Clear Plastic Molding

Molding Clear Plastic at Conovawell

Clear injection molded products are popular and innovative, finding widespread applications in daily life, including automotive lighting, medical containers, lenses, packing boxes, and more. However, producing transparent parts through injection molding is significantly more challenging than manufacturing ordinary components.

Conovawell offers top-quality and cost-effective custom transparent plastic services. The production of transparent injection molds is just as crucial as the injection of transparent plastic parts. High-quality injection molds serve as the foundation for producing superior transparent injection molded parts.

Selecting the appropriate injection molding material is key for Clear Plastic Molding. Conovawell provides a range of transparent plastics, including PC, PMMA, PVC, PS, GPPS, etc., for your injection molded parts. The choice of material can significantly impact the functionality and performance of the final product. For instance, PC/PMMA are preferred for automotive lighting due to their excellent finishing and light transmittance.

Switching to transparent PP would result in a completely different performance. Before proceeding with processing, it’s essential to thoroughly consider both the material and transparent plastic injection molding technology. Transparent mold manufacturing technology holds great importance!

At Conovawell, you can obtain the certified transparent plastic you desire. Feel free to contact our engineering team at info@conovawell.com to select the right injection molding material for your project and receive a complimentary project evaluation.

Molding Clear Plastic
PC material property:

PC material property:

PMMA material property

PMMA material property:

PVC material property

PVC material property:

GPPS material property

GPPS material property:

High transparency, excellent gloss, easy to tint, amorphous plastic; exhibits dimensional stability (shrinkage around 0.4%), but has poor wear resistance, necessitating high-quality packaging to prevent scratching; sensitive to internal stress, brittle, lacks ductility, low impact strength, prone to cracking and forming sharp edges.

Mold materials for clear plastic molding are: NAK80,1.2344ESR, S136, S-STAR

Use the right steel for clear plastic part mold building. 

Mold materials

NAK80, P20HH, 1.2344ESR, S136, S136H, and S-STAR are among the most commonly used steels at Conovawell for cavities and cores. For small batches of transparent plastic molded parts, including covers, we typically recommend NAK 80, S-STAR, P20HH, or S136H as mold inserts. Even without heat treatment, they offer a mold life exceeding 300,000 cycles. For larger volume requirements, heat-treated S136 and 1.2344ESR offer advantages over NAK 80, S-STAR, P20HH, or S136H, providing enhanced durability.

NAK80 is hardened steel known for its high hardness, excellent dimensional stability, and superb machinability. It is capable of achieving a mirror polish up to #8000, making it suitable for medium-volume injection molding production. NAK80 is ideal for applications requiring simple structures and high performance and precision, such as automotive fascia boards, copiers, printers, and products made from corrosive plastics like POM and PVC that cannot tolerate flame retardants.

S-STAR (DAIDO): Known for its high purity, uniform structure, exceptional mirror polish, and corrosion resistance. With proper heat treatment, its hardness can reach HRC50-52, enhancing its polish, corrosion resistance, and wear resistance. Ideal for low to medium-volume injection molding of high mirror finish parts using materials like PC and PMMA. Also suitable for molding corrosive plastics such as PVC, PA, and POM, as well as plastics with fire retardants. Additionally, it is suitable for mechanical components in the food industry requiring corrosion resistance.

S136 (ASSAB): Recognized for its high purity, uniform structure, outstanding mirror polish, and corrosion resistance. Through appropriate heat treatment, it can achieve a hardness of HRC50-52, improving its polish, corrosion resistance, and wear resistance. Suitable for long-term high mirror finish injection molding of materials like PC and PMMA. Also suitable for molding corrosive plastics such as PVC, PA, and POM, as well as plastics with fire retardants. Additionally, it is suitable for mechanical components in the food industry requiring corrosion resistance.

Mold materials
1.2344ESR(Groditz) 

1.2344ESR(Groditz) 

P20HH(FINKL)

P20HH(FINKL)

For clear parts, mold gate design options include the use of hot nozzle needle valve gates for direct gating, as well as sub gates and side-feeding fan gates.

Hot sprue valve gate: This option offers good liquidity, allowing for a high degree of position flexibility and small gate size. It is suitable for thick or large clear products lacking appropriate gating areas. However, it may leave behind gating traces. Hot sprue valve gates are commonly chosen for car taillight products.

Sub gate: Sub gates can be designed on mold ribs in the cavity, core, side walls, and ejector. They offer flexibility in gate point selection and automatically separate from the part, leaving slight gate marks. However, they are prone to pulling out material powder and causing pits on the surface, as well as leaving drying marks in the gate point area.

