Injection molding of transparent plastic products requires precision and specialized techniques to achieve clarity and uniformity, making it vital in industries ranging from optics to consumer electronics.

To mold transparent plastics effectively, maintain precise temperature and pressure, choose suitable mold materials and plastic resins, ensuring clarity and minimizing defects like bubbles and haziness.

Grasping the specifics of transparent plastic molding not only improves product quality but also enhances efficiency. Delve deeper to understand how different techniques and materials can optimize your transparency-focused production process.

How Does Raw Material Preparation Affect Injection Molding of Transparent Plastic Products?

Proper raw material preparation is crucial for achieving optimal transparency and quality in injection-molded plastic products.

Proper preparation of raw materials, including drying, purity, and particle size, is crucial for consistent quality in injection-molded plastics, preventing defects and ensuring clarity in automotive and electronics products.

Wybór surowców

• Poliwęglan (PC)¹:It has excellent optical transparency, high impact strength, and good heat resistance, and can maintain stable physical properties over a wide temperature range. It is suitable for the production of products with high requirements for transparency and strength, such as optical lenses, transparent housings for high-end electronic products, and automobile lampshades. However, its processing fluidity is relatively poor, and the injection molding process requires precise control of process parameters.

• Polymethyl methacrylate (PMMA)²: Also commonly known as acrylic, PMMA has excellent light transmission, comparable to glass, and good weather resistance. It is commonly used in the production of display racks, advertising light boxes, decorative ornaments, and various types of lamps and lanterns, etc. It can present a very good visual effect. However, its surface hardness is relatively low, making it easy to scratch. Therefore, attention should be paid to protection during subsequent use and processing.

• Polyethylene terephthalate (PET):It is commonly used in food, pharmaceuticals, and other transparent packaging. It has good chemical stability, transparency, and certain barrier properties. The cost is relatively reasonable. However, its crystallization characteristics have a certain impact on the injection molding process. The processing should reasonably control the temperature and other parameters based on the crystallization needs.

• Other transparent plastics:Polystyrene (PS) is a cheap, transparent material that is often used to make simple, see-through products like disposable tableware and stationery. Cyclic polyolefin (COC) is used a lot in the optical industry, especially for making high-precision lenses. It’s great for this because it doesn’t bend light much, it’s really pure, and stuff like that. But it’s more expensive to make.

Raw Material Drying

• Drying equipment selection³:Common drying equipment includes hot air circulation ovens, vacuum ovens, and dehumidification dryers. If you’re doing a lot of drying and you need to get the moisture content of your clear plastic raw materials really low, then a dehumidification dryer is the way to go. It will keep giving you a nice, low humidity drying environment and it will get the moisture out of your material. If you’re doing small to medium-sized batches or you’re drying materials that don’t have to be super dry, then a hot air circulation oven is the way to go.

• Drying parameter settings:For PC raw materials, we usually use a dehumidifying dryer. The drying temperature⁴ is usually set at 110-130 degrees Celsius, and the drying time is 4-8 hours. The specific drying time should be adjusted according to the initial moisture content of the raw materials and the size of the batch. At the same time, the humidity of the drying environment should be strictly controlled, and it should be kept below 10% to ensure that the moisture content of the final raw materials can be reduced to within 0.02%. For PMMA raw materials, the drying temperature can be set at 70-90 degrees Celsius, and the drying time is about 3-6 hours. Its sensitivity to humidity is relatively low, but it is also necessary to ensure that the moisture content after drying meets the requirements to avoid defects such as silver wire and bubbles in the product.

What is the Impact of Injection Molding Machine Selection and Debugging on the Injection Molding of Transparent Plastic Products?

Selecting the right injection molding machine and precise debugging are key to achieving optimal transparency and quality in plastic products.

Selecting the right injection molding machine and precise debugging are crucial for clear plastics, impacting transparency and quality by reducing defects such as haze and discoloration, essential in automotive, medical, and consumer products.

Wybór maszyny do formowania wtryskowego

• Screw structure: When you’re molding clear plastic, you want to use a screw that has a taper. The screw should have a taper that gets smaller as you go from the back of the screw to the front of the screw. This will help you melt the plastic better and make the temperature and viscosity of the plastic more consistent. This will help you make better parts that are more clear. Also, you want to use a screw that is long. You want to use a screw that is about 20 to 25 times longer than it is wide. A longer screw will give the plastic more time to melt. This is especially important when you’re molding clear plastic that has a high viscosity.

