Analyzing whether structure of plastic part meets requirements of mold forming and ejection can be carried out from following aspects: demoulding angle, rubbing position, slider, lifter, sharp thin steel position, and ejection.

Plastic part must have sufficient demoulding slope to avoid topping, and dragging. Demolding slope is related to properties of plastic material, shape of plastic part, and surface requirements.
Recommended value of minimum draft angle of commonly used plastic materials . For parts that do not require demolding inclination in 3D file of plastic parts, refer to general demolding inclination requirements in technical description. Appearance surface of plastic parts requires smooth or textured surface, and its demolding slope is also different. Slope values are as follows:
(1) Demolding slope of small plastic parts with smooth outer surface is 1˚, and demolding slope of large plastic parts is 3˚;
(2) External surface etched surface Ra <6.3, draft angle of 3˚, Ra6.3 draft angle of 4˚;
(3) Fire pattern surface of outer surface Ra <3.2 has a draft angle of 3˚, and Ra3.2 has a draft angle of 4˚.
For plastic parts of built 3D model, use Pro/E to check demolding angle. Steps are as follows:
Analysis ® Surface Analysis ®
Draft Check®[Given maximum slope value, select analysis Part or Surface, and determine direction surface corresponding to analysis] ® Compute.

 

It should be noted that when modifying demoulding angle of plastic part, requirements of assembly relationship and appearance of plastic part must be ensured, as shown in Figure 3.2.1.

Mold rubbing and meeting are shown in Figure 3.2.2. Rubbing surface of mold should have a slope, and rubbing slope has two functions:

(1). Prevent glue overflow, because vertical bonding surface cannot be preloaded;

 

(2) Reduce wear.
Analysis of rubbing and meeting can be considered from following aspects:

(1). Ensure structural strength. As shown in Figure 3.2.3, in order to avoid deformation or breakage of protruding parts of mold, value of B/H in design is more than or equal to 1/3.

 

(1). Prevent occurrence of flashes. As shown in Figure 3.2.3, surface fit value E is 1.2mm. As shown in Figure 3.2.4 and Figure 3.2.5, ensure that surface clearance value e is 0.25mm. If considering slope of rubbing surface, when h is 3mm, slope α is 5˚; when h> 3mm, slope α is 3˚; when some plastic parts have specific requirements on slope, height of rubbing surface h is 10mm , allowable slope α is 2˚. Sealing position at the tip of rubbing surface should have rounded corners R0.5 or more.

 

(3). Facilitate mold processing and maintenance. As shown in Figure 3.2.6 and Figure 3.2.7, inserts are made on rotating shaft mold.

 

When side wall of plastic part has concave and convex shapes, side holes and buckles, side core must be drawn out before mold is opened to eject plastic part. This mechanism is called slider. As shown in Figure 3.2.8, outer hole of plastic part needs to be pulled out of back mold. As shown in Figure 3.2.9, if inner groove of plastic part is ejected with an lilfter, top spacing is not enough, and inner slider must be adopted.

 

 

In addition, ejection mechanism that uses diagonal ejection, ejection and core pulling is completed at the same time is called inclined ejection. For parts on plastic parts that need to be core-pulled, when space for positioning is insufficient, lifter mechanism can be used to complete. In lifter mechanism, diagonal ejection distance should be greater than core pulling distance (B> H) as shown in Figure 3.2.10 to prevent ejection interference.
As shown in Figure 3.2.11, inner and outer side walls of plastic part are concave, there are bone obstructions and insufficient height on inner side. Anterior mold of outer side wall must be aligned, and inner side wall must be ejected obliquely.

 

As shown in Figure 3.2.12, there should be no clamping line around side hole of plastic part, side hole must be core-pulled in front mold position, buckle position must be ejected from mold.

Regardless of whether parting surface is specified in plastic parts data, mold designer must specifically determine; if there is an unreasonable part of specified parting surface, feedback to the other party.

 

Pay attention to following points when analyzing parting surface of plastic parts:
(1) Determine surface clamping position according to appearance requirements, as shown in Figure 3.2.13.
(2) Place part of plastic part that requires coaxiality or is easy to be misplaced on same side of parting surface, as shown in Figure 3.2.14 and Figure 3.2.15.

 

(4). Consider size difference between large and small ends of plastic part caused by draft angle, as shown in Figure 3.2.16.

 

(5). Determine position of plastic part in mold so that parting surface formed should try to prevent side holes or undercuts to avoid use of complex mold structures, as shown in Figure 3.3.16 and Figure 3.3.17.

 

Avoid sharp and thin steel positions that affect strength and service life of mold. Generally, sharp and thin steel positions are not easy to reflect on plastic parts, so analyze mold conditions of plastic parts. There are two reasons for sharp and thin steel position on mold-structure of plastic part and structure of mold.

(1). Sharp and thin steel bits produced by structure of plastic parts. As shown in Figure 3.2.18 and Figure 3.2.19, double wishbone of plastic part produces sharp and thin steel positions on mold; it can be changed to a single wishbone or middle width is enlarged to avoid sharp and thin steel positions on mold.

 

(2). Sharp and thin steel positions produced by mold structure. As shown in Figure 3.2.20, at rounded corners of plastic part, sharp steel is easy to appear on mold; mold structure is shown in Figure 3.2.21, this method splits mold, sharp steel appears; Figure 3.2.22 shows that parting surface extends direction of arc normal to avoid sharp steel.

 

 

 

Plastic parts are usually ejected from mold using thimble, cylinder and push plate. If plastic part has special structure or surface finish requirements, other methods should be used to eject mold, such as ejector block ejection, diagonal ejection, screw rotation ejection, secondary ejection, etc. For ejection of certain transparent plastic parts, it must be noted that ejection marks cannot be exposed.
As shown in Figure 3.2.23, multi-cavity thin shell small plastic parts are ejected by push plate.

 

As shown in Figure 3.2.24, plastic part is a transparent sheet. In order to avoid pinching traces, top block is used for ejection; note that there should be no rounded corners on the bottom of such plastic parts to prevent ejection marks from leaking.

 

As shown in Figure 3.2.25, plastic part has an inner concave arc, and a secondary ejection mechanism is adopted to realize plastic part ejection.