Plastic technology is developing at a rapid pace. The development of new materials for new applications, the improvement of performance for existing material markets, and the improvement of performance for special applications are several important directions for new material development and application innovation.
New high thermal conductivity bioplastics
Nippon Electric Corporation has newly developed a bioplastic based on plants, and its thermal conductivity is comparable to that of stainless steel. The company mixed carbon fiber with a length of several millimeters and a diameter of 0.01mm and a special adhesive into polylactic acid resin made from corn to produce a new type of bioplastic with high thermal conductivity. If 10% carbon fiber is mixed in, the thermal conductivity of bioplastic is comparable to that of stainless steel; when 30% carbon fiber is added, the thermal conductivity of bioplastic is twice that of stainless steel, and the density is only 1/5 of stainless steel.
In addition to good thermal conductivity, this bioplastic also has the advantages of light weight, easy molding, and low environmental pollution. It can be used to produce thin and light outer frames of electronic products such as computers and mobile phones.
Color-changing plastic film
The University of Southampton, UK, and the Institute of Plastics, Darmstadt, Germany, jointly developed a color-changing plastic film. This film combines natural optical effects with artificial optical effects, and is actually a new way to allow objects to change color accurately. This color-changing plastic film is a plastic opal film, which is composed of plastic balls stacked in a three-dimensional space. The plastic balls also contain tiny carbon nanoparticles, so that light is not only transmitted between the plastic balls and the surrounding material. It reflects from the edge zone between them, and also reflects from the surface of the carbon nanoparticles filled between these plastic balls. This greatly deepens the color of the film. As long as the volume of the plastic sphere is controlled, light substances that only scatter certain spectral frequencies can be produced.
Plastic blood
Researchers at the University of Sheffield in the UK have developed an artificial "plastic blood" that looks like a thick paste and can be dissolved in water. Giving blood to patients can be used as a blood substitute during first aid. This new type of artificial blood is made of plastic molecules. There are millions of plastic molecules in a piece of artificial blood. These molecules are similar in size and shape to hemoglobin molecules. They can also carry iron atoms and transport oxygen throughout the body like hemoglobin. Since the manufacturing raw material is plastic, this kind of artificial blood is light and easy to carry, does not need to be refrigerated, has a long shelf life, is more efficient than real artificial blood, and is less expensive.
New bulletproof plastic
A Mexican research team developed a new type of bulletproof plastic in 2013. It can be used to make bulletproof glass and bulletproof clothing, and its quality is only 1/5 of traditional materials. to 1/7. This is a specially processed plastic substance that is super bulletproof compared to plastics with normal structures. Tests have shown that the new plastic can withstand bullets up to 22mm in diameter. Normal bulletproof materials will be damaged and deformed after being hit by bullets, making them unable to continue to be used. This new type of material will temporarily deform after being impacted by a bullet, but it will quickly return to its original shape and can continue to be used. In addition, this new material can evenly distribute the impact of the bullet, thereby reducing damage to the human body.
Plastics that reduce car noise
Polymer Group Inc. (PGI) uses renewable polypropylene and polyethylene terephthalate to create a new base material , used in moldable automotive parts to reduce noise. This material is mainly used in body and wheel well liners to create a barrier layer that can absorb sound in the car compartment and reduce noise by 25% to 30%. PGI has developed a special one-step production process , organically combine recycled materials and untreated materials, and make the two materials into a whole through lamination and needle punching.
1. Shrinkage
The form and calculation of thermoplastic molding shrinkage are as mentioned above. The factors that affect the molding shrinkage of thermoplastics are as follows:
1.1 Plastic varieties During the molding process of thermoplastic plastics, due to factors such as volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation, etc., compared with thermosetting plastics, the shrinkage rate is larger. The rate range is wide and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity control treatment is generally larger than that of thermosetting plastics.
1.2 Plastic part characteristics When molding, the molten material contacts the surface of the cavity and the outer layer is immediately cooled to form a low-density solid shell.
Due to the poor thermal conductivity of plastic, the inner layer of the plastic part cools slowly to form a high-density solid layer that shrinks greatly. Therefore, those with thick walls, slow cooling, and thick high-density layers will shrink more. In addition, the presence or absence of inserts and the layout and quantity of inserts directly affect the material flow direction, density distribution and shrinkage resistance. Therefore, the characteristics of plastic parts have a greater impact on shrinkage size and directionality.
1.3 The form, size, and distribution of the feed inlet directly affect the material flow direction, density distribution, pressure-holding and shrinking effect, and molding time. Direct feed inlets and feed inlets with large cross-sections (especially those with thicker sections) have smaller shrinkage but greater directionality, while feed inlets with wider and shorter lengths have less directivity. Those that are close to the feed inlet or parallel to the direction of material flow will shrink more.
1.4 Molding conditions: The mold temperature is high, the molten material cools slowly, has high density, and shrinks greatly. Especially for crystalline materials, the shrinkage is greater due to high crystallinity and large volume changes. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the shrinkage and directionality of each part. In addition, holding pressure and time also have a greater impact on shrinkage. If the pressure is high and the time is long, the shrinkage will be small but directional. The injection molding pressure is high, the viscosity difference of the molten material is small, the interlayer shear stress is small, and the elastic rebound after demoulding is large, so the shrinkage can be appropriately reduced. The material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting various factors such as mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of the plastic part.
When designing the mold, based on the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the form, size and distribution of the inlet, the shrinkage rate of each part of the plastic part is determined based on experience, and then the cavity size is calculated . For high-precision plastic parts and when it is difficult to control the shrinkage rate, it is generally advisable to use the following method to design the mold:
① Set a smaller shrinkage rate for the outer diameter of the plastic part and a larger shrinkage rate for the inner diameter to leave room for testing. Room for post-mold corrections.
② Trial mold to determine the form, size and molding conditions of the pouring system.
③ The dimensional changes of the plastic parts to be post-processed must be determined after post-processing (measurement must be done 24 hours after demoulding).
④Correct the mold according to the actual shrinkage situation.
⑤Try the mold again and change the process conditions appropriately to slightly correct the shrinkage value to meet the requirements of the plastic part.