Moldability of thermoplastics

Moldability of thermoplastics

The various properties of plastics related to the molding process and molding quality are collectively referred to as the technological properties of plastics. Understanding and mastering the process performance of materials is directly related to whether the plastic can be formed smoothly and the quality of the plastic parts can be guaranteed, and it also affects the design requirements of the mold. The main process performance and requirements of thermoplastics are introduced below.

In addition to thermodynamic properties, crystallization, and orientation, the molding process performance of thermoplastics also includes shrinkage, fluidity, heat sensitivity, water sensitivity, hygroscopicity, and compatibility.

(1) Shrinkage

Plastic is usually formed by filling the mold cavity in a high-temperature molten state. When the plastic part is taken out of the mold and cooled to room temperature, its size will be smaller than the original size in the mold. This characteristic is called shrinkage. It can be expressed as a percentage of plastic shrinkage per unit length, that is, shrinkage (S). Because this shrinkage is not only caused by thermal expansion and contraction of the plastic part itself. Moreover, it is also related to various molding process conditions and mold factors, so the shrinkage of plastic parts after molding is called molding shrinkage. It is possible to adjust the process parameters or modify the mold structure to reduce or change the size of the plastic part. Molding shrinkage is divided into two forms: size shrinkage and post-shrinkage, and both of them are directional.

①The size of the plastic part shrinks. Due to thermal expansion and contraction of plastic parts and physical and chemical changes inside the plastic part, the size of the plastic part shrinks after it is demolded and cooled to room temperature. Therefore, it must be considered when designing the molded parts of the mold. Make compensation to avoid out-of-tolerance in the size of plastic parts.

② Post-shrinkage of plastic parts. When the plastic part is formed, a series of stresses are generated due to factors such as internal physical, chemical and mechanical changes. After the plastic part is formed and solidified, there is residual stress. shrinking phenomenon. Generally, the post-shrinkage of general plastic parts is relatively large within 10 hours after demoulding, and it takes a long time for the basic shape to reach the final shape after 24 hours. Generally, the post-shrinkage of thermoplastics is greater than that of thermosetting plastics. The post-shrinkage of injection molded plastic parts is greater than that of compression molded plastic parts.

In order to stabilize the size of plastic parts after molding, sometimes according to the performance and process requirements of plastics, plastic parts need to be processed after molding. After heat treatment, the size of plastic parts will also shrink, which is called post-processing shrinkage. In the mold design of high-precision plastic parts, errors caused by post-shrinkage and post-processing shrinkage should be compensated.

③The directionality of shrinkage. The orientation effect of polymers along the flow direction during the molding process of plastics will lead to anisotropy of the plastic parts, and the shrinkage of the plastic parts will inevitably be different due to different directions: usually the shrinkage is large and the strength is high along the direction of the material flow, and it is different from the material flow The shrinkage in the vertical direction is small and the strength is low. At the same time, due to the uneven distribution and density of additives in various parts of the plastic part, the shrinkage is also uneven, resulting in poor shrinkage of the plastic part, which is likely to cause warping, opening and even cracking of the plastic part.

The molding shrinkage rate of plastic parts is divided into actual shrinkage rate and calculated shrinkage rate. The actual shrinkage rate indicates the difference between the size of the mold or plastic part at the molding temperature and the size of the plastic part at room temperature, and the calculated shrinkage rate indicates the size at room temperature. Size The difference between the size of the mold and the size of the plastic part.

Calculated as follows

S-actual shrinkage;

S--Calculation of shrinkage;

Lc - the size of the plastic part or mold at the molding temperature;

 Ls - the size of the plastic part at room temperature;

 Lm - the size of the mold at room temperature.

Because the difference between the actual shrinkage rate and the calculated shrinkage rate is very small, the calculated shrinkage rate is often used to calculate the size of the cavity and core in the design of ordinary medium and small molds. For large and precision mold design, the actual shrinkage rate is generally used to calculate the size of the cavity and core.

In actual molding, not only different types of plastics have different shrinkage rates, but also different batch numbers of the same type of plastic, or different parts of the same plastic part often have different shrinkage values. There are four main factors affecting the change of shrinkage rate.

a. The variety of plastic. All kinds of plastics have their own range of shrinkage, but even the same plastic has different shrinkage and anisotropy due to different relative molecular weights, fillers and proportions.

b. Plastic structure. The shape, size, wall thickness, presence or absence of inserts, number and layout of the plastic parts have a great influence on the shrinkage rate value. Generally, the larger the wall thickness of the plastic part, the greater the shrinkage rate, and the plastic parts with complex shapes are smaller than the shape. The shrinkage rate of simple plastic parts, the shrinkage rate of plastic parts with inserts is reduced due to the obstruction of inserts and chilling.

