Injection Molding
By Rodrigo Garcia de Alba Hernandez
November 26, 2002

This web page shows a description of this process, its importance to the industry and the some of its variants.

OBJECTIVE:
To talk about this process involves, like most of the manufacturing processes, a lot of variables, but my intention is to resume it in the most simple and plain way. That will make other people, unfamiliar to it, feel comfortable and understand all the different steps and things involved in it.
INTRODUCTION:
The Injection Molding (IM) process is an important process in the plastics industry. It process approximately 6% of the LDPE (Low Density Polyethylene), 16% of the HDPE (High Density Polyethylene), 5% of the PVC (Polyvinyl Chloride) and 30% of the PP (Polypropylene), which represent the four most sold plastics in the world, they represent 74% of the sales of plastics industry, world wide; among other plastics processed in this industry. Like the experts like to say, it “…is one of key production methods for processing plastics.”1  It is capable of producing parts of great complexity and different shapes.

(images provided by www.badgercolor.com)

(image provided by www.plastics1.com)

(image provided by www.engelmachinery.com)

What is Injection Molding?

IM is the process of melting raw plastics inside a barrel and injecting the melt into a mold cavity, where it cools until it keeps the shape of the cavity. We call this cycle.


Cycle Sequence:

The cycles are discontinuous; this means that the cycle has to come to an end to start the next one.

First the solid raw material, (that can be in different shapes but mostly in a form called pellets, with the shape of small seeds) is disposed inside of the feeding hopper, which is the first part of the plasticating unit. The plasticating unit or injection unit is the one in charge of the melting of the plastic and injecting it into the mold. The hopper is over the barrel and the feed throat connects them. The barrel is a cylinder heated with electric heating bands in different zones that allow melting the plastic better without burning it. Inside of the barrel is the screw, “which is the heart of the plasticating unit.”2  It has been proven that most of the heat used in melting the plastic comes from the shear caused by the rotation of the screw inside of the barrel. The screw also pushes the plastic to its front until it reaches the desired amount of melted plastic.



A typical injection machine.
(image provided by www.plasticsone.com)

The injection unit also injects the melted material inside of the mold trough the nozzle. The desired amount of plastic needed to fill the whole mold cavity is known as shot size. Once the desired shot size is in front of the screw, oil or some hydraulic fluid is pumped by one or two pumps (old machines have two) and it pushes the screw forward which pushes the melted material through the nozzle into the mold.

The clamping unit is in charge of holding the mold, open, close it and ejecting the parts. It has to hold the pressure that the injection of the plastic applies to the mold. In other case flash will occur. This is called clamp tonnage, which is the maximum force that the machine can apply to the mold and this one has to be greater than the pressure that the plastic applies to the mold.

The mold also cools the injected material until it reaches a certain temperature when is going to keep the shape of the cavity. During cooling, the screw stays at the front of the barrel applying what is called holding pressure which pushes more plastic inside of the mold, because the plastic shrinks when its been cooled and more plastic has to be injected to fill the mold.

When the plastic is cool enough the mold is opened and the part is ejected of the mold. Then the cycle starts again.

“It is common to group the basic steps of this process as a time cycle and they are these four elements:
1. Fill time, is the time it takes to displace the air in the mold cavity with plastics material.
2. Pack time, is the time required to maintain enough pressure to fill out the part and to achieve gate freeze.
3. Cooling or Dwell time, is the time required to cool or set enough for safe removal from the mold cavity.
4. Dead time, is the time required to open the mold, remove the molded part and close the mold.”2

All Rights Reserved to Rodrigo Garcia de Alba Hernandez
Figure 1: Flow chart on a typical injection molding cycle

Difference between injection of Thermoplastics and Thermosets:

The main difference in the process is that the thermoplastics are heated to melt and then cooled to solidify and take the shape of the mold cavity; thermosets have to be heated to a certain temperature to melt and heated in the mold to react chemically and crosslink taking the shape of the mold cavity; which is called curing.

This implies certain changes in the mold and in the temperature controls to avoid premature crosslinking of the thermosets. The molds for thermoplastics have cooling channels where water is pumped continuously to control the mold temperature and cool the part. On the other hand, the molds used for thermosets have heating systems to cause the chemical reaction or curing when the plastic is injected inside of the mold.

The machine also needs better temperature controls for injection for thermosets. This is accomplished by having water jackets along the barrel, instead of heating bands, divided in three different zones and being “the principal source for melting the plastic”3. Also the screw has zero compression, which eliminates shear as a heat source.

