How to do injection molding

- Sep 26, 2018-

                              How to do injection molding

   Injection molding is a wide range of knowledge, technical and practical strong industry.Injection molding process need to use plastic raw materials, pigments, shuikou material, mold, injection molding machine and peripheral equipment, fixture, spray, all kinds of auxiliary materials and packaging materials, etc., the injection molding workshop management to bring very great workload and certain difficulty, compared with other industries or departments, higher requirement for injection molding workshop management personnel at all levels.

   Injection molding production needs 24 hours of continuous operation, generally for two - sided or three - shift work mode, injection molding workshop in the work room, the division of labor is complex, different job requirements for different personnel skills.To make injection molding workshop production run smoothly, need to each link and each post personnel, materials, equipment, tools and so on carries on the management, mainly includes: raw material room, broken material room, mixing room, production site, after processing, tool room, semi-finished products, office and other regional operation and coordination management work.

   Type of injection molding

1. Rubber injection molding: rubber injection molding is a production method of injecting rubber materials directly into the mold from the cylinder for vulcanization.The advantages of rubber injection molding are: although it is intermittent operation, the molding cycle is short, the production efficiency is high, and the preparation process of blank material is cancelled, the labor intensity is small, and the product quality is excellent.

2. Plastic injection molding: plastic injection molding is a method for plastic products. Molten plastic is injected into the mold of plastic products by pressure, and various plastic parts are formed through cooling.There is a special injection molding machine for mechanical injection molding.The most commonly used plastics today are polystyrene.The resulting shape is often the final product, and no additional processing is required before installation or use as the final product.Many details, such as bumps, ridges, and threads, can be moulded in one step.

   Factors affecting the

   Due to the volume change of crystallization in the process of injection molding of plastic varieties, high pressure during injection molding, small difference in melt viscosity, small interlaminar shear stress, large elastic rebound after demoulding, injection molding shrinkage can also be appropriately reduced, high temperature, large shrinkage, but small directivity during injection molding.

   When pressure is increased during injection molding, the molten material is subjected to shear action and liquidity is also increased, especially PE and POM are more sensitive. Therefore, pressure of injection molding is adjusted timely during molding to control liquidity.During molding, material temperature, mold temperature, pressure and speed of injection molding can also be controlled to properly adjust filling conditions to meet the needs of molding [1].

   Injection molding process

   Injection molding is a processing method used in mass production of some complex shaped parts.Specifically, the material melted by heat is injected into the mold cavity by high pressure, which is then cooled and solidified to form the product.

   Temperature control

1. Barrel temperature: the temperature to be controlled in the injection molding process includes barrel temperature, nozzle temperature and mold temperature.The first two temperatures mainly affect the plasticization and flow of plastics, while the second one mainly affects the flow and cooling of plastics.Each kind of plastic flow with different temperature, the same kind of plastic, due to different sources or brand, the flow temperature and decomposition temperature is little different, this is due to the different average molecular weight and molecular weight distribution, plastic in different type of injection machine of plasticizing process is also different, so choose the cylinder temperature is not the same.

2. Nozzle temperature: the temperature of the nozzle is usually slightly lower than the maximum temperature of the cylinder, which is to prevent possible "salivation phenomenon" of molten material in the straight-through nozzle.The temperature of the nozzle must not be too low, otherwise the nozzle may be blocked by the premature coagulation of the molten material, or the product performance may be affected by the injection of the early coagulation into the mold cavity.

3. Mold temperature: the mold temperature has a great influence on the internal performance and apparent quality of products.The mold temperature depends on whether the plastic crystallization exists, the size and structure of the products, the performance requirements, and other process conditions (melting temperature, injection speed, injection pressure, molding cycle, etc.).

   Pressure control

   Pressure in injection molding process includes plasticizing pressure and injection pressure, and directly affects plastics plasticizing and product quality.

1. Plasticizing pressure :(back pressure) when the screw injection machine is used, the pressure on the top of the screw fuse when the screw is rotating backward is called plasticizing pressure, also known as back pressure.The pressure can be adjusted by the overflow valve in the hydraulic system.In injection, the size of the plasticizing pressure is depending upon the design of the screw, the product quality requirements, and the type of plastic and needs to change, if the situation and the screw rotation speed is constant, increase plasticizing pressure will strengthen shearing action, which will raise the temperature of the melt, but will reduce the plasticizing efficiency, increase the current and leakage flow, increase the driving power.

   In addition, increasing the plasticizing pressure can often make the melt temperature uniform, the color mixture uniform and discharge the gas in the melt.In general operation, the decision of plasticizing pressure should be under the premise that assures product quality is good lower better, its specific numerical value is different with the breed of the plastic that USES, but often little exceeds 20 kilograms/square centimeter.

2. Injection pressure: in the current production, the injection pressure of almost all injection machines is based on the pressure applied to the plastics by the plunger or screw top (which is converted from the oil way pressure).Injection pressure in the injection molding role is to overcome the plastic flow from the barrel to the cavity flow resistance, to give the rate of melt filling mold and the melting compaction.

   Iii. Molding cycle

   The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle.It actually includes the following parts: molding cycle: molding cycle directly affects labor productivity and equipment utilization.Therefore, in the production process, on the premise of quality assurance, should shorten as far as possible in the molding cycle of each relevant time.

   During the whole molding cycle, injection time and cooling time are the most important.The filling time in injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3-5 seconds.

