(1) Average wall thickness: This concept is crucial for product design. Finished shell should have a uniform wall thickness, and internal rib thickness must also be maintained at a certain proportion of average wall thickness. Otherwise, severe surface shrinkage is likely, as shown in Figures 1-6 and 1-7.

 

(2) As shown in Figure 1-8, shrinkage marks of cross-section with uneven wall thickness will remain on punch surface and will not affect appearance.
(3) Generally, if height of screw column exceeds 10mm, mold will use a sleeve (thread tube) to eject it directly using ejector plate without adding draft angle. Sleeve structure can be determined by whether top surface of screw post has an acute angle, as shown in Figure 1-9.

 

(4) Anti-shrinkage ring is a material reduction measure taken when screw post is extremely high and root wall thickness is relatively thick due to draft angle, making surface prone to shrinkage, as shown in Figure 1-7.
(5) If molding material is transparent and appearance does not allow for any traces of punch ejector pin, mold structure must utilize an ejector plate to eject the entire finished product. If finished product is too thick, an average wall thickness reduction must be made, and fillets (the larger the better) must be added at height differences to eliminate projection lines of material loss, as shown in Figure 1-10.

 

Figure 1-10 Rounded corners on the inside of acrylic parts
(6) If finished product structure design has a side convex (concave) design and cannot be demolded by a convex and concave mold, a slanted pin mold design must be used. Slanted pin's stroke must not have any structural elements and must be cleared (clearing means making room; in plastic molds, if two parts do not need to be tightly fitted, clearing should be used). As shown in Figure 1-11, calculations are as follows:

 

Spatial pin space = a + b, finished product demolding, slant pin travel = c.

 

Figure 1-11 Side-convex design with clearance
①Maximum limit angle of slant pin is 10°. The larger angle, the easier it is for finished product to deviate during ejection. A cross hook ejector rod must be used to position finished product.
②In calculation example, slant pin thickness is 6mm, and a 6° demolding ejection is used.
(7) Basic structure of front and rear shells—stop. Function of stop:
①Prevents gaps from appearing at parting line of shell due to poor sealing, allowing internal parts to be seen.
②Keeps parting line of shell flat, preventing scratches from step differences.
③Prevents external forces from causing shell to collapse, damaging electronic components, or causing product malfunctions. Draft angle of concave mold sidewall is shown in Figure 1-12:
①Polishing treatment ≥ 1.5°.
②Fine surface etching > 3°, side etching depth = front depth x 70%.
(8) If plastic product is very deep, as shown in Figure 1-13, using a regular round ejector pin for ejection can easily cause ejector pin to bulge. Therefore, it is necessary to use a flat ejector pin (or B-pin) to eject from parting surface, and pins must be evenly distributed to prevent uneven stress.

 

Molds can be divided into side gate and point gate series according to gate type.
(1) Single-point gate (pin gate): Commonly known as a two-plate mold, it directly injects material from concave surface of finished product, as shown in Figure 1-14.

 

Advantages: Material flow in cavity is uniform, and its position is generally located in the center of finished product, so there is no concern about weld lines (watermarks).
Disadvantages: ① Material head must be processed twice, and milled flat with a milling cutter.
② Injection position must be covered with a nameplate or sticker.
(2) Multi-point pin gate: Commonly known as a three-plate mold, it has an additional ejector plate, as shown in Figure 1-15.

 

Figure 1-15(a) Multi-point gate (three-plate mold)

 

Figure 1-15(b) Example of multi-point casting
Advantages: No need to trim gate; gate is ejected and broken off by ejector plate. It is often used for finished products where material leakage is difficult or where aesthetics are a concern.
Disadvantages: Due to multi-point injection, more weld lines are formed. These can be eliminated by using molding conditions or mold temperature control.
(3) Side gate: As shown in Figure 1-16, depending on mold structure, system can be divided into four types, including two-plate and three-plate molds.

 

(4) Pin gate: Depending on mold structure, it can be divided into following types.

(5) Submarine gate: Mold has a three-plate structure, with sprue entering through "D"-shaped boss of punch, as shown in Figure 1-17.

 

Figure 1-17 Submerged gate
Purpose and Design of Mold Vent Holes
When mold is closed and nozzle is ready for injection molding, if there is no appropriate vent hole design in cavity, gas inside cavity cannot escape, and finished product is prone to forming bubbles, localized surface burning, and poor appearance. Mold vent hole design is as follows:
(1) Utilize parting surface of mold module as a venting groove, as shown in Figure 1-18.
(2) Add a permeable metal round rod (mesh) insert to punch.
(3) Utilize ejector pin clearance as a venting hole.

 

Figure 1-18 Vent design
Based on experience of mold makers, if A < 0.1mm, finished product will still not have burrs.