Introduction of transparent plastic injection molding process

Time：2021-08-31 15:48:38 / Popularity： / Source：

1. Characteristics of commonly used transparent raw materials

Due to high light transmittance of transparent plastics, strict requirements on the surface quality of plastic products must be required, there should be no defects such as markings, pores, whitening, haze, black spots, discoloration, and poor gloss. Therefore, in the entire injection molding process, we must pay great attention to, put forward strict or even special requirements for design of raw materials, equipment, molds, and even products.
Secondly, because transparent plastics mostly have high melting point and poor fluidity, in order to ensure surface quality of product, it is often necessary to make fine adjustments at higher temperature, injection pressure, injection speed and other process parameters, so that mold can be filled when plastic is injected, no internal stress will cause product deformation and cracking.

2. Problems that should be paid attention to in process

In order to reduce internal stress and surface quality defects, following issues should be paid attention to injection molding process.
• 1) Injection temperature should be at a higher injection humidity under premise that plastic resin does not decompose;
• 2) Injection pressure: generally higher to overcome defect of high melt viscosity, but too high pressure will cause internal stress, cause difficult demoulding and deformation;
• 3) Injection speed: In the case of satisfying mold filling, generally it should be low, and it is better to use slow-fast-slow multi-stage injection;
• 4) Holding pressure time and molding cycle: Under condition that product is filled with molds, does not produce dents or bubbles; it should be as short as possible to minimize residence time of molten material in barrel;
• 5) Screw speed and back pressure: on the premise of meeting plasticization quality, they should be as low as possible to prevent possibility of degrading;
• 6) Mold temperature: Cooling of product has a great impact on quality, so mold temperature must be able to accurately control process. If possible, mold temperature should be higher.
• 7) In order to prevent deterioration of surface quality, generally use as little release agent as possible during injection molding; when using recycled materials, it should not be more than 20%

3. Injection molding process of commonly used transparent raw materials

In addition to above common problems, transparent plastics also have some technological characteristics, which are described as follows:
1. PMMA has high viscosity and poor fluidity. Therefore, high material temperature and high injection pressure must be used for injection molding. Influence of injection temperature is greater than injection pressure, but increase in injection pressure will help improve shrinkage rate of product.
Injection temperature range is wide, melting temperature is 160℃, and decomposition temperature is 270℃, so material temperature adjustment range is wide and manufacturability is better. Therefore, to improve fluidity, start from injection temperature. Impact resistance is poor, wear resistance is not good, it is easy to scratch, and it is easy to be brittle. Therefore, it is necessary to increase mold temperature and improve condensation process to overcome these defects.
2. PC has high viscosity, high melting temperature and poor fluidity. It must be injected at a higher temperature (between 270-320T). Relatively speaking, temperature adjustment range of material is narrow and manufacturability is not as good as PMMA. Injection pressure has little effect on fluidity, but due to high viscosity, injection pressure is still higher. Correspondingly, in order to prevent internal stress, pressure holding time should be as short as possible.
Shrinkage rate is large, size is stable, but product has a large internal stress and is easy to crack. Therefore, it is better to increase temperature instead of pressure to improve fluidity, increase mold temperature, improve mold structure and post-processing to reduce possibility of cracking. When injection speed is low, defects such as ripples are likely to occur at gate, temperature of radiating nozzle must be controlled separately, temperature of mold must be high, resistance of runner and gate should be small.
3. PET molding temperature is high, material temperature adjustment range is narrow (260-300℃), but after melting, it has good fluidity, so manufacturability is poor, and it is often necessary to add anti-spreading devices in nozzle. Mechanical strength and performance are not high after injection, performance must be improved through stretching process and modification. Mold temperature is accurately controlled to prevent warpage. It is an important factor of deformation, so it is recommended to use hot runner molds. Mold temperature is high, otherwise it will cause poor surface gloss and difficulty in demolding.

4. Defects and solutions of transparent plastic parts

• 1. Silver streak: Affected by anisotropy of internal stress during mold filling and condensation process, stress generated in vertical direction causes resin to undergo flow upward orientation, non-flow orientation produces a different refractive index to produce a flashing silk pattern. After its expansion, it may cause cracks in product. In addition to paying attention to injection molding process and mold, it is best to make product annealed. For example, PC material can be heated to above 160℃ for 3-5 minutes, then cooled naturally.
• 2. Bubbles: Because water vapor and other gases in resin cannot be discharged, (during mold condensation process) or due to insufficient mold filling condensation surface condenses too quickly to form a "vacuum bubble".
• 3. Poor surface gloss: Main reason is large roughness of mold. On the other hand, premature condensation prevents resin from copying state of mold surface. All these make surface produce small unevenness and make product lose its gloss.
• 4. Shock marks: refers to dense ripples formed from straight gate as center. Reason is that melt viscosity is too large, front end material has condensed in cavity, then material breaks through condensing surface, causing surface to appear shock lines .
• 5. Whitening and haze: It is mainly caused by dust falling into raw materials in air or raw materials have too much water content.
• 6. White smoke and black spots: mainly formed by decomposition or deterioration of resin in barrel due to partial overheating of plastic in barrel.

Gud Mould Industry Co., Ltd