【Abstract】With continuous development of science and technology, molding simulation has become an important link in development of injection molds. Injection pressure is an important parameter in injection molding process and is often used to evaluate molding performance of plastic parts. First, empirical formulas are used to qualitatively derive key factors affecting injection pressure, then MoldFlow software is used to verify impact of each factor on injection pressure. Then, through statistical theory and actual molding pressure data, rules of two are found, finally accurate estimation of injection pressure can be achieved in the early stage of mold development.

Automobile headlights are not only important functional components of automobiles, but also important exterior surfaces. As shown in Figure 1, headlight is composed of a headlight lampshade, a decorative frame, a lamp housing, and electrical components. In this article, previous lamp shade is taken as an example, injection pressure is accurately estimated by applying rheological theory and MoldFlow software. Lampshade of headlight is generally made of PC material, gating system mostly adopts form of heat transfer to cold, and fixed mold side has a single point of glue, as shown in Figure 2.

 

Figure 1 Headlight composition

 

Figure 2 Front lamp shade pouring system

Plastic material is a typical non-Newtonian fluid, that is, shear stress of fluid has a nonlinear relationship with shear rate. This paper uses Newtonian slit linear viscous flow model to qualitatively derive key factors affecting injection pressure.
Model assumes that fluid flows in a slit of length l, height h, and width w, and flow mode is a stable simple shear flow, as shown in Figure 3. Under condition of w/h>20, lateral viscosity and wall resistance are ignored.

 

Figure 3 Slit flow model

 

Material viscosity is a measure of polymer's ability to flow under external pressure. It is affected by polymer structure, temperature, shear, and pressure. Headlight lamp cover material fixes PC, so material, shear, and pressure are not considered, only influence of mold temperature and melt temperature on injection pressure is analyzed.
Verification object 1 (small): size: 100*50*30mm, wall thickness: 2.0mm, volume: 21.6cm3.
Verification object 2 (big): size: 520*190*100mm, wall thickness 2.5/2.8mm, volume 577.5cm3.
Material selection: Makrolon AL2447.
Verification conditions: Recommended mold temperature of headlight lampshade is 80℃~120℃, and material temperature is 280℃~320℃. Single variable principle is adopted, and injection pressure is measured at intervals of 5℃.
Verification results: relationship between injection pressure and mold temperature is shown in Figure 4, relationship between injection pressure and melt temperature is shown in Figure 5.

 

Figure 4 Relationship between injection pressure and mold temperature

 

Figure 5 Relationship between injection pressure and melt temperature
Interpretation of curve: injection pressure decreases with increase of temperature, which is basically proportional. Different sizes of plastic parts have different effects on injection pressure, and large plastic parts have a larger gradient; melt temperature has a greater impact on injection pressure than mold temperature.
Verification conclusion: increasing molding temperature can improve injection pressure. Considering current situation, temperature adjustment is about 5℃~15℃, and injection pressure is reduced by 2.5~7.5MPa for mold temperature, accounting for 2%~6%; for melt temperature, pressure is reduced by 5.7~17MPa, accounting for 4.5%~13.5%. .

Verification object: size: 520 * 190 *100mm, wall thickness 2.5/ 2.8mm, volume 577.5cm3.
Material selection: Makrolon AL2447. Verification condition: Injection speed is measured at an interval of 10cm3/s.
Verification result: relationship between injection pressure and injection speed is shown in Figure 6.
Curve interpretation: When injection speed is low, contact time between material and mold is long, heat loss is large, thickness of frozen layer increases, and flow resistance increases, resulting in an increase in molding pressure. When injection speed is high, as speed increases, shear heat of material increases, viscosity decreases, and molding pressure decreases. However, after speed increases significantly, a larger pressure is required to maintain a high shooting speed. Injection speed of headlight lampshade is generally 20~35mm/s, corresponding to flow rate of 157~275cm3/s, pressure of comparison curve has no significant change (viscosity of PC material is not sensitive to shear.
Verification conclusion: In the process range of headlight lampshade, injection pressure has almost nothing to do with injection speed, so influence of injection speed is not considered.

 

Figure 6 Relationship between injection pressure and injection speed

Verification object: size: width 50mm, height 30mm, wall thickness: 2.0mm.
Material selection: Makrolon AL2447.
Verification condition: Injection pressure is measured at 25mm intervals for length of plastic part.
Verification result: Relationship between injection pressure and flow length is shown in Figure 7.
Interpretation of curve: injection pressure increases with increase of flow length, and relationship is basically linear.
Verification conclusion: Injection pressure of headlight lampshade increases as length of plastic part increases. Under normal circumstances, shape of headlight plastic part does not change, so flow length is only compared with pressure of different plastic parts with same wall thickness.

 

Figure 7 Relationship between injection pressure and flow length

Verification object 1 (small): size: 100*50*30mm.
Verification object 2 (big): size: 200*100*30mm.
Material selection: Makrolon AL2447.
Verification conditions: single variable principle is adopted, and injection pressure is measured at 0.5mm intervals of wall thickness.
Verification result: Relationship between injection pressure and wall thickness is shown in Figure 8.
Curve interpretation: Injection pressure decreases exponentially as wall thickness increases. When wall thickness increases to a certain value, pressure tends to balance. Under normal circumstances, wall thickness of headlight lampshade is 2.0~3.5mm, and increase in wall thickness can significantly reduce injection pressure.

 

Figure 8 Relationship between injection pressure and wall thickness

By analyzing and verifying four factors that affect injection pressure of headlight lampshade, corresponding relationship curve is obtained, then ranking of factors affecting injection pressure is obtained: plastic part wall thickness > flow length > melt temperature > mold temperature > injection speed.

Injection molding machine panel displays pressure as injection hydraulic pressure, which refers to pressure of hydraulic oil acting on injection cylinder, namely Ph.
Injection pressure calculated by MoldFlow software is pressure exerted on melt by screw of injection molding machine, that is, Pm. Panel pressure Ph of injection molding machine can be converted to injection pressure Pm through conversion formula to facilitate theoretical comparison.

Collect panel pressure of different injection molding machine numbers and convert it to injection pressure according to enhancement ratio, then compare it with test pressure of MoldFlow software, use average ratio of multiple sets of data as pressure compensation factor. Pressure compensation factor can accurately injection pressure with an accuracy rate of more than 90%, as shown in Table 1.

 

Table 1 Pressure compensation factor (average ratio)

In this paper, key factors affecting injection pressure are approximated by empirical formula, and MoldFlow software is used to verify order of factors affecting injection pressure: plastic part wall thickness>flow length>melt temperature>mold temperature>injection speed. By introducing a compensation factor, accurate prediction of injection pressure of headlight lampshade is realized.