Transparent material characteristics and injection molding process
Because plastic has a light weight, toughness is good, easy to shape. Low cost, return to this in modern industry and daily use products, more and more plastic instead of glass, especially for optical devices and packaging industry, developing rapidly. However, due to demand better of their transparency, wearable pieces should be of high impact firmware is better to return to the right plastic components, injection molding process the whole process and equipment. Mold, they have to make a lot of work to ensure that these be used instead of glass, plastic (hereinafter referred to as transparent plastic), the surface of good quality, so as to achieve the use of requirements. Currently on the market for general use with transparent plastic polymethyl methacrylate (commonly known as Acrylic or perspex, codenamed pmma), polycarbonate (symbol pc). Polyethylene terephthalate glycol resin (code-named pet), transparent nylon. as (a styrene-acrylonitrile copolymer), polysulfone (codenamed psf) and so on, which we have contacted most is pmma. pc and the pet three kinds of plastic, because he edited sites are limited, Here's an example of these three plastic to discuss the features and transparent plastic injection molding process.
First, the performance of transparent plastic
Transparent plastic, there must be transparent, then to have a certain strength and wear resistance, can shock, heat-resistant parts is better to be excellent chemical resistance, water absorption should be small, the only way to be in use, to meet the transparency request a long time constant, are listed in the table below, compare pmma, pc, and pet performance.
(1) the variety, this is just taking the average, the actual data on different species are different.
(2) pet information (mechanical aspects) as amended by drawing upon data.
2, transparent, plastic injection molding process should pay attention to common problems
Transparent plastic due to higher light transmission rate, which inevitably calls demanding surface quality of plastic products, can not have any markings, pores, whiten. Fog halo, black spots, color, luster is not so good a result, the raw material throughout the injection molding processes and equipment. Mold, and even product design, we must pay great attention to a strict and even special requirements. Second, because of transparent plastic, mostly high melting point, poor mobility, so as to ensure the product surface quality, often in the machine high-temperature, injection pressure, injection speed and other parameters to make minor adjustments to make when both full of plastic injection mold, not will produce internal stress caused by deformation and cracking products.
Here's preparations for its raw materials. Requirements for equipment and tooling, injection molding processes and products, material handling aspects to talk about precautions.
(A) the preparation and drying of raw materials due to any point in the plastic contains impurities, may affect the product transparency, and storage, and transportation. Feeding process, must pay attention to sealing to ensure a clean raw material. In particular, raw materials contain moisture, heat, might cause deterioration of raw materials, they must be dry, and the injection, the feeding must be used dry hopper. Would also like to note that the drying process, the type of air should preferably be through a filter, dehumidification, in order to ensure no contamination of raw materials.
(B) the barrel, screw and its annexes clean
In order to prevent contamination of raw materials and in the screw and accessories depression there is old stuff, or impurities, in particular the existence of thermal stability of the resin, so before use, stopping applications after they screw cleaning agent to clean the parts, so that it can not stick with impurities, when there is no screw when the cleaning agent can be used pe, ps and other resin cleaning screw. When the temporary shutdown, in order to prevent the raw materials at high temperatures to stay a long time, causing lower solution should be drying machines and barrel temperature decreases, such as pc, pmma such as barrel temperature should be reduced to below 160 ℃. (Hopper temperature should be lowered to 100 ℃ for the pc or less)
(C) It should be noted on the mold design issues (including product design)
To prevent back flow of poor or uneven cooling caused by plastic molding bad, resulting in surface defects and deterioration in general in the mold design, should pay attention to the following points.
a) wall thickness should be uniform, stripping slope be large enough;
b) the transition part should be gradually. Smooth transition, to prevent any sharp corners. Produce sharp edges, especially the pc product must not have gaps;
c) gate. Flow channel as large, chunky, and the condensation process of contraction set in accordance with gate location, if necessary, add cold material should be well; d) die surface should be smooth, roughness low (preferably less than 0.8);
e) Vent. Trough must be sufficient, as well as air and melt when the exhaust gas;
f) In addition to pet, the wall thickness should not too thin, generally not less than lmm.
(D) injection molding process should pay attention to the issues (including injection molding machine requirements)
In order to reduce internal stress and surface quality defects in the injection molding process should pay attention to the following aspects.
a) should use special screws, with a separate temperature-controlled radio Tsui injection molding machine;
b) injection of the temperature in the plastic resin is not decomposed under the premise of injecting suitably high humidity;
c) Injection pressure: a general high melt viscosity in order to overcome the shortcomings, but the pressure is too high will produce stripping difficulties caused by internal stress and deformation;
d) Injection speed: In the case of filling to meet in general should low, it is best to adopt a slow-slow a fast multi-level injection;
e) holding pressure time and molding cycle: to meet the product filling and do not produce depression, the case of air bubbles; should as short as possible to minimize the melt residence time in the barrel;
f) screw speed and backpressure: in meeting the quality of plastics under the premise should be as low as possible and prevent the possibility of lower solution;
g) Mold temperature: cooling products, good or bad, a great impact on quality, so mold temperature must be able to precisely control the process, if possible, die Wenyi higher good.