Safety in Injection Molding:
Many safety devices described next assure the safety in this process:
·   "Mechanical drop bars: They respond to the motion of the front gate. In case that electrical and hydraulic safety devices fail this will stop the mold from closing.

· Electrical interlock: This device disables the electrical circuit that controls the clamping unit and prevents mold closing when the door is open. It is a small rod, attached to the front door, connected to a limit switch.

· Hydraulic interlock: The usual interlocks are hydraulic switches and actuating arms. Compared to the electrical interlock, which functions right or not at all, the advantage of this device is that it can function partially.

· Purge guards: Purging the machine is dangerous because hot plastic may spray. These guards enclose the nozzle of the machine.

· Rear door safety systems: This device consists in a switch attached to the rear door that shuts off the machine when the door is open.

· Safe molding practices: The objective of this is to avoid the build up of pressure inside the barrel by backing of the carriage when the machine is not working, even if it is just for a few minutes.

· Safety for the machine: Almost all injection molding machines use shear pins or shear keys connecting the screw with its motor and they will break when barrel is not hot enough or a hard object blocks the motion of the screw to avoid damage of the screw."2

Advantages and Disadvantages:
This is a comparison to the rest of the plastic processes.
ADVANTAGES:
· “Direct route from raw material to finished parts,
· Very little finishing, or none at all, of molded parts,
· Full automatability,
· High reproducibility,
· Low piece costs for large volumes.”1
· “High output rates
· Fillers (generally used to decrease the density of the material and reduce costs) and inserts may be used
· Small, complex parts with close dimensional tolerances can be molded
· More than one material may be injection molded (co-injection molding)
· Thermoplastics scrap may be ground and reused.”2
DISADVANTAGES:
· “Not practical for short production runs, because of the cost of running the machine and the wearing of the equipment.
· Costly machines
· Complicated process.”2
Special Injection Molding Processes:
· Co-injection Molding:
“Is a process in which two or more materials are injected into the mold cavity.”2 One of the main advantages of this process is that it can produce an outside layer or skin with some properties, usually the desired aesthetic properties, and have a cellular center core, usually with the desired mechanical properties. The intern core has to have blowing agents to expand and obtain the desired cellular density.
One of the restrictions of this process is the restriction in materials. Not all the materials can be used in this process; the skin material has to be palatable or pigmented. Used to produce all kinds of products, from ketchup bottles to poles for construction.
· Reaction Injection Molding (RIM):
Also known as liquid reaction molding or high-pressure impingement mixing. In this process several chemicals are mixed and forced into the mold cavity and they react inside of it, polymerization reaction.
Only some materials can be used in this process, such as polyols, isocyanates and some polyurethanes, polyester, epoxies and polyamide monomers.
The materials are mixed in a special chamber before they are injected into the mold.
The major users of this process are the automotive (bumpers) and the furniture industries (tables and chairs for outdoors).
· Reinforced Reaction Injection Molding (RRIM):
This is the use of short fibers or flakes to produce a more isotropic product. The process is the same as RIM, the only difference is the one mentioned. The fiber makes the monomer viscosity increase resulting in a much harder and abrasive resistant product.
· Turret Molding:
Is the molding of preforms for blow molding, usually used to produce bottles. They look like test tubes with thick walls and they have the open of the bottle already finished.
· Gas Assisted Injection Molding (GAIN):
This process gives the manufacturer the opportunity to reduce the amount of plastic needed, weight and even increase the mechanical strength of the part. This process advantages also include the chance to make thick-walled parts extent of sink marks and air bubbles economically.
A simple way to explain this process is that the injection machine has a gas injection unit attached full with nitrogen. The machine injects a certain amount of plastic into the mold, filling it partially. Then the gas injection unit starts injecting nitrogen into the cavity. The gas pushes the plastic along the cavity until is completely full and also pushes the material to the walls of the cavity so the gas gets trapped inside of the plastic.
This process has many advantages besides the ones cited before. The pressure applied by the gas to the plastic reduces the packing pressure and the cooling time needed, because the gas cools the part; which also reduce the cycle time needed to produce it. Resulting in less cost and less wear of the machine.
The only disadvantage is the cost of the equipment needed and the more complicated mold and part design. However, the savings can be much more greater and the profits too.