Side-feeding fan gate: In this design, molten plastic flows evenly through the gate laterally, reducing stress and minimizing the chance of air entering the cavity, which helps prevent streaks and bubbles. However, the pouring gate does not automatically separate from the part, resulting in sprue marks on the part’s edges. Additional tools are required to flatten the pouring gate. Clear PC and PMMA typically use a side-feeding fan gate, which facilitates proportional injection and pressure holding. This gate design is also beneficial for improving airlines and flow marks. The runner should be an S-shaped and cold runner, avoiding sharp corners to reduce material flow speed, minimize jetting, and tone down plastics. When positioning the fan gate, the goal is to make the gate point as visible as possible based on the assembled drawing.v

Side-feeding fan gate
Demolding draft requirements for clear parts

Demolding draft requirements for clear parts: The demolding draft on the side face of clear parts should be larger compared to ordinary parts. This is because clear parts are more susceptible to scratching, and consideration must be given to wall thickness and material properties. Generally, materials with greater shrinkage require a larger demolding draft. PC, known for its high viscosity and greater holding force, is prone to scratching, hence suggesting a demolding draft of 1.5-2°. Similarly, PMMA and GPPS, which also have high viscosity and brittle tendencies, require a demolding draft of no less than 2°.

Mold ejectors for clear parts include scrap ejectors, slides, ejector bars, and blade ejectors. Scrap ejectors are suitable for products that cannot tolerate downward ejection. They assist in ejecting without leaving marks on the product surface, and the scrap ejector position requires flat cutting with tools.

Slides and ejector bars are used for products where appearance is not crucial, as they are not easily visible after assembly. However, there may be parting line marks between the push plate and the direct ejector. Blade ejectors are used when a product has many ribs, high holding force, and requires a mark-free surface. They can be designed at the bottom of ribs to evenly eject the product. Blade ejector holes should be processed smoothly to avoid burrs, as rough holes can lead to breakage and burning of the mold.

Mold ejectors

Thickness requirements for clear parts are crucial to avoid issues such as shrinkage marks, bubbles, and swash marks. Excessive thickness requires increased injection pressure and cooling time during molding, resulting in longer lead times. Ideally, clear parts should have a thickness of 1.5-3mm to mitigate these issues. Additionally, the length and width proportions should be balanced to prevent pressure weakening over injection distance, which can lead to internal stress and product deformation. Controlling and improving product transformation involves adjusting injection parameters, pressure holding time, and cooling. If parameters cannot be optimized, separate runners and staggered water running temperatures between the cavity and core can help address deformation issues.

Polishing requirements for clear parts typically involve using tools such as an oil strip, wool wheel, or grinding paper to remove surface imperfections and achieve a smooth finish. Hand polishing is commonly employed to ensure precise results. For surfaces requiring high performance, high-precision grinding and polishing methods are used, particularly in applications like automotive lighting. This process utilizes specialized grinders and polishing liquids with abrasive materials to achieve a surface roughness of Ra0.008 μm. International polish standards such as SPI A1, A2, and A3 are commonly followed. It’s essential to conduct rough polishing and fine polishing in separate locations and carefully clean the surface to remove any residues from the previous polishing stages.

After rough polishing with an oil strip and 1200# grinding paper, the parts should be transferred to a dust-free environment for further polishing to ensure no dust particles adhere to the mold surface. Precision polishing, with tolerances of 1 μm or less (including 1 μm), should be conducted in a clean polishing room. For even higher precision polishing, an absolutely clean environment is required to prevent damage to the polished surface from dust, smoke, dandruff, or saliva droplets. Care must be taken to avoid undercuts or rounded corners on the parting face during the polishing process, which can be achieved by marking the parting area or using polishing jigs to prevent cross-boundary polishing and rounding of corners. After polishing, dust protection measures should be taken. Before ceasing polishing operations, all abrasives and lubricants must be thoroughly removed to ensure a clean surface, followed by the application of transparent anti-rust oil to the part surface.

Summary of experience

  • The polishing machine requires a high rotation speed.
  • Utilize a small-diameter, soft polishing wheel, such as a pure silk polishing wheel.
  • Use polishing paste with the finest grind, such as reputable brands of green polishing paste (commonly known as green oil).
Injection molding
Injection molding

Injection molding requirements for clear parts are stringent due to the need for high light transmittance and top-quality plastic surfaces, which must be free from streaks, porosity, whitening, blurring, pits, dark spots, discoloration, or poor gloss. Throughout the injection molding process, special attention must be given to product design and strict requirements should be placed on raw materials, machinery, and molds. Additionally, clear plastics typically have high melting points and poor flowability, necessitating slight adjustments to technological parameters such as temperature, injection pressure, and speed to ensure that the mold is filled with plastic material without deformation or breakage due to internal stress. Raw materials must be free from impurities, and during the injection molding process, material addition should occur using a dry hopper while ensuring that the air is filtered and dehumidified to prevent material contamination. Injection molding screws should be cleaned with detergent before and after production to ensure the absence of impurities, and alternative cleaning methods using PE and PS materials can also be employed.

