Bottom frame products of auto parts are shown in Figure 1. Maximum dimensions of product are 147.00 mm * 98.40 mm * 39.09 mm, average thickness of plastic part is 2.00 mm, material of plastic part is ABS, shrinkage rate is 1.005, and weight of plastic part is 24.30 grams. Technical requirements for plastic parts are that there should be no defects such as peaking, underfilling, flow lines, pores, warping deformation, silver streaks, cold material, and spray lines.

 

Figure 1 Product picture of bottom frame of auto parts
Bottom frame of auto parts is internal support of Volkswagen's navigation system, which has high requirements. Specific technical requirements are as follows: In addition to being free of various defects, material of plastic parts must meet TL527A standard, dynamic test must meet TL823 45, environmental cycle test shall be in accordance with PV2005 auto parts climate change stability test, flame retardant performance shall be TL1010, and light resistance shall be in accordance with PV1303. Quality of Volkswagen is excellent, and its technical standards have many typical representatives, which are worthy of reference for engineers who design automobile molds.
It can be seen from Figure 1 that basic structure of plastic part is a rectangular irregular narrow frame plastic part. Plastic parts do not have sliders and lifters, and mold structure is relatively simple. Due to irregular shape of plastic part, its dimensional tolerance is strict. Tolerance is shown in Figure 2. In Figure 2, some important assembly dimensions and center distance dimensions are marked with tolerances. SC in box behind dimension indicates special characteristics, which can also be understood for important dimensions.
When designing mold, it is first necessary to analyze plastic part to have a preliminary and reasonable judgment on mold structure. This set of molds does not have a complicated structure, main difficulty is analysis of mold opening direction, and further is parting surface design of plastic parts. There are 6 U-shaped columns on the edge of plastic part, facing front mold and back mold of mold respectively, resulting in a complicated parting surface here. There are two flat cylindrical columns on one side of plastic part, which are easy to stick to mold.
Analysis of mold opening direction is closely related to ejection of plastic parts, gate design and parting surface design. Selection of mold opening direction determines the entire mold structure, which is related to mold manufacturing cost, mold life, injection molding cycle, size of injection molding machine, and whether full-automatic injection molding can be used, which will generally affect injection molding cost. Parting surface of this set of molds is not complicated. Difficulty lies in choice of mold opening direction, that is, which side of plastic part is placed on movable mold and which side is placed on fixed mold. Correct judgment will leave side with high mold sticking force in movable mold to facilitate ejection and demoulding of plastic parts, and design gating system on the other side. In most cases, mold opening direction can be determined by using factors such as wrapping force, and plastic part is left in movable mold. Inner circle of this set of molds is reserved for rear mold, and 4 U-shaped columns are reserved for rear mold, which is beneficial for plastic parts to be reserved for rear mold. Two flat cylindrical columns are left in the front formwork, where tight force on front formwork is very large, so front formwork must be designed with a first-drawing structure.

 

Figure 2 Product Tolerance Requirements

 

Figure 3 Analysis of front mold extraction
Structure of first drawing of front mold is shown in Figure 3 and Figure 6. Gating system of mold adopts latent gate, as shown in Figure 7. In order to adapt to mold structure where front mold is drawn first, panel of mold is separated from A plate by a certain distance, and front mold is drawn first to reduce tightness of insert on flat cylinder. Therefore, end surface of sprue sleeve needs to form a separation movement with A plate, so a 2° taper needs to be ground on its circumference. A spring is designed between panel and A-plate as driving force for separation, and plug screw is used as limit. A nylon buckle is designed to be tightened between plate A and plate B, so that panel and plate A are separated first, so that front mold can be pulled out first.

 

Figure 4 3D mold design drawing

 

Figure 5 Basic structure of mold
Structure of plastic part is a narrow frame structure, which has a large tightening force for movable mold, so ejection force is very large. In order to prevent plastic parts from being ejected and deformed, enough thimbles and flat thimbles must be designed around plastic part, and at the same time, rigidity of fixed mold and movable mold must be increased to prevent insufficient rigidity of mold. At the same time, gap between reset lever and B plate is appropriately reduced to improve its installation accuracy, and screw is supported in the design of ejector plate to increase ejection stability.
Two slender inserts of front mold are drawn first, which need to be sloped and directly embedded in fixed plate. Both mold plate and mold core should be designed to carry water through to prevent inserts from burning after mold heats up.

 

Fig. 6 Design drawing of gating system of front mold first extraction mechanism