When you receive a product, do not rush into mold splitting. The most important first step is to check the product structure, including molding issues such as draft angles and wall thickness. Of course, this can be quite difficult for someone new to mold structure design, as they may not yet know what constitutes a product suitable for mold design. This is perfectly normal—it is simply a process of daily accumulation and experience.

Once you have analyzed the product’s draft angles, wall thickness, and undercuts in the ejection direction, you will generally understand the orientation of the mold parting line and the possible gate locations. Ultimately, however, these details must be confirmed with the customer.

Some may ask: Once I finish analyzing the product structure, can I start designing the mold? The answer is definitely NO.
To avoid detours in the design process, you must confirm several key factors that affect the mold structure.

The details are as follows:

1. Tonnage and model of the customer’s injection molding machine
If this is not confirmed, you cannot determine the diameter of the sprue bushing inlet, the locating ring diameter, the size and position of ejection holes, the depth the machine can extend into the mold, or even the mold base size and shut height. You might spend hours designing a complex mold with hydraulic cylinder core-pulling, feeling proud of your work—only to find the mold cannot run at the customer’s facility because they only have electric injection machines without auxiliary hydraulic cores. At that point, you will probably feel helpless.

2. The customer’s mold clamping method
Common methods include pressure plate clamping, screw clamping, hydraulic clamping, and magnetic clamping. Only after confirming this can you decide whether you need to design clearance holes for clamping screws or clamping slots on the mold.

3. Product issues identified earlier, parting lines, material, and shrinkage rate
Do not assume that PP shrinkage is always 1.5%. You must confirm the exact material grade and any modifiers the customer will use for mass production.

If possible, fully understand the product’s assembly relationship and function. This information is extremely helpful for mold design, as it tells you which surfaces are cosmetic, which are non-cosmetic, which areas can have larger draft angles, and which cannot be modified. You may even find that some undercuts can be eliminated or simplified if you understand the actual assembly and usage.

Always remember: Mold making is about simplifying complex problems.
I often see people proud of designing overly complex structures, which I consider ignorant. Product designers sometimes create unreasonable structures due to limited experience. If we, as downstream engineers, do not point these out, they may never improve.

You must also confirm external connection parameters for water, oil, air, and electrical circuits. Only by knowing the customer’s requirements in advance can you design these circuits predictably. Otherwise, you may finish the entire layout—only to find the customer requires series-connected oil lines inside the mold, forcing you to completely rearrange water lines, ejector pins, and screws.

The general design priority for these four circuits is:
1. Oil circuits first – to ensure balanced flow, especially for cylinder ejection. Unbalanced oil flow causes asynchronous ejection.
2. Water circuits second – to ensure cooling efficiency.
3. Air and electrical circuits last.

On the mold, the typical arrangement from top to bottom is: electrical connections, water circuits, air circuits, and oil circuits at the bottom.

Other details.
Once you have all this information, you can begin mold design—starting with mold splitting, which most designers enjoy because it gives a strong sense of accomplishment.

The principle of creating parting surfaces is simple is best. Use extrusion whenever possible instead of scanning or advanced commands. Maintain a holistic view and keep parting surfaces as clean as possible. For non‑precision molds, avoid shut-off inserts within 0.1–0.5 mm if you can. Parting surfaces should also follow the natural shape of the product for a clean, logical layout.

As a side note: When learning 3D software, make sure you understand the logic behind each command. The key is not knowing the software, but applying it flexibly—especially in UG. In short: Mindset matters more than the tool.

When creating parting surfaces:
- Maximize the angle of shut-off surfaces
- Maximize the area of touch-off surfaces
- Maximize the width of lands

Always ask yourself: Will the fitter who bench-fits this mold curse me later? If the answer is no, proceed. If you do get complaints, those become valuable experience.

While creating parting surfaces, you must plan the layout of slides and lifters, as they also affect the parting line.
A typical slide uses trigonometric relationships. Keep the angle of the angle pin below 30°. Use the largest feasible diameter for angle pins, since they bear force.

Slides have many variations: upward slides, downward slides, internal slides, hydraulic cylinder slides, cavity-side slides, slide‑in‑slide, slides with reverse ejection, slides with lifters, etc. All use trigonometry to release undercuts and ensure normal mold opening and closing.

For lifters, keep the angle preferably below 15°. Angles over 20° risk poor durability and rough motion, based on trigonometry and theoretical mechanics. Lifters also come in many forms: upward, downward, offset, large lifters with inserted rods, small lifters with locking tabs, lifters under ejector blocks, lifters on slides, lifters on lifters, etc. All serve one purpose: using trigonometry to form undercuts.

Once mold splitting is complete, you move on to mold layout.
Based on the product, select the appropriate structure: two-plate mold, three-plate mold, hot runner, IMD, IML, two-shot mold, stack mold, etc. All structures serve stable mass production. When choosing a mold base, refer to the four key points confirmed earlier.

After selecting the mold base, consider insert design.
The principles are: simplify machining, save material, aid molding and venting. Any extremely thin section should be a separate insert for easy replacement. Consider insert strength, machinability, and future water circuit layout.

Next, add standard components, following the principle: prioritize critical areas, maintain symmetry and balance.
Start with ejector pins, considering water circuit layout. Usually, place ejector pins first, rough out water circuits, then adjust both to reach a balanced layout.

Ejection should be placed in areas with high holding force and on rigid features such as ribs, bosses, and edges to avoid ejector marks or unbalanced ejection.
Cooling should target hot spots, which often coincide with high holding force areas—balance this carefully.

Choose ejector pins, ejector blocks, or stripper plates based on the product. If the product tends to stick to the cavity, consider undercuts on the core or ejection on the cavity. This comes from fully analyzing product behavior during injection.

Finally, add remaining standard components, keeping layout balanced.

After completing the entire mold design, always perform these checks:
1. Draft analysis on all inserts to check for undercuts
2. Interference check – the most critical step
3. Mold open/close motion simulation
4. Machinability and assembly feasibility of all components

If all pass, congratulations—the basic process is complete.

Mold design is full of trade-offs. Better quality often means higher cost; simpler design may require product changes or compromise mold strength and life. There is no single perfect solution.
Finding the right balance makes a successful mold design.

The structures others use may not suit your project. By following the principles above, you will be able to design reasonable, reliable mold structures.