In many ways, plastics have replaced glass in several applications. Think display structures, sample bottles, tool parts, cosmetics containers, and Tupperwares. These amongst many others are examples of applications of clear injection molded parts. Clients want clear parts for different reasons. This could be to make the product more visible to customers. Some equipment parts need to be clear for functionality or aesthetics. For a while, glass has been the go-to for clear objects. They are durable, unreactive, and inexpensive. The primary raw material for glass is silicon dioxide, sand. A very abundant resource on earth. But glass comes with the disadvantage of brittleness, heavyweight, and limited processability. This is the main reason why plastics have been a successful alternative. From everyday products like polycarbonate screens to high-end applications like solar panels. The processing of clear plastics calls for some special considerations. The goal is clear plastic without visible flaws. A discoloration that might be well hidden in colored plastic will not pass in clear ones. This article covers the injection molding of clear plastics. Here we explore how this is different from molding colored plastics. You’ll learn what precautions to take and the types of products that get made this way.
Types of Plastics Used as Clear Products
The type of plastic to use as a raw material in injection molding depends on how the plastic gets produced. So first let us look at plastics production and their properties. This gives a better understanding of why some plastics work better.
Plastics belong to a class of materials known as polymers. Other nonplastics which belong to this class include rubbers and fibers. Materials like crosslinked polyisoprene or polybutadiene are rubbers. Common names for these are latex and spandex to give a better picture. So generally speaking, stretchy and resilient materials are rubbers. Cellulose, silk, or wool are fibers. Think of products like paper and silk fabrics. These are fibers. You can think of fibers as polymers that are not stretchy. A single fiber has a high length to diameter ratio. A typical single fiber is not visible with the naked eye. And now plastics. Plastics have properties intermediate between rubbers and fibers. They will stretch until they reach a yield point. But the yield point is very low compared to rubber. This is the point beyond which they will not return to their original form. At their yield strength, they continue to stretch until they reach a breaking point. Another characteristic is that they are rigid at room temperature. Rubbers are nonrigid.
Plastics, like other polymers, get made through the polymerization process. This is a process where hundreds or thousands of molecules react with each other. They do so to form bigger molecules called polymers. This molecule has a repeating sequence of the smaller molecules. The reaction is polymerization. There are different types of polymerization reactions. They differ in the type of reaction mechanisms and the type of system used. Some polymerization reactions need certain solvents and catalysts. Some do not need a solvent. There those which need heat to start. There are those initiated catalyzed by radiation. So different types of plastics get produced in different processes and environments.
Now let us look at the color of the plastics. That is the original color before any dye or pigment gets added. The color of a material is generally determined by how it absorbs and reflects light. The color we see is to do with how the human eye perceives the light. How its molecules absorb and reflect light to do with the spatial arrangement. Also, the components used in its preparation are important. So catalysts of particular color or solvents also contribute to the final color. Other factors like degradation and radiation also contribute to the color of plastic. Now having covered this, what plastics get used to achieving clear products?
With the above in mind. The following are plastics that are common for use in clear products. The first three are the most common.
- Polycarbonate (PC)
- Polymethylmethacrylate (PMMA)
- Polyethylene Terephthalate (PET)
- Nylon/polyamide (PA)
- A copolymer of acrylonitrile styrene (AS)
- Polystyrene (PS)
- Polypropylene (PP)
- Polyethylene (PE)
Injection Molding as an Option for Clear Plastic Products
To appreciate the advantages of injection molding you should be aware of alternatives. So let’s weigh other methods for processing clear plastics against injection molding. The alternatives include CNC machining, 3D printing, and vacuum casting. Here we look at the key aspects of these processes. Note that this focuses on processes that achieve complex shapes. Blow molding or film-forming are not included here. For obvious reasons that they are only used for bottles and films.
CNC involves a programmed hand cutting and drilling into an object. The motion of the arm gets controlled by a computer. The pattern is already programmed by the operator into the computer. Good precision gets achieved using this method using part with the right resolution. Compared to injection molding this is a much slower process. The time dedicated also means a single piece costs more. It is more suited to prototype production. Machining of plastics causes lots of abrasions. The result of this process depends on the abrasion resistance of the material. The clarity of the final product is often altered. Post-processing methods like polishing can improve clarity. Extra process means more cost. A plastic with poor abrasion resistance will result in a hazy or cloudy appearance. For a part to get machined it needs to be rigid enough. This way it withstands the force exerted. PMMA achieves clear products via CNC machining. But this is after polishing. Another rigid plastic is polycarbonate (PC). Although rigid enough for machining does not achieve a clear product. Even after extensive polishing, the end product has poor clarity. CNC also often requires the making of the parts as separate runs. These are then joined together by gluing to achieve a more complex shape. Joining adds costs and introduces weak points.
3D printing is another option. Transparent parts usually cost more to 3D print than colored parts. This is due to the added precautions needed to print clear parts. The main drawback here is the limitation of SLA 3D printing. SLA or stereolithography apparatus uses light to solidify polymers. Solidification occurs as they form into long-chain polymers. It is generally known that 3D printing is an expensive process. So there is little need to compare it to low-cost high output injection molding. The goal here is to establish how Injection molding stands out from such methods. So although 3D printed clear parts are possible. It comes at a considerable cost. It is also limited to polymerization or curing by light.
