Injection moulding process considerations

injection molding¹ is a popular manufacturing process that can be used to produce a variety of parts and products. However, several factors must be considered when selecting this process.

For broader context, compare this topic with injection mold design, and supplier sourcing guide.

For readers comparing injection molding options, this article connects the injection mold², plastic material behavior, supplier³ evaluation, and quality control decisions that determine whether a project can move from design to repeatable production.

First, the type of material to be molded must be considered. Some common injection molding materials are better suited for injection molding than others, and certain types of materials may require special handling or processing.

Second, the size and complexity of the desired part must be taken into account. Injection molding is well suited for producing large numbers of relatively simple parts, but more complex parts may be better suited for other manufacturing processes.

Finally, the cost of injection molding types of equipment and tooling must be considered. In many cases, the initial investment in injection molding equipment can be significant, but the cost per part is usually lower than for other types of manufacturing processes.

Injection molding is a manufacturing process involving the injection of molten material into a mold cavity. The material cools and hardens to take the shape of the mold cavity. Injection molding is used in a wide variety of industries, from automotive to consumer products.

What Are the Key Design Considerations for Injection Molded Parts?

The most important design consideration is achieving uniform wall thickness throughout the part.

First, the wall thicknesses of the parts must be uniform.

Second, the material must be compatible with the injection molding process. Some materials, such as glass or metal, cannot be injected into the mold cavity.

For complex geometries, this may require coring out thick sections with ribs or gussets to maintain structural integrity while achieving uniform wall thickness throughout the part.

Third, the dimensions of the part must be within the tolerance of the injection molding machine. If the size is too large or too small, the part may not eject properly from the mold or may not meet the customer’s specifications.

Finally, the injection mold must be designed for proper cooling and ventilation to prevent defects in the finished part. By considering all of these factors, the designer can produce a high-quality injection molded part that meets the customer’s requirements.

The performance of plastic products is determined by the interaction of material properties and molding process parameters. The choice of material has a significant impact on the product properties, as different plastics have different physical and chemical properties.

The molding process also plays an important role, as different parameters can lead to significant variations in the final product. To achieve the desired properties, materials and molding processes must be carefully selected. By doing so, it is possible to produce high-quality plastic products that meet the specific needs of the application.

Plastic product properties are influenced by material properties and molding process parameters, and different plastics require process parameters tailored to their properties to obtain the best physical properties.

The key points of injection molding are as follows:

Why Does Plastic Shrinkage Matter in Injection Moulding?

Shrinkage is the reduction in part dimensions after cooling, and it is one of the most critical variables in injection moulding.

a. Plastic species thermoplastic molding process because there is also the crystallization of the volume of the shape of the change, internal stress, frozen in the plastic parts of the residual stress, molecular orientation and other factors, so compared with thermosetting plastics are larger shrinkage, shrinkage rate range, directional obvious.

In addition to the shrinkage after molding, annealing, or moisture conditioning treatment shrinkage is generally larger than in thermosetting plastics.

b. Characteristics of plastic parts When molding, the molten material, and cavity surface contact the outer layer and immediately cool to form a low-density solid shell.

Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part cools slowly and forms a high-density solid layer with large shrinkage. Therefore, the proper wall thickness, slow cooling, and high-density layer are thick shrinkage.

In addition, the presence or absence of inserts and the layout and number of inserts have a direct impact on the direction of material flow, density distribution, and shrinkage resistance size, so the characteristics of the plastic parts on the size of shrinkage, directional impact.

c. Inlet form, size, and distribution of these factors directly affect the direction of material flow, density distribution, pressure-holding and shrinkage effect, and molding time.

Direct inlet, inlet cross-section large (especially thicker cross-section) is small shrinkage but directional, the inlet wide and short length is small directional. Those close to the inlet or parallel to the direction of material flow will have large shrinkage.

d. Molding conditions mold temperature is high, the molten material cooling slow, high density, shrinkage, especially for crystalline materials due to high crystallinity, volume change, so shrinkage is greater.

