Injection molding is a cycle process, each cycle mainly includes: quantitative feeding - melting and plasticizing - pressure injection - mold filling and cooling - mold opening and parts removal. And then take out the plastic parts and then close the mold for the next cycle.
The resin molecular chains in thermoplastics are linear or branched. There is no chemical bond between the molecular chains. When heated, they soften and flow. The process of cooling and hardening is physical change. Therefore, injection molding is suitable for almost all thermoplastics. Nylon (nylon), polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polystyrene (PS), Polyoxymethylene (POM), polycarbonate (PC), polyurethane (PU), polytetrafluoroethylene (PTFE) are all thermoplastic injection materials, and they are common plastics for injection molding.
2. Why are most thermosetting plastics not suitable for injection molding
When thermosetting plastics are heated for the first time, they can soften and flow. When heated to a certain temperature, they will produce chemical reaction and harden by crosslinking. This change is irreversible. After that, when they are heated again, they can no longer soften and flow. The resin of thermosetting plastics is linear or branched before curing. After curing, chemical bonds are formed between the molecular chains to form a three-dimensional network structure, which can no longer be fused and dissolved in solvent. Phenolic, formaldehyde, melamine formaldehyde, epoxy, unsaturated polyester, silicone and other plastics are thermosetting plastics, so this kind of plastics cannot be the common plastics for injection molding.
Founded in 1978, Zhongde is a professional manufacturer focusing on OEM & ODM customized services. Based on 40 years technology deposition, we have developed a one-stop solution from mold designing and rapid prototyping to mass production. Our manufacturing capabilities include plastic injection molding service, compression molding service, CNC machining service, metal stamping service, 3D printing service, etc.
What are the common design rules in plastic design?
The following is a description of the key points of the plastic product structure design
The structural design of plastic product development should follow the following principles:
a. The structure design should be reasonable: the assembly gap is reasonable, and all plug-in structures should be reserved for gaps; ensure sufficient strength and rigidity (safety test), and properly design a reasonable safety factor.
b. The structural design of plastic parts should comprehensively consider the manufacturability of the mold, and simplify the manufacturing of the mold as much as possible.
c. The plasticity of the structure of plastic parts should be considered, that is, the injection molding production efficiency of the custom plastic injection molding companies should be high, and the rejection rate of injection molding should be minimized.
d. Consider facilitating assembly production (especially not conflicting with assembly).
e. The structure of stamping plastic parts adopts standard and mature structure as much as possible, so-called modular design.
g. taking into account the cost
Points of Attention to Common Problems in the Design and Structure of Plastic Products
1. About the demoulding angle of plastic parts:
Generally speaking, for any side of the molded product, a certain amount of demolding slope is required to allow the product to be ejected from the mold smoothly. The size of the draft angle is generally between 0.5 degrees and 1 degree. You will get more details in advantages and disadvantages of compression moulding. Pay attention to the following points when selecting the draft angle:
2. About the wall thickness determination and wall thickness treatment of plastic parts:
It is very important to determine the wall thickness of plastic parts reasonably. The wall thickness of a plastic part is first determined by the use requirements of the plastic part: including the strength of the part, quality cost, electrical performance, dimensional stability, and assembly requirements. Generally, the wall thickness has an empirical value, which can be determined by referring to similar
3. Regarding the strengthening of plastic parts:
In order to ensure the strength and rigidity of the plastic part without causing the wall thickness of the plastic part to be too thick, reinforcement ribs can be set at the appropriate part of the plastic part. Reinforcing ribs can also avoid the deformation of plastic parts. In some cases, reinforcing ribs can also improve the flow of plastic during the molding process of plastic parts. Making the surface arched and waved is also one of the ways to increase strength and rigidity.
4. About the rounded corner design of plastic parts:
In the design process of plastic parts, in order to avoid stress concentration, increase the strength of plastic parts, improve the flow of plastic parts and facilitate demoulding, excessive arcs should be adopted on all sides or internal connections of plastic parts. In addition, the rounded corners on the plastic parts are also indispensable for mold manufacturing and machining, and to improve mold strength. When there are no special requirements on the structure of the plastic part, each corner of the plastic part should have a rounded corner with a radius of not less than 0.5~1mm. Where possible, the fillet should be as large as possible.
For the corners of the inner and outer surfaces, the outer fillet should be the inner fillet plus the wall thickness to reduce internal stress and ensure uniform wall thickness.
