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1. What is Injection Molding?
Injection molding is the most widely used polymeric fabrication process. It evolved from metal die casting, however, unlike molten metals, polymer melts have a high viscosity and can not simply be poured into a mold. Instead a large force must be used to inject the polymer into the hollow mold cavity. More melt must also be packed into the mold during solidification to avoid shrinkage in the mold. Identical parts are produced through a cyclic process involving the melting of a pellet or powder resin followed by the injection of the polymer melt into the hollow mold cavity under high pressure.

Injection molding can be used to form a wide variety of products. Complexity is virtually unlimited, sizes may range from very small to very large, and excellent control of tolerances is also possible. Most polymers may be injection molded, including thermoplastics, fiber reinforces thermoplastics, thermosetting plastics, and elastomers. Structural injection molding is also possible in which a core and skin may be made of different polymers. Reaction injection molding and liquid injection molding, which differ in the manner of mixing ingredients, involve the injection of liquid polyurethane systems that polymerize within the mold.

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2. Basic Injection molding cycle
Mold close 每 injection carriage forward 每 inject plastic 每 metering 每 carriage retract 每 mold open 每 eject part(s) Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels that assist in cooling the mold and the heated plastic solidifies into the part. Improper cooling can result in distorted molding. The cycle is completed when the mold opens and the part is ejected with the assistance of ejector pins within the mold.

The resin, or raw material for injection molding, is most commonly supplied in pellet or granule form. Resin pellets are poured into the feed hopper, a large open bottomed container, which is attached to the back end of a cylindrical, horizontal barrel. A screw within this barrel is rotated by a motor, feeding pellets up the screw's grooves. The depth of the screw flights decreases toward the end of the screw nearest the mold, compressing the heated plastic. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Electric heater bands attached to the outside of the barrel assist in the heating and temperature control during the melting process.

The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection.
The sprue and runner system can be cut or twisted off and recycled, sometimes being granulated next to the mold machine. Some molds are designed so that the part is automatically stripped through action of the mold.

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3. Common Molding defects
Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)

Molding Defects




Raised or layered zone on surface of the part

Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater

Burn marks

Black or brown burnt areas on the part located at furthest points from gate

Tool lacks venting, injection speed is too high

Color streaks

Localized change of color/colour

Masterbatch isn't mixing properly, or the material has run out and it's starting to come through as natural only


Thin mica like layers formed in part wall

Contamination of the material e.g. PP mixed with ABS , very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond


Excess material in thin layer exceeding normal part geometry

Tool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.

Embedded contaminates

Foreign particle (burnt material or other) embedded in the part

Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection

Flow marks

Directionally "off tone" wavy lines or patterns

Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times)


Deformed part by turbulent flow of material

Poor tool design, gate position or runner. Injection speed set too high.

Polymer degradation

polymer breakdown from hydrolysis , oxidation etc

Excess water in the granules, excessive temperatures in barrel

Sink marks

Localized depression (In thicker zones)

Holding time/pressure too low, cooling time too short, with sprueless hot runners this can also be caused by the gate temperature being set too high

Short shot

Partial part

Lack of material, injection speed or pressure too low

Splay marks

Circular pattern around gate caused by hot gas

Moisture in the material, usually when hygroscopic resins are dried improperly


String like remain from previous shot transfer in new shot

Nozzle temperature too high. Gate hasn't frozen off


Empty space within part (Air pocket)

Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness).

Weld line

Discolored line where two flow fronts meet

Mold/material temperatures set too low (the material is cold when they meet, so they don't bond)


Distorted part

Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool)

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4.  Properties and Applications of Thermoplastics




(High Density)

Flexible, translucent / waxy, weatherproof, good low temperature toughness (to -60'C), easy to process by most methods, low cost, good chemical resistance. 

Chemical drums, jerricans, carboys, toys, picnic ware, household and kitchenware, cable insulation, carrier bags, food wrapping material.


Semi-rigid, translucent, good chemical resistance, tough, good fatigue resistance, integral hinge property, good heat resistance.

Flexible and rigid packaging, automotive Bumpers, cladding, and exterior trim, consumer products, and industrial uses. PP fiber is used in tape and strapping.