Mold temperature distribution and cooling inside and outside the plastic parts and density uniformity is also related, directly affecting the size and direction of the shrinkage of each part.

In addition, the holding pressure and time also have a greater impact on the shrinkage, the pressure is large, and the time is long shrinkage is small but directional.

High injection pressure, the molten material viscosity difference is small, the interlayer shear stress is small, the elasticity after the mold jump, so the shrinkage can also be moderately reduced, high material temperature, shrinkage, but the direction of small.

Therefore, adjusting the mold temperature, pressure, injection speed, and cooling time during molding can also change the shrinkage of plastic parts.

When designing the mold, according to the shrinkage range of various plastics, the wall thickness, and shape of the plastic part, the size and distribution of the inlet, and the shrinkage rate of each part of the plastic part are determined empirically, and then the cavity size is calculated.

For high-precision plastic parts and difficult grasp the shrinkage rate, it is generally appropriate to use the following methods to design the mold.

1. Take a smaller shrinkage rate for the outer diameter of the plastic part and a larger shrinkage rate for the inner diameter to leave room for correction after the trial mold.

2. Test mold to determine the form, size, and molding conditions of the pouring system.

3. To, post-process the plastic parts by post-processing to determine the size change (measurement must be after 24 hours after demolding).

4. Correct the mold according to the actual shrinkage

5. Try the mold again and slightly correct the shrinkage value by changing the design process conditions appropriately to meet the requirements of the plastic part.

What Factors Affect Thermoplastic Molding Shrinkage?

The four main factors are plastic variety, part geometry, gate design, and molding conditions.

2. The size and structure of the plastic molding mold. If the uniform wall thickness of the molded part is too large or the cooling system is not good will affect the shrinkage rate. In addition, the presence or absence of inserts and the layout and number of inserts directly affect the direction of material flow, density distribution, and shrinkage resistance size.

3. The form, size, and distribution of the material mouth. These factors directly affect the direction of material flow, density distribution, pressure-holding and shrinkage effect, and molding time.

4. Mold temperature and injection pressure. High mold temperature and high melt density during molding will result in high plastic shrinkage, especially for plastics with high crystallinity. The temperature distribution and density uniformity of plastic parts also directly affect the size and direction of shrinkage.

Holding pressure and holding time also have an impact on shrinkage. If the pressure is high and the time is long, the shrinkage is small but the directionality is large. Therefore, adjusting the mold temperature, pressure, injection speed, and cooling time during molding can also change the shrinkage of plastic parts.

When designing the mold, according to the shrinkage range of various plastics, the wall thickness, and shape of the plastic part, the size and distribution of the inlet, and the shrinkage rate of each part of the plastic part are determined empirically, and then the cavity size is calculated.

For high-precision plastic parts and difficult grasp the shrinkage rate, it is generally appropriate to use the following methods to design the mold.

a) Take a small lower shrinkage rate for the outer diameter of plastic parts and a larger shrinkage rate for the inner diameter, to leave room for correction after the mold trial.

b) Test mold to determine the form, size, and molding conditions of the pouring system.

c) The plastic parts to be post-treated will be post-treated to determine the dimensional change (measurement must be done after 24 hours after demolding).

d) Correct the mold according to the actual shrinkage.

e) The mold is tested again and the shrinkage value can be slightly corrected to meet the requirements of the molded part by changing the process conditions as appropriate.

How Does Plastic Material Fluidity Affect the Moulding Process?

Material fluidity is the ability of molten polymer to flow and fill the mold cavity.

Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, small performance viscosity, flow ratio is good, the same name of the plastic must check its instructions to determine its liquidity is suitable for injection molding.

According to the requirements of mold design, the fluidity of commonly used plastics can be roughly divided into three categories.

1. Good fluidity PA, PE, PS, PP, CA, poly (4) methyl garlicene.

2. medium fluidity polystyrene series resins (such as ABS, AS), PMMA, POM, polyphenylene ether.

3. poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, polyaryl sulfone, fluoroplastics.

b. The fluidity of various plastics also changes due to various molding factors, the main factors affecting the following.

1. Temperature material temperature increases the fluidity, but different plastics also vary, PS (especially impact-resistant and MFR value higher), PP, PE, PMMA, modified polystyrene (such as ABS, AS), PC, CA, and other plastics fluidity with temperature changes. For PE and POM, the temperature increase or decrease has less effect on its liquidity. So the former in the molding of the temperature should be adjusted to control the fluidity.

2. Pressure injection pressure increases, the molten material is subject to shear, liquidity also increases, especially PE, POM is more sensitive, so it is appropriate to adjust the injection pressure to control the liquidity when molding.

3. Mold structure pouring system form, size, layout, cooling system design, molten material flow resistance (such as surface finish, channel cross-section thickness, cavity shape, exhaust system), and other factors directly affect the actual liquidity of the molten material in the cavity, where the molten material to reduce the temperature and increase the flow resistance of the liquidity will be reduced.

Mold design should be based on the fluidity of the plastic used, and choose a reasonable structure. When molding, we can also control the material temperature, mold temperature and injection pressure, injection speed, and other factors to properly adjust the filling situation to meet the molding needs.

Why Does Crystallinity Matter in Plastic Injection Moulding?

Crystallinity is the degree of structural order in a solid polymer, which determines shrinkage behavior and thermal requirements.

The so-called crystallization phenomenon is the plastic from the molten state to condensation, molecules from independent movement, completely in a state of no order, into molecules to stop free movement, according to a slightly fixed position, and a tendency to make the molecular arrangement into a regular model of a phenomenon.

As a criterion to distinguish the appearance of these two types of plastics depending on the transparency of the thick-walled plastic parts, the generally crystalline material is opaque or translucent (such as POM, etc.), and amorphous material is transparent (such as PMMA, etc.).

However, there are exceptions, such as poly (4) methyl garoulein is crystalline plastic but has high transparency, and ABS is amorphous material but is not transparent.

In the mold design and selection of injection molding machines, the following requirements and considerations should be noted for crystalline plastics.

1. More heat is needed to raise the material temperature to the molding temperature, so use equipment with a large plasticizing capacity.

2. The heat released during cooling and tempering is large and should be fully cooled.

3. The specific gravity difference between molten state and solid-state is large, molding shrinkage is large, easy to occur shrinkage, porosity.

4. Fast cooling, low crystallinity, small shrinkage, and high transparency. The crystallinity is related to the wall thickness of plastic parts, the wall thickness is slow cooling, high crystallinity, high shrinkage, and good physical properties. So the crystalline material should be required to control the mold temperature.

5. Significant anisotropy and high internal stress. The uncrystallized molecules tend to continue to crystallize after demolding and are in a state of energy imbalance, prone to deformation and warpage.

6. The crystallization temperature range is narrow, and it is easy to inject the unmelted material into the injection mold or block the inlet.

What Are Heat-Sensitive and Hydrolysis-Prone Plastics?

Heat sensitivity is when plastics degrade under prolonged high-temperature exposure or excessive shear.

Thermosensitive plastics produce monomers, gases, solids, and other by-products during decomposition, especially some decomposition gases that are irritating, corrosive, or toxic to the human body, equipment, and molds.

Therefore, the mold design, selection of injection molding machine and molding should pay attention to, should use screwmolding injection molding machine, pouring system cross-section should be large, mold and barrel should be chromium-plated, there should be no * corner stagnant material, must strictly control the molding temperature, plastic to add stabilizers to weaken its heat-sensitive performance.

b. Some plastics (such as PC) will decompose under high temperature and pressure even if they contain a small amount of water, this property is called easy hydrolysis, which must be heated and dried in advance.

How Do Stress Cracking and Melt Rupture Occur?

Stress cracking occurs when internal molding stresses combine with external chemicals or mechanical loads.

