Plastic parts layout refers to arrangement of one or more kinds of plastic parts through reasonable injection moulding process and mold structure according to requirements of customer. Layout of plastic parts complements mold structure and plastic processability, directly affects later injection moulding process. When aligning, corresponding mold structure must be considered, position is adjusted under condition that mold structure is satisfied.
Consider following points from perspective of injection moulding process:
(1) Flow length. Flow length of each compound is different. If flow length exceeds process requirements, plastic parts will not be filled. (See Chapter 2 for details)
(2) Flow path waste. Under premise of satisfying filling of each cavity, length of flow channel and size of section should be as small as possible to ensure minimum of flow path waste.
(3) Gate location. When gate position affects position of plastic parts, gate position must be determined first and then ranked. In the case of a multi-cavity mold, gate location should be uniform.
(4) Feeding balance. Injection balance means that compound fills cavity at the same time under substantially same conditions.
(5) In order to meet feeding balance, following methods are generally used:
A. According to feeding arrangement (as shown in Figure 5.1.1), it is suitable for case that size of plastic parts is basically same.

 

B. According to large plastic parts close to main runner, small plastic parts are arranged away from main runner, then plastic and gate size are adjusted to meet feeding balance.
Note: When weight ratio of plastic parts is greater than 8, it should be discussed with product designer. In this case, it is difficult to adjust runner and gate size to meet balance requirements.
(6) Cavity pressure balance. Cavity pressure is divided into two parts, one is axial pressure parallel to mold opening direction; second is lateral pressure perpendicular to mold opening direction. Displacement stress is to balance axial pressure and lateral pressure with respect to center of mold to prevent peak of overflow.
Ways to meet pressure balance:

A. Position is uniform and symmetrical. Axial balance is shown in Figure 5.1.2; lateral balance is shown in Figure 5.1.3.

 

B. Using mold structure balance, figure 5.1.4. This is a commonly used method for balancing side pressure.

 

Consider following points from perspective of mold structure

Position should be such that runner and nozzle are at a certain distance from edge of front cavity to meet sealing requirements. Generally, D1≥5.0mm, D2≥10.0mm, as shown in Figure 5.1.5.

 

Distance between row slot and edge of seal should be greater than 15. mm.

When aligning, it should meet space requirements of mold structural parts, such as wedge, slide, and lifter. At the same time, following points should be guaranteed:
A. Mold structure has sufficient strength;
B. No interference with other mold base parts;
C. When there are moving parts, stroke must meet ejector requirements. When there are multiple moving parts, no mutual interference. See Figure 5.1.6
D. Position of ejector sleeve is required to avoid position of K.O. holes.

 

In order to achieve a better cooling effect of mold, attention should be paid to effect of screw and ejector pin on cooling water hole when ranking, position of cooling water hole is reserved.

When ranking, it should be as compact as possible to reduce size of mold, length and width ratio should be appropriate, and also consider installation requirements of injection molding machine.

Surface that open mold and remove plastic parts or casting system, called parting surface. In addition to influence of displacement, parting surface is also affected by various factors such as shape, appearance, precision, gate position, row position, ejection, and processing of plastic part. A reasonable parting surface is a prerequisite for integrity of plastic part. Generally, it should be considered from following aspects:
(1) According to basic requirements of release of plastic parts, plastic parts can be taken out from mold smoothly, position of parting surface should be set at projection edge of the largest part of stripping direction of plastic parts.
(2) Make sure that plastic parts are left on the side of back mold, and it is good for ejection and thimble marks are not exposed on design surface.
(3) Parting line does not affect appearance of plastic parts. Parting surface should try not to damage smooth outer surface of plastic part.
(4) Ensure quality of plastic parts. For example, plastic parts with coaxiality requirements are placed on same side of parting surface.
(5) Selection of parting surface should avoid formation of side holes and undercuts. If slide forming is required, slide structure should be simple and front slide position should be avoided as much as possible.
(6) Reasonably arrange gating system, especially gate location.
(7) Meet locking requirements of mold, direction in which projection area of plastic parts is large is placed in mold clamping direction of front and rear molds, and direction in which projection area is small is used as lateral parting surface; When it is a curved surface, it should be locked with a bevel.
(8) Conducive to mold processing.

Generally, horizontal distance between top surface of step and root is D≥0.25, as shown in Figure 5.2.1. To ensure requirements of D, size of angle "A" is generally adjusted. When angle affects product structure, it should be determined in consultation with relevant person in charge. When there are several step faces in parting surface, and H1≥H2≥H3, angle "A" should satisfy A1≤A2≤A3, and try to take same angle to facilitate processing.