• Precision of injection system:The injection system of the injection molding machine should be able to control the injection volume with high precision. The repeatability of the injection volume should be able to reach within ±1%. This is to ensure that the volume of plastic melt injected into the mold cavity is accurate in each injection. This is also to avoid quality problems such as dimensional deviation, lack of material, or flying edges of the products due to the deviation of the injection volume.

• Clamping system stability:The clamping system needs to have enough clamping force to make sure the mold cavity can be closed tightly during the injection process, to prevent the plastic melt from overflowing from the parting surface under high pressure, forming a flying edge. At the same time, the opening and closing action of the clamping system should be smooth and precise, so as to avoid unstable factors such as violent impact of the mold affecting the molding quality of the products and the service life of the mold.

Parameter Adjustment

• Prędkość wtrysku: Don’t inject too fast, or the plastic will melt and create turbulence, trapping air and causing bubbles. This will affect the transparency and appearance of the product. For simple, evenly-walled transparent products, the injection speed can be stable, usually around 10-30cm³/s. But for complex, thin-walled transparent products, the injection speed needs to be stable. For complex products with thin walls, reinforced walls, or transitions from thin to thick walls, you often need to use multi-stage injection speeds. For example, at the beginning of the injection, fill at a slower speed (5-10cm³/s) so the melt can smoothly enter the main flow channel and the main part of the cavity. When filling the thin-walled or complex parts, increase the speed to 20-40cm³/s to ensure the melt can completely fill these parts. Then, when you get close to the end of the cavity, reduce the speed to avoid flash.

• Pressure maintaining speed:The speed of the pressure holding stage is relatively slow, mainly for the smooth replenishment of the melt, and its speed is generally set at 1 – 5cm³/s, which should be adjusted according to the specific requirements of the products and the parameters such as pressure holding pressure, so as to ensure that the quality of the products is stable during the pressure holding process.

• Ciśnienie wtrysku:When setting the ciśnienie wtrysku⁵, you need to consider the shape of the product, the size, the wall thickness, the mold runner system, and other factors.For example, for thin-walled transparent plastic products, because the melt in the cavity cools quickly, you need to use a higher injection pressure to make sure the melt can quickly and completely fill the entire cavity, so you don’t get a short shot. You might need to use 100-150MPa or even higher. For thick-walled products, the injection pressure is lower, usually around 50-100MPa. You need to be careful not to use too much pressure, because that can cause a lot of residual stress inside the product, which can cause the product to crack or deform later. When you’re actually setting up the mold, you usually have to try a few different molds to find the best injection pressure.

• Ciśnienie trzymania:Holding pressure is usually lower than the injection pressure. Its main role is to supplement the plastic melt during the cooling and shrinkage of the product, so that the density of the product is uniform, to avoid shrinkage marks. For PC products, the holding pressure is generally 60% – 80% of the injection pressure, and the holding time is mostly between 5 – 20 seconds according to the wall thickness of the product; the holding pressure of PMMA products is about 50% – 70% of the injection pressure, and the holding time is about 3 – 10 seconds; the holding pressure of PET products is roughly 70% – 90% of the injection pressure, and the holding time may be 8 – 20 seconds, the specific parameters have to be optimized by trial molding. The specific parameters should be optimized and adjusted through trial molding.

• Temperatura beczki:For PC raw materials, the barrel temperature is usually set at 260℃ – 280℃ in the front section, 240℃ – 260℃ in the middle section, and 220℃ – 240℃ in the back section. This temperature gradient helps the raw material to plasticize gradually and evenly. The temperature will soften from the charging section, and then reach a good flow state suitable for injection in the front section.For PMMA raw materials, the barrel temperature is usually set at 200℃ – 230℃ in the front section, and 200℃ – 230℃ in the middle section, and 200℃ – 230℃ in the middle section. The temperature of the cylinder of PMMA raw materials is generally in the range of 200℃ – 230℃, 180℃ – 200℃ in the middle section, and 160℃ – 180℃ in the back section. It should be finely adjusted according to its relatively narrow processing temperature window to prevent decomposition caused by too high a temperature, which affects the transparency and color of the products.The temperature of the cylinder of PET raw materials in the front section is mostly in the range of 270℃ – 290℃, the temperature of the middle section is in the range of 250℃ – 270℃, and the temperature of the back section is in the range of 230℃ – 250℃, which should be paid attention to. PET in the process of crystallization, through temperature control to regulate the degree of crystallinity, which in turn affects the performance and appearance of products.

