Plastic

Application of injection plastic parts in the automotive field

Injection molding is one of the most commonly used production processes for plastics. This is true because it provides a viable solution for the mass production of high-quality injection-molded automotive parts from a variety of polymers. In the automotive industry, where consistency, safety and quality are paramount, automotive plastic injection molding is an important manufacturing process.

Advantages of Injection Molding in Automotive Applications

Injection molding is an established production process in which automotive moldmakers inject molten plastic material into mold cavities. The molten plastic then cools and hardens, and the manufacturer extracts the finished product. Although the mold design process is critical and challenging (a poorly designed mold can lead to defects), injection molding itself is a reliable method of producing solid plastic parts with a high-quality finish.

Here are a few reasons why this process is beneficial for the production of automotive plastic parts:

1. Repeatability

In the automotive industry, repeatability—or the ability to consistently produce the same parts—is critical. Because automotive plastic injection molding typically relies on a solid metal mold, the final molded automotive part produced using that mold is virtually identical. Certain factors affect injection molding, but injection molding is a highly repeatable process if the mold is well designed and finished.

2. Scale and cost

The injection mold manufacturing process can be an expensive process due to the cost of the mold. However, it is still a highly scalable process whose overall cost decreases as manufacturers make more parts. For mass production applications, injection molding is therefore beneficial to manufacturers. However, for anything not mass-produced, injection mold costs can inhibit the cost-effectiveness of the process.

3. Material Availability

A significant advantage of using injection molding in automotive production is that the process is compatible with a wide range of rigid, flexible and rubber plastics. Manufacturers use a variety of different polymers for a variety of applications in the automotive industry, including ABS, polypropylene, acrylic, acetal, nylon, polycarbonate, and more.

4. High precision and surface finish

Injection molding is ideal for producing plastic parts with relatively simple geometries, enabling high surface finish quality. Manufacturers have many surface treatment options when producing parts, including various surface textures (such as glossy, rough, or matte) that are applied directly to the mold rather than molding the part. However, different plastic materials can also affect the final surface finish.

5. Color Options

In automotive plastic injection molding, it is easy to modify the color of the molded auto part to fit the vehicle's color scheme. Unlike other processes, injection molding allows you to mix dyes with raw material pellets before manufacturing begins. This results in a solid, consistent coloration after molding without the need for painting or coloring.

6. Rapid Prototyping Using Rapid Tools

While automakers widely use injection molding to mass-produce auto parts, they also use it as a prototyping tool. By making fast, low-cost aluminum molds using rapid tooling—often through additive manufacturing or CNC machining—automotive moldmakers can produce low-volume prototype auto parts faster than traditional (steel) molds.

Production application of automobile injection molding

In the automotive sector, injection molding is one of the main methods manufacturers use to form plastic parts. However, it's difficult to list the plastic parts in a car produced using injection molding, so we'll take a look at some of the main ones.

1. Components under the hood

Over the past two decades or so, manufacturers have transitioned to plastic for many under-hood components that were previously made of metal. For these applications, robust polymers such as ABS, nylon and PET are common. However, manufacturers now use injection molding to make parts such as cylinder head covers and oil pans. This method is less weight and cost than metal parts.

2. External Components

Injection molding is an established process for many automotive exterior components, including fenders, grilles, bumpers, door panels, floor rails, lamp shades, and more. The splash guard is a good example of the durability of injection molded parts. Additionally, the parts that protect the car from road debris and minimize splashing are often made of rubber or other durable and flexible materials.

3. Internal Components

Manufacturers also use automotive plastic injection molding to produce many automotive interior parts. They include instrument assemblies, interior surfaces, dashboard panels, door handles, glove boxes, air vents, and more. In addition, they use injection molding to produce decorative plastic components.

Alternative to Injection Molding for Low-Cost Automotive Prototypes

In many cases, molded plastic can be used as a substitute for metal. Previously, manufacturers used only metal for things like brackets, trunk lids, seat belt modules and airbag containers. Today, injection molding is the preferred production method for these plastics.