   The pressure holding time in the injection time is the pressure time of plastics in the mold cavity, which takes up a large proportion in the entire injection time, generally about 20-120 seconds (extra thick parts can be as high as 5-10 minutes).Before the melting at the gate is frozen, the amount of holding time has an impact on the accuracy of the product size, if in the future, there is no effect.It is known that it depends on material temperature, mold temperature and the size of main channel and gate.If the size and process conditions of the main runner and gate are normal, the pressure value with the minimum fluctuation range of product shrinkage is usually taken as the criterion.

   The cooling time is mainly determined by the thickness of the product, the thermal and crystalline properties of the plastics, and the mold temperature.The end point of the cooling time shall be based on the principle that the product shall not change when it is unmoulded. The cooling time is generally about 30~120 seconds, and the cooling time is too long to be necessary, which not only reduces the production efficiency, but also makes it difficult for complex parts to be unmoulded.

Other times in the molding cycle are related to whether the production process is continuous and automatic and the degree of continuous and automatic.

   Injection molding industry development

(I) development and innovation is a strategy for the overall, long-term and fundamental development of the injection molding industry.We should not only focus on planting trees and cultivating forests, we should not only focus on the present and future, but also on the last and fundamental aspects. We should not only focus on experience, knowledge and instructions, but also need no strategy. Otherwise, we will lead the injection molding industry to narrow roads, canyons, detours and dead ends.

(2) don't be too cautious.To do well in injection molding industry depends on various techniques, such as technology, business, management and public relations.Also should rely on a variety of strategy, such as technology development strategy, talent development strategy, marketing strategy, financing strategy.Art is small slightly, slightly is big art, art and slightly the same is to pay attention to plan, different point is the scope that plan has size, the time that plan has length.Heavy art light is heavy small light big, heavy short light long, heavy technology is more general.Here, the biggest technique is development and innovation, and development and innovation is the injection molding industry development strategy.

   In order to realize the development and innovation of the injection molding industry, it is necessary to re-analyze the market demand highlights and trends, re-analyze the challenges and opportunities, re-analyze their advantages and disadvantages, and re-analyze the key links and steps.Why it is necessary to reanalyze these basic facts because many of the analyses of the past are outdated, or because the analyses of the time are inherently flawed and wrong.

   To achieve the development and innovation of injection molding industry, new business contents, new business means, new personnel framework, new management system and new business strategies should be formulated.Strategy is open-ended.In terms of essential characteristics, the development strategy formulated by each enterprise should be consistent;In terms of content, focus and structure, the development strategies of different enterprises have little in common.Recently, China injection molding net visited several plastic molding factories in Singapore and felt a lot. They have already distanced themselves from us. They have gone from simple injection molding processing to product design and development to a one-stop production mode.

   China's injection molding industry to the direction of OEM and ODM development.That's the way to go.

   Factors influencing injection molding process

   The influencing factors of thermoplastic plastic molding shrinkage is as follows: plastic varieties of thermoplastic plastic molding process because there are crystal taxiing on volume change, internal stress is strong, frozen inside the plastic residual stress is large, molecular orientation factors such as the gender is strong, so compared with thermosetting plastic shrinkage is bigger, wide range of shrinkage and clear direction, also after molding shrinkage, shrinkage rate after annealing or wet processing generally are larger than the thermosetting plastic.

   The molten material contacts the surface of the mold cavity and immediately cools to form a low density solid shell.Due to the poor thermal conductivity of plastics, the inner layer of plastic parts is slowly cooled to form a dense solid layer with large contraction.Therefore, the wall thickness, slow cooling, high density layer thickness of shrinkage.In addition, the layout and quantity of embedded parts and embedded parts have a direct impact on the direction of material flow, density distribution and the size of contraction resistance, etc., so the characteristics of plastic parts have a significant impact on the size and directivity of contraction.

   The shape, size and distribution of feed inlet directly affect the direction of feed flow, density distribution, compression and shrinkage and forming time.Large cross sections (especially thick ones) of direct feed inlet and feed inlet are smaller and more directional.Close to the feed inlet or parallel to the feed flow direction, the contraction is large.

   The mold under the forming condition has high temperature, slow cooling of the molten material, high density and large contraction, especially for the crystallization material because of high crystallinity and large volume change, so the contraction is greater.The mold temperature distribution is also related to the inner and outer cooling and density uniformity of the plastic parts.In addition, maintaining pressure and time also have a greater impact on contraction.

   High injection pressure, small viscosity difference of molten material, small interlaminar shear stress, large elastic rebound after demoulding, so the contraction can also be reduced moderately, high temperature, large contraction, but small directivity.Therefore, it is also possible to adjust the mold temperature, pressure, injection speed and cooling time during the molding process.The mold is designed according to the shrinkage range of various plastics, the thickness and shape of the plastic part, the form size and distribution of the feed inlet, the shrinkage rate of each part of the plastic part is determined according to experience, and then the size of the mold cavity is calculated.

   For high-precision plastic parts and difficult to master shrinkage, the following methods are generally used to design the mold:

   The outer diameter of plastic parts is smaller than that of plastic parts.

   To determine the form, size and molding conditions of the casting system.

   The size change of the plastic parts to be reprocessed is determined after reprocessing (measurement must be made 24 hours after demoulding.

   Adjust the mold according to the actual shrinkage.

   Die retest and change process conditions to slightly correct shrinkage value to meet the requirements.

   The fluidity of thermoplastic plastics can be generally analyzed from a series of indexes such as molecular weight size, melting index, archimedean spiral flow length, apparent viscosity and flow ratio (process length/component wall thickness).Low molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long screw flow length, low apparent viscosity, high flow ratio, good liquidity. For plastics with the same product name, the specification must be checked to determine whether its liquidity is suitable for injection molding.


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