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What is Injection Molding? | Processes, Types and Materials

Sudarshan Naik

Summary

Plastic products are present everywhere around us which includes combs, toothbrushes, knobs, toys, chairs, etc. Most of these products are manufactured using Injection Molding machines. The injection molding machine is a versatile machine. It can manufacture plastic products at a breakneck speed and can produce a high volume of products annually. The article covers the components of this versatile machine and important aspects related to manufacturing of plastic products.

Introduction

vintage injection moulding machine
a news paper with injection moulding news

An article from Popular Mechanics Magazine September 1970 titled “What can you do with an Injection Molding Machine?” The editor talks about a machine that can help a person to mass produce items at home. The editor says “specialty items like bottle cappers, telephone dialers, golf tees, fishing lures, screwdriver handles, keychains, etc. can be mass produced through this machine.

The many different molds available give a wide choice to make things for fun or profit.” The editor also tells how this machine produces a product. The person needs to fill the cylinder of this machine with plastic pellets. Then pull down the pipe-handle lever in a steady, uninterrupted manner. The lever operates a ram which forces molten plastic into the mold. Once the mold is filled, ram returns to the top of it’s stroke, the mold is unlocked, opened and the finished part is removed. The process is again repeated to produce further quantities. 

Can this machine produce a product at such a pace? Yes, it can. Then let us know more about the injection molding machine, the materials that can be used and its advantages.

Table of Contents

  1. What is Injection Molding?
  2. Components of Injection Molding Apparatus
  3. How fast can a product be produced?
  4. But, there is a catch
  5. Add On: Price Per Part
  6. Can multiple parts be produced in a Single Cycle?
  7. Materials
  8. Products that can be produced using Injection Molding
  9. Advantages

What is Injection Molding?

Injection Molding Apparatus

The Injection Molding Machine or the Injection Molding Apparatus was developed to achieve an idea. 

What’s the idea? First, a negative impression of the desired shape will be produced which is known as mold. As the name suggests, the molten plastic material is injected with a pressure into the cavity of the mold. The material cools down and then takes the shape of the mold. Then, the mold will open and the plastic product will be ejected from the machine. 

Components of Injection Molding Apparatus

This blog deals with general configuration of the machine. It can help us in understanding both the hobby machines (as shown in the advertisement) and the commercial ones. The Injection Molding Apparatus consists of three units. They are:

  1. Injection Unit

The task of this unit is to 

  1. Take material inside the apparatus
  2. Melt the material
  3. Inject the molten material into the mold

To do these three tasks, Hopper and Barrel are used. The first task is done through Hopper while the second and third tasks are done through Barrel

Barrel consists of a reciprocating screw (inside the barrel) and heater bands (outside the barrel). As the material enters the barrel, the screw rotates and the material moves forward through the grooves of the screw. Plastic granules rub against each other as the material moves forward and the heat is generated. The material completely melts as it reaches the tip of the screw. Heater bands are also provided to heat the material. When enough volume of material is available at the tip of the screw, the screw stops rotating, it moves forward and injects the material inside the mold.

  1. Mold Unit

Mold unit consists of Mold. The task of this unit is 

  • Transport the molten material to the cavities
  • Allow the material to cool down and take the shape of the mold

The transportation of material is done through the Sprue-Runner-Gate system. Sprue is the contact point between the injection unit and the mold unit. Molten material enters from the sprue, then it is distributed by a runner and finally the material enters the cavity through the gate.

drawing with white background

For the second task, the mold consists of coolant channels which allows the material to cool down. 

Watch the video below to visualize Transportation (0:53 to 1:06) and Cooling (1:16 to 1:36)

  1. Clamping Unit
drawing with white background

The mold is composed of two halves known as Core and Cavity. The core is usually the negative impression of the interior side of the part. 

The task of Clamping Unit is 

  • To close the two halves of the mold right from the start till the solidification of the material. The Clamping Unit applies a force against the injection pressure to keep the mold closed.
  • Retract the core so that the material can be ejected from the mold.

How fast can a product be produced?

