【Abstract】Analyzed injection model and structure of injection mold of plastic bowl, designed cooling system by using Tian-shaped and s-shaped cooling water channels; compared two cooling schemes, analyzed temperature field distribution of plastic bowl, surface and internal temperature field of steel mold block after cooling for 10 minutes, S-shaped cooling scheme is better than Tian-shaped cooling scheme. In order to achieve a better cooling effect, an optimization model is proposed. Analyze pipe flow and heat transfer equations to obtain cooling influence factors, use optimization model to analyze temperature field distribution and verify that hydraulic flow rate, pipe material, and surface roughness play a decisive role in cooling.

Plastic has become an indispensable basic material in industry. In 2018, plastic products reached 60.42 million tons. Compared with traditional metal products, plastic products have advantages of lower price, better plasticity, light weight and corrosion resistance, making plastic products more and more widely used in life. In recent years, plastic molding processing technology has also been continuously improved, development of high-end plastic molding processing technology can better meet economic development needs of domestic plastic products. Plastic bowls are necessities in life. Injection molding process is mainly used in preparation process. Plastic raw material melt is injected into mold under a certain pressure and speed by an injection molding machine and cooled, thereby solidifying into a plastic bowl. Plastic bowl injection process is greatly affected by cooling, cooling speed and uniform arrangement will improve efficiency of preparation. For plastic bowl injection molding, a variety of cooling schemes are proposed, surface roughness, flow rate and layout scheme are compared. With the help of Comsol software, cooling process is analyzed and the best scheme is determined. At the same time, it provides references for design of injection molds for plastic products.

Plastic bowl has a circular structure with an external dimension D*h of 160*60mm and a wall thickness of 2.4mm. Structure is relatively simple, so accuracy requirements are low, and surface is mainly required to be bright. PP polypropylene material is used for injection molding. Because material of PP is light in weight and less dense than water, final heat resistance and toughness are strong, which is beneficial to improve performance of plastic bowl. However, requirements for mouth of bowl are higher, there should be no traces of pouring port. In addition, wall thickness of bowl is relatively thin, if simple traditional injection is used, it is difficult to succeed, and it is easy to produce a plastic bowl lacking material. In order to improve injection molding, injection pressure must be increased, but it will cause large internal stress and make it difficult to demold. During injection process, plastic parts are required to have no common injection surface defects, and no deformation is allowed. However, it was found that during injection process, cooling rate was too fast, causing mold temperature to be lower than 50℃, causing surface of plastic part to be not smooth, leaving marks above 90℃ would easily cause plastic to warp and deform. Therefore, hot runners and cooling structures are used to improve injection, filling and cooling problems of plastic bowl mold.
Injection membrane structure is shown in Figure 1. Ejection method is adopted for demolding, plastic melt is injected into cavity 6 of mold base 3 through sprue sleeve 1 during injection, pressure is maintained, cooling water channel 5 is used for cooling. When mold is fully opened, it is ensured that workpiece remains in core 4, ejector plate 7 is pushed up by ejector rod, thereby pushing out model. In order to realize reset, ejector rod 8 moves downward, ejector plate 7 is reset under action of return rod spring 9.

 

Figure 1 Injection mold structure
1. Sprue sleeve 2. Positioning ring 3. Mould base 4. Core 5. Cooling channel 6. Cavity 7. Ejector plate 8. Ejector 9. Return lever spring

Cooling is an important process in plastic bowl injection molding. First, cooling time may take up half of production cycle time, or even more; second, uniform cooling is used to avoid defects in manufacture of plastic bowls. Because plastic material cools down uniformly and slowly during mold injection process, residual stress can be avoided, which leads to risk of distortion and cracks in final plastic part. Therefore, cooling is very important for molding of plastic bowls, positioning and properties of cooling passage have become an important factor in mold design. Layout of cooling water channel directly affects cooling efficiency and temperature distribution of plastic bowl. The most important thing is to ensure that cooling of plastic bowl is more uniform and reduce shrinkage deformation. Two types of cooling water circuits are used. As shown in Figure 2, Tian-shaped water circuit uses a traditional cooling water circuit. Mold steel is drilled and pipe heads are drilled to form a Tian-shaped water circuit. Pipes are connected diagonally, all pipes are designed in a straight line. S-shaped waterway is manufactured by rapid prototyping and is evenly distributed on upper and lower sides of mold. Water pipes are all round pipes with an inner diameter of ϕ 10mm.

