Precautions in Polystyrene Molding

Polystyrene (PS) is widely used in various industries due to its excellent transparency, lightweight, and processability. However, to obtain high-quality transparent PS products, strict attention must be paid to every link of the molding process. This blog will focus on the key precautions in PS molding, helping you avoid common problems and improve production efficiency.

Cleaning Requirements

Transparency is one of the core advantages of PS products, and any contamination during the molding process will directly affect the transparency and appearance quality of the final products. Therefore, the following cleaning requirements must be strictly followed:

(i) Cleanliness of Resin Pellets

Resin pellets must be clean and free of impurities. Due to the electrostatic effect, pellets are prone to absorbing dust in the air, which will be exposed in the products due to light refraction, affecting transparency. Before opening the packaging, it is necessary to clean the outer surface of the packaging, the surrounding environment, and the container used to hold the pellets. Operators should also pay attention to personal cleanliness to avoid introducing contaminants.

For opened resin packages, it is necessary to inspect the pellets regularly; the remaining pellets should be carefully sealed to facilitate reuse and prevent contamination.

(ii) Cleanliness of Equipment

For PS molding, no other resins or impurities are allowed in the hopper, barrel, or nozzle, as they will seriously affect the transparency and mechanical properties of PS products. If conditions permit, special equipment, barrels, screws, and nozzles should be used for PS processing to avoid cross-contamination.

If special equipment is not available, the equipment must be thoroughly cleaned before use. The cleaning method can be disassembly cleaning or resin flushing. Recycled PS resin can be used for flushing, but it is necessary to ensure thorough cleaning; after flushing out all other materials in the barrel, flush it several times with new PS resin.

In addition, the hopper gate is often overlooked, so it is recommended to disassemble and clean this part thoroughly to avoid residual impurities.

(iii) Cleanliness of Molds

The presence of oils (such as cutting oil, lubricating oil, and anti-rust oil) will not only reduce the transparency of PS products but also increase the risk of product cracking. Before molding, all parts of the mold must be carefully wiped clean, including the mold cavity, runner, root of the molding hole, corners, parting surface, ejector pins, and ejector plate.

Ensure that there is no oil stain, dust, or other contaminants on the mold surface to ensure the smoothness and transparency of the product surface.

Use of Recycled Resin

Uncontaminated recycled PS resin can be mixed with new resin for use. Generally, the dosage of recycled resin should be controlled at 15% to 30% of the new resin. For products with low quality requirements, the proportion can be appropriately increased to about 50%.

However, it should be noted that repeated recycling will lead to discoloration and degradation of PS resin, which will affect the product quality. Therefore, recycled resin should not be used for products with high transparency requirements.

Release Agent

The use of release agent is generally not recommended in PS molding. Because release agent will reduce the light transmittance of PS products and increase internal stress, which may lead to product cracking. If it is really necessary to use a release agent, the dosage must be minimized to avoid affecting the product quality.

Post-Treatment of Products

Post-treatment of PS products can be carried out in air or water. The process parameters of post-treatment vary according to the wall thickness of the products:

• When the product wall thickness is ≤ 6mm, the post-treatment temperature is 60～70℃, and the treatment time is 30～60 minutes;

• When the product wall thickness is ＞ 6mm, the post-treatment temperature is 70～78℃, and the treatment time is 120～300 minutes.

The internal stress of PS products can be detected by polarized light transmission or polar solvent immersion test (using kerosene as the medium). Timely detection and elimination of internal stress can effectively reduce product deformation and cracking.