How Are Injection Molds Made?

Table of Contents

Whether you’re just getting started or looking for a new way to manufacture your products, knowing how injection molds are made can help you choose the right tool for your next project.

The manufacturing process of injection molds includes a DFM report, mold design, mold processing, mold assembly, and mold trial. These five indispensable steps.  This article will learn more about how injection molds are made.

The First Step: DFM report

DFM means to design for manufacturing, Design for manufacturability.

That is, starting from improving the manufacturability of injection molded parts, making parts and various processes easy to manufacture, low manufacturing cost, high efficiency, and low-cost ratio.


DFM report includes these details:

1: Surface requirements

2: Structural distribution: Schematic diagram of mold layout

3: Parting analysis: parting line of front and rear models

4: Parting analysis: front mold insert pin, insert the parting line

5: Parting analysis: back mold insert pin, insert the parting line

6: Type analysis: row position type line

7: Gating system: gate size and location

8: Product analysis: product thickness analysis

9: Product Analysis: Product Thickness Modification Suggestions

10: Draft analysis: draft analysis of front and rear molds

11: Draft analysis: draft analysis of front and rear molds

12: Draft analysis: position draft analysis

13: Ejection system

14: Marking and Engraving

15: Carrying water in the front model

16: Carrying water after the model

In a nutshell, these details are divided into these details of product mass production: 

What marks can the mold leave on the products, the thimble marks, the selection of the parting line position, the parting line can be seen on conventional injection molded parts, and what else? It is the adjustment of the wall thickness of the products.

The Second Step: Mold Design

In this step, the mold engineer specifically designs the 2D and 3D drawings of the injection molded parts to prepare for the next step of mold making.

Mold structure design includes these details:

(1) Placement of plastic parts in the mold and selection of parting surfaces;

(2) Determine the number of mold cavities, the arrangement of the cavities, etc.;

(3) Design of mold gating system, including runner layout and gate type, size, and location;

(4) The structural design of the injection molded parts, mainly the structural form of the molded parts;

(5) The design of the part ejection mechanism;

(6) Design of side parting and core pulling mechanism (if necessary);

(7) Exhaust method design;

(8) The overall size of the mold is determined, and the mold base is purchased.

The mold base has been gradually standardized, and the mold base is selected according to the mold base atlas provided by the manufacturer.


The Third Step: Mold Making

The main processing technology of injection mold includes standard machining and Electrical discharge machining.

Standard machining

Historically, standard machining was used to create injection molds. It involves shaping metal and cutting it to specified dimensions.

It also includes heat treatment. Besides the cutting, machining, and finishing, a part can be machined to remove blemishes and improve its surface quality. 

Surface finishes are an important aspect of part design because they enhance aesthetic appeal and improve grip quality.


Electrical discharge machining

Amongst the many types of plastic mold machining, electrical discharge machining is one of the most commonly used. 

Its main advantages include high precision, precision shape and depth, and a superior surface finish. 

It is also highly suitable for machining complex profiles in exotic molten plastic materials. It is especially useful for deep cavity applications.

Electrical discharge machining, also called spark machining, is a machining technique that uses a series of electrical sparks to erode materials. 

The process involves heating a tool and a workpiece until an electrical spark is developed. This spark then vaporizes a tiny piece of the material. This process produces tiny cuts on the material’s surface.

Electrical discharge machining is widely used for machining cavity shapes in tooling and die programs.

It is also used to produce cavity shapes in custom plastic injection molds. The process begins by machining a graphite electrode into the desired cavity shape.

The electrode then is placed in a Ram EDM. This electrode is used to erode a hole profile in the workpiece. The electrode is made of graphite, brass, copper, or tungsten.

Dielectric fluid is pumped through a sparking gap. This helps to keep the electrode and workpiece apart. It also serves as an electrical insulator until a voltage is applied. 

A steady flow of dielectric fluid is then pumped through the cutting zone to remove the excess plastic material. The dielectric fluid is generally deionized water. It also helps to cool the electrode.