 

Angle "A" should be selected as follows:
When H ≤ 3mm, slope α ≥ 5 ̊; 3mm ≤ H ≤ 10mm, slope α ≥ 3 ̊; H > 10mm, slope α ≥ 1.5 ̊;
When some plastic parts have special requirements, they should be selected according to product requirements.

 

When selected parting surface has characteristics of a single curved surface (such as cylindrical surface), as shown in Figure 5.2.2, it is required to construct a parting surface according to pattern of Figure 5.2.2a, that is, according to curvature direction of curved surface. Otherwise, an unreasonable structure as shown in Fig. 5.2.3a is formed, which produces a sharp steel and a sharp-shaped sealing surface, and pointed sealing position is not easy to seal and is easily damaged.

 

When parting surface is a more complex space surface and cannot extend a certain distance according to curvature direction of surface, surface cannot be directly extended to a certain plane, which will produce a step and pointed seal surface as shown in Figure 5.2.4a. and should construct a smoother seal surface in the direction of curvature, as shown in Figure 5.2.4b.

 

In mold, care should be taken to ensure an effective seal distance on same surface. As shown in Figure 5.2.3a, 5.2.3b, general situation requires D≥3mm.

 

When constructing a parting surface, if there are one or more parting surfaces with height differences such as step type and curved type, it is necessary to construct a reference plane, as shown in Figure 5.2.5a and 5.2.5b.
Purpose of reference plane is to provide placement planes and machining references for subsequent machining.

Turning position here refers to a step surface formed by parting surface at different heights in order to be in contact with reference plane.
Step surface is required to be as flat as possible. Size of figure "A" is generally required to be greater than 15°, which is allowed to be avoided when clamping. Corner R gives priority to machining tool radius, generally R≥3.0 mm.

 

Since lateral pressure generated by cavity cannot be self-balanced, it is easy to cause front and rear molds to be displaced in the direction of force. Generally, inclined surface is used to lock, rigidity of front and rear molds is used to balance lateral pressure. As shown in Figure 5.2.7, locking ramp requires a complete fit when clamping.

 

Angle A is generally 15°, and the greater slope, the worse balance effect.

When constructing parting surface, if parting surface near nozzle has a height difference, it must be connected with a flat surface. Range of flat surface is larger than diameter of nozzle. Effective area should be greater than Ø18mm, as shown in Figure 5.2.6.

 

Regardless of whether original hole is left or needle is inserted, following methods are generally adopted to construct hole position. In order to make mold simple, it is recommended to insert needle at hole position, subject to designer's permission.

 

A. Direct contact. As shown in Figure 5.2.9, it is suitable for a structure with a relatively flat collision. However, for keyholes of "keyboard" class (see Figure 5.2.10a), in order to change direction of "flash" that may be generated, structure and size of insertion form are often used, as shown in Figure 5.2.10b.

 

B. Intermediate plane collision. As shown in Figure 5.2.11a, structure that is used for steeper collision structure with intermediate plane can effectively shorten height of steel at the perforation and improve stress of steel. In order to avoid misalignment of front and back mold, it is recommended to use size and structure shown in 5.2.21a. In structure shown in Figure 5.2.11b, due to lateral component force at the impact, when impact perforation is small, under action of alternating stress, steel position at perforation is prone to break, which affects life of mold.

 

C. Insertion. Generally not used and only used in the cases shown below.
(1) When height difference between "a" point and "b" is less than 0.5 mm, as shown in Fig. 5.2.12a, insertion structure is adopted.
(2) When "a" point is higher than "b" point, as shown in Figure 5.2.12b, insertion structure is adopted.

 

When using a plug-in structure, structure and dimensions shown in Figure 5.2.12c are often used. Minimum distance of sealing surface shall be 1.0mm; inclination of guiding part shall be A≥5o and length H≥2.5mm.

When parting line has to be split into a curved surface, in order to avoid sharp steel, direction of parting surface should be normal direction of any point on parting line, as shown in Figure 5..2.13.

 

For a single product, when there are multiple choices for parting surface, it is necessary to consider appearance requirements of product and select a more subtle parting surface. For finished product with slide parting, slide parting line must consider structure of adjacent finished product. If adjacent finished product also needs slide parting, then slide parting line should be adjusted and aligned; as shown in Figure 5.2.14a; 5.2.14b; 5.2.14c; if adjacent finished product does not need slide parting, in the case of satisfying structure, slide parting line should be shortened as shown in Figure 5.2.7d.