When you get a part and prepare to start Plastic injection mold design, what you should to do?

Maybe you will say i can outer sourcing for it, but there have some key factor on plastic mold design you need to know and understand. Here have some injection mold design basics info. Lets dig it out.

Plastic Part Analysis.

  1. Check the part size,part weight,product requirements (like Plastic materials, Mold cavity numbers, dimension tolerance, critical dimension ). design requirements, the use of plastic parts with complex shapes and precision requirements, assembly and appearance requirements.

How to check Part size in nx?

check part size in nx

How to check Part weight in nx?

set part density in nxcheck part weight in nx

how to check draft angle in nx?

check part draft angle in nx

2. then study the product structure. For example, what structure needs to be made on the product when we build the mold, where to put Slider, lifter, shut-off, hydraulic cylinder? Is these mold component reliable? Is it work smoothly?

how to check plastic part wall thickness in nx

3.According to the productspecification, in past real case, some products have only one mold opening direction, but some products have multiple mold opening directions, Parting surface. Even in same mold, there also can have different parting surface to make the part out. The better of the parting surface will reduce the mold cost and extend the mold life.

Choose better and economy Part surface, easy to machining and meet customers requirements, Analysis of the possibility and economy of the molding process of plastic parts.

It is clear that the production batch (production cycle time, production efficiency) of the plastic parts is indicated in the general customer order.

How to calculate Production cycle time?

Here is a sample, customer require 500kpcs part per month,

Each month have 26days working days

Each day:22 working hours

Each hour: 60 Minutes

How many parts need to injected per minutes


Cycle time:60/14.56

Means we need produce arround 14pcs/ per minutes, it is hard to meet this requirement if we only build 1Cavity mold, so we have to add more cavity, consider of the mold balance, we consider 8 cavity


Cycle time= 60/1.82

The above analysis is mainly to select injection equipment, improve equipment utilization, determine the number of mold cavities and the size of the mold feeding chamber.

B: Analyze the molding process of plastic:

Molding methods, molding equipment, material types, mold categories, etc.

4.Master the actual production situation of the manufacturer:

A: The technical level of the factory operators

B: Manufacturer’s existing equipment technology

C: Technical specifications of molding equipment

D: Ordering and processing methods for mold materials and accessories commonly used by manufacturers (preferably in our factory)

5.Determine the mold structure:

Generally ideal mold structure:

A: Process technical requirements: geometric shape, dimensional tolerance, surface roughness and other in line with international standards.

B: Economic requirements for production: low cost, high productivity, long tool life and easy manufacturing.

C: Product quality requirements: meet all requirements of the customer’s drawings.

Molding Shrinkage Meaning

Molding Shrinkage refers to the percentage of the shrinkage value of the room temperature dimension to the original uncooled size after the plastic part is taken out from the mold and cooled to room temperature.


injection molding shrinkage calculation

The formula for the mold shrinkage is: α = (D-M) / D;

Among them, the mold size D at room temperature and the product size M after molding.

Injection molding shrinkage determines the success or failure of the whole product and the plastic injection mold. If the shrinkage rate of one product does not lead ,will cause the whole product size smaller. if the result is serious, the entire mold will be rebuilding.

Injection Mold Clamping Force

The clamping force is simple and straightforward. In order to restrain the tension of the mold during injection molding, the force applied to the mold is called the clamping force. This thing is important. In the daily design process, when choosing an injection molding machine, this is one of the considerations.

When moulding, if the clamping force is not enough, the product may have defects such as flash and burs. However, if the clamping force is too large, there is a direct damage to the mold, causing problems such as deformation of the template, uneven support of the support Pillar and the guide pin. It is also easy to cause poor product exhaust.

How to calculate injection molding clamping force when designing?

To calculate the clamping force, first of all, we have to know what factors affect the clamping force?

The tension caused by the mold is mainly the injection pressure at the time of injection molding. The magnitude of this force is directly related to the size of the projection surface of the product. Under the same conditions, the larger the projected area of the product, the greater the clamping force required. Knowing this will be easy.

Next, let’s calculate it.

1: The actual clamping force should be greater than the injection pressure

injection moulding clamping force calculation formula

2: Injection pressure (kg) = product projected area (cm2) x cavity pressure (kg / cm2) x safety factor

The projected area of the product can be calculated in software.

Cavity pressure, good fluidity, 300kg/cm2

Medium fluidity of 400kg/cm2

Poor fluidity: 600 kg/cm2

Safety factor: generally takes about 1.2~1.3

Here is an example.

At present, there is a product in hand, using the software to calculate the projected area of 83027mm2, the material is ABS, one out of one.

Check part projected area

Injection pressure = 119.25 * 400 * 1.2 = 57240 kg = 57T

Then, the actual selection of injection molding machines should be greater than 57 tons of machines.

Of course, this is only one of the values. It is also necessary to select the injection molding machine based on the actual required mold size, part weight, mold thickness and other comprehensive information.

Mold layout

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