Compression Molding

By: Jeff Butcher

December 4, 2003


This is a Wabash Compression Molding Machine, owned by Ball State University.

Table of Contents:

    - Objective
    - Introduction
    - History
    - Thermosetting Plastics
    - Basic Process
    - Process Pictures
    - Parameters
    - Molds
    - Advantages / Disadvantages
     - Product Examples
    - Terminology
    - References

 

Objective: 
    The design of this website is to provide information to individuals in a way that they will have a greater insight of what compression molding is.  It also aims to explain it in a basic way, so the average person will be able to comprehend and learn further knowledge about it. 

Introduction: 

    7Compression molding is a process of applying heat and pressure to a plastic resin in matched dies.  The resin melts due to the heat, then the pressure causes it to form into a desired shape.  This is done in a compression molding press.  4It is mainly an application used for molding thermoset materials, and it is the most common.  7"Thermoset plastic parts are made from polymeric resins that are capable of forming chemical crosslinks."  Curing is another term that crosslinking is often called.  Polymer molecules bind together through crosslinking creating a complex network that will not melt.  Thermoplastic materials can also be used in the compression molding process. 

History:

    3Molding of thermosetting materials can probably be traced back to 1909 when Leo Hendrik Baekeland, produced phenol-formaldehyde resins.  2Chemistry played a big part in the early years with thermosetting plastics.  A large amount of  pressure, stipulations for heating, and elaborate molds were needed  to achieve a part.  Therefore, rough apparatuses were made and resulted in very long cycle times.  Zelov was one of the first people to get a patent on a automatic compression machine.  The patent was issued in the 1930's, and soon after hundreds of these machines were being sold to plastics companies.  Though these early machines were insufficient and time consuming, they paved the way for modern compression molding. 

 

Thermosetting Plastics:

    5Thermosets are synthetic materials that contain natural polymers molecules of high molecular weight.  The molecules are cross-linked to form elaborate three dimensional structures.  Because of the complex networks made by thermosets, plastics made by this material are very hard to break down.  They do not lose their shape or structure until the temperature is raised unreasonably high.  Then the result is burning or charring. 
    Some thermosetting materials are:  Phenolics, Ureas, Malamines, Alkyds, Polyesters, and Epoxies.
 Phenolics offer the widest range of use because they have good mechanical, electrical, and thermal stability.  Ureas and Melamines have good electrical insulation properties and have many color capabilities.  Some advantages of using alkyds are: less likely to corrode, have good chemical and moisture resistance, cheap, and cure quickly.  Polyesters have a very small and manageable shrinkage rate from the mold.  Epoxies are used when producing delicate electrical components. 

Basic Small Laboratory Process:
              (Summarized version of Dr. Rex Kanu's ITMFG 225 lab activity)
1)  Follow all safety rules and practice extreme caution, this is a high temperature process.
2) Turn machine on and set to appropriate pressure and temperature.
3)  Have mold inserted into the machine.
4)  Heat up mold to appropriate temperature required.
5)  Check temperature with digital/analog thermometer
6)  When desired temperature is reached, extract mold from the machine.
7)  Open mold into two halves.
8)  Put appropriate amount of resin into one of the halves.
9)  Close the mold halves.
10)  Place back into the machine.
11)  Press the button that will cause the platen to clamp together
12)  Pressure is now being applied to the mold.
13)  The heat and pressure begins to liquefy the resin and fills the mold.
14)  When the appropriate amount of time has passed, the platens release pressure from the mold. 
15)  Wearing gloves, remove the mold from machine.
16)  Separate the mold halves.
17)  Push the ejector pin to knock finished product out of mold. 
18)  Repeat these steps to make additional molds.
19)  Temperatures and time setting may vary with different resins or molds.
20)  When done, shut the machine down appropriately and clean work area.
Process Pictures

 

Parameters:

    Things to keep in mind when performing compression molding:

    - 7The quantity of resin put into the mold.
This is a factor because if not enough resin is placed in the mold cavity, the part will not be formed completely.  If too much material is applied into the mold, it may not melt completely or excess flash may be produced. 

    - 7Pressure of the molding process.
2"The fluid plastic is held under pressure, often ranging upwards from 2000 psi, for a sufficient length of time for the material to undergo polymerization or cross linking, which renders it hard and rigid."  7Different materials require different forces to mold them.  High-viscosity materials require more pressure than materials with low-viscosities. 

