metal gears are prone to corrosion and degradation, such as water meters and chemical equipment control.

Compared with unlike metal gear, plastic gear can absorb impact load by deflection deformation, and can better disperse the local load changes caused by shaft deflection and mistook.

The inherent lubrication characteristics of many plastics make them ideal gear materials for printers, toys, and other low-load operating mechanisms, not including lubricants. In addition to operating in a dry environment, gears can also be lubricated with grease or oil.

The enhancement of materials

In the specification of internal gear and structural materials, the important role of fibers and fillers in the properties of resin materials should be taken into account.

For example, when acetal copolymer is filled with 25% short glass fiber (2mm or less), its tensile strength increases by two times, and its hardness increases by three times at high temperatures.

The use of long glass fiber (10 mm or less) fillers improves strength, creep resistance, dimensional stability, toughness, hardness, wear, and much more.

LFRP-reinforced materials are becoming an attractive candidate for large gear and structural applications because of the required hardness and good controlled thermal expansion properties.

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