We mentioned earlier that choice of materials for plastic parts is crucial to product design. So what properties of specific materials can provide guidance and reference for our design?
First, let’s take a look at the two ways to obtain material information:
① Official website of material suppliers; such as Sabic, Bayer, DSM, LG, DuPont, etc. all provide professional performance TDS downloads of their plastic raw materials on their official websites.
② Through Internet resources; such as https://www.ulprospector.com, http://www.matweb.com, http://www.campusplastics.com. Some of these websites not only provide material properties of plastic parts, but also include inquiries on properties of other materials such as metals. Following is information about PBT-GF30 material provided by matweb.

 

Secondly, performance parameters of materials usually include following aspects: physical performance parameters, mechanical performance parameters, thermal performance parameters, environmental performance parameters, electrical performance parameters and flame retardant performance parameters.
Next, we will explain one by one what they represent and impact they have on our design from these aspects.

Density is a measure of weight within a specific volume. Density is equal to weight of an object divided by its volume. It can be represented by symbol ρ (pronounced [rəʊ]). In International System of Units and Chinese legal measurement units, unit of density is kilograms per cubic meter. Meter, symbol is kg/m3.
Density is an important physical property that describes density of matter. It not only helps us understand properties and behavior of matter, but also has a wide range of applications in various fields. Density range of plastic materials is generally 1~2g/cm³, while density of common steel is about 7g/cm³, and density of aluminum is 2.7g/cm³. Therefore, if plastic can replace metal, it can bring visible effects to weight reduction of products; at the same time, density is also a key factor affecting cost.
In addition, density is also an important indicator of compressibility of a material. How to understand?
"Density" can be understood literally as density of matter. The greater density, the closer molecules of substance are, the stronger interaction force, and the smaller compressibility; conversely, the smaller density, the looser molecules of substance, the weaker interaction force, and the greater compressibility.

Melt flow rate, formerly known as melt index, is defined as: amount of thermoplastic material extruded within a certain period of time under specified conditions, that is, weight of melt passing through standard die capillary every 10 minutes, expressed in MFR, and unit is g/10min. Melt flow rate can characterize viscous flow characteristics of thermoplastic plastics in molten state. It has important guiding significance for ensuring quality of thermoplastic plastics and their products, adjusting production process.
This parameter affects pressure and time of plastic injection molding. It can also be understood as the extrusion rate of plastic. Some suppliers use viscosity coefficient to characterize.
Its physical meaning is: the larger value, the better processing fluidity of material and the more convenient molding process, that is, the smaller viscosity and the smaller molecular weight.

Plastic shrinkage refers to percentage difference between size of plastic part at molding temperature and size after it is removed from mold and cooled to room temperature. It reflects degree of size reduction of plastic parts after they are removed from mold and cooled.
Plastic parts are processed and formed by molds. After cooling, product will shrink compared with size of mold.
Factors that affect plastic shrinkage include: plastic type, molding conditions, mold structure, etc. Different polymer materials have different shrinkage rates. Secondly, shrinkage rate of plastic is also closely related to shape of plastic part, complexity of internal structure, whether there are inserts, etc.
Among thermoplastics, crystalline plastics generally show greater shrinkage than amorphous plastics. Shrinkage rate of crystalline plastics is generally 1% to 3%, while shrinkage rate of amorphous plastics is generally 0.4% to 0.8%.
According to provisions of German standard DIN16901, at room temperature of 23±2℃ and relative humidity of 50±5%, size of injection molded product is measured after leaving it at room temperature for 16 hours, and then shrinkage rate of material is calculated:
Shrinkage S={(M-P)/M}×100%
Among them: S (Shrinkage) - shrinkage rate; M (Mold) - mold size; P (Product) - plastic part size.
Therefore, from above calculation formula, we can also deduce dimensions when designing mold cavity:
M=P/(1-S)
Of course, some companies will have some similarities or empirical formulas based on their own experience, but basic methods are similar to this.
In addition, shrinkage rate of material is divided into two directions: transverse (along direction of material flow) and longitudinal (perpendicular to direction of material flow). Performance parameters are also given on TDS of following materials:

 

Both crystalline materials and glass fiber/carbon fiber filled materials will have anisotropy, that is, shrinkage rate in flow direction is small, and shrinkage rate perpendicular to flow direction is large. Generally, the higher crystallinity, the longer glass fiber length, and the more obvious orientation. This is determined by orientation of material.
Other factors that affect product shrinkage:
Molding conditions: For example, an increase in injection pressure and holding pressure will reduce shrinkage of material;
Product structure: Holes, ribs and other similar structures will reduce shrinkage rate; thicker or more uneven wall thickness will increase shrinkage rate;
Mold structure: The further away from gate position, the greater shrinkage rate.
Significance of shrinkage rate to product design:
1. If product has strict dimensional tolerance requirements, then amorphous plastic materials with small shrinkage rates can be preferred;
2. We will choose crystalline materials for many auto parts, which cannot avoid a large shrinkage rate. Therefore, when designing product structure, we need to add more holes, reinforcement ribs and other structures, and mold gates need to be set away from important control dimensions. Structure is more recent, control accuracy of material temperature and mold temperature is tightened, even mold needs to counter-compensate for warpage, injection and holding pressure must also be controlled to increase during molding. As mentioned above, after product mold design is completed, it must be judged through mold flow analysis to determine whether supplier's mold plan can be accepted.
3. Since materials have different shrinkage rates, unless they are exactly same, be sure to confirm material of product before opening mold. Because when mold remains unchanged, changing raw materials will cause dimensional inconsistencies, which will not meet design requirements, causing structural assembly of other products and other problems.