    - 7Down stroke or Upstroke of the machine.
 7A down stroke machine consists of a compression press in which the platen moves downward to close the mold. 
An upstroke machine consists of a compression press where the platen moves upward to close the mold. 

     - 7Mold temperature.
 2The mold is subjected to high levels of heat, approximately around 300ºF to 375ºF.  Appropriate heating is crucial to allow for the resin to soften for it to flow into the mold cavity.  If the temperature is too low the plastic material my not melt completely and produce an unfinished part.  If the temperature too high, it can cause the part to get burned or warped.      

    - 7Molding cycle time.

4

 

 

Mold Closure Types:

    7There are three basic types of molds used when performing compression molding.  These are flash molds, positive molds, and semi positive molds.

    Flash Molds are used for production of items when:   1 " variation in wall thickness, or eccentricity, is permissible; the radius is comparatively small or, preferably, the corner is square on the outer edge of the open end of the molding; the material is not full fabric; and the displacement of the mold cavity is sufficiently large to contain the required molding material in powder form, or in some cases preforms."

    1Positive Molds were used in the past when tools were made up of the die shell, and the top and bottom plunger.  According to Dr. Rex  Kanu, precise amounts of material is needed to make parts.  1Occasional use of positive molds are used when test pieces need to be made.

    Dr. Rex Kanu stated that Semi Positive Molds  are a combination of flash and positive molds, and that the flash is separated from the material.  1"Semi positive molds are designed for three purposes: to provide a powder cavity for bulky material; to provide a powder cavity for bulky moldings in any material; and to provide a means of escape for surplus material." 

 

Advantages:

    There are several advantages of compression molding.  7For instance, there is little waste produced in the process.  This is because there is no gates, sprues, or runners.   Because the molds lack these features, there is no extra flash after the product is made.  Therefore; the molds are easy to make, which results in a much cheaper cost than other processing forms.  Other advantages due to the mold not having gates, sprues, or runners are:  Less likely to disturb insert, be stressed, or erode.  The waste that is formed from thermoset materials can be recycled as fillers.

Disadvantages: 

    There are also disadvantages of performing compression molding to make products.  7It is difficult to create intricate parts because the molds need to be simple.  Ejector pins or inserts are easily damaged when removing the part from the mold.  The time that some mold cycles need may be very long.  Because thermoset materials are typically used, rejected parts or scrap cannot be reprocessed.  (i.e., it cannot be melted down to be reused for the same purpose) 

 

 Product Examples:7

- Dinnerware
- Buttons
- Knobs
- Appliance Housings
- Radio Cases

 

Terminology:

7Compression Molding - molding process in which a thermoset resin is placed between matched dies and then squeezed, under heat, until cured.

7Compression Press - machine in which compression molding is done.

2Flash - extra plastic attached to a mold; must be removed for the part to be considered complete.

2Gate - in injection and transfer molding, it is the narrow opening that material is injected to reach the cavity.

2Runner - secondary feed channel that runs from the inner end of the sprue in an injection or transfer mold to the cavity gate. 

2Sprue - main feed channel that runs from the outer face of an injection or transfer mold to the gate in a single-cavity mold or to the runners in a multiple cavity mold. 

7Thermoset - polymers in which cross linking occurs.

7Viscosity - resistance of materials to flow.

 

References:\

1 Bebb, R.H. (1962). Plastics mould design:  Compression and transfer
     moulds (1st ed.). London: The Chapel River Press.

 

2 (1971).  Compression and transfer molding. In N. M. Bikales (Ed.),
     Molding of plastics (pp. 11-25). Canada: John Wiley & Sons,
     Inc.

3 Retrieved October 28, 2003 from website.                                           http://www.infoplease.com/ipa/A0767080.html

 

4 Intelligent Systems Lab under the Technology Reinvestment Program.  Retrieved October 23, 2003 from website.
 http://islnotes.cps.msu.edu/trp/cmp/index.html

5 Limited, G.G. (1981).  Some considerations when choosing specific
     materials. In J. F. Monk. (Ed.), Thermosetting plastics (pp.
     8-11). London: Biddles of Guildford.

 

6 Sors, L., Bardocz, L., & Radnoti, I. (1981). Plastic molds and
     dies. New York: Van Nostrand Reinhold Company and Akademiai
     Kiado.



 7 Strong, Brent.  (2000).   Plastics  materials and processing (2nd ed.).  New Jersey:           Prentice Hall  Inc.