Guidelines for Using Thermoplastics

Table of Contents

A thermoplastic is a type of polymer that can be melted through exposure to heat then processed using processes such as injection molding where they harden when cooled.

This transition of thermoplastic does not affect the chemical properties after being heated and cooled. ThermoplasticsThey are widely used in the manufacture of parts in industries such as automotive and electronic industry.

Thermoplastics offer benefits such as they can be recycled, light in weight, good chemical resistance, and promote aesthetics. They are highly versatile materials that offer various advantages in manufacturing and design across various industries. These attributes it essential in modern industries driving efficiency and sustainability in product development and manufacturing.

Here we are going to explore thermoplastics in details and get to understant how to use thermal plastics.

Understanding Thermoplastics

 These attributes make thermoplastics important in modern industries.

To better understand thermoplastics let’s dive into the dynamics of thermal statics.

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Thermoplastic Injection molding machine (Image source: Iqsdirectory)

Understanding Thermoplastics

Thermoplastics are formed through joining small molecules called monomers using a process called polymerisation. Strong covalent bonds hold together the monomers. 

This class of polymer can be softened and melted through exposure to heat which later cools to solidify during the injection molding process. Thermoplastics have loose polymer chains that break apart when heat is applied. They are the most commonly used type of plastics.

Thermosets are formed when two or more components are joined together to form a highly cross-linked network. The formation of thermosets is irreversible.

Thermoplastics and Thermosets are two main categories of polymer, each with distinct features and characteristics. Here are some of the distinguishing properties.

Change in State 

One distinguishing feature between them is their reversibility. Thermoplastics soften when exposed to heat and harden when cooled without undergoing any chemical change. However thermosets plastics undergo an irreversible chemical reaction when exposed to heat.

Processing Methods 

Thermoplastics can be manufactured using a wide range of techniques such as injection molding and 3D printing due to their reversible nature. Thermosets on the other hand are processed using techniques such as compression molding or casting which involve curing reactions to form a permanent solid structure.


Thermoplastics are made from recyclable material due to their reversible behavior. Thermoset plastics however are not recyclable.

Common types of thermoplastics include.

  • Polyethylene (PE) : They are used in packaging, pipes, toys, automotive parts and household goods.
  • Polypropylene (PP) : Can be used in automotive parts, packaging and medical equipment.
  • Polyvinyl chloride (PVC) : They are used in construction in making pipe, window profiles and medical devices 

Material Selection Guidelines 

When selecting thermoplastics for specific applications, several factors need to be considered. They include.

Mechanical Properties 

First you need to consider the required tensile strength and the impact resistance based on the application stress bearing requirements. Determine the materials hardness to resist distortion, which is crucial for wear resistant application.

Another mechanical property is the stiffness of the material which is associated with the flexibility. The material needs to be able to withstand deformation and maintain its structural integrity.

Thermal Resistant

Evaluate the type of temperatures the material will be exposed to during application to make sure it can maintain dimensional stability and mechanical properties. Identify the temperature at which the materials deform under load stress to make the necessary adjustment to meet the requirement of the part.

Chemical Compatibility

First you need to identify the types of chemicals the material will be exposed to during operation. Consider the materials resistance to acids, bases and other chemicals to ensure durability and enhanced performance in a chemically aggressive environment.

Considering the environmental impact and recyclability of thermoplastics is crucial for several reasons.

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Injection Molding Material (Image source: Shutterstock)

Waste Reduction

Through promoting the use of recyclable thermoplastics and establishing efficient recycling infrastructure, environmental challenges such as pollution of oceans, landfills and ecosystems are reduced.

Resource Conservation

Thermoplastics are produced from non-renewable resources such as petroleum. Through choosing recyclable thermoplastics valuable resources can be conserved and reduce demand for raw materials.

Energy Saving

Recycling thermoplastic processes requires less energy input compared to manufacturing new products from raw materials. This leads to reduced greenhouse gas emission and environmental impact.

Regulatory Compliance 

Increased regulatory pressure and consumer demand for sustainable products are driving manufacturers to consider the environmental impact of their materials and manufacturing process. Through this manufacturers can enhance their reputation and meet market expectations.

Design Considerations for Thermoplastic Parts

When designing thermoplastic parts, several considerations must be taken into account to ensure optimal performance and cost effectiveness.

Wall Thickness

When designing thermoplastic parts, maintain a uniform wall thickness throughout the part to ensure consistent cooling and minimize the risk of warping and sink marks. 

Abrupt changes in thickness can lead to differential cooling and uneven shrinkage resulting in part distortion. Use thicker walls to distribute stress evenly on load bearing areas to enhance structural integrity.

Rib Design

Ribs need to be incorporated in the design to reinforce thin sections and improve stiffness without increasing the overall part weight and material usage. Make use of filet radii at the intersection of ribs and walls to distribute stress more evenly and reduce the risk of stress concentration and part failure.

Draft Angles

Include draft angles in part geometry to facilitate a smooth and effective ejection process preventing sticking or damage of part. Draft angles should depend on material properties, surface finish requirement and part complexity. 

Ensure that draft angles are consistent across all part surfaces to promote consistent part quality.

Material flow analysis plays a crucial role in predicting and optimizing the manufacturing process. Here are some of its significance.