For this reason, in addition to adding additives to the raw materials to improve the crack resistance, the raw materials should pay attention to dry, reasonable choice of molding conditions to reduce internal stress and increase the crack resistance. And should choose a reasonable shape of the plastic parts, and should not set the inserts and other measures to minimize the stress concentration.

Mold design should increase the slope of the mold release, choose a reasonable feed port and ejector mechanism, molding should be appropriate to adjust the material temperature, mold temperature, injection pressure, and cooling time, and try to avoid plastic parts too cold and brittle when the mold release, moldingplastic parts should also be post-treatment to improve anti-cracking, eliminate internal stress and prohibit contact with solvents.

b. When a certain melt flow rate of the polymer melt, at a constant temperature through the nozzle hole when its flow rate exceeds a certain value, the melt surface occurs in lateral cracks called melt rupture, the appearance and physical properties of plastic parts. Therefore, in the selection of high melt flow rate of polymer, etc., should increase the nozzle, sprue, inlet cross-section, reduce the injection speed, and increase the material temperature.

How Do Thermal Performance and Cooling Speed Affect Quality?

Thermal performance is the primary factor determining cycle time, part quality, and hot-runner feasibility.

The cooling time of plastics with high heat deformation temperature can be short and the mold can be released early, but the cooling deformation should be prevented after the mold is released.

The cooling speed of plastics with low thermal conductivity is slow (such as ionic polymers, etc. cooling speed is extremely slow), so it must be fully cooled, and the cooling effect of the mold should be strengthened.

Hot sprue molds are suitable for plastics with low specific heat and high thermal conductivity. Plastics with high specific heat, low thermal conductivity, low heat deflection temperature, and slow cooling rate are not suitable for high-speed molding, so an appropriate injection molding machine must be used and the cooling of the mold should be strengthened.

b. According to the characteristics of various types of plastics and the shape of plastic parts, it is necessary to maintain the appropriate cooling speed. Therefore, the mold must be set up with a heating and cooling system according to the molding requirements to maintain a certain mold temperature.

When the material temperature makes the mold temperature rise, it should be cooled to prevent the deformation of plastic parts after demolding, shorten the injection molding cycle and reduce the crystallinity.

When the residual heat of plastic is not enough to keep the mold at a certain temperature, the mold should be equipped with a heating system to keep the mold at a certain temperature to control the cooling rate, ensure fluidity, improve filling conditions or control the plastic parts to make them cool slowly, prevent uneven cooling inside and outside the thick-walled plastic parts and improve crystallinity, etc.

For good fluidity, large molding area, and uneven material temperature, it is sometimes necessary to use alternate heating or cooling or partial heating and cooling according to the molding situation of plastic parts. For this reason, the mold should be equipped with the corresponding cooling or heating system.

Why Is Moisture Absorption Critical in Injection Moulding?

Moisture absorption is a critical factor affecting dimensional stability, surface quality, and mechanical strength.

The common hygroscopic⁴ plastics with strong moisture absorption are PMMA, PA, PC, ABS, and POM. These materials must be dried to specific moisture levels before processing. Poor moisture-absorbing plastics such as PE, PP, and POM (when unfilled) have lower drying requirements, but should still be monitored for surface moisture.

The water content in the material must be controlled within the allowable range, otherwise, the water will become gas or hydrolysis under high temperature and pressure, making the plastic resin blistering, liquidity decline, and appearance and mechanical properties bad.

Therefore, moisture-absorbing plastics must be preheated according to the requirements of appropriate heating methods and specifications to prevent re-absorption of moisture when in use.

Conclusion

The injection molding process involves the injection molding machine equipment, the design of the injection molding product, the design and production of the injection mold, the information related to the injection molding material, and the debugging of the injection molding production process, etc. Each link needs to be considered thoroughly to ensure the final product is of high quality.

Need a Quote for Your Injection Molding Project?

Get competitive pricing, DFM feedback, and production timeline from ZetarMold’s engineering team.

