Процесс литья под давлением для оптических материалов

Процесс литья под давлением для оптических материалов

Because the plastic with light weight, good flexibility, easy. Low cost, in modern industry and household products, therefore, increasingly using plastic instead of glass, special used in optical instruments and packaging industries, the development is particularly rapid. But because of its better transparency requirements, high wear resistance, toughness, and composition of plastics, injection molding process technology, equipment and supplies. Die, have to do a lot of work, to ensure that they are used in place of glass plastic (hereinafter referred to as transparent plastic), good surface quality, which meet the requirements of use.

Currently on the market for general use of the transparent plastic polymethyl methacrylate (commonly known as acrylic or plexiglass, codenamed PMMA), поликарбонат (code PC). Polyethylene glycol terephthalate resin (codenamed PET), transparent nylon. AS (acrylonitrile-styrene copolymer), polysulfone (code PSF), which most of us are PMMA. PC and PET are three plastic, due to the limited length, the following is a case study of the three plastic, discuss the characteristics of transparent plastic and injection molding processes.

Properties of transparent plastics

Clear plastic must first have a high degree of transparency, secondly, to have a certain strength and wear resistance, impact resistance, heat is better, excellent chemical resistance, water absorption less, only in this way can be used, can meet the requirements of transparency and long-term change, table l listed below, compare performance of PMMA, PC and PET.

Note: (1) because of a wide variety, it's just average, different varieties of real data is different.

(2) PET data (mechanical) for drawing data.

From the data in table 1 show that PC is an ideal choice, but its raw material prices and the injection process is more difficult, it is still dominated by using PMMA, (General requirements for the products), PPT due to pass through the stretch to get a good mechanical performance, so much packaging, containers are used.

Second, transparent plastic injection molding process, we should pay attention to common issues

Transparent plastic due to the high rate of transmission is necessary for plastic products surface of strict quality requirements, without any markings, blowhole, white. Fog Halo defects such as poor, black, color, gloss, and throughout the process of raw materials, injection molding equipment. Molds, and even product design, strict or special request to pay great attention to and. Secondly due to the transparent plastic for high melting temperature, poor mobility, so in order to ensure the quality of products, often in the barrel temperature, injection pressure, velocity and other parameters make small adjustments, make the injection material at both full mode without internal stress caused deformation and cracking.

Following its raw material preparation. Requirements for equipment and tooling, injection molding processes and products in terms of materials handling, to talk about matters needing attention.

(A) preparation and drying of raw material because the plastic contains any impurities, may affect the transparency of the products, and storage and transport. In the feeding process, attention must be sealed to ensure clean raw materials. Especially raw materials containing water, heating will cause the deterioration of raw materials, so make sure you dry, and at the time of injection, feed hopper must be used. Note also that the drying process, enter the best filtering and dehumidifying the air in order to ensure that does not contaminate raw materials. The drying process as shown in table 2,

(B) barrel, screw and accessories clean

In order to prevent the contamination of raw materials and the screw hollows and accessories save old materials or impurities, especially hot stability of resin, before use, stop screw after cleaning and washing the parts, making it not having impurity, when there is no screw cleaning agent, PE, PS resin cleaning screw available. When a temporary outage in order to prevent the material residence time at high temperature long cause solution reduced, dryer and barrel temperatures should be lower, such as PC, ПММА, barrel temperature must be below 160 deg. (Hopper PC temperature should drop to below 100 c)

(C) in die and mould problems in design (product design)

In order to prevent back flow is impeded, the plastic molding adverse or uneven cooling, surface defects and deterioration, mold design, the following points should be noted.

A) wall thickness shall be as uniform as possible, ejection gradient is large enough;

B) transition should gradually. Smooth transition to prevent sharp edges. Sharp edges, especially PC product must not have gaps;

C) gate. Channel large, thick and short as possible, and set the gate according to the contraction and condensation process and, if necessary, should be added to cold slug well;

D) mold surfaces should be smooth, low roughness (preferably less than 0.8);

E) vent. Tanks should be sufficient to exhaust gases in the air and melt in a timely manner;

F) among PET things, wall thick not too thin, not being less than the LMM.

(D) the problems should be noticed in injection molding process (including injection molding machine required)

In order to reduce internal stresses and surface quality defects in injection molding process should note the following problems.

A) special screw should be used, with separate temperature control of injection molding machine nozzle;

B) injection in a plastic resin decomposition temperature of the premise, to use the high temperature;

C) injection pressure: generally high melt viscosity in order to overcome the shortcomings, but the pressure is too high would cause internal stress release

Something deformation;

D) injection speed: meet the filling of cases generally should be low, preferably using slow-fast-slow multi-stage injection;

E) holding time and cycle time: meet the product filling, no dents, bubbles circumstances; should be as short as possible, melt in the barrel to minimize the residence time;

F) screw speed and back pressure: the premise of meeting the plastification quality should be as low as possible, prevent