Rigid, opaque, glossy tough, good low temperature properties, good dimensional stability and easily electroplated, low creep. 

Telephone handsets, rigid luggage, domestic appliance housings (food mixers), electroplated parts, radiator grills, handles, computer housings.


PBT, PET and PBT Blends are engineering plastics with excellent processing characteristics and high strength and rigidity for a broad range of applications.

Engineering polymers are used in the manufacture of a wide range of components, including under bonnet parts, exterior parts (window wiper holders and exterior mirror housing).

Polyphenylene Sulphide

Rigid, opaque non-burning continuous use at 240'C, good chemical resistance, good electrical insulator, moisture resistant, rarely used unfilled.

Chemical pumps, hair dryer grills, non-stick cookware (alloyed with PTFE), medical equipment, lamp bulb bases, TV and automotive components.


Polycarbonates are strong, stiff, hard, tough, transparent engineering thermoplastics that can maintain rigidity up to 140oC and toughness down to -20∼C or special grades even lower.

In recent years Polycarbonate blends have become increasingly commercially important. PC is widely used in blends due to its excellent compatibility with a range of polymers


compatibility with many different kinds of additives - PVC can be clear or colored, rigid or flexible, formulation of the compound is key to PVC's "added value".

Window frames, drainage pipe, water service pipe, medical devices, blood storage bags, cable and wire insulation, resilient flooring, roofing membranes, stationary, automotive interiors and seat coverings, fashion and footwear, packaging, cling film, credit cards, synthetic leather and other coated fabrics.

Aramids PI
Aromatic Polyamide

Rigid, opaque, high strength, exceptional thermal and electrical properties (up to 480'C), resistant to ionizing radiation, high cost. 

Aerospace components, reinforcing fibers, high temperature resistance foams, chemical fibers and arc welding torches.

(High Impact)

Hard, rigid, translucent, impact strength up to 7 x GPPS, other properties similar.

Yoghurt pots, refrigerator linings, vending cups, bathroom cabinets, toilet seats and tanks, closures, instrument control knobs.


The majorities of nylons tends to be semi-crystalline and are generally very tough materials with good thermal and chemical resistance. The different types give a wide range of properties with specific gravity, melting point and moisture content tending to reduce as the nylon number increases.

Nylon fibers are used in textiles, fishing line and carpets. Nylon films is used for food packaging, offering toughness and low gas permeability, and coupled with its temperature resistance, for boil-in-the-bag food packaging.

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5. Availability of custom colours

Product #
Product Name
Product #
Product Name
Bright White
Cherry Red
Cloud Gray
Cornflower Blue
Deep Black
Leaf Green
Mid Blue
Mid Gray
Midnight Blue
Powder Blue
Racing Green
Seville Orange
Soft Gray
Transparent Amber
Transparent Blue
Transparent Green
Transparent Red

• Plastic injection is not able to do exact colour matching. Added colourants are approximate. Customers are responsible for verifying actual parts meet their colour requirements. Colours shown here depend on your computer*s colour calibration.
• These additives come in pellet form and are mixed with the resin you choose if it is compatible. Final resin colour depends many factors including shades of the "natural" coloured resin, and there will be some amount of swirling where the colourant is not completely mixed.
• If you need accurate colour matching and consistent colour throughout the part, you can send us the actual products, or show use the color code, then we produce the sample for you to confirm.

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8. What is extrusion Molding?

Extrusion moulding is a manufacturing process used to make pipes , hoses , drinking straws , curtain tracks, rods , and fibres .

The machine used to extrude materials is very similar to an injection moulding machine. A motor turns a screw which feeds granules of plastic through a heater. The granules melt into a liquid which is forced through a die , forming a long 'tube like' shape. The shape of the die determines the shape of the tube. The extrusion is then cooled and forms a solid shape. The tube may be printed upon, and cut at equal intervals. The pieces may be rolled for storage or packed together. Shapes that can result from extrusion include T-sections, U-sections, square sections, I-sections, L-sections and circular sections. One of the most famous products of extrusion moulding is the fiber optic .

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9.Plastic Extrusion Molding Design Considerations

•  Here's some of the design consideration in plastic extrusion process.