Request a Free Quote → See our injection molding for a comprehensive overview.

Frequently Asked Questions

Frequently Asked Questions

What Is the Most Important Factor in Injection Moulding Process Design?

Shrinkage determines whether your final part matches the designed dimensions. Crystalline plastics like POM and PA can shrink 1.5–2.5%, while amorphous materials like ABS shrink only 0.4–0.7%. If the mold is not compensated for the correct shrinkage rate, parts will be undersized or warped. Shrinkage also varies within the same part — flow-direction shrinkage differs from cross-flow shrinkage, and thicker sections shrink more than thin walls. The practical solution is to run a test mold first, measure actual shrinkage, and then adjust the production mold accordingly. This iterative approach saves money compared to guessing and re-cutting steel.

Why Must Plastic Pellets Be Dried Before Injection Moulding?

Many engineering plastics — particularly PC, PA, PET, and PMMA — absorb moisture from the air through hygroscopic action. If these materials enter the barrel with excess moisture, the water reacts with the polymer at high temperature, causing hydrolysis that breaks molecular chains and permanently reduces mechanical strength. Visually, you will see splay marks (silver streaks) on the part surface. Drying requirements vary: PC typically needs 3–4 hours at 120°C to reach below 0.02% moisture, while PA6 may need 4–6 hours at 80°C. Skipping or shortening the drying step to save cycle time is a false economy that leads to scrap and customer complaints.

What Causes Stress Cracking in Injection Molded Parts?

Stress cracking occurs when residual internal stress from the molding process combines with an external chemical agent or mechanical load. Materials like PC and PMMA are especially susceptible. The root causes include fast injection speeds that create orientation stress, inadequate packing pressure that leaves voids, uneven cooling that creates thermal gradients, and aggressive ejection that adds mechanical stress. In practice, parts may pass initial inspection but crack days or weeks later when exposed to solvents, cleaning agents, or sustained load. Prevention requires optimized process parameters, proper gate placement, adequate draft angles for ejection, and post-molding annealing for critical applications.

How Can ZetarMold Support Your Injection Moulding Project?

ZetarMold brings over 20 years of injection molding and tooling experience from our Shanghai factory, operating 47 machines from 90T to 1850T. We provide full DFM (Design for Manufacturing) feedback before tooling begins, in-house mold manufacturing supporting 100+ mold sets per month, and a six-step quality workflow from IQC through OQC. Our engineers can advise on material selection, shrinkage compensation, process parameter optimization, and thermal management to prevent defects like warpage, sink marks, and stress cracking. Whether you need a single prototype mold or high-volume production with 400+ material options, we deliver consistent quality backed by ISO 9001, ISO 13485, ISO 14001, and ISO 45001 certifications.

What Is the Difference Between Crystalline and Amorphous Plastics in Moulding?

Crystalline plastics like PA, POM, and PEEK form ordered molecular structures as they cool, resulting in higher shrinkage of 1.5 to 2.5 percent, opacity, and better chemical resistance. Amorphous plastics like PC, ABS, and PMMA cool into a random molecular arrangement with lower shrinkage of 0.3 to 0.7 percent, transparency, and easier processing. The key processing difference is that crystalline materials need more precise temperature control and longer cooling times. For mold designers, crystalline materials demand larger cavities to compensate for shrinkage, while amorphous materials are more forgiving dimensionally but require attention to stress cracking risks.

1. injection molding: injection molding refers to is the production process that melts plastic, injects it into a mold cavity, cools the part, and repeats the cycle for stable volume manufacturing. ↩

2. injection mold: injection mold refers to an injection mold is the precision tool that defines part geometry, cooling behavior, ejection, gating, surface finish, and repeatability. ↩

3. supplier: A supplier is a manufacturing partner evaluated by tooling capability, process control, material knowledge, inspection discipline, communication, and reliability. ↩

4. hygroscopic: Hygroscopy is a property where a material absorbs moisture from its surrounding environment, critical for drying engineering plastics before molding. ↩