Mold design for demoulding mechanism

Time:2019-11-11 10:32:01 / Popularity: / Source:

Demoulding of plastic parts is the last step in injection molding process. Quality of demoulding will finally determine quality of plastic parts. When mold is opened, plastic parts must be left on half mold half with demoulding mechanism(usually on moving mold). using demoulding mechanism to pull out plastic parts.

Demoulding design principles:

(1) In order to prevent plastic parts from being deformed due to demolding, thrust arrangement is as uniform as possible, and as close as possible to shrink-wrapped core, or part that is difficult to demould. Such as elongated column of plastic parts, demoulding by E.P. sleeve.
(2) Thrust point should be applied to part with the highest rigidity and strength of plastic part, avoiding action on thin position, and action surface should be as large as possible, such as flange, bone position, shell wall edge, etc. Cylindrical plastic parts are mostly demolded by push plates.
(3) Avoid demoulding marks affecting appearance of plastic parts. Demolding position should be set on concealed surface (internal) or non-appearance of plastic parts; especially for transparent plastic parts, selection of demolding position and demoulding form must be noted.
(4) Avoid whitening and deformation of plastic parts due to vacuum adsorption. It can be used for compound demoulding or exhaust with ventilated steel. For example, demoulding by ejector pin and push plate or ejector rod and push block, and ejector pin is appropriately enlarged to fit gap venting, an intake valve may be provided if necessary.
(5) Demoulding mechanism should be reliable and flexible, have sufficient strength and wear resistance. For example, demoulding by swinging rod and inclined roof, strength and wear resistance of sliding surface should be improved, lubricating groove should be opened on sliding surface; surface hardness and wear resistance can also be improved by nitriding treatment.
(6) Mold return needle should be in contact with front mold plate after mold clamping, and length is less than 0.1mm, as shown in Figure 8.1.1.
(7) Spring return is often used for ejector plate return; because spring reset is not reliable, it can not be used as a reliable first reset.
demoulding mechanism 

8.1 Demoulding by E.P., Rectangular E.P.

Commonly used methods for demoulding of plastic parts include ejector pin, rectangular E.P., push plate demoulding; because price of E.P. sleeve and rectangular E.P. is higher (8 to 9 times more expensive than ejector pin), push-plate demoulding is mostly used in cylindrical thin-shell plastic part. Therefore, the most used demoulding is ejector pin. When ejector pin cannot be arranged around plastic parts. For example, if surrounding area is mostly deep bone position and bone depth is 15 mm, rectangular E.P. can be used for demoulding. Surface hardness of ejector pin and rectangular E.P. is above HRC55, and surface roughness is less than 1.6. Ejector pin and rectangular E.P. demoulding mechanism are shown in Figure 8.1.1. Main points are as follows:
(1) When diameter of ejector pin is d£Ø2.5mm, stepped E.P. is selected to increase strength of ejector pin.
(2) Rectangular E.P., stepped E.P. K/H.
(3) Ejector surface is a sloped surface, dowel must be added on fixed end of ejector pin; To prevent ejector slip, slope can process a plurality of R slots, as shown in Figure 8.1.2.
demoulding mechanism 
(4) Match length of rectangular E.P., ejector pin and hole is L=10~15mm; for small diameter thimble, L takes 5 to 6 times diameter.
(5) Ejector pin is at least 0.15 mm from edge of cavity, as shown in Figure 8.1.2.
(6) Avoid collision between ejector pin and front mold, as shown in Figure 8.1.3. This result is easy to damage front mold or peak.
Principle of layout of ejector pin position.
8.1.1 Matching clearance of ejector pin and rectangular E.P.
Matching parts of ejector pin, stepped E.P. and rectangular E.P. are shown in Figure 8.1.4, Figure 8.1.5, Figure 8.1.6. Matching requirements are as follows:
Rectangular E.P. 
Figure 8.1.4
Rectangular E.P. 
(1) Ejector pin head diameter d and rectangular E.P. matching size t, w and rear mold are matched with matching clearance of £0.04mm
(2) Size of ejector pin and rectangular E.P. hole is d10.8mm or d110.8mm in remaining non-mating sections, gap between fixed end of step and facet plate is 0.5mm.
(3) Bottom end of ejector pin and rectangular E.P. must be flush with bottom surface of needle plate.
(4) As shown in Figure 8.1.7, top end surface of ejector pin should be flush with back mold surface, which is higher than back mold surface by e£0.1mm.
E.P. sleeve 
8.1.2 fixation of ejector pin
(1) Ejector pin is generally fixed in the step of needle plate, as shown in Figure 8.1.4. In order to prevent ejector from rotating, there are two common methods: one is ejector pin axial step edge positioning by dowel, as shown in Figure 8.1.8; the other is lateral dowel positioning as shown in Figure 8.1.9.
ejector pin 
(2) Fixed by headless screw as shown in Figure 8.1.10. This method is used when there is no pad at the end of ejector pin. It is usually used on fixed ejector pin and three plate mold ball pull rod.
ejector pin 

8.2 Demoulding by E.P. Sleeve

E.P. Sleeve demoulding as shown in Figure 8.2.1. E.P. Sleeve is often used for demoulding of cylinders with a length equal to 20 mm.
Standard E.P. Sleeve surface hardness is HRC/60 and surface roughness is £Ra1.6. In addition, wall thickness of E.P. Sleeve should be 1mm; when cylinder is arranged, fixed position of E.P. Sleeve cannot interfere with K.O.
E.P. sleeve 
8.2.1 E.P. Sleeve matching requirements
Mating relationship of E.P. Sleeve demoulding is shown in Figure 8.2.2. Figure 8.2.3, matching requirements are as follows:
E.P. sleeve 
(1) Length of matching section between E.P. Sleeve and rear mold is L=10~15mm, matching gap of its diameter D is £
(2) Remaining unmatched section size is D10.8mm.
8.2.2 fixation of large E.P. Sleeve
E.P. Sleeve is fixed to bottom plate, usually using a headless screw as shown in Figure 8.2.1. When diameter of cylinder needle is d$8mm or 5/160, fixed end is fixed by a spacer, as shown in Figure 8.2.4.
E.P. sleeve 

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