(image provided by www.gasassist.com)
· Powder Injection Molding:
This one involves the injection of ceramic or metal powder to the plastic to produce complex design parts or harder (metal) or insulating (ceramic) parts.
It consists in three steps. The first one is to add the ceramic or metal powder to the plastic. Waxes are usually added too. The plastic is just what is called a carrier of the powder. Then the mixture is injected into the mold. Then the part is ejected and placed into an oven where the plastic is removed from the part and leaving the part in an approximate shape but very fragile. The last step is a sintering process where the part is placed in another oven with higher temperatures than the first one. These three steps increase the shrinkage of the part to 25 to 30%.
The other disadvantage of this process is that the mixture literally “... eats up ...”1  the barrel, the screw, the cavities, the cores and other parts of the injection machine. This draws the manufacturers to make all these parts of special wear resistant materials that are more expensive.
Some of the products made by this process are bodies of TVs and other electronic devices with “perfectly square corners and various holes and ribs in the parts.”3
· Others:
They are known as Liquid resin molding techniques (LRM). These include Resin transfer molding (RTM), Vacuum injection molding (VIM) and Thermal-expansion resin transfer molding (TERTM). These are processes where liquid resins are injected by low pressure and rapidly cured inside the mold; and mixing is mechanical rather than by impingement.
There is another process called Two color molding that consists in injecting a layer of plastic of one color and rotate the mold 180º to inject the second layer over the first one, used to produce cups with the inside of a different color than the outside.


The Future of Injection Molding:

Some of the new tendencies and technology in injection molding are the electric injection machines and the gas assisted injection molding.
The electric machines have several advantages over the old design of the conventional injection machine. It runs silent, its operating cost is less, and they are more accurate and stable.

An all-electrical Injection Machine
(image provided by www.mhi.co.jp)

Conclusions:
        Injection molding is one of the most important processes for plastics and it has a very
        wide list of kinds of products it can produce, which makes it very versatile.

References and Related links:

1  MENGES / MICHAELI / MOHREN; How to Make Injection Molds; Third Edition; Hanser; Cincinnati, USA; 2001
2  RICHARDSON & LOKENSGARD; Industrial Plastics, Theory and Applications; Third Edition; Delmar Publishers Inc.;
    Albany, NY, USA; 1997
3  BERNIE A. OLMSTED & MARTIN E. DAVIS; Practical Injection Molding; SPE; MarcelDekker; New York, USA; 2001

- Injection Molding Recovery Remains Brisk; Wood, Bill; Modern Plastics v. 79 no10 (Oct. 2002) p. 56-7; USA
- Comparison of Structure Development in Injection Molding of Isotactic and Syndiotactic Polypropylenes; Choi, Dongman. ; White, James L.; Polymer Engineering and Science v. 42 no8 (Aug. 2002) p. 1642-56; USA
- New methods expand roles of gas-assist molding. ; Two papers from the Molding 2002 Conference, New Orleans, La., March, 2002; Plastics Technology v. 48 no6 (June 2002) p. 37-9; USA
- Electric machines and new processes catch fire.; Knights, Mikell; Plastics Technology v. 48 no1 (Jan. 2002) p. 34-9; USA
- Gas-assisted injection molding--with internal liquid cooling.; Lee, Jun Seok. ; Cha, Sooyoung. ; Lai, Francis.; Plastics Engineering v. 57 no12 (Dec. 2001) p. 42-5; USA
- Advanced materials at the powder metallurgy conference.; Capus, Joseph M.; Advanced Materials & Processes v. 159 no10 (Oct. 2001) p. 43-6; USA
- Two-stage injection unit adds large-shot capability to electric molding machines.; Klaus, M. Barr.; Modern Plastics v. 78 no11 (Nov. 2001) p. 48-51; USA
- Reaction Injection Molding of Polyurethane Foam for Improved Thermal Insulation.; Koo, Myung Sool. ; Chung, Kwansoo. ; Youn, Jae Ryoun.; Polymer Engineering and Science v. 41 no7 (July 2001) p. 1177-86; USA
- Designing molds for external gas-assist injection molding.; Pearson, Terry.; Modern Plastics v. 78 no8 (Aug. 2001) p. M8-M9; USA
- CO2 process claimed to enhance part finish.; Moore, Stephen.; Modern Plastics v. 78 no3 (Mar. 2001) p. 60; USA
- Gas-assist is tops with golf cart manufacturer.; Automotive Plastics v. 1 no2 (Oct. 2000) p. 50-1; USA
 


This research is dedicated to my Dad, who introduced me to this fascinating world of plastics and taught me to work hard. Thanks Dad, I love you and miss you a lot.

    Ball State University