Plastic Injection Molding Companies: 3 Fundamental Rules for Molding

Jul 02, 2021

The fundamentals of plastic injection

Plastic Injection Molding Companies

Design a part of constant thickness to limit its deformation and internal stresses during post-injection cooling is something must for plastic injection molding companies.

Define a parting line to determine the part's demolding direction;

Define drafts to ensure good ejection of the part when it comes out of the mold. The draft designates the angle that is given to a surface so that it can be released from the mold. 

Conversely, generating undercut consists in designing a shape that goes against the mold release for technical reasons, for example the realization of specific technical functions such as clips.

Plastic injection molding constraints and functions

Molding constraints can influence 2 categories of functions:

  • Technical functions

As plastic injection is a means of obtaining which, by definition, makes it possible to design thin (shells) and demouldable parts, not all parts can be produced in this way.

This therefore implies the presence of undercuts to allow demoulding and the absence of undercuts preventing demolding .

The production of plastic injection molding companies is sometimes possible, but it requires providing additional functions to the mold, which increases the cost of the tooling. In this context, the definition of certain technical functions must be carried out in correlation with the means of obtaining … and the validation of the mold maker.

How to enhance plastic injection molding companies development phase?

 It is important to always anticipate the development phase of the mold and to define the geometry of the part in the direction of retouching of the latter. It will always be better to machine the mold than to have to add material to “reload” the mold. 

The development of a clip is an example of the development. It is recommended to orient you on a clip with a “weaker” stiffness, in order to “stiffen” it when focusing.

Moreover, this method of obtaining inevitably generates more or less significant deformations on the final part. The level of geometrical precision is obviously not as important as in the case of the machining of a metal part for example ... This is why, here again, it is important to identify the functional constraints potentially impacted upstream of design in order to implement modifications to limit them.

Aesthetic functions of plastic injection molding companies

The main impacts of molding constraints are aesthetic:

  • Traces of injection points,
  • Shrinkage (materials contract on cooling; however, the thickest areas tend to contract more than those of low thickness),
  • Ejector brands,
  • Streak marks,
  • Welding lines,
  • Burrs,
  • Burns,
  • Deformation,
  • Textured shading,
  • Non-uniform color ...

The origin of these defects is varied in plastic injection molding companies. They may result from the design of the injection mold and the molding conditions.

The challenge is that many plastic parts are trim parts, the main function of which is precisely the aesthetic function.

This is why to work on the cosmetic aspect of these parts, it is extremely important to integrate these constraints upstream of the project, during the design of the part. It is then necessary to define the material, the thicknesses, the undercuts, the parting line ...

Plastic injection molding graining

For example, the injection process makes it possible to produce a texture which is called “ graining ”. Even if this operation is carried out in the mold after geometric validation of the part, it requires a specific angle depending on the depth of the grain. Therefore, this draft angle will have to be taken into account well upstream of the design.

Plastic Injection Molding Companies 2021

Again, at this stage, collaboration with the plastic injection molding companies is very profitable because the information and advice gathered often saves time and money.

A double challenge for the plastic injection molding companies of the Design Office

Within the AMETRA Group, the work of the engineers in the design office will therefore have to combine a functional reasoning (the parts must carry out X and Y technical functions) with a reflection on the aesthetic rendering.

In aeronautics and the automotive industry, for example, plastic parts are often associated with trim. They are seen by the end-user, so their appearance is inevitable.

What is plastic injection molding companies bad design?

A bad plastic injection molding companies design can generate many aesthetic defects, it is important to use our know-how very early in order to make sure that everything goes well during the manufacture of the plastic part, which will go through physical phenomena of changes. state ultimately having a very significant impact on its rendering.

Problem of burn points on the injected part.

Possible causes can be:

  • - Clogged gas leaks.
  • - Injection speed too high.
  • - The material temperature is too high.


  • - Clean up gas leaks.
  • - Reduce the injection speed.
  • - Reduce the temperature of the material.

Problem of spontaneous burrs in the assembly areas.

Possible causes can be:

  • - Foreign body in the parting line.
  • - On injection of the part.
  • Similarly, the material temperature is too high.
  • Moreover, the machine does not have sufficient closing force to compensate for the injection pressure.
  • - Control of the mold cooling temperature


  • - Checking the adjustment zones makes it possible to check that no foreign element has been introduced or that there are no plastic residues from previous injections.
  • - Checking the surfaces of the partition to detect any damage.
  • Furthermore, check the injection process: speeds, temperatures, load and pressures.
  • - Regulation of the closing force of the machine.
  • In addition, check the cooling of the mold.

Continuous burrs in the adjustment areas.

Possible causes can be:

  • - Wear of the adjustment zones.
  • - Foreign bodies or plastic residues deposited on the seals.
  • - Machine chucks out of adjustment.
  • Moreover, incorrect injection parameters: speeds, load and pressures.
  • Similarly, the material temperature is too high.
  • - The solutions :


  • - Repair the gaskets and readjust the injection mold.
  • - Remove any foreign objects or plastic debris in the clamping area.
  • Similarly, Repair the gaskets and readjust the injection mold.
  • - Review of the injection process: speeds, temperatures, loads and pressures.
  • In addition, Regulation of the closing force of the machine.
  • - Check the cooling of the mold.

Possible causes can be:

  • - Plastic temperature too low.
  • - The material entries section is different.
  • - Injection points blocked.
  • - Non-uniform heating of the hot runner.
  • Furthermore,  Nozzles at different temperatures.
  • In addition, Unequal evacuation of gases due to leaks.
  • - Lack of uniformity in cooling.


  • - Checking the injection temperatures.
  • - Check the condition of the injection nozzles.
  • - Moreover, Check the operation of the shutters.
  • - Check the resistances of the hot runners.
  • -Similarly, Check for gas leaks.
  • - Check the cooling of the mold.


To take into account the plastic injection molding companies constraints from the design of the part, there are several solutions. Either we have significant experience feedback, or we are able to involve the company which will produce the mold later. Unfortunately, the mold maker is not always known and chosen from the start by the customer for whom the part is designed.