• Temperatura dyszy: The nozzle temperature is usually a little lower than the temperatura beczki⁶. For PC, the nozzle temperature can be set at 250-270°C. This can ensure that the plastic melts smoothly and is injected into the mold through the nozzle. It can also prevent drooling caused by the nozzle temperature being too high. For PMMA, the nozzle temperature is suitable at 190-210°C. For PET, the nozzle temperature is 260-280°C. The specific temperature should be set reasonably according to the melt viscosity and flow characteristics of different plastics.

Mold Design and Manufacturing Points How to Influence the Injection Molding of Transparent Plastics?

Effective mold design and manufacturing are critical for optimizing the injection molding of transparent plastics, ensuring clear and high-quality end products.

Transparent plastic molding is influenced by material selection, mold polish, and design precision. Optimal conditions enhance clarity and reduce defects, benefiting automotive, electronics, and packaging industries.

Exhaust Design

• Exhaust grooves:The depth, width, and length of the exhaust grooves need to be carefully designed. The depth is usually between 0.02mm – 0.05mm. If it’s too shallow, the exhaust effect will be poor. If it’s too deep, the plastic melt will easily overflow and form a flying edge. The width is usually around 3 – 8mm, depending on the size and structure of the mold. The length of the venting channel should extend to the outside of the mold cavity so that the air can be smoothly discharged out of the mold. The location of the air venting channel should be chosen at the last part of the melt to be filled, such as the corner of the cavity, the end of the reinforcement, and other places where air is likely to accumulate, in order to improve the efficiency of air venting.

• Gas permeable steel inserts⁷:Gas-permeable steel is a material with good gas permeability, which will be made into part of the mold inserts to assist the exhaust effect is significant. When using air-permeable steel inserts, pay attention to its installation position and other parts of the mold with, inserts of the porosity is generally between 15% – 30%, the pore size is uniform, so as to ensure that the air passes through smoothly, and at the same time will not make the plastic melt a large number of infiltration and clogging of the pore. The number and distribution of air permeable steel inserts in the mold should be determined according to the shape and size of the product as well as the exhaust demand, for example, for large and complex structure of the transparent product molds, it may require more than one air permeable steel inserts distributed in different parts to ensure the exhaust effect.

• Other exhaust methods:You can also use a vacuum pump to help with the venting process. By connecting a vacuum pump to the mold, you can pull the air out of the cavity before injecting the plastic. This puts the cavity in a negative pressure state, making it easier for the plastic melt to fill the cavity and reducing the chance of air bubbles. Another simple way to vent is to use the natural gap at the parting line. However, this method has limited effectiveness and is usually used in combination with other venting methods. Also, be careful to control the size of the parting line gap to prevent the melt from overflowing.

Optymalizacja układu chłodzenia

• Cooling channel layout⁸:The cooling channel layout should be designed according to the shape and wall thickness of the product. For products with regular shapes and uniform wall thickness, the cooling channels can be arranged in a straight line or in a circle for uniform distribution. For products with irregular shapes and large differences in wall thickness, the cooling channels should be partitioned. The cooling channels in the thicker parts of the wall should be denser, or the cooling channels should be closer to the surface of the cavity to accelerate the cooling of these parts and ensure the overall cooling of the product. For example, for a transparent product mold with a thin-walled reinforcement and a thick-walled body structure, the spacing of the cooling channels below the thick-walled body can be set to 20-30mm, while the spacing of the cooling channels near the thin-walled reinforcement can be narrowed to 10-15mm. The direction of the cooling channels should be reasonably designed to avoid the formation of cooling dead zones.

• Cooling channel size:The diameter of the cooling channel is usually between 8mm – 12mm. If the diameter is too small, it will affect the flow of coolant and cooling efficiency. If the diameter is too large, it will make the mold structure too complex, increase costs, and take up too much space in the mold. The distance between the cooling channel and the cavity surface should also be adjusted according to the product wall thickness and other factors. Usually, it is around 10 – 20mm. If the distance is too close to the surface of the product, it is easy to produce cold marks, which affects the appearance quality. If the distance is too far, the cooling effect is poor, resulting in a longer cooling time for the product and a longer production cycle.

