Sandwich injection process optimization and plastic parts development based on Moldflow software

Time：2022-11-11 08:36:01 / Popularity： / Source：

1. Analysis of rheological mechanism of sandwich injection molding

Different from traditional injection molding process in which a single material is injected at one time, sandwich injection molding process generally uses two materials for injection, which mainly includes three steps, as shown in Figure 1. First inject shell melt (material A), then inject core melt (material B) when injection volume of shell melt meets process requirements, shell melt injected first continues to fill cavity forward under push of core melt; when injection volume of core melt is close to filling cavity, shell melt can be re-injected according to process requirements to close gate area.

Figure 1 Sandwich injection molding process
Sandwich injection molding is a special plastic injection molding process, and polymer material is subjected to external forces during molding process, including shear stress and tensile stress. Since in sandwich injection molding, not only flow of melt in cavity, but also flow of core layer melt in shell layer melt, relative flow between shell layer melt and core layer melt are included. Bulk flow situation is relatively complex, further analysis and discussion of rheological behavior of polymer melts in sandwich injection molding are required.
As shown in Figure 2, when shell melt is initially injected, shell melt is subjected to shear stress and tensile stress from gate and cavity. During subsequent injection of core layer melt, core layer melt pushes shell layer melt to continue to flow forward. At this time, core layer melt and shell layer melt generate relative flow and mutual shear force.

Figure 2 Melt flow pattern of sandwich injection molding
It can be seen that when mold factors are fixed, flow state of shell melt in the first injection is only related to factors that can affect flow behavior of shell melt, such as shell melt temperature, viscosity, injection rate (that is, shear force) and sensitivity of viscosity to shear force changes; under condition that both mold state and shell melt material remain unchanged, flow state is only related to melt temperature and injection rate. In second injection, core layer melt and shell layer melt both flow forward and also flow relatively. Relative viscosity and relative volume of layer melt are also important influencing factors. Because in process of generating core-shell structure in second injection, flow states of core-layer melt and shell-layer melt influence each other. On the one hand, flow state and accumulation of shell melt determine resistance to forward flow of core melt; on the other hand, flow state of core melt also determines force of the shell melt being penetrated and carried forward. Therefore, in sandwich injection molding process, in addition to injection process parameters, core/shell viscosity ratio and injection switching volume are also parameters that need to be considered.

2 Analysis and process optimization of sandwich injection molding plastic parts

2.1 Plastic information and mold design

In view of rheological behavior characteristics of sandwich injection molding summarized above, development of sandwich injection plastic parts is carried out with square box-shaped plastic parts as research object. Model of plastic part is shown in Figure 3. External dimensions of plastic part are 130 mm * 90 mm * 58 mm, and wall thickness is 2.25 mm. In addition to good strength and good appearance, plastic part requires that penetration distance of core layer should be as sufficient as possible, and thickness distribution of core layer should be uniform to ensure that plastic part has good comprehensive performance.

Figure 3 Square box-shaped plastic parts
According to above characteristics and requirements of plastic parts, considering characteristics of sandwich injection molding, core and shell materials of square box are all PP materials. Among them, shell material is polypropylene with grade of PP-TC16G80, and core material is recycled PP plastic. Material information is shown in Table 1.

Material	Material grade	Material viscosity/Pa*s (200℃, 1000S-1)
PP	PP-TC16G80	115.9
Recycle PP	-	91.68

Table 1 Material information of square box-shaped plastic parts
Corresponding mold structure design is carried out for square box-shaped plastic parts, and mold structure is shown in Figure 4. Cavity is set as 1 mold and 1 cavity, parting surface is based on principle of "maximum cross section", opening surface of square box is selected as parting surface, and gate form is a straight gate. Mold structure includes basic structures such as fixed template, movable template, core, cavity plate, push rod, etc., without lifter and slider structure.

