Advantages And Disadvantages of Two-shot Injection ...

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There are a variety of manufacturing methods used to create products using plastic polymers, including two-shot injection molding, compression thermoset molding and extrusion. While all of these are viable manufacturing processes, there are several advantages to this process that make it the top choice for many plastics manufacturers. The process is relatively simple; one material is injected into a mold in order to make the initial section of the product, followed by a second injection of a secondary material that is compatible with the original material. There are three good reasons many manufacturers use this method of manufacturing plastics or polymers.

Advantages of two-shot injection molding

 

Two-shot injection molding is cost-effective

The two-step process needs only one machine cycle, rotating the initial mold out of the way and putting the secondary mold around the product so that the second, compatible thermoplastic can be inserted into the second mold. Because the technique uses only one cycle instead of separate machine cycles, it costs less for any production run and requires fewer employees to make the finished product while delivering more items per run. It also ensures a strong bond between the materials without the need for further assembly down the line.

Enhanced product quality

Two-shot injection molding enhances the quality of most thermoplastic items in several ways:

ㆍImproved esthetics: Items look better and are more appealing to the consumer when they are crafted of different colored plastics or polymers. The merchandise looks more expensive if it utilizes more than one color or texture
ㆍImproved ergonomics: Because the process allows for the use of soft-touch surfaces, the resulting items can have ergonomically designed handles or other parts. This is particularly important for tools, medical devices, and other hand-held items.
ㆍEnhanced sealing capabilities: It provides for a better seal when silicone plastics and other rubbery materials are used for gaskets and other parts that require a strong seal.
ㆍCombination of hard and soft polymers: It lets you combine both hard and soft polymers for outstanding comfort and utility for even the smallest of products.
ㆍReduced misalignments: It can greatly reduce the number of misalignments when compared to over-molding or more traditional insert processes.
ㆍComplex mold designs: It enables manufacturers to create more complex mold designs using multiple materials that can’t be effectively bonded using other processes.
ㆍExceptionally strong bond: The bond created is exceptionally strong, creating a product that is more durable, more reliable, and with longer life.

Versatility

Product manufacturers favor a wide range of applications for two-shot injection molding, including automotive interior parts, medical equipment, tools, and toys. It allows manufacturers to combine various materials and colors to create a strong and attractive final product. Some materials can be effectively combined with this process, including silicone and thermoplastics, nylon and thermoplastic elastomers, or hard nylon and soft-touch materials.

Two-shot injection molding can solve your company’s product production difficulties. An experienced plastic manufacturer can guide you from concept to finished product and ensure a cost-effective solution.

Producing an assembly with multiple components

Compared to other methods of plastic molding, two-shot is ultimately a more cost-efficient way of producing an assembly with multiple components. Here’s why:

Part Consolidation: Two-shot injection molding reduces the number of components in a finished assembly, eliminating an average of $40K in development, engineering, and validation costs associated with each additional part number.

Improved Efficiency: Two-shot molding allows multiple components to be molded with a single tool, reducing the amount of labor needed to run your parts and eliminating the need to weld or join components after the molding process.

Improved Quality: Two-shot is carried out within a single tool, allowing for lower tolerances than other molding processes, a high level of accuracy and repeat-ability, and reduced scrap rates.

Complex Moldings: Two-shot injection molding allows for the creation of complex mold designs that incorporate multiple materials for functionality that cannot be achieved through other molding processes.

Disadvantages of two-shot injection molding

1) High tooling costs and long setup lead times. Up-front costs are high due to the design, testing, and tooling required. There is the initial design and prototyping (probably via CNC or 3D printing), then the design of a prototype mold tool to produce replicas of the part in volume. Lastly, and only after extensive testing during both stages, you can finally inject mold a part.

2) Part design restrictions. Plastic parts must be designed with injection molding consideration and must follow the basic rules of injection molding, for example:

Avoid undercuts and sharp edges as much as possible

Use uniform wall thicknesses to prevent inconsistencies in the cooling process resulting in defects like sink marks.

Draft angles are encouraged for better de-molding.

Don’t forget, because tools are typically made from steel or aluminum, it can be difficult to make design changes. If you need to add plastic to the part, you can make the tool cavity larger by cutting away steel or aluminum. But in order to take away plastic, you need to decrease the size of the tool cavity by adding aluminum or metal to it. This is extremely difficult and in many cases might mean scrapping the tool (or part of it) and starting over.

Also, the weight and size of the part will determine the tool size and necessary press size. The larger the part, the more difficult and expensive it will be.

3) Small runs of parts can be costly. Due to the complexity of tooling, and the necessity to rid the machine of all previous material before the next product can be made, the setup time can be quite lengthy. Therefore small runs of parts have traditionally always been thought of as too expensive to injection mold.

 

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Generally speaking, the automobile lamp is made up of reflector, lens, shell/ light source, electronic circuit, installation parts and so on. The main Plastic raw material for automobile lamps is our country are PC, PP, ABS, PMMA and a few other plastic materials. But do you know how to choose these materials?

Today, let me together with you to understand the mystery of the selection of it.

1. Why the lamp shell is plastic?

There are many reasons.

(1) The elasticity of plastic is stronger than that of glass. Not in the case of severe impact will not be easily broken, even if it breaks down on the road, it will not cause great harm to pedestrians or animals.

(2) Light quality, compared to the glass lamp housing, the plastic lamp housing is lighter about 0.3~1.4 kg

(3) With the advantages of good corrosion resistance, good impact resistance, high transparency, good insulation, low thermal conductivity, good mechanical performance, strong plasticity, easy molding and processing.