c. Die structure. The parting surface of the plastic mold, the direction of pressure, and the structural form, layout, and size of the gating system directly affect the direction of material flow, density distribution, pressure-holding and feeding, and molding time, and have a great impact on shrinkage and directionality, especially Extrusion and injection molding are more prominent.

d. Molding process conditions. Molding conditions such as mold temperature, injection pressure, and holding time have a great influence on the shrinkage of plastic parts. The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for crystalline plastics, because of the large volume change, the shrinkage is greater, and the uniform temperature distribution of the mold also directly affects the size and direction of the shrinkage of each part of the plastic part. The injection pressure is high, the viscosity difference of the melt is small, and the elastic recovery after demoulding Larger, less shrinkage. The longer the holding time, the smaller the shrinkage, but the directionality is obvious.

Since the shrinkage rate is not a fixed value, but fluctuates within a certain range, the change in the shrinkage rate will cause the size of the plastic part to change. Therefore, the shrinkage range of the plastic, the wall thickness of the plastic part, the shape of the feed port The shrinkage rate of each part of the plastic part is determined by comprehensive consideration of form, size, position and molding factors. For high-precision plastic parts, plastics with a small shrinkage fluctuation range should be selected, and there is room for mold repair. After the mold test, the mold should be gradually corrected to meet the size and accuracy requirements of the plastic part.

(2) Liquidity

During the molding process, the ability of the plastic melt to fill the mold cavity at a certain temperature and pressure is called the fluidity of the plastic. The quality of plastic fluidity directly affects the parameters of the molding process to a large extent, such as molding temperature, pressure, cycle, the size of the mold gating system and other structural parameters. When determining the size and wall thickness of plastic parts, the influence of fluidity should also be considered.

The size of the fluidity is related to the molecular structure of the plastic, and the resin with linear molecules but little or no cross-linking structure has high fluidity. Adding fillers to plastics will reduce the fluidity of the resin, while adding plasticizers or lubricants can increase the fluidity of the plastics. Reasonable structural design of plastic parts can also improve fluidity, for example, when the rounded structure is used at the corners of runners and plastic parts, the fluidity of melt is improved.

The fluidity of plastic has a great influence on the quality of plastic parts, mold design and molding process. Plastics with poor fluidity are not easy to fill the cavity, and are prone to defects such as lack of material or weld lines, so a large molding pressure is required to form. On the contrary, plastics with good fluidity can fill the cavity with less molding pressure. However, if the fluidity is too good, severe flashing will occur during molding. Therefore, in the molding process of plastic parts, when selecting plastic part materials, appropriate fluidity plastics should be selected according to the structure, size and molding method of the plastic parts to obtain satisfactory plastic parts. In addition, when designing the mold, the parting surface, gating system and feeding direction should be considered according to the fluidity of the plastic; the fluidity of the plastic should also be considered when selecting the molding temperature.

According to the mold design requirements of injection molding machines, the fluidity of thermoplastics can be divided into the following three categories.

Plastics with good fluidity: such as polyamide, polyethylene, polystyrene, polypropylene, cellulose acetate and polymethylpentene.

Plastics with medium fluidity: such as modified polystyrene, ABS, AS, polymethyl acrylate, polyoxymethylene and chlorinated polyether.

Plastics with poor fluidity: such as polycarbonate, hard polyvinyl chloride, polyphenylene oxide, polyalum, polyarylsulfone and fluoroplastics. There are three main factors affecting the fluidity of plastics:

①The material temperature is high, the plastic fluidity increases, but the material temperature has different effects on the fluidity of different plastics, polystyrene, polypropylene, polyamide, polymethyl methacrylate, ABS, AS, polycarbonate The fluidity of plastics such as ester and cellulose acetate has a greater impact on temperature changes; while the fluidity of polyethylene and polyoxymethylene is less affected by temperature changes.

②Pressure When the injection pressure increases, the melt will be greatly sheared and the fluidity will also increase, especially polyethylene and polyoxymethylene are very sensitive. However, excessive pressure will cause stress in the plastic part, and will reduce the viscosity of the melt and form flash.

③ The form, size, layout, cavity surface roughness, sprue section thickness, cavity form, exhaust system, cooling system design, melt flow resistance and other factors of the mold structure gating system directly affect the fluidity of the melt.

(3) Thermal sensitivity

The chemical structure of various plastics may change under the action of heat. Some plastics with poor thermal stability will have the characteristics of degradation, decomposition and discoloration when the material temperature is high and the heating time is long. The degree of sensitivity is called the thermal sensitivity of the plastic. Plastics with strong heat sensitivity (that is, plastics with poor thermal stability) are often referred to as heat-sensitive plastics for short.