A machinist designs electrodes around the part. A coated carbide cutter is used for making the electrodes, as graphite is highly abrasive.

Electrical discharge machining is used for highly demanding machining applications. It is a cost-effective and precise method for machining complex profiles and deep cavities. It also allows product teams to work with high-cost metals.


The Step Four: Injection Mold Assembly

Before assembly, you should carefully study and analyze the general assembly drawing and part drawing.

Understand the function, character, eristics, and technical requirements of each part, and determine the assembly benchmark. 

Through the assembly, the quality indicators of the product, the accuracy of the mold movement, and safety during use should be fully achieved. Various technical requirements.


1. Assembly datum

(1) The main working parts in the plastic mold, such as the core, cavity and insert, are used as the reference parts for assembly, and other parts of the mold have assembly reference parts for preparation and assembly.

(2) Use the guide post guide sleeve or the template side base surface of the mold as the assembly reference plane for trimming and assembly.

2. Mold assembly accuracy

(1) The mutual accuracy of each part and component, such as distance dimension accuracy, coaxial, equality, perpendicularity, etc.

(2) Relative motion accuracy, such as transmission accuracy linear motion, and rotary motion accuracy.

(3) Fitting accuracy and contact accuracy, such as fit clearance, interference contact status, etc.

(4) The wall thickness of the plastic injection molding. When making a new mold, the wall thickness of the molding should be less than the lower limit of the size.

3. Repair principle

(1) To repair the demoulding slope, in principle, the cavity should ensure that the size of the large end is within the tolerance range of the part size, and the core should ensure that the size of the small carbonyl is within the tolerance range of the part size.

(2) The radius of the fillet at the corner, the cavity should be smaller, and the core should be larger.

(3) When the plastic injection mold has both a horizontal parting surface and a vertical parting surface when correcting, the horizontal parting surface should leave a slight gap when the vertical parting surface contacts. 

The gap value is more than 0.01mm; small molds only need to be painted with red and then touch each other; the gap of large molds is about 0.02mm.

(4) For molds that are clamped with inclined surfaces, after the inclined surfaces are closed, there should be a gap of 0.02-0.03mm at the parting surface.

(5) The connection between the arc and the straight line on the repaired surface should be smooth, no dents are allowed on the surface, and the filing pattern should be consistent with the mold opening direction.

The Five Four: Mold Trial

The main steps of mold trial

Check whether the temperature of the barrel and injection mold is suitable for processing raw materials, and adjust the pressure and injection volume.

Because the filling rate does not change much, it may cause the filling volume to change, when the mold is pushed by the screw when it is heated, the time should be extended appropriately for the impact finished product.

Reasonable adjustment to reduce the total processing cycle. After successful debugging, a reasonable control tolerance can be obtained. 

When comparing the measured dimensions of each sample, it is necessary to pay attention to whether the dimensions of the product are stable and whether certain dimensions tend to increase or decrease. 

It shows that the processing conditions are still changing, such as poor temperature control or oil pressure control, and whether the dimensional changes are within the tolerance range.

During the mold test, care should be taken to make the processing run longer to stabilize the melt temperature and hydraulic oil temperature. 

If the shrinkage rate is too large and the finished product seems insufficient, you can also refer to increasing the gate size. 

According to all finished products that are too large or Adjust injection molding machine conditions for undersizing.

If the cavity size is still correct, try to change the machine conditions, if the cavity size is too large or too small, please correct, such as mold filling rate, mold temperature, and various pressures, and check if some cavity is filled slowly, check and correct faults of injection machines.

Such as defects in oil pumps, oil valves, temperature controllers, etc., will cause changes in processing conditions, and even perfect injection molds cannot exert good efficiency in poorly maintained machines.

Properly keep all records of sample inspection during mold trial, and keep all data that will help to successfully establish the same processing conditions in the future to obtain products that meet quality standards.


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