The task done by the injection unit, molding unit and clamping unit constitutes one single cycle. The cycle time is as short as a few seconds to as long as five minutes. The average time falls between 20 seconds and 60 seconds for a well designed injection mold with a proper temperature control system.

But, there is a catch

Building a mold with varying complexity can take as little as 2 weeks to as long as 12 weeks. An injection mold can cost from thousands of dollars to ten thousands of dollars. Simple, low volume 3D printed molds can cost as little as $100. More complex high-production, multi-cavity molds can cost $25000 - $50000 or more.

Once a mold is available, this speed and mass production power can be achieved.

Add-On: Price Per Part

With injection molding, the cost of making a mold is high but a high volume of plastic parts can be produced through the mold. Thus the cost per part reduces as the quantity of parts increases. Have a look at the example

Can multiple parts be produced in a Single Cycle?

Yes. But in two different ways:

  1. Multi-Cavity Mold

Multiple cavities of one single design are present in the mold. Thus, multiple quantities of the same product can be manufactured in a single Injection Molding Cycle.

multi-cavity mold of injection molding
  1. Family Mold

In the family mold, multiple cavities are present in the mold. But, each cavity has a different design. Thus, different products can be produced in a single Injection Molding Cycle.

diagram of family mold

Materials

There are about 85 thousand plastic materials for the injection molding process. The plastic materials that can be used on an injection molding machine may vary from machine to machine. The common plastic materials used for injection molding process are

  1. Polyamide (Nylon)
  2. Acrylonitrile Butadiene Styrene (ABS)
  3. High Density Polyethylene (HDPE)
  4. Acrylonitrile Butadiene Styrene + Polycarbonate (ABS + PC) Blend
  5. Polycarbonate (PC)
  6. Polypropylene (PP)
  7. Acrylic (PMMA)
  8. Acetal (POM)
  9. Low Density Polyethylene (LDPE)
  10. General Purpose Polystyrene (GPPS)
  11. High Impact Polystyrene (HIPS)
  12.  Polyetheretherketone (PEEK)
  13. Polyvinyl Chloride (PVC)

Products that can be produced using Injection Molding

  1. Automotive: car bumpers, dashboards, plastic cup holders, seats, door and locking systems, knobs, steering wheels, roof modules, air vents, etc.
  1. Aerospace: housing, chassis components, bezels, battery housings, circuit boxes, knobs, turbine blades, panels, etc.
  1. Consumer Goods: toys, kitchenware, toothbrush, furniture, outdoor chairs, mouth guards, beverage glasses, combs etc.
  1. Electronics: instrument housings, cell phone components, audio equipment, multiple pronged plugs, keyboards, mouse, computer cases etc
  1. Medical: surgical equipment and components, beakers, test tubes, housings and casings for laboratory equipment, dental instruments, prosthetics, etc.
  1. Agriculture: animal troughs, brushes and cleaning equipment, tubs, baskets, bins, crates, pallets etc.

Advantages

  1. Little Waste

The waste produced during injection molding like sprue and runners from the molded part can be reground and recycled to be used again. 

  1. Consistency

Since the same mold is used, each cycle can produce identical products.

  1. Materials

As mentioned before, a wide range of material is available for injection molding. 

Conclusion

Injection Molding Machines are the best when it comes to mass-manufacturing plastic products with a faster pace and in a consistent manner. Products can be manufactured for a wide range of applications due to availability of a wide range of materials and with reduced price per part.

Karkhana.io - Your Ally for On-demand, Injection Molding

Karkhana.io is India’s foremost on-demand manufacturing company. We partner with start-ups, small & medium enterprises and large businesses, and address their prototyping and mass production requirements. We have a large number of trusted vendors with manufacturing capabilities spanning injection molding, 3D printing services, vacuum casting, sheet metal fabrication and CNC machining

Karkhana.io’s vetted network of Injection Molding partners can offer high-quality parts embedded with complex features.

In a highly fragmented and inefficient vendor ecosystem such as it exists in India, our platform digitalizes procurement, project management, and logistics. We adhere to robust processes in terms of IP protection, materials & engineering, quality control and logistics. Karkhana.io takes the pain out of the vendor management process - ensuring a seamless experience for our customers.

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