• Coolant selection and control:Coolant is commonly used in injection molding machines, including water, oil, and special mold coolant. Water has good cooling effect and low cost, but it is easy to cause mold rust, so it needs to add rust inhibitor and other protection; oil has relatively slow cooling speed, but it has good rust and lubrication properties, suitable for some molds with higher temperature control requirements and strict precision requirements. The temperature of the coolant should be adjusted according to the type of plastic products and process requirements, generally controlled between 5 ℃ – 30 ℃, and at the same time, the temperature control system is used to realize the precise control of the coolant temperature to ensure that the cooling process is stable and uniform.

Design of Demolding Mechanism

• Selection of demolding method:There are several common demolding methods, such as push plate demolding, inclined top demolding, push rod demolding, etc. Push plate demolding is suitable for transparent products with simple and flat shape. The products are pushed out of the mold cavity as a whole through the push plate. The demolding force is uniform and it is not easy to damage the surface of the products. Inclined top demolding is mostly used for products with inverted buckle structure. The inclined top can be used to smoothly remove the products with inverted buckles through the inclined movement during the process of opening and closing the mold. Push rod demolding is more flexible. According to the specific shape of the product and the need to set the position of multiple pusher, but pay attention to the design of the contact part between the pusher and the product to avoid leaving obvious traces of the pusher on the surface of the product.

• Demolding tilt setting⁹:It is important to have a demolding angle when demolding transparent plastic products. Generally, the demolding angle is between 1° and 3°. For products with high surface precision requirements and soft texture, the demolding angle can be increased to 3° to 5°. When designing the mold cavity, we must consider the setting of the demolding slope, so that the product can be cooled and contracted along the slope direction for smooth demolding. At the same time, we must ensure that the dimensional accuracy and appearance quality of the product are not affected by the slope. Especially for transparent products with optical requirements, the design of the demolding slope should be more delicate to avoid changes in optical properties caused by problems with light refraction due to the slope.

• Surface treatment of demoulding parts:The surfaces of the parts that come into contact with the products, such as the push plate, the inclined top, the push rod, etc., should be polished to a fine degree, so that the surface roughness is below Ra0.8μm, so that the scratches on the surface of the products can be minimized during the demolding process, ensuring that the products have a high degree of finish and transparency. In addition, you can also use surface treatment methods such as hard chrome plating on the surface of the demolding parts to improve their wear resistance and corrosion resistance, extend the service life of the demolding parts, and further protect the smooth demolding process and the quality of the products.

How Does Injection Process Control Affect the Injection Molding of Transparent Plastics?

Injection process control is crucial for molding high-quality transparent plastics, ensuring clarity and mechanical strength throughout various applications.

Controlling injection process conditions enhances transparency and quality in plastics by ensuring consistent temperature and pressure, crucial for optics and electronics industries.

Multi-Stage Injection Control

When you use a multi-stage injection process¹⁰, you usually divide it into 3-5 stages for control. At the beginning of the injection process, which is the first stage, you inject at a very slow speed (like 3-5 cm³/s) and low pressure (about 30-40% of the final injection pressure) to let the melt fill the main flow paths and gates of the mold cavities slowly and smoothly, and to avoid vortexing and air entrainment caused by the high-speed impact of the melt.

In the second stage, when the melt starts to enter the main part of the cavity, according to the size and wall thickness of the cavity, the injection speed should be increased appropriately to 8-15cm³/s, and the pressure should be increased to 50%-60% of the final injection pressure to ensure that the melt can be evenly distributed and filled around the cavity. In the third stage, if there are thin-walled products, the injection speed should be increased to.

In the third stage, if there are thin-walled areas, reinforcing bars, and other structural complexities, the speed is further increased to 15 – 25cm³/s and the pressure is increased accordingly to ensure that the melt can fully fill these difficult-to-fill parts. For some large or particularly complex products, it may also be necessary to set up the fourth and fifth stages to further fine-tune the injection speed and pressure to achieve perfect filling.

Melt Flow Control

When you’re injecting, always pay attention to the flow of the plastic melt in the cavity. You can watch the pressure, speed, and other parameters of the injection molding machine’s curve and use the mold’s visualization window (like using transparent inserts or opening observation holes, etc.) to see the flow of the melt state intuitively.

If the melt is found to have turbulence, stagnant flow, or an unstable front surface, the injection speed, pressure, and other parameters should be adjusted in a timely manner to ensure that the melt can fill the cavity smoothly in a laminar flow. This is very important to ensure the transparency and internal quality of the product.