(V) other aspects of the
In order to prevent the upper surface due to deterioration in the quality, generally when you minimize the use of injection mold release agent; When we go back to the materials used shall not be greater than 20%.
For the addition to pet, the product should be carried out post-processing to eliminate stress, pmma should be 70-80t hot air circulation drying 4 hours; pc should be clean air, glycerin. Liquid paraffin and other heating 110-135 ℃, the time determined by product, the maximum need for more than 10 hours. The pet must be bi-oriented processes in order to get good mechanical properties.
3, transparent plastic injection molding process
(1) Transparent plastic Technological Characteristics: In addition to the above common problems, transparent plastic will also have a number of technology features, are set out below:
1, pmma process characteristics pm-ma viscosity, mobility somewhat less material must therefore be a high-temperature, high injection pressure injection molding because as the influence of temperature is greater than one injection of injection pressure, injection pressure increase will help to improve product shrinkage. Injection of a wide temperature range, melting temperature of 160 ℃, while the decomposition temperature reaches 270 ℃, so a wide range of material temperature regulation, technology is better. Therefore, to improve the mobility, can be started from injection temperature. The impact of poor, wear well, easy to draw flowers, easy to embrittlement, it should raise the mold temperature, to improve the condensation process, to overcome these deficiencies.
2, pc of Technological Characteristics
pc viscosity, high temperature melting materials, poor mobility, return to this must be based on high temperature injection molding (270-320t between), the relative temperature adjustment range is expected to narrow, craft-oriented than pmma. Injection pressure less impact on the flow, but viscosity, still higher injection pressure, the corresponding internal stress in order to prevent production, holding pressure time should be as short as possible. Shrinkage rate is high, dimensionally stable, but the product inside the stress of big, easy to crack, it is appropriate to raise the temperature rather than pressure to improve mobility, and from increased die temperature, improved mold structure and post-processing to reduce the possibility of cracking. When the injection speed is low, the gate at prone to defects such as corrugation, radiation alone control the nozzle temperature, mold temperature must be high flow channel, the gate resistance is smaller.
3, pet of the Technological Characteristics
pet molding temperatures are high, and the material temperature regulation scope is narrow (260-300 ℃), but the melt, the flow is good, and therefore poor technique, and often shot Tsui extension is necessary to add anti-flow devices. After injection, mechanical strength and performance is not high, you must be modified by stretching processes and improve performance. Precise control of mold temperature is to prevent warping. Deformation of the important elements back to return to the recommended hot runner molds. Mold Temperature official high, otherwise it would cause the surface gloss difference, and stripping back hard.
4, transparent plastic parts of the defects and solutions
Due to space relations, here is the discussion of transparency affect product defects, other defects please refer to the company's product specifications or other information?. Its shortcomings about the following:
(A) Silver grain: from filling and condensation process, the internal stress anisotropy effects of the stress generated by the vertical direction, so that the flow of resin occurs orientation, while the non-flow direction resulting in different refractive index born flash wire pattern, when the After its expansion, it may make the product cracks. In addition to injection molding process and mold the attention of foreign (preferably product for annealing. If pc material can be heated to 160 ℃ for more than 3-5 minutes to keep, and then natural cooling can be.
(B) the bubble: Master in the resin, the water gas and other gas emissions do not go out, (in the mold condensation process) or because of inadequate filling, condensation, condensation to form the surface of another fast "vacuum bubble."
(C) Surface gloss difference: the main mold roughness on the large, on the other hand condensation prematurely, so that resin can not be copied mold surface state, all of which make the surface of the cattle-tiny uneven, leaving products tarnish.
(D) earthquake patterns: refers to the center from the straight gate the formation of dense wave, the reason for the melt viscosity is too large, front-end material has been condensed in the cavity, and later they broke through the condensation surface material, leaving the surface of shock pattern.
(V) whiten. Fog halo: mainly due to the dust in the air, in or into raw materials caused by too much moisture.
(Vi) white smoke. Black spots: mainly due to plastic in the machine barrel, due to local overheating Ershi resin produced barrel formed by decomposition or deterioration. In order to clearly overcome the lack of the intake measures.
This information comes from 【Wishsino Plastic Mold Making Company Website】