 

Figure 2 Cooling water circuit of two schemes
a —Tianzi waterway b —S-shaped waterway

Analyze cooling process through comsol and view distribution of temperature field of plastic bowl, as shown in Figure 3. After injection of plastic material, average temperature of plastic mold is 473K (199.85℃). Water at room temperature is used as cooling fluid and flows through pipe at a speed of 10L/min to simulate cooling process of 10min. After cooling for 10 minutes, temperature difference between the hottest part and the coldest part is about 100℃; average temperature of plastic bowl after Tian-shaped cooling is 421.78K (148.63℃), and average temperature of plastic bowl after S-shaped cooling is about 410K (136.85℃) ; It is found that surface temperature of plastic bowl after Tian-shaped cooling is higher than that of S-shaped plastic bowl, more concentrated at about 450K (176.85℃); from analysis of temperature of cooling pipe, it can be seen that temperature at outlet and middle section of Tian-shaped pipe does not change much, which means that temperature taken away by flow medium is not much, its efficiency is low; from S-shaped pipe, it can be seen that outlet temperature is relatively high, middle section goes from low to high to realize heat transfer. By comparison, it is found that S-shaped cooling pipe arrangement is more conducive to heat transfer of plastic bowl. It can be seen from Figure 4 that in Tian-shaped layout scheme, because there are fewer cooling pipes arranged up and down in plastic bowl, it can be seen that temperature of steel molds above and below plastic bowl is relatively high. However, temperature around steel mold is relatively low. Cooling pipe around plastic bowl is conducive to heat transfer around steel mold. In S-shaped layout scheme, it can be seen that there are more corresponding cooling pipes arranged on upper and lower sides of plastic bowl, so cooling at corresponding positions is more uniform, but heat of mold on both sides of water flow inlet and outlet is greater. But generally speaking, maximum temperature of S-shaped cooling solution is 460K (186.85℃) lower than that of Tian-shaped 468K (194.85℃), heat dissipation around plastic bowl is more uniform and efficiency is higher.
It can be seen from Figure 5 that temperature field distribution inside mold after cooling for 10 minutes, it is found that Tian-shaped cooling scheme has a higher temperature in the center of plastic bowl, which is higher than 400K (128.85℃), indicating that plastic bowl has less cooling corresponding to upper and lower sides; while maximum temperature of S-shaped cooling scheme is 350K (76.85℃), which is lower than Tian-shaped cooling, but it better guarantees temperature around plastic bowl, which is relatively uniform, temperature is lower than 345K (71.85℃), which has a better cooling effect for plastics.

 

Figure 3 Distribution of temperature field of plastic bowl after cooling for 10 minutes

 

Figure 4 Distribution of temperature field on the surface of mold block after cooling for 10 minutes

 

Figure 5 Distribution of temperature field inside mold block after cooling for 10 minutes

A large amount of heat is generated during injection process, main heat comes from injected plastic melt injected into mold and then radiated. If heat cannot be radiated in time, it will directly affect quality of plastic bowl and its production efficiency. Injection mold mainly transfers heat through three aspects. First, heat of plastic melt is in contact with core and cavity to transfer heat to mold; second, mold is transferred through cooling medium designed by cooling system; third, outer surface of mold contacts external environment to conduct heat convection and dissipate heat. Model is cooled by these three methods, among which 90%-95% of heat of injected melt is taken away by second method. Next, through pipeline flow and heat transfer equation, influencing factors of cooling heat are analyzed.