Pressure Maintaining Curve Optimization

The holding pressure process¹¹ is not a single pressure to keep constant, but can be set to achieve more accurate control of the holding pressure curve. Usually at the beginning of the holding pressure, a higher holding pressure (such as the upper limit of the holding pressure range described above), with the cooling and contraction of the product, gradually reduce the holding pressure to form a decreasing pressure curve, which can better adapt to the product contraction process of the melt to replenish the demand for changes to avoid defects due to the holding pressure is unreasonable leading to the product shrinkage marks, internal voids and other defects.

When determining how long to hold pressure, you need to consider the wall thickness, the size of the part, the shrinkage characteristics of the plastic, and other factors. You’ll have to run some test molds and look at the part after it comes out of the mold to see how it looks and how accurate it is dimensionally. Then you can make adjustments to optimize it.

Pressure Maintaining Process Monitoring

When you’re holding pressure, you need to keep an eye on the pressure and time. You also need to watch the pressure feedback from the mold cavities (you can put pressure sensors in the molds). You want to make sure the holding pressure is stable and accurate. If the holding pressure is all over the place or the holding time isn’t long enough, you need to make adjustments. You want to make sure the parts are the same density and have a complete internal structure when you’re done holding pressure. That way, they’ll cool and come out of the mold right.

Cooling Time Determination

Cooling time calculation is more complicated, it is related to the wall thickness of the product, the thermal conductivity of the plastic, the cooling efficiency of the mold, and the ambient temperature. Generally speaking, the cooling time can be initially estimated by theoretical formula combined with practical experience, and then further determined accurately through the test mold.

For example, for a PC product with a wall thickness of 2mm, the preliminary estimation of the cooling time may be around 10 – 15 seconds, but the actual test mold may need to be increased or decreased according to the final quality of the product. When determining the cooling time, it is necessary to observe the deformation, dimensional accuracy and surface quality of the product after demolding to ensure that the product is sufficiently cooled and shaped so that warping, deformation and dimensional deviation will not occur after demolding due to insufficient cooling.

Cooling Uniformity Guarantee

To make sure the products cool evenly, you need to do more than just optimize the cooling system layout, size, and other things I mentioned earlier. You also need to make sure the coolant flows evenly and is the same temperature everywhere.

You can do this by putting flow control valves and temperature sensors and other stuff in each cooling channel. That way, you can control and watch the coolant flow in each channel. This will make sure each part cools the same way. It will also keep some parts from cooling too fast or too slow. That way, you won’t have any problems with the parts getting too much stress or getting bent out of shape.

What Post-Treatment Methods are Used for Transparent Plastic Products After Injection Molding?

Post-treatment methods enhance the quality and durability of transparent plastic products, crucial in industries requiring clarity and precision.

Post-treatment methods for transparent plastics like annealing, UV coating, and polishing enhance optical clarity, surface finish, and environmental resistance, ensuring product longevity and performance after injection molding.

Wyżarzanie

When you make transparent plastic products, they can get stress inside when you shoot them into the mold. This stress can make cracks and make the plastic look bad when you use it. You can get rid of the stress by annealing the plastic. For example, if you make PC products, you can put them in an oven at 120-130 degrees C for 1-2 hours. Then you can let them cool down slowly to room temperature. You have to change the temperature and time depending on how thick the plastic is, how big the plastic is, and what kind of plastic you use.

Obróbka powierzchni

To make the products look better and smoother, we can do some surface treatment. For example, we can use grinding to remove the small defects on the surface of the products after they are taken out of the mold, and then use polishing paste to polish the surface of the products to make them look like a mirror; for some products with higher optical requirements, we may need to do some special surface treatment, such as coating, to make them have better light transmittance, wear resistance and other properties.

Wnioski

This paper is about injection molding of transparent plastic products. First, raw material selection is crucial for transparency and strength. Commonly used transparent plastics include polycarbonate, polymethyl methacrylate and polyethylene terephthalate.

Secondly, the selection and commissioning of the wtryskarka¹²and the fine adjustment of the process parameters, especially the injection speed, pressure and holding time, have a significant impact on the molding quality.

In addition, mold design, exhaust design and cooling system optimization are also crucial to ensure stability and product quality during the injection process. Finally, post-treatments such as annealing and surface treatment help eliminate internal stresses and improve product transparency.