Zhongde is an industry leading custom injection molding manufacturer in China and has been widely recognized by many Fortune 500 companies. Based on rich industry experience, complete production line, professional technical team and thoughtful service team, we promise to provide better quality products with shorter lead time. Custom machined plastic/metal parts, custom rubber prototype and injection molded rubber parts are avaliable.
Compression molding is the operation of first putting powdered, granular, or fibrous plastic into the mold cavity at the molding temperature, and then closing the mold to press to shape and solidify it. Compression moulding method can be used for thermosetting plastics, thermoplastics, and rubber materials.
Advantages of injection moulding over compression moulding
(1) In the custom plastic compression moulding process, the loss of raw materials is small and will not cause too much loss (usually 2% to 5% of product quality).
(2) The internal stress of the product is very low, and the warpage deformation is also small, and the mechanical properties are relatively stable.
(3) The wear of the mold cavity is small, and the maintenance cost of the mold is low.
(4) The cost of molding equipment is low, and its mold structure is simpler, and its manufacturing cost is usually lower than that of injection molds or transfer molding molds.
(5) Larger flat products made by compression molding can be formed. The size of the product that can be molded by molding is only determined by the clamping force of the existing molding machine and the size of the template.
(6) The shrinkage of the product is small and the repeatability is good.
(7) A mold with a large number of cavities can be placed on a given template, with high productivity.
(8) Comap with high precision plastic injection molding, it can be adapted to automatic feeding and automatic removal of products.
(9) High production efficiency, easy to realize specialized and automated production.
(10) The product has high dimensional accuracy and good repeatability.
(11) The surface is smooth and clean without secondary modification.
(12) Products with complex structures can be formed at one time.
(13) Mass production and relatively low price.
What are the disadvantages of compression moulding
(1) The molding cycle in the entire manufacturing process is long, the efficiency is low, and the staff has a large physical consumption.
(2) It is not suitable for molding complex products with dents, side slopes or small holes.
(3) In the production process, it is difficult to completely fill the mold, and there are certain technical requirements.
(4) After the curing stage is over, different products have different stiffness, which affects product performance.
(5) For products with high dimensional accuracy requirements (especially for multi-cavity molds), the process is somewhat short.
(6) The flash of the final product is thick, and the workload of removing the flash is large.
(7) The disadvantage of compression molding lies in the complexity of mold manufacturing, large investment, and the limitation of the press, which is most suitable for mass production of small and medium composite materials, such as rubber compression molding and compression molding plastic.
In the daily production of modified plastic injection molding, some problems are likely to occur. The more common defects mainly include dark spots, gloss differences, and surface wrinkles (also known as orange peel). Generally speaking, these defects are often Occurs near the gate, so today we will analyze the causes of the defects from the mold and molding process.
Gloss difference
For injection molding plastic products, the difference in gloss on the surface of the textured product is most obvious. Even if the surface of the mold is very uniform, an irregular gloss may appear on the product. That is to say, the mold surface effect of some parts of the product is not well reproduced.
As the distance of the melt from the gate gradually increases, the injection pressure of the melt gradually decreases. If the distal end of the gate of the product cannot be filled, the pressure there is the lowest, so that the texture of the mold surface cannot be correctly copied to the surface of the product. Therefore, the area where the cavity pressure is the greatest (half of the fluid path from the gate) is the area where the gloss difference is the least.
To change this situation, you can increase the melt and mold temperature or increase the pressure, while increasing the holding time can also reduce the difference in gloss.
Good design of custom plastic parts can also reduce the chance of gloss differences. For example, drastic changes in the wall thickness of the product can cause irregular flow of the melt, which makes it difficult for the surface texture of the mold to be copied to the surface of the product. Therefore, the design of uniform wall thickness can reduce the occurrence of this situation, and excessive wall thickness or excessive ribs will increase the chance of gloss difference. In addition, insufficient venting of the melt is also a cause of this defect.
Dark spots
Dark spots appear near the gate, like a dim halo. It is especially obvious when producing plastic products with high viscosity and low fluidity materials, such as PC, PMMA, or ABS. When the cooled surface layer resin is taken away by the center-flowing resin, such visible defects may appear on the surface of the product.
It is generally believed that such defects frequently occur in the filling and holding pressure stages. In fact, dark spots appear near the gate, usually at the beginning of the injection cycle. Experiments show that the occurrence of surface slip is actually due to the injection speed, more precisely the flow speed at the front end of the melt flow.
