One of the questions teams developing a new product ask most often is this: how is an injection mold made, and why does the process take weeks? Mold making is not a job that finishes on a single machine; it is a chain of steps that starts with reviewing the part design and matures through engineering decisions, precision machining and trial shots. In this article we walk through, step by step, how a plastic injection mold gets from drawing to series production.
1. Part Analysis and Design for Manufacturing (DFM) Review
Every mold project begins with a review of the 3D part data supplied by the customer. The goal at this stage is to find out, before touching any mold steel, whether the part can be produced by injection molding without problems. The main topics in a DFM (Design for Manufacturing) review are:
- Wall thickness: Abrupt thickness changes mean sink marks, warpage and long cooling times.
- Draft angles: Vertical surfaces with no draft cause the part to scratch or stick as it is ejected from the mold.
- Undercuts: Features such as clips, side holes or snap hooks require cores or sliders and directly affect mold cost.
- Material and shrinkage allowance: Materials such as PP, PA6 GF30, ABS or PC have different shrinkage rates; the mold dimensions are scaled up accordingly.
- Tolerances and cosmetic surfaces: Critical dimensions and cosmetic surfaces are identified, and features that could leave marks are positioned accordingly.
A mold flow analysis is usually carried out at this stage as well. The analysis shows fill balance, weld line locations, air trap areas and possible warpage in advance. A small wall thickness correction made here can prevent a mold revision that would otherwise take weeks.
2. Mold Design: The Engineering Backbone of the Mold
Once the part is approved, mold design begins in a CAD environment. Mold design is the sum of the decisions that determine how the part fills, how it cools and how it is ejected.
Parting Line
The parting line is where the two halves of the mold meet. If it is not chosen correctly, the part suffers from flash, visible marks or sticking in the mold. It is placed on hidden edges and simple geometry wherever possible.
Runner and Gate System
Whether to use a cold runner or a hot runner depends on the part quantity, the material and the cycle time target. The gate type (edge gate, tunnel gate, pin-point gate, etc.) and its location determine both fill quality and how visible the gate mark is on the part. In multi-cavity molds, a balanced runner is essential so that all cavities fill at the same time and at the same pressure.
Cooling Channels
Most of the injection cycle is spent on cooling. Distributing the channels evenly and at an equal distance from the part surface both shortens the cycle time and reduces warpage. For deep cores, baffles, bubblers or, where necessary, conformal cooling solutions are used.
Ejection System, Cores and Sliders
To eject the part from the mold without damage, ejector pins, stripper plates or ejector blades are positioned. For undercut features, angled lifters or sliders that move sideways are designed. These moving components are the most delicate parts of the mold and the ones that need the most maintenance.
3. Mold Steel Selection
Steel is selected according to the expected number of shots, how abrasive the plastic material is, the surface quality requirements and the budget. The general characteristics of commonly used mold steels are summarized below:
| Steel | Delivery condition | General characteristic | Typical use |
|---|---|---|---|
| 1.2311 | Pre-hardened (approx. 28–34 HRC) | Good machinability, no heat treatment required | Medium-volume molds, mold plates |
| 1.2738 | Pre-hardened (approx. 28–34 HRC) | More uniform hardness in large sections thanks to nickel addition | Large molds, automotive interior trim parts |
| 1.2344 | Used in hardened condition (approx. 48–52 HRC) | High wear and thermal fatigue resistance, good polishability | High-volume production, glass-fiber reinforced materials |
For corrosive materials such as PVC, or for transparent parts that require a mirror finish, stainless mold steels are evaluated separately.
4. CNC Machining, EDM, Heat Treatment and Polishing
When the design is complete, tool paths are generated in the CAM environment and manufacturing begins. The typical workflow in mold making is as follows:
- Roughing: Steel blocks are machined close to their final shape on CNC machines, leaving a machining allowance on parts that will be heat treated.
- Heat treatment: Steels such as 1.2344 are hardened and tempered; this step is usually skipped for pre-hardened steels.
- Finishing: Mold cavities and parting surfaces are brought to their final dimensions with precision CNC machining.
- EDM (sinker EDM and wire EDM): Sharp corners and deep, narrow ribs that a milling cutter cannot reach are eroded with copper or graphite electrodes; wire EDM is used to cut inserts and ejector pin pockets.
- Polishing and surface treatment: Cosmetic surfaces are polished to the required level or textured.
5. Assembly, Trial Shots (T0/T1) and Sample Approval
All machined components are fitted, the cooling circuits are tested for leaks and the mold is assembled. The mold is then mounted on the injection molding machine and the first trial shot, known as T0, is run. The purpose of T0 is to confirm that the mold works correctly mechanically and that filling and part ejection go smoothly.
Common problems encountered in trial shots and their likely solutions are as follows:
| Defect | Likely cause | Corrective approach |
|---|---|---|
| Flash | Gap at the parting line, insufficient clamping force | Fitting the surface, process adjustment |
| Short shot | Insufficient venting, narrow gate | Cutting vents, enlarging the gate |
| Sink mark | Thick section, insufficient packing pressure | Adjusting wall thickness, improving cooling |
| Warpage | Unbalanced cooling, fiber orientation | Correcting cooling balance, revising the gate location |
After the identified corrections are made, T1 and, if necessary, further trials are carried out. The sample parts are presented to the customer together with a dimensional report; in sectors such as automotive, the first article inspection report and process documentation are also prepared at this stage.
6. Moving to Series Production and Total Lead Time
With sample approval, the mold is ready for series production. Process parameters are recorded, a mold maintenance plan is set up and production begins. For a mold of medium complexity, the time from design to sample approval can vary depending on the part geometry and the number of moving components, [PLACEHOLDER] e.g. 6–10 weeks. The factor that shortens lead time the most is a part design that has been settled during the DFM stage.
If you are planning a mold for a new part, Altınel Kalıp can support you with design review, mold flow analysis and mold making. You can share your part data to request a technical evaluation and a quote.