In injection molding design, in addition to general mold design considerations, the following aspects should be given special attention:

1. To achieve the desired product dimensional tolerance, appropriate mold dimensional tolerances must be considered.
2. Preventing fluctuations in molding shrinkage rates should be accounted for.
3. Preventing molding deformation must be addressed.
4. Avoiding deformation during demolding should be taken into consideration.
5. Minimizing mold fabrication errors is essential.
6. Controlling mold precision fluctuation should also be considered.


**1. Appropriate Mold Dimensions and Tolerances**
**1.1 Relationship between Product Dimensional Accuracy and Mold Dimensional Accuracy**
A product drawing should be created, considering the mold design, mold fabrication, and molding process.

The mold dimensions can initially be derived from the product drawing. Based on these dimensions, the mold is fabricated to achieve the actual mold dimensions. Using this mold, the actual molded product can be obtained, and its dimensions can be checked against the required dimensional tolerances.

**1.2 Appropriate Shrinkage Rate**
As noted, even when using the same resin with the same colorant, the shrinkage rate can vary depending on the molding conditions. In precision molding, shrinkage rate variations should be minimized, and predicted and actual shrinkage should ideally align closely. The typical approach is to reference the actual shrinkage rates of similar past products to estimate shrinkage, though experimental molds can also be used to obtain actual shrinkage rates, which are then refined for the production mold.

However, it is nearly impossible to perfectly predict the shrinkage rate, so mold adjustments are often necessary after trial molding. As a result, adjustments tend to increase dimensions for recesses and reduce dimensions for projections. Therefore, for recess dimensions, a smaller shrinkage rate should be used, while for projections, a larger rate should be applied. For gear outer diameters, a smaller shrinkage rate should be set to avoid interference, as larger shrinkage will only increase the backlash.

**2. Preventing Fluctuations in Molding Shrinkage Rate**
In precision injection molding, producing a mold to meet exact dimensions is essential. However, even with fixed mold dimensions, the actual product size may vary due to shrinkage differences. Therefore, controlling shrinkage rate is crucial in precision injection molding.

The suitability of mold design largely determines shrinkage rate, and this may also vary by resin batch. A change in colorant can also affect shrinkage. Furthermore, differences in molding machines, variability in molding conditions, and the reproducibility and consistency of each cycle all impact actual shrinkage, making control challenging.

**2.1 Primary Factors Affecting Shrinkage Rate**
Mold dimensions can be derived by adding shrinkage to product dimensions, so major factors influencing shrinkage must be considered in mold design.

Primary factors influencing molding shrinkage include: (1) resin pressure, (2) resin temperature, (3) mold temperature, (4) gate cross-sectional area, (5) injection time, (6) cooling time, (7) product wall thickness, (8) reinforcement content, (9) orientation, and (10) injection speed. These factors vary depending on resin type and molding conditions.

- **(1) Resin Pressure**: Resin pressure significantly affects shrinkage; higher pressure reduces shrinkage and increases product dimensions. Even within the same cavity, resin pressure may vary due to differences in product shape, resulting in shrinkage variations. In multi-cavity molds, resin pressure differences among cavities lead to varying shrinkage rates.

- **(2) Mold Temperature**: For both amorphous and crystalline resins, higher mold temperatures increase shrinkage. Maintaining mold temperature at a specific level is essential in precision molding, and attention should be given to the cooling circuit during mold design.

- **(3) Gate Cross-Sectional Area**: Generally, shrinkage varies with changes in gate size. Shrinkage decreases as gate size increases due to resin flow characteristics.

- **(4) Product Wall Thickness**: Wall thickness affects shrinkage. For amorphous resins, larger wall thickness increases shrinkage, while smaller thickness reduces it. In crystalline resins, excessive variations in wall thickness should be avoided. In multi-cavity molds, differences in cavity wall thickness also lead to shrinkage variations.

- **(5) Reinforcement Content**: With glass fiber-reinforced resins, higher glass fiber content reduces shrinkage, and shrinkage along the flow direction is smaller than across it. Careful consideration of gate design, location, and quantity is necessary to prevent warping.

- **(6) Orientation**: All resins exhibit orientation to some degree, but it is particularly significant in crystalline resins. It varies based on wall thickness and molding conditions.

Post-molding shrinkage is also influenced by factors such as (i) internal stress relief, (ii) crystallization, (iii) temperature, and (iv) humidity.

**2.2 Measures to Take**
- **(1) Runner and Gate Balance**: As discussed, shrinkage rate depends on resin pressure. To achieve uniform filling in single-cavity, multi-gate, or multi-cavity molds, gate balance is essential. Achieving flow balance within the runner is recommended before gate balancing.

- **(2) Cavity Arrangement**: To simplify setting molding conditions, cavity arrangement should be carefully planned. In typical cavity arrangements, mold temperature distribution forms concentric circles around the gate. When selecting cavity arrangement for multi-cavity molds, concentric ring arrangements centered on the gate are optimal.


**3. Preventing Molding Deformation**
Molding deformation results from internal stress due to uneven shrinkage, so uneven shrinkage should be minimized. For circular products with a central hole, a center gate should be used. However, if shrinkage differs significantly between the flow and perpendicular directions, an elliptical shape may result. For high precision, three or six-point gating may be necessary, with careful gate balance.


**4. Preventing Deformation During Demolding**
Precision products are generally small with thin walls and sometimes thin ribs. Mold design should minimize product deformation and ensure easy demolding. For high-pressure molding, attention is needed to prevent products from sticking to the mold cavity. When molding gears with low-shrinkage resins, it is ideal to place gear cavities on the ejection side template. When ejector pins are used, their number and ejection pressure points must prevent deformation.


**5. Minimizing Mold Fabrication Error**
- **5.1 Appropriate Mold Structure for Desired Processing Method**: Achieving desired product dimensional accuracy requires corresponding mold dimensions and high-precision machining. High wear resistance is essential for maintaining mold accuracy, requiring quenching. Grinding machines and EDM can achieve precision within 0.01 mm.

- **5.2 Modular Molds**: Modular molds are used to achieve high precision in quenched parts using grinding. Features of these molds include:
- (1) Ability to choose suitable materials with appropriate hardness.
- (2) High corrosion and wear resistance.
- (3) Separate heat treatments, facilitating optimal treatment conditions.
- (4) Good polishability, enhancing mirror finish.
- (5) Small draft angles, facilitating easy grinding.
- (6) Hardness retention, extending mold life.
- (7) Easy vent positioning, simplifying cavity filling.
- (8) Easy grinding, improving component accuracy and interchangeability.


**6. Preventing Mold Precision Error**
To ensure consistent positioning of sliding components in each cycle, fluctuations in mold precision must be minimized. Quenching and grinding of sliding components are necessary to maintain precision, and side core sliding parts should have recesses for positioning alignment.