 Such as rigid polyvinyl chloride, polychlorotrifluoroethylene, polyoxymethylene, polychlorotrifluoroethylene, etc. This kind of plastic is easy to thermally decompose, thermally degrade or degrade when heated for a long time during the molding process, which will affect the performance and surface quality of the plastic part.

When heat-sensitive plastic melt undergoes thermal decomposition or thermal degradation, various decomposition products will be produced, some of which will irritate, corrode or have certain toxicity to the human body, molds and equipment; Catalysts for the decomposition of plastics, such as the decomposition of polyvinyl chloride to produce hydrogen nitride, can further intensify the decomposition of polymers.

In order to avoid thermal decomposition of heat-sensitive plastics during processing and molding, heat stabilizers can be added to the plastics during mold design, injection molding machine selection and molding; appropriate equipment (screw injection molding machines) can also be used to strictly control molding Temperature, mold temperature, heating time, screw speed and back pressure, etc., remove decomposition products in time, and anti-corrosion measures should be taken for equipment and molds.

(4) Water-sensitive plastics. The water sensitivity of plastic refers to its sensitivity to water degradation under high temperature and high pressure. Even if it contains a small amount of water, it will decompose under high temperature and high pressure. Therefore, the moisture content must be strictly controlled before water-sensitive plastics are molded and dried.

(5) Hygroscopicity

Hygroscopicity refers to the degree of affinity of plastics to moisture. In this way, plastics can be roughly divided into two categories: one is plastics that absorb water or adhere to moisture, such as polyamide, polycarbonate, polyester, ABS, etc.; the other is plastic that neither absorbs water nor easily adheres to moisture. Such as polyethylene, polypropylene, polyoxymethylene, etc.

For any plastic with a tendency to absorb water, if the moisture is not removed before molding and the content exceeds a certain limit, then during the molding process, the moisture will turn into gas and promote the decomposition of the plastic, resulting in foaming and reduced fluidity of the plastic, resulting in molding Difficult, and reduce the surface quality and mechanical properties of plastic parts. Therefore, in order to ensure the smooth progress of molding and the quality of plastic parts, plastics with a high tendency to absorb water and adhere to moisture must be dried before molding to remove moisture. If necessary, infrared heating should be installed in the hopper of the injection molding machine. .

(6) Compatibility

Compatibility refers to the fact that two or more different types of plastics do not produce phase separation in the molten state.

ability.If the two plastics are not compatible, surface defects such as delamination and peeling will appear on the parts when they are mixed and melted. The compatibility of different plastics has a certain relationship with their molecular structures. Those with similar molecular structures are easier to be compatible, such as high-pressure polyethylene, low-pressure polyethylene, and polypropylene, etc.; when the molecular structure is different, it is difficult to be compatible. For example, blending between polyethylene and polystyrene. The compatibility of plastics is also commonly known as blending. Through this property of plastics, the comprehensive properties similar to copolymers can be obtained, which is one of the important ways to improve the properties of plastics.

The water sensitivity of plastic refers to its sensitivity to water degradation under high temperature and high pressure. Polycarbonate is a typical water-sensitive plastic. Even if it contains a small amount of water, it will decompose under high temperature and high pressure. Therefore, the moisture content must be strictly controlled before water-sensitive plastics are molded and dried.

(5) Hygroscopicity

Hygroscopicity refers to the degree of affinity of plastics to moisture. In this way, plastics can be roughly divided into two categories: one is plastics that absorb water or adhere to moisture, such as polyamide, polycarbonate, polyester, ABS, etc.; the other is plastic that neither absorbs water nor easily adheres to moisture. Such as polyethylene, polypropylene, polyoxymethylene, etc.

For any plastic with a tendency to absorb water, if the moisture is not removed before molding and the content exceeds a certain limit, then during the molding process, the moisture will turn into gas and promote the decomposition of the plastic, resulting in foaming and reduced fluidity of the plastic, resulting in molding Difficult, and reduce the surface quality and mechanical properties of plastic parts. Therefore, in order to ensure the smooth progress of molding and the quality of plastic parts, plastics with a high tendency to absorb water and adhere to moisture must be dried before molding to remove moisture. If necessary, infrared heating should be installed in the hopper of the injection molding machine. .

(6) Compatibility

Compatibility refers to the ability of two or more different types of plastics to not produce phase separation in the molten state.

If the two plastics are not compatible, surface defects such as delamination and peeling will appear on the parts when they are mixed and melted. The compatibility of different plastics has a certain relationship with their molecular structures. Those with similar molecular structures are easier to be compatible, such as high-pressure polyethylene, low-pressure polyethylene, and polypropylene, etc.; when the molecular structure is different, it is difficult to be compatible. For example, blending between polyethylene and polystyrene. The compatibility of plastics is also commonly known as blending. Through this property of plastics, the comprehensive properties similar to copolymers can be obtained, which is one of the important ways to improve the properties of plastics.

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