It helps predict how molten thermoplastic material flows and spreads within the mold cavity. Through simulating the flow behavior, manufacturers can identify potential flow restrictions and areas of high shear stress allowing them to make informed design adjustments.

It also contributes and helps optimize gate design by evaluating different gate placements and sizes to achieve optimal material flow. Material flow analysis helps validate process parameters such as melt temperature, injection speed, pressure and the cooling time.

Furthermore it helps improve mold design by providing insight into how the geometry, cooling system and surface finishing affect material flow and part quality.

Processing Techniques for Thermoplastics

Thermoplastics can be processed using various techniques depending on application requirement and part geometry. Here are some common techniques.

Injection Molding 

This is a common manufacturing process that involves the injection of molten thermoplastic into the mold cavity under high pressure, where it cools and solidifies to form parts. 

Injection molding is highly suitable for high volume production, complex parts with tight tolerances such as automotive components, medical devices and electronic components.


This is the process in which molten thermoplastics are forced through a series of dies to create desired shapes. A die is the desired shape or finish to a material. The extruded material is cooled and solidified to form the final part. 

Extrusion is used to manufacture various products such as pipes, sheets and films. It is suited for producing long uniform parts with consistent dimensions.


This is the process of transforming a plastic sheet into a 3-Dimensional shape by using heat and pressure. Thermoforming is a favorable process because it is efficient, cost effective, has short lead time and freedom of design. 

This process can be used in manufacture of trays, medical packaging and automotive interior components. It is best suited for large shallow parts with simple geometries.

Optimizing processing parameters is crucial for achieving desired part quality. Below are some tips to achieve this.

Choose the appropriate thermoplastic material based on the specified requirement of application such as mechanical properties, thermal and chemical resistance. Maintain precise control over temperature rates to ensure proper flow behavior and prevent overheating or premature cooling. 

Make adjustments on the injection speed and pressure to ensure optimal fill and packing of the mold cavity. Optimize the cooling system design and cycle time to ensure rapid and uniform cooling of the part while maintaining geometry accuracy. By following these tips manufacturers are guaranteed to achieve the desired part quality.

Post-Processing and Finishing Techniques

Post processing enhances the properties of thermoplastics parts preparing them for their intended end use. It involves corrective measures. Below are some common post processing methods.


Machining involves the use of cutting tools such as drills, mills to remove extra materials from thermoplastic parts to achieve specific dimensions, features and surface finishes. It is used to produce accurate holes, slots and surface finishes that are not possible to achieve during the molding process.


These techniques are used to join thermal plastic parts together permanently.this helps create strong bonds without the need for mechanical fasteners. Common preferred thermoplastic welding methods include, hot plate welding, vibration welding and solvent welding. 

It is used to assemble thermoplastic parts into larger components. It is widely used in automotive, aerospace, electronics and medical devices. It is important to consider parameters such as temperature and pressure to ensure proper fusion and strength of the welding joints.

Surface finishing

This technique is used to improve the appearance and texture of thermoplastic parts by refining their surface. Some of the methods used here are polishing, sanding, painting, coating and buffing. 

This technique enhances the aesthetics, durability and performance of thermoplastic parts. It is applicable in consumer products, automotive and electronics.

Troubleshooting Common Issues with Thermoplastics

In ensuring successful manufacturing process and high quality parts it is essential to address some common issues such as:

Warping and Distortion

Uneven cooling or incomplete filling can lead to warping and distortion of thermoplastic parts, affecting the geometry accuracy and aesthetic appeal. This can be minimized by optimizing the mold design, use of draft angles, ribs, gate placement and cooling systems.

Flash and Parting Line

Poor mold alignment can result in flash and parting line defects leading to additional trimming. This can be resolved through ensuring proper mold alignment  and clamping force.

Material Degradation 

Exposure to high temperatures, shear force or prolonged processing times can degrade thermoplastic material. This reduces the mechanical properties and part quality. This can be minimized through controlled processing parameters such as temperature and injection speed.

Dimensional Inaccuracies

Variation in material shrinkage and processing conditions can result in inconsistent dimensions of parts. This can be avoided through use of simulation tools to validate part dimensions before production.

Sink Marks

Poor packing of the mold cavity can cause sink marks on thermoplastic parts affecting the performance. To avoid this, ensure there is complete mold cavity filling and proper packing of materials. Use proper gate design and mold venting to minimize voids.

Stress Cracking

This occurs when tensile stress causes a material to crack  leading to a part failure. This is a common issue encountered when thermoplastics are exposed to certain chemicals, solvent or environmental conditions.

Factors such as material selection, design optimization  and processing parameters can be put into consideration to avoid stress cracking.

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Thermoplastic injection molding machine (Image Source: Shutterstock)


We have seen that to effectively use thermoplastics in manufacturing and design projects we need to consider material selection. 

Select a thermoplastic material based on the specific application requirement, putting in mind factors such as mechanical properties, chemical resistance and thermal stability. You have to also consider the design. Optimize part geometry, wall thickness, rib design to improve part performance .

Post processing and finishing is another crucial factor to consider. Ensure use of machining, welding and surface finishing techniques to enhance aesthetic properties of the final part. Conduct simulation and analysis  to conduct quality control and material testing to identify and address any defects.

By following these key guidelines manufacturers can optimize product performance, efficiency and cost effectiveness in manufacturing and design projects.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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