Liquidity
The ability of plastic in molten state after being heated to fill the whole cavity under pressure is called fluidity, which is generally tested by professional fluidity testing dies.
The fluidity ofplastics of the same kind and different brands is different. Plastic parts with different structures have different requirements for the fluidity of plastics. The fluidity of plastics is closely related to molding temperature and pressure.
Usually, if the fluidity is too large, on the one hand, it will make the melt filling not tight, resulting in loose products and affecting product quality; On the other hand, the fluidity is too great, and it is easy for the injection-molded parts to stick to the mold and cast with nozzles around them, resulting in nozzle blockage. However, the fluidity of plastics is too small, and for the plastic parts with complex structure and long process, it is difficult to flow during injection molding, and it is easy to appear the phenomena of glue shortage and shrinkage, resulting in a large number of waste products; Or it is necessary to use high-pressure injection molding, which will easily lead to excessive internal stress of plastic parts. We should choose plastic molding with appropriate fluidity according to product structure, size and thickness.
Content of water-absorbing volatiles
Thermoplastics contain more or less moisture and volatiles, and a proper amount of moisture has plasticizing effect.
If the moisture and volatiles in plastic exceed a certain proportion, many problems (such as degradation, fogging, strength reduction, etc.) will occur during injection molding. In severe cases, bubbles (silver lines) and rough surface will occur, and the transmittance of transparent products will be damaged (turbid). It is difficult to guarantee the precision of precision plastic parts.
However, absolutely dry plastic will cause the fluidity to decrease and brittleness to increase, and it is difficult to fill the mold during molding, which should be paid special attention to. Some people think that the more fully dried the plastic, the better, which is a wrong idea.
There are three main reasons that cause more moisture and volatiles in plastics.
A, the average molecular weight of plastic resin is low;
B, the plastic resin is not fully dried during production;
C, the water-absorbent plastic absorbs the moisture in the surrounding air due to improper storage, and different plastics have different drying temperature and drying time regulations
Shrinkage rate
There are three main reasons for the shrinkage rate:
A, plastic has much larger thermal shrinkage than metal (one order of magnitude larger, about ten times larger).
B, plastic products are not rigid bodies after hardening, and plastic parts have certain elastic recovery after demolding.
C, just demoulding, the pressure began to decrease, but the plastic parts were still stuck on the mold wall, and the plastic deformation occurred.
The factors that affect the shrinkage are the properties of plastics, molding conditions, mold and product design. The shrinkage of amorphous plastics is less than 1%, and that of crystalline plastics exceeds 1%. The products injected with crystalline plastics have post-shrinkage, so it is necessary to measure their dimensions after cooling for 24 hours, and the accuracy can reach 0.02 mm.
Processing temperature of plastics
The processing temperature of plastic is the temperature at which it reaches the viscous state, and the processing temperature is not a point but a range (from melting point to decomposition temperature). When thermoforming plastic, the appropriate processing temperature should be selected according to the size, complexity, thickness, insert condition, temperature tolerance of colorant used, machine performance and other factors.
Why can the temperature reflected by thermometer often change in injection molding production, and the temperature set when the same product (same mold) is put into different machines for production may be different?
In fact, the hot forming temperature of plastics is relatively fixed, which is only caused by the difference of temperature measuring methods, layout of temperature measuring points and performance of temperature sensors. The temperature displayed on the temperature indicator controller is not the actual temperature of the melt in the charging barrel, but an indirect and local temperature.
Properties of plastics to be known before injection molding
When setting the injection molding process conditions, every injection molding worker needs to thoroughly understand the related properties of the plastics used in order to scientifically set the process conditions and analyze the problems in the injection molding process.
Types and grades of plastics (factors considered when understanding the composition and performance of plastics);
Density of plastic (factors considered when setting the position of multi-stage glue injection);
Hygroscopicity and allowable water volume of plastics (factors considered when setting drying conditions);
Glass transition temperature, melting point and decomposition temperature of plastics (factors considered when setting barrel temperature);
Melt index FMI of plastics (factors considered when setting injection pressure and back pressure);
Crystallinity of plastics (factors considered when setting mold temperature/material temperature);
Permissible injection pressure range of plastics (factors considered when setting injection pressure);
Permissible residence time of plastics in the material barrel (factors to be considered when setting the residual amount and stopping the machine);
Molding shrinkage of plastics (factors considered when setting mold temperature/material temperature/pressure);
The mold temperature range during plastic molding (factors considered when setting mold temperature);
Other properties (such as chemical resistance, heat distortion temperature, etc.) are considered in the post-processing of plastic parts.