On the other hand, manufacturers can sometimes replace molded plastic parts with 3D printed plastic auto parts. This is especially the case in prototyping, where there is less need for extreme durability or a smooth surface finish. Many moldable plastics are available as FDM 3D printer filament or as SLS 3D printer powder for nylon. Some professional and high-temperature 3D printers can also print reinforced composites for high-strength parts.

For one-off prototypes, especially non-mechanical parts, 3D printing can be a cost-effective alternative to molding. Since there is no mold cost, the production price is not that high.

In some cases, manufacturers may even 3D print auto parts for a handful of end-uses. They might use SLM 3D printing to make fluid handling components such as valves (usually not injection molding). Another option, however, is to use SLS 3D printing to create parts such as bumpers, trim and windbreakers, which are sometimes injection-molded.

As we all know, the engine operating conditions are complex and harsh, and it needs to adapt to various external environments, so there are generally high requirements for engine parts. Especially in the aspect of resin materials related to engines, because of their high requirements on working temperature and pressure, it is necessary to use resin materials with superior performance.

In the engine of the car, thermoplastic engineering plastics are commonly used. The following takes the engine as an example to analyze the plastic parts, mainly including:

•PUR material:

Fuel pipe protection cover, high pressure fuel rail protection cover, high pressure fuel pump protection cover, high pressure fuel pump frame cover. These parts are used to protect the components while eliminating noise, and the foamed plastic is used because the material retains its shape when molded, is softer, and costs less.

•PA66 (with PA66-GF reinforcement):

Oil dipstick, intake manifold, cylinder head cover, oil pan wire harness bracket, chain tensioner arm, engine carbon canister solenoid valve muffler, etc. This material is mainly used because PA66 has higher fatigue strength and rigidity, and its blended modified material has higher wear resistance and better heat resistance.

•PA46 (with PA46-GF reinforcement):

It is mainly used for some parts of the throttle valve. This material has excellent heat and wear resistance, but due to its high cost, it is generally used less.

•PA6:

Used in oil filter, it has the advantages of high strength and wear resistance, and has good durability and mechanical properties after glass fiber reinforcement.

•PPAs:

It is used for the engine water jacket. Since the water jacket has been in high temperature and high humidity for a long time, PPA is used. It has high tensile strength, strong heat resistance and strong coolant resistance under high temperature and high humidity. this material.

• PBT:

It is mainly used for some parts of the ignition coil, because the material has excellent electrical properties and flame retardant ability, and at the same time, the heat-resistant aging temperature is high, which can meet the working conditions of the ignition coil.

•PPS:

It is mainly used for some parts of ignition coils. The main advantages are that the water absorption rate is extremely low, the flame retardancy is good, the heat resistance and electrical properties are good, and the creep resistance under load is strong.

Important properties of plastics

•Fluidity

The fluidity of thermoplastics can generally be analyzed from a series of indices such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (process length/plastic wall thickness).

•Crystallinity

Thermoplastics can be divided into crystalline plastics and amorphous plastics according to whether there is crystallization during condensation. The so-called crystallization phenomenon is that when the plastic changes from the molten state to the condensation, the molecules move independently, completely in a disordered state, and the molecules stop moving freely, according to a slightly fixed position, and there is a tendency to make the molecular arrangement a regular model. .

•Thermal performance and cooling rate

Different plastic varieties have different thermal properties such as specific heat capacity, thermal conductivity, and heat distortion temperature. Plastic varieties with high specific heat capacity require a large amount of heat when plasticizing, and an injection molding machine with large plasticizing capacity should be selected. The cooling time of plastics with high heat distortion temperature can be shortened and demolded early, but cooling deformation should be prevented after demoulding. Plastics with low thermal conductivity have a slow cooling rate, so they must be fully cooled to enhance the cooling effect of the mold.