Figure 4 Mold structure of square box-shaped plastic parts

2.2 Optimization of process parameters

By analyzing rheological behavior of sandwich injection molding, it is concluded that switching volume of sandwich injection molding is an important process parameter. Inappropriate switching volume may cause melt front of core layer to penetrate melt front of shell layer, resulting in unqualified molding quality of plastic part. Using Co-injection module of Moldflow software, simulation analysis of sandwich injection molding is carried out according to different first switching volume parameters of plastic part, appropriate first switching volume parameters were obtained to ensure molding quality of developed square box-shaped plastic part. Settings of different switching volume parameters are shown in Table 2, and other basic molding process parameters are shown in Table 3.
Table 2 Different switching volume parameter settings

Group	First switch volume	Second switch volume
1	40%	90%
2	50%	90%
3	60%	90%

Table 3 Basic molding process parameters

Process parameters	Numerical value
Melt temperature of material A/℃	200
Melt temperature of material B/℃	200
Injection time/s	1
Mold surface temperature/℃	40
Holding time/s	10
Holding pressure/MPa	100
Cooldown time/s	20

Results of "B material thickness factor" for different switching volumes are shown in Figure 5. It can be seen from Figure 5 that when the first switching volume is set to 40%, a dark red area appears at the end of cavity filling (see circle in Figure 5(a)), that is, core melt front penetrates shell melt front, and thickness of core material at the end of melt flow is close to 100% , this phenomenon not only destroys appearance quality of plastic parts, but also affects mechanical properties of plastic parts to a certain extent. When the first switching volume is set to 50%, melt flow end of core layer of plastic part has better penetration, and no melt front breakthrough occurs; when the first switching volume is set to 60%, although there is no breakthrough phenomenon in melt flow front, melt penetration of the overall core layer is insufficient, penetration is poor, and melt content of core layer in plastic part is small.

Fig.5 Thickness factor results of B material with different switching volumes
In summary, in subsequent development process of plastic parts, switching volume setting of 50% of first switching volume and 90% of second switching volume is selected, design of plastic parts is further optimized according to penetration of B material to ensure molding quality of plastic parts.

2.3 Optimization of design parameters of plastic parts

Figure 6 shows penetration analysis results of B material with original wall thickness of square box. It can be seen from Figure 6 that melt flows from bottom surface to side of plastic part to be formed, and core layer melt on long side of plastic part basically penetrates to the end, and effect is good, but penetration distance on short side is lacking, and penetration effect on corner surface is poor, resulting in uneven distribution of thickness of core layer of plastic part and penetration effect of core layer. This is because flow length of melt on long side and short side is inconsistent, which will adversely affect mechanical properties and service strength of plastic part. In view of this phenomenon, on the basis of not changing shape design of plastic part, wall thickness optimization design of square box-shaped plastic part is carried out to improve melt flow balance of square box-shaped plastic part, improve uniformity of distribution of core layer material and penetration effect of core layer melt, improve performance and strength of plastic parts.

Figure 6 Penetration result of original wall thickness of square box B material
Aiming at inconsistency of original wall thickness on long and short sides of core layer melt penetration, optimal design of scheme shown in Table 4 was carried out. Analysis results of penetration length of B material for four schemes are shown in Figure 7. By adjusting wall thickness of plastic parts, mainly by thickening corner surface and short side surface, plan 1 to plan 4 gradually realize uniform distribution of core layer melt, core layer melt distribution in plan 4 has the best uniformity, which meets industrial production requirements of plastic part.

Wall Thickness Location	Underside	Corner side	Long side	Short side
Original plan	2.25	2.25	2.25	2.25
Plan 1	2.25	2.30	2.25	2.25
Plan 2	2.25	2.50	2.25	2.25
Plan 3	2.25	2.50	2.25	2.50
Plan 4	2.25	2.70	2.40	2.50

Table 4 Optimization scheme design of box-shaped plastic parts ( mm )

Fig.7 Analysis results of penetration length of B material for 4 plans

2.4 Mold test verification of box-shaped plastic parts

According to optimization results of square box-shaped plastic parts, plan 4 is taken as optimization plan of plastic parts, and sandwich injection production is carried out. Sandwich injection sample is shown in Figure 8. Results show that optimized square box-shaped sample has good surface quality, no flash, warpage, melt front breakthrough, etc. Surface material of plastic part is all A material, core layer material is evenly distributed, and basically penetrates to edge of plastic part place. Total weight of sample is 77 g, of which shell material is about 45.8 g, accounting for about 59.5%, and core material is about 31.2 g, accounting for about 40.5%.

Figure 8 Sandwich injection sample

Gud Mould Industry Co., Ltd