PC exhibits high viscosity, a high melting point, and poor flowability, requiring injection temperatures between 270°C and 320°C, with a relatively narrow adjustable temperature range compared to PMMA. Injection pressure has a limited impact on flowability due to its high viscosity, necessitating higher injection pressures. To prevent internal stress, it is advisable to minimize pressure-holding time. Although PC has low shrinkage and stable dimensions, it is prone to significant internal stress and breakage. Improving flowability by increasing temperature is preferred over increasing pressure while enhancing mold stability, structure, and finish can reduce breakage. Low injection speeds may result in wrinkles around the gate, underscoring the importance of minimizing resistance in the runner and gate for optimal mold stability.

PC
PMMA

PMMA exhibits high viscosity and poor flowability, requiring high temperatures and injection pressures, with injection temperature having a greater impact than injection pressure. Increased injection pressure can help reduce product shrinkage. With a melting point of around 160°C and a decomposition temperature exceeding 270°C, PMMA offers a wide material temperature range and favorable processing properties. Liquidity can be enhanced by adjusting the injection temperature. However, PMMA is prone to poor shock and abrasion resistance, making it susceptible to scratching and fragility. These shortcomings can be addressed by raising the mold temperature and optimizing the cooling process.

For inspection requirements on clear parts, all product features must be thoroughly checked for accuracy. Dimensions should be inspected according to the tolerances specified in the product drawing, with at least three product trials conducted for verification. Parts should be examined for deformation, surface joint lines, gas marks, black spots, grease, wave marks, sticking, bubbles, scratches, misalignment, cavities, and cut marks, among other defects.

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clear parts

Packing requirements on clear parts

  • Before packaging, it is essential to inspect the parts to ensure they meet quality standards. Additionally, remove any material chips, trimmings, or broken filaments from the parts and keep them clean by applying a protective film.
  • During packaging, care must be taken to avoid scratches between parts, especially for structural components that are susceptible to extrusion or breakage.
  • When placing parts in packaging boxes, ensure they are positioned securely and reliably. Parts can be arranged vertically, horizontally, diagonally, or back-to-back, depending on the specific product requirements. It is crucial to prevent products from tipping over or being inverted during transportation.
  • After packing, ensure that the products do not protrude beyond the surface of the box, and the surface of the box remains flat. When stacking boxes, ensure that the stress is on the boxes themselves rather than the products inside them.
  • Products of the same type should be packed in boxes of the same specifications to maintain consistency across each batch. The quantities of boxes should also be consistent between day and night shifts.
  • Only one remnant box is allowed for each batch after packing, ensuring that each batch has minimal leftover inventory.
  • Material identification cards must be clearly marked with the date, shift, product name, specifications, quantity, and serial numbers of the boxes. Each packing box should have only one material identification card affixed uniformly to the upper middle area, and multiple identification cards are not allowed. The boxes should be placed in designated areas with the identification cards facing outward for easy access.
clear parts
clear parts

Common defects and improve solutions on clear parts

Streaks occur during the filling and cooling process due to internal stress affecting resin flow and orientation, resulting in variations in refractive indices and visible marks. If left unaddressed, streaks can lead to cracks in the parts.

To mitigate streaking, it is important to clean grease pits, thoroughly dry the resin, control resin temperature, increase injection pressure, adjust counter-pressure, reduce screw speed, optimize gate size and position, appropriately increase venting size and position, and inspect for blockages in nozzles, runners, and gate locations.

Bubbles occur primarily due to inadequate venting of air within the resin. During mold cooling, vacuum bubbles may form due to short shots or excessively rapid cooling.

To address bubbles, ensure the raw material is thoroughly dried, reduce resin temperature, increase injection pressure, speed, and duration, increase the wall thickness of the gate point, adjust mold temperature appropriately, extend cooling time, enhance the venting system, and inspect for blockages in nozzles, runners, and gate locations.

Poor surface glossiness typically results from excessive roughness of the mold surface. Additionally, premature cooling prevents the resin from accurately replicating the mold surface, leading to surface imperfections such as sagging and poor gloss.

To improve surface glossiness, elevate resin temperature, increase injection speed and duration, design an appropriate gating system, raise mold temperature, and extend cooling time.

clear parts
clear parts

Whitening typically occurs due to airborne dust contaminating the raw material or excessive moisture content in the raw material.

To address whitening, remove impurities and dirt, regulate resin temperature, increase injection pressure, reduce production lead time, adjust counter-pressure, and elevate mold temperature.

Joint lines result from the convergence of two or more gate points, causing increased thickness in holes, recessed areas, and joint regions.

To mitigate joint lines, adjust product thickness, relocate joint line positions, incorporate sprue wells, reposition joint lines to less noticeable areas by controlling flow speed via needle valve sequencing or changing gate locations, adjust product design to eliminate joint lines, and increase resin temperature, mold temperature, injection speed, and pressure.