Introduction to Compression Molding

Compression molding is one of the original processing methods for manufacturing plastic

parts developed at the very beginning of the plastics industry. In fact, it was widely used in

the bakery industry for cookie or cake molding before plastic materials existed.

Aalthough it is also applicable to thermoplastics, compression molding is commonly used in

manufacturingthermoset plastic parts. The raw materials for compression molding are

usually in the form of granules, putty-like masses, or preforms. They are first placed in an

open, heated mold cavity. The mold is then closed and pressure is applied to force the

material to fill up the cavity. A hydraulic ram is often utilized to produce sufficient force

during the molding process. The heat and pressure are maintained until the plastic material

is cured.

1. Molding compound is placed in an open, heated mold cavity.

2. The mold is closed and pressure is then applied to force the material to fill

up the entire mold cavity. Excess material is channelled away by the

overflow grooves. The heat and pressure are maintained until the plastic

material is cured.

3. The final part after the mold is removed.

There are two different types of compounds most frequently used in compression molding:

Bulk Molding Compound (BMC) and Sheet Molding Compound (SMC). SMC costs higher but

can be pre-cut to conform to the surface area of the mold. The more evenly distributed

material over the mold surface usually results in less flow orientation during the

compression stage and, therefore, higher product consistency.

Compression molding is commonly used for manufacturing electrical parts, flatware, gears,

buttons, buckles, knobs, handles, electronic device cases, appliance housing, and large

container.

Common plastics used in compression molding processes include

Polyester

Polyimide  (PI)

Polyamide-imide  (PAI)

Polyphenylene Sulfide  (PPS)

Polyetheretherketone  (PEEK)

Fiber reinforced plastics

There are four primary factors in a successful compression molding process:

Amount of material

Heating time and technique

Force applied to the mold

Cooling time and technique

Pros and Cons of Compression Molding

Pros

Low initial setup costs

Fast setup time

Capable of large size parts beyond the capacity of extrusion techniques

Allows intricate parts

Good surface finish (in general)

Wastes relatively little material

Can apply to composite thermoplastics with unidirectional tapes, woven fabrics,

randomly orientated fiber mat or chopped strand

Compression molding produces fewer knit lines and less fiber-length degradation

than injection molding.

Cons

Production speed is not up to injection molding standards

Limited largely to flat or moderately curved parts with no undercuts

Less-than-ideal product consistency

Trouble Shooting Guide - Compression Moulding

Defects and Causes Possible Remedies

Blistering

1. Component undercured a)b)c)

Increase cure timeIncrease mould temperatureIncrease preheating

2. Cure hardening of moulding surface (burning) a) Decrease mould temperatures3. Gas trapped within Moulding a)

b)Employ a breathe during cyclePreheat powder

Short mouldings

1. Material charge Insufficient Increase charge weight to mould2. Material distributed unevenly in tool Adjust distribution of powder3. Closing speed of press too low – moulding cures

before tool closedIncrease closing speed

4. Available pressure too low Use higher pressure5. Material flow too stiff a) 

b)Use preheatingSelect easier flowing material

6. Tool temperature too high Reduce tool temperature

Porosity

1. Material charge insufficient Increase weight load to tool2. Lack of venting on blind ribs Add vents or vented ejector pins3. Mould temperature too high Reduce temperature4. Flash line too tight Ease flash line5. Material flow too easy Select stiffer flowing materials

Dull finish

1. Component undercured Increase cure time2. Lack of pressure on moulding due to use of insufficient

powderIncrease charge weight to mould

3. Poor finish on tool a) b)

Polish toolChromium plate tool

Rough, Uneven patches

1. Material preheated too long Reduce preheat time2. Tool opening too far on “breathing” Restrict opening

Flow marks on surface

1. Material distributed incorrectly giving asymmetric flow Adjust distribution2. Insufficient preheating Increase preheat time3. Flash line uneven encouraging flow in one direction Modify flash line4. Material too easy flowing Select stiffer flow powder

Sink marks over ribs

1. Moulding undercured Increase cure time2. Mould under packed a) 

b) c)

Increase charge weightIncrease dwell timeSelect stiffer flowing material

Warped mouldings

1. Moulding overcured a) b)

Decrease mould temperatureReduce cure time

2. Temperature differential between punch and die Adjust temperature3. Ejections not applying even pressure Check ejectors4. Design of component not satisfactory Modify design to increase rigidity

Thick flash

1. Tool overloaded (if component is complete) Reduce weight of powder added to tool2. Material flow too stiff Select easier flowing material

Bulk Factor

Definition: (1) The ratio of the thickness of uncured, prepreg materials to their thickness when fully cured. The bulk factor is also the ratio of the density of the solid plastic object after molding to the apparent density of the loose molding powder.