Following momentum and mass conservation equations describe flow in cooling channel:

 

Where u — —average fluid velocity of cross section in tangential direction of centerline of pipeline, mm/s
A — —Cross-sectional area of pipeline, mm2
ρ — —Density, kg/mm3
p — —pressure, N/mm 2
d h — —pipe hydraulic diameter, mm
t — time, s
f D — friction factor, given by following formula:

 

It can be seen from above equation (2) that friction factor f D depends on value e/d of surface roughness e divided by pipe diameter d.

a. Cooling pipes.
Energy equation of cooling water in pipeline is:

 

T — —Cooling water temperature, K
k (W/ (m · K)) — — Thermal conductivity
Second term on right represents heat dissipation caused by internal friction of fluid, which is negligible for short channel used in this model. Q wall is source term (W/m) that expresses heat exchange with surrounding mold block, and it exerts a heat balance function through line heat source at position of pipe.
b. Mold blocks and polyurethane parts.
Heat transfer in steel mold block and molded PP material plastic bowl is controlled by conduction:

 

Where T 2 — temperature of steel block, K
c. Heat exchange.
Heat exchange term Q wall (W/m) couples two energy balances obtained by equation (3) and equation (4) respectively, heat transfer through tube wall is expressed by following formula:

 

In formula, Z — circumference of tube, m
h — —Heat transfer coefficient, W/ (m 2·K)
T ext — —External temperature of tube, K
Q wall — is source term in tube heat transfer equation
Heat transfer coefficient h depends on physical properties and flow characteristics of water, and is calculated according to Nusselt number:

 

Where k-thermal conductivity of material
Nu - Nusselt number
d h — —Hydraulic diameter of pipe, mm
From calculation of above equations, it can be seen that fluid velocity u, pressure p, source term Q wall in tube heat transfer equation, friction factor f D and other factors in design of cooling system are related. Among them, the greater friction factor, the greater radial heat transfer.

According to Tian-shaped and S-shaped cooling schemes, a mixed cooling method of the two is proposed, as shown in Figure 6. And analyze temperature field of optimized model. It can be seen from Figure 6 that temperature of cooled plastic bowl is not much different from that of S shape, but surface and internal temperature of steel mold is further reduced. It can be found that the highest temperature of steel mold is 380K (106.15℃) concentrated on mold side. In steel mold, temperature at the bottom of corresponding plastic bowl is obviously more uniform and temperature is lower about 345K (71.85℃). Cooling effect of optimized model has been improved to a certain extent.

 

Figure 6 Temperature field distribution of optimized model
a — — Optimized model waterway layout plan b — — Distribution of temperature field of plastic bowl after cooling for 10 minutes
c — —Distribution of temperature field on the surface of mold block after cooling for 10 minutes d — —Distribution of temperature field in mold after cooling for 10 minutes

(1) In the design of cooling system, Tian-shaped and S-shaped waterway schemes are proposed. Distribute Tian-shaped and S-shaped waterways evenly on the top and bottom of mold. Comparing Tian-shaped and S-shaped cooling, analyzing distribution of temperature field after cooling for 10 minutes, it can be seen that S-shaped cooling scheme has more uniform cooling and better cooling efficiency, but temperature of mold at inlet and outlet of waterway is higher than that of Tian-shaped. Scheme and S-shaped scheme propose optimized schemes to improve cooling effect.
(2) In heat transfer analysis of cooling system, through pipe flow and heat transfer equation, influencing factors of cooling heat are analyzed, fluid velocity and pressure in cooling system design are related to source term and friction factor in tube heat transfer equation. Among them, the greater friction factor, the greater radial heat transfer.
(3) In optimization model and verification, a combination of Tian-shaped and S-shaped schemes is proposed. After further verification, it is found that mixing scheme is more beneficial to injection cooling molding of plastic bowl.