•Hygroscopic

Plastics can be roughly divided into two types: hygroscopic, adhering to water, and non-absorbing and not easy to adhere to water. The water content in the material must be controlled within the allowable range. Under high temperature and high pressure, the water turns into gas or hydrolyzes, so that the resin bubbles, fluidity Deterioration, poor appearance and mechanical properties.

Process Parameters

•Temperature control

One is the control of barrel temperature. The temperature of the plastic is controlled by the barrel, so the temperature of the barrel is related to the plasticizing quality of the plastic. When selecting the barrel temperature, the main focus is to ensure that the plastic is well plasticized and can be injected smoothly without causing local degradation of the plastic.

The second is the control of nozzle temperature. In order to prevent the molten plastic from being cast, the nozzle temperature should usually be adjusted to be slightly lower than the maximum temperature of the barrel during operation, but not too low, generally five to ten degrees Celsius lower than the maximum temperature of the barrel. The third is to control the temperature of the sol. The sol temperature is the actual temperature reached by plastic plasticization, which can be measured at the nozzle or by the air jet method.

•Pressure control

Injection pressure refers to the melt pressure at the screw head during injection. The function of injection pressure is to overcome the resistance of the melt flowing from the barrel to the cavity, give the melt a certain filling speed and compact the melt.

The holding pressure refers to the pressure that needs to continue to maintain the melt flow in order to compensate for the shrinkage of the material in the mold. The holding pressure has a great impact on the quality of the molded product. Back pressure refers to the pressure experienced in the plasticizing process of plastic, which is the pressure that needs to be generated and surpassed before the screw retreats, also known as plasticizing pressure. Appropriate back pressure plays an important role in improving product quality.

Clamping pressure is an important parameter of the injection molding machine and the clamping force applied to the mold. Calculation of clamping force: The clamping force is equal to the clamping force constant multiplied by the projected area of the product.

•Time control

Injection molding production cycle, including mold clamping, injection, pressure holding, product, cooling, mold opening and product ejection time.

The injection time should not be too long in general. After the cavity is full, it is equivalent to maintaining the pressure under the injection pressure. If the time is too long, the orientation pressure of the product will increase. Generally, the filling time of the product is 3~10S. Generally speaking, in the molding cycle process, the cooling link is the most time-consuming link, so reasonable control of the cooling time can greatly save production costs, usually the cooling time range is 5~120 seconds.

Common injection molding defects and countermeasures

•Material shortage

Lack of material means that the mold is not full, which mainly occurs in places far from the gate and thin walls. The main reason is that the injection pressure or speed is too low, or the holding pressure is insufficient. The improvement method is mainly in terms of equipment: arranging the injection molding machine with the appropriate plasticizing capacity, setting the appropriate plasticizing temperature, shortening the injection molding cycle, etc.:

In terms of mold: avoid too long and too thin runners, and avoid too thin mold walls.

In terms of technology: adjust enough feed amount, increase injection pressure, increase injection rate, and increase material temperature.

•Void

The voids inside the article appear as rounded or elongated bubbles, and voids tend to occur in articles with relatively thick walls and where they are thickest. Therefore, the holding pressure should be increased, the holding time should be increased, the mold temperature should be increased, and the melt temperature should be reduced.

•Flash

Spill is also known as flash. Mostly occur at the opening and closing of the mold. The main reasons are: the clamping force is too low, the mold has gaps, the molding temperature is too high, the injection pressure is too high, and the exhaust is improper. The improvement methods include equipment direction (clamping force, mold parallelism, wear), mold direction (parting surface accuracy, mold design rationality), and process direction (injection pressure reduction, reduction of feeding amount, barrel temperature).

•Top white

If the demoulding force is too high, deformation will occur, eventually causing whitening of the ejection part, insufficient cooling time, and insufficient strength when the product is demolded, resulting in whitening of the top. Improvement method: molding process (reduce the holding pressure, shorten the holding time, advance the switching of the holding pressure, and prolong the cooling time) mold design (select the demoulding slope according to the specifications, and polish the mold in the demolding direction).