2. The application of plastics in car light 

PC (Polycarbonate)

According to the relevant data, PC is widely used on car lights, PC series materials in the use of plastic materials in headlight accounted for about 50% (weight ratio), In the automotive lights, the PC is mainly used in headlamps and fog lamps’ lens and external lens

Why choose PC?

PC, Polycarbonate, has excellent impact and transparency, good mechanical and electrical insulation properties, wide temperature range, high dimensional stability and high creep resistance.

The internal structure of the car led to a certain temperature difference in different parts, and the temperature of some part of the reflectors and decorative rings may reach 100 0 ~ 190 0, so PC and PC-HT are usually chosen in the selection of materials.

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PP (Polypropylene)

Most of the PP used in car lights are modified PP, accounting for about 30% of the plastic material in the lamp (weight ratio). For example, the rear lamp housing, the trim panel and the wiring harness guide slot (PP-T20), the headlight housing and the back cover, generally use PP. For the moment, the PP is the main material for the headlight housing.

PP has good electrical insulation, chemical corrosion resistance and good fatigue resistance. It has the highest mechanical and thermal resistance in general plastics. Besides, it has a price advantage over other plastics.

PBT/PET (Polybutylece Terephthalate)

The PBT is usually used for the decorated ring and the adjusting brackets in headlights. Modified PBT and ASA, plus glass fiber reinforcement, can be used to make the shell of fog lamps. Mechanical properties similar to those of PA, POM, small friction coefficient, good self-lubrication, good thermal stability and chemical stability, excellent electrical insulation, high rigidity and hardness, good impact, high dimensional stability, good heat resistance.

Decorative ring material has been dominated by heat-resistant PC material in past years, gradually replaced by PBT/PET in recent years. Compared to the heat-resistant PC, PBI has the prominent features of high heat resistance, good processability, low cost. The material has high surface gloss, excellent in processing stability and good hydrolysis resistance, excellent flowability and heat resistance, material price compared with the heat resistance PC, can greatly reduce the cost of about 40%.

PA (Nylon)

The PA used in the headlights is usually PA+GF, the ordinary adjusted screws or brackets in the headlights, using PA6+GF or PA66+GF.

Aromatic PA has higher heat resistance temperature and strength than ordinary PA. Some important regulating structures and internal brackets of the lamp will also use some aromatic PA plus glass fiber.

PMMA

PMMA, commonly known as acrylic or plexiglass, is a highly transparent, amorphous thermoplastic material, it has excellent optical performance, light transmittance up to 90%~92%, refractive index 1.49, good weather resistance, high surface hardness and excellent overall performance. It is mainly used for optical transparent products. Taillight lens and interior atmosphere lamp optical materials.

Well, here comes the question. Why does the headlight of the car use PC instead of PMMA?

Both PC and PMMA have high transparency and good processing characteristics, but there are some differences in the use of lights.

Firstly, in the heat resistance temperature, PC generally can reach 130 degrees Celsius, while PMMA is only about 100 degrees Celsius.

Secondly, the toughness of PC material is better, and PMMA material is very crisp, it’s easily broken by impact.

Finally, the unit price of PC is higher than PMMA, and the liquidity at the time of forming is not better than that of PMMA.

In general, the headlight lamp is PC, which is mainly due to the high brightness and heat of the headlights, and in front of the vehicle, which requires a heat shock resistant material. Taillight lampshade is generally PMMA, in addition to cheap factors, and the taillights are generally single signal lamp assembly, to red and transparent double color alternating, so the good fluidity PMMA is a choice.

ABS、PC/ABS

In the automotive field, ABS is generally used after modification with PC material, depending on the temperature and impact properties of the ABS to determine the proportion of PC content. ABS and PC/ABS are commonly used in taillight housing and trim panels.

BMC, PEI, PEEK, PPS, LCP and other materials

Lamp reflector mainly adopts the material such as BMC, PET etc.

BMC is a new type of material, which has the advantages of thermosetting plastics and reinforced engineering plastics. When Injection molding, it can flow evenly at the same time and can be formed in a very short time. It has high rigidity and high strength, good molding shrinkage and dimensional accuracy stability. On the basis of the characteristics of BMC above, generally headlight reflector adopts the BMC. The main function of the headlight is lighting the road ahead, ensure the vehicle in normal driving, the reflector with the corresponding bulb can shoot two kinds of light beam, near beam and high beam, the near beam should have clear boundaries and high brightness illumination.Therefore, BMC's good molding shrinkage and dimensional accuracy stability can meet this point.

Tail lamp generally use the materials such as PET, PC, or directly from the lamp on the hub molding, Tail lamp is light function combination, dimensional accuracy is not as high as headlights’ requirements, but if you can see the reflector directly from the lamp-chimney, the reflected surface on the reflector must be beautifully designed, ensure that the light of the unique shape after lighting.

The development trend of the Automotive Light materials

The future lights will develop toward simplicity, light weight, high heat resistance and environmental protection. For the selection of materials for automotive lights, the following 3 points are as the main points to strengthen and breakthroughs in the future:

Heat dissipation, the infrared material can be used as the shell of the headlamp or fog lamp.

Materials (coatings) are required to have better weather resistance and temperature resistance.

Contact us to discuss your requirements of structural foam injection molding. Our experienced sales team can help you identify the options that best suit your needs.