The dark spots around the gate and the dark spots that appear after the formation of a sharp corner are caused by the initial injection speed being too high, and the cooling surface is displaced by the internal fluid. Increasing the injection speed gradually and injecting it step by step can serve this defect.
Even if the injection speed of the melt is constant when it enters the mold, its flow speed will change. When entering the mold gate area, the melt flow rate is very high, but after entering the mold cavity, that is, the filling stage, the melt flow rate begins to decrease. This change in the velocity of the front end of the melt flow can cause defects on the surface of the product.
Reducing the injection speed is one way to solve this problem. In order to reduce the speed of the front end of the melt flow at the gate, the injection can be divided into several steps and the injection speed is gradually increased. The purpose is to obtain a uniform melt flow rate throughout the filling stage.
Low melt temperature is another cause of dark spots on plastic products. Increasing the barrel temperature and screw back pressure can reduce the probability of this phenomenon. In addition, the temperature of the mold is too low to cause surface defects, so increasing the mold temperature is another feasible way to overcome the surface defects of the product.
Mold design defects can also produce dark spots near the gate. Sharp corners at the gate can be avoided by changing the radius. Pay attention to the position and diameter of the gate during design to see if the gate design is appropriate.
Dark spots not only occur at the gate location but also often appear after sharp corners of custom plastic custom injection moulding products are formed. For example, the sharp corner surface of an article is generally very smooth, but behind it is very dull and rough. This is also caused by the excessively high flow rate and injection speed causing the cooling surface layer to be replaced by internal fluid and sliding.
It is again recommended to use step injection and gradually increase the injection speed. The best method is to allow the melt to only start to increase in velocity after it has flowed over the sharp edge.
In the area far from the gate, sharp changes in the angle of the product can also cause this defect. Therefore, when designing products, it is necessary to use smoother rounded transitions in those areas.
Orange peel
"Orange peel" or surface wrinkling defects generally occur at the end of the runner when a thick-walled product is molded with a high-viscosity material. During the injection process, if the melt flow rate is too low, the surface of the plastic product will quickly solidify. As the flow resistance increases, the front flow of the melt will become uneven, so that the first solidified outer layer material cannot fully contact the cavity wall, resulting in wrinkles.
These creases become indelible defects after curing and holding pressure. For this defect, the solution is to increase the melt temperature and increase the injection speed.
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Nowadays, the trend of processing technology is developing with high and new technology. These technologies include micro injection molding, high filling compound injection, water assisted injection molding, mixing and using various special injection molding processes, foam injection molding, mold technology, simulation technology and so on.
The injection manufacturing process mainly includes four stages: filling, pressure maintaining, cooling and demoulding. These four stages directly determine the quality of the products. It is a complete continuous process.
1. Filling stage
Filling is the first step in the whole process of injection manufacturing process, from the beginning of injection molding when the mold is closed to about 95% of the mold cavity filling. Theoretically, the shorter the filling time is, the higher the molding efficiency is. But in practice, the molding time or injection speed is restricted by many conditions.
2. Pressure maintaining stage
The function of the packing stage is to apply pressure continuously, compact the melt and increase the density of the plastic to compensate the shrinkage behavior of the plastic. In the packing process, the back pressure is high because the cavity has been filled with plastic. In the process of packing compaction, the screw of the injection molding machine can only move forward slowly, and the plastic flow speed is also relatively slow. At this time, the flow is called packing flow.
3. Cooling stage
In the introduction of injection and mold, the design of cooling system is very important. This is because only when the plastic products are cooled and solidified to a certain rigidity, can the plastic products avoid deformation due to external force after demoulding. Because the cooling time accounts for 70% - 80% of the whole injection molding manufacturing process, a well-designed cooling system can greatly shorten the molding time, improve the injection molding productivity and reduce the cost.
4. Demoulding stage
Injection molding manufacturing cycle consists of clamping time, filling time, holding time, cooling time and demoulding time. Cooling time accounts for the largest proportion, about 70% - 80%. Therefore, the cooling time will directly affect the plastic product molding cycle length and output size. In the demoulding stage, the temperature of plastic products should be cooled to lower than the thermal deformation temperature of plastic products, so as to prevent the relaxation of plastic products caused by residual stress, or warping and deformation caused by external demoulding force.