Geometry of the part
In a plastic extrusion process material melts down in a gradual manner as it travels through the barrel, and subsequently out of the die. Since it is typically in the form of a liquid as it exits the die, the wall thicknesses must remain uniform, along with the symmetrical shape. Otherwise, there is every chance that a greater pressure on one of the side can force the profile sideways, thus creating a "bow", in place of a straight part. A perfectly balanced shape permits maximum running speed, at a low cost. Hollowness in the profile can create knitlines, in the place where there is separation and rejoining of the material.

Material used
Most of the thermoplastics can be extruded, the long list includes materials like HDPE, LDPE, ABS, acetates, polystyrene, polypropylene, butyrates, acetals, nylons, polyphenylene sulfides, polycarbonates, thermoplastic polyesters and rubbers among others. Materials like nylon, are difficult to extrude as they become very fluid as they are melted. Others like acetal or butyrate comes with an objectionable odour thus resulting in very few takers for them.

As plastic extrusions are not contained fully by metal tooling, tolerances as a rule must typically be looser than the other types of molding processes. Specialized tooling can hold tighter tolerances, however the following are the "normal tolerances":

•  Wall thickness: ㊣.005

•  Cut length: ㊣ .062 or more

•  Height or width: ㊣ .010 per inch of width

•  Straightness: 0.045 bow per foot

Basic Die Design
A tooling system for the basic die design comprises:

•  Flat Plate Die
Plate that has the shape of the part cut through the die. However for guiding the material there is no transition. The flat plate die design is considered a low cost alternative for low volume production.

•  Semi-streamlined Die
Here at the back of the die the corners and edges have radii for helping in the transition of material into the die.

•  Fully Streamlined Die
In this case the die forcefully channels the material uniformly throughout the die. This design however is most expensive and well suited for high volume jobs.

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10. Plastic Extrusion Process

Extrusion techniques can be used to process most thermoplastics and some thermoset plastics. The resins most commonly extruded for medical applications include polyethylene, polypropylene, polyurethane, polystyrene, fluoropolymers, polyamide, polyester, and flexible polyvinyl chloride. A characteristic that often differentiates extruded from injection-molded plastics is the viscosity of the plastic at normal processing temperatures. Extruded plastics often have a higher melt viscosity, which allows the extrudate to retain the shape imparted to it by the die while the extrudate is in the quenching stages.

Combinations of various resins can be used to gain special physical, biological, or chemical properties. Many additives can be used during the extrusion process to enhance processing characteristics of the polymer or to alter product properties. Such additives include lubricants, thermal stabilizers, antioxidants, radiopacifying agents, and colorants.

Because extrusions are not 100% contained by metal tooling, tolerances must generally be looser than other molding processes. While specialized tooling can hold tighter tolerances, generally these are the "normal tolerances" you should expect:  
Wall thickness: ㊣.005  
Cut length: ㊣ .062 or more  
Width or height: ㊣ .010 per inch of width
Straightness: .045 bow per foot

Advantages of Extrusion

  • Low cost tooling and short lead times
  • Low Cost parts.
  • Use of multiple materials in a variety of durometers.
  • Reinforcement via fiber wrapping.
  • Color matching.

Compare to other processes:
Often, extruded parts may be injection molded or thermoformed, depending on their shape. The attraction of extrusion is the low tooling cost, and shorter lead times. For high volume parts, such as sawed-off short clips, injection molding may be a lower cost option. While the piece price may be lower, design changes are required to create draft angles, and tooling is much higher. A "trough" shape can be vacuum formed, but, if it can be extruded, it will be cheaper, as the vacuum formed material has been extruded into sheet already. In profile extrusion, you convert raw resin directly to a finished part.

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11.Plastic Extrusion Design

When looking to design a plastic extrusion, you may want to consider the following design tips.

Basic Die Design

  • Flat Plate Die 每  Plate with the shape of the part cut through the die with no transition guiding the material. This is a low cost design for low volume production.
  • Semi-streamlined Die 每  At the back of the die the edges and corners have radii to help the material transition into the die.
  • Fully Streamlined Die 每  The die aggressively channels the material evenly throughout the die. This design is most expensive and ideal for high volume jobs.