The third option is vacuum casting. This process begins with clear plastic in liquid form. This is then placed into a mold. The vacuum gets passed through and the part gets molded by suction. This part achieves complex shapes. This process gets limited to plastics like polyurethanes. The liquid form is usually a combination of the two parts that make up the polyurethane polymer. So the actual plastic gets formed in the reaction that results in the hardening. The vacuum removes trapped air to achieve a clear product with a good finish. A variation of this is vacuum thermoforming. But this form of vacuum forming can only get used to forming products with thin walls. The process is quite expensive due to the need for vacuum and mold. Compared to the low production volume this means the cost is not met by quantity. Although it results in a good clear finish, it is expensive. Injection molding is a superior option compared to those discussed above. This is for several reasons. The next section discusses some key considerations for working with clear plastics.
Special Requirements For Molding Clear Plastics
Precautions needed to start from the raw materials to the final product. Here we look at the whole process from the beginning to the end. Here are some of the special considerations for achieving clear products. The general aim is to prevent any distortion to the clarity of the plastic.
Cleaning
With colores plastics, a few impurities can blend in and never get noticed in the product. You must likely have used a lot of clear plastic products. So you would know that any tiny spec will immediately get noticed. This is even more important in certain products. Examples are tools or packaging for food, medicine, or hygiene products. So injection molding facilities for clear plastics need a high standard of cleanness.
The cleaning gets done with cleaning agents. A run with clear plastic is also used to clean. Here you allow a clear plastic to go through the injection molding machine. This mixes with any impurities. In a way, it is like flushing the system with clear plastic first before the real run. The plastics for the flush can get cleaned and recycled in colored plastics.
Raw materials Processing
Whether using pellets or liquid resin these need to stay clean. The raw materials should remain sealed to prevent impurities from surrounding. Dust particles, droplets, or microbes can get to unsealed raw materials. To a large extent keeping them sealed prevents a lot of this. If the materials need drying the drying air gets filtered. This prevents airborne contaminants. The surfaces or containers used for drying also need cleaning.
Well polished Machine Parts
The parts of the machine which will contact the plastic need to be well polished. They should be of hard polished steel with a high-quality surface finish. Uneven surfaces cause haziness on the product surface. This is particularly important for the mold cavity surface. Well, the polished finish is important for clear product formation.
Dedicated Machine
The best way to prevent debris is to dedicate a machine for processing clear products. This reduces the chance of staining from other runs. It also reduces the amount of effort in cleaning. Where possible only some parts get dedicated. Other parts get used for other processes. For example, the parts not in direct contact with the plastic can go to other use.
Temperature control
This is in both heating and cooling. Light transmittance gets affected by the arrangement of the molecules as they cool. Uneven cooling leads to a hazy appearance. The process of injection molding exerts a lot of stress on the material. In thermoplastics injection molding this exerts high temperature to melt the plastics. Some thermosets also need high temperatures. This gives the energy required for the curing process to occur. Overheating results in degradation. This causes spots and discoloration. The temperature in the barrel gets monitored and controlled to prevent this.
Wide Gates Help
This affects how the mold gets filled. A wider gate reduces shear on the melt. This prevents the distortion of light transmittance. It also allows more melt to get into the mold at the same time. This reduces the chances of uneven cooling. Uniform cooling helps the formation of a clear product.
Vents
Any air trapped in the feed will end up in the melt. Likewise, any air trapped in the melt will end up in the product. Vents are important features in the injection molding machine. They get placed in the barrel. It is common to have a decompression zone after the vent. The change in pressure forces out trapped air. Air bubbles also form from moisture in the feed.
Injection Speed
A slow injection rate means the flow is more orderly. This means the flow is closer to lamina than turbulence. A calmer more steady flow allows for more ordered alignment of molecules. Hence a clear product formation.
Mold Release Agent
Some manufactures add mold release agents to aid product ejection. This should not get used in clear plastics. The mold release agent can cause coloration or distort the clarity of the surface. Instead of a mold release take advantage of the shrinkage of the plastic. Allow the product to cool completely and use ejectors to aid product release.
Advantages of Injection Molding for Clear Plastics
Based on the issues pointed out within the article so far. The below list summarizes the advantages of using injection molding for clear plastics.
- Parts get processed in their molten state. This reduces abrasion.
- A wider variety of plastics can get processed. This is unlike other methods like CNC that only works for rigid plastics
- A better finish gets achieved with many types of plastics
- There is hardly a need for polishing to improve clarity
- Complex shapes get achieved in one process
- Higher volume production rate
- Less expensive compared to other methods
Some Example Applications of Injection Molded Clear Plastics
Examples of injection products. The image on the right also shows the mold used.
Conclusion
Injection molding of clear plastics needs a high level of cleanliness. Special care is necessary to prevent the inherent build-up of color. This results from degradation or oxidation. Avoiding the introduction of external impurities also improves quality. Although many plastics can achieve products. The best plastics options are those that withstand injection molding. and keep their clarity.