Part Design

  • Importance of Consistent Wall Thickness 每  Consistent wall thickness is the most important aspect in the design of your part. It allows for an even flow of material through the die that produces more controlled parts with a lower tooling cost.
  • Hollow Section Design 每  Avoid designing profiles with hollow sections, they add significantly to the cost of both the part and the tooling.

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12.Types of plastic extrusion

  • Sheet Extrusion
    Sheet extrusion is a technique for making flat plastic sheets from a variety of resins. The thinner gauges are thermoformed into packaging applications such as drink cups, deli containers, produce trays, baby wipe containers and margarine tubs. Another market segment uses thick sheet for industrial and recreational applications like truck bed liners, pallets, automotive dunnage, playground equipment and boats. The third primary use for extruded sheet is in geomembranes, where flat sheet is welded into large containment systems for mining applications and municipal waste disposal.
  • Profile Extrusion
    Rubber Profile Extrusion is accomplished by forcing uncured rubber through a die, under heat and pressure, to form a part with a uniform cross section. This uncured rubber is then run through a heating unit to initiate the chemical cross linking reaction that causes the rubber to cure.
  • Pipe extrusion
    Pipe extrusion is defined as a process of forcing the polymer melt through a shaping die (in this case: circular). The extrudate from the die is sized, cooled and the formed pipe is pulled to the winder or a cut off device with the aid of haul off device.
  • Co-extrusion
    The process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling.
  • Blown Film Extrusion
    In film blowing a tubular cross-section is extruded through an annular die (usually a spiral die) and is drawn and inflated until the frost line is reached. The extruded tubular profile passes through one or two air rings to cool the material.
  • Cast Film Extrusion
    The cast film process differs from the blown film process through the fast quench and virtual unidirectional orientation capabilities. These characteristics allow a cast film line to operate at higher production rates while producing amazing optics. Applications in food and retail packaging take advantage of these strengths.
  • Foam Extrusion
    During the chemical foam extrusion process plastic resin and chemical foaming agents are mixed and melted. The chemical foaming agent decomposes liberating gas which is dispersed in the polymer melt and expands upon exiting the die. Typically foamed profile extrusions require more intense cooling than solid profiles due to the insulation properties of the foam structure.
  • Pultrusion
    Similar to extrusion but with much higher Strengths- even used to make road bridges. Glass or other fibres are incorporated into the extrusion and so loadings of up to 60% glass can be achieved with very good fibre alignment. Materials are generally thermosetting type materials such as epoxy.
  • Calendering
    Calendering is a process that usually uses four heated rolls rotating at slightly different speeds. Again the material is fed into the rolls, heated and melted, and then shaped into sheet or film. PVC is the most commonly calendered material.

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13.Materials for Extruded Plastic Profiles

If we categorize the plastic materials used for the extrusion it would be found that Flexible and rigid PVC remains the most popular plastic material used for extrusion. Among others the major plastics used are HDPE, LDPE, Poly propylene, PMMA, and PS. But it must be kept in mind that the different thermoplastics that are used meet different performance and cost requirements. The table that follows takes a closer look at some of the materials that are typically used in making extruded profiles from plastic material.





•  Ability to withstand high temperatures.
•  Good adhesive properties.

Pipes, hosing, refrigerator, fridge and freezer parts.


•  Excellent resistance to hot water and organic chemicals.
•  Very good friction and abrasion qualities.

Display stands, light diffusers, point of sale holders etc.


•  Crystal clarity
•  Exceptional resistance to chemicals, weather and environment.


Polystyrene (HIP)

•  Good adhesive properties.

Packaging and coving


•  Can be supplied both in high or low density.
•  Good resistance to chemicals and low temperatures.

Packaging, gas and water piping, protective covers


•  Tough material
•  Good resistance to chemical environments.
•  Ability to withstand high temperatures.

Tubing and hinge components.


•  Good flexibility for different applications.

Trims, hosing sleeps etc.

UPVC (rigid PVC)

•  Extremely versatile
•  Wide range of colours
•  Low cost

Windows, Tubes, glazing

Clear PETG

•  Good clarity
•  Excellent resistance to chemicals
•  High gloss



•  High temperature performance
•  Extremely strong
•  Choice of wide range of grades

Light diffusers, Low voltage light tubes for marking etc.

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