Acrylic Injection Molding

Acrylic Injection Molding: The Ultimate Guide

This guide explores all the vital aspects of acrylic injection molding process.

Whether you want to learn about the benefits, techniques or mold types – you will find all information you need right here.

Take a look:

1.  What Is Acrylic Injection Molding?

Acrylic is a thermoplastic homopolymer which is very clear and can sometimes be used instead of glass.

It is stronger and more resistant compared to glass and can be molded easily.

The process of forming and creating different shapes to produce different parts with acrylic is known as acrylic injection molding.

acrylic injection molding

In other words, it is the process of making or creating different complex parts and components using acrylic as the material.

2.  Benefits Acrylic Injection Molding

Below is a detailed discussion on some of these benefits.

The optical clarity is excellent – This material is very transparent and does not turn into yellow colour even after a long time.

This clarity allows 92% transmission of light and can be used in applications that require sufficient light transmission.

It can excellently resist sunlight – Acrylic can withstand being exposed to too much UV light.

UV light does not penetrate acrylic even if the material sheet is as thin as 4mm.

The acrylic material also offer resistance to weathering – The material does not degrade as a result of weathering.

The material is strong and can withstand impact – The material has more strength than glass and it does not break easily.

The mold shrinkage of this material is very low – This is a great advantage for the part designers.

It has a long-lasting dimensional stability – This is possible because it can withstand corrosion and other elements that can cause deformation.

It offers resistance to almost all aqueous solutions – it can withstand the presence of most bases and acids without being corroded.

The material does not absorb water easily – the rate of water absorption for materials made from acrylic is very low.

It is not toxic – The chemical composition of acrylic is made up of none toxic elements and easy to handle.

It comes with less or no waste during production processes – The process of injection molding only accommodates the amount of the plastic needed.

This is why the acrylic material does not go to waste.

If the waste happens to be there, then this material can be recycled and be used again and again.

The material used is less expensive – Generally, using acrylic in injection molding is not as expensive because there is reduced labour cost.

Another reason is that the material itself is cheap.

3.  Acrylic Properties For Plastic Injection Molding

Acrylic properties are grouped under four categories as described below.

However, these properties will vary and this will be determined by the type of acrylic material you intend to use.

Thermal properties: – Its linear thermal expansion is 5 to 9 degrees F and thermal conductivity of 3.9 BTU

The deflection temperature is at around 65 to 100⁰C and a melting point of about 130-140⁰C

The operating temperature at 65-93⁰C.

Mechanical properties: – The tensile strength is at about 800-1100psi and the tensile modulus at around 350000-500000 and the tensile elongation at 2%.

Flexural modulus and flexural strength at 350000-500000psi and 12000-17000psi respectively.

The comprehensive strength therein is about 11000-19000psi.

Electrical properties: – The dissipation factor is about 60Hz and the Dielectric Strength is 400 V/mil and the Dielectric constant is 60Hz.

Optical properties: – The material can transmit light up to 92% and its refractive index is at around 1.48 to 1.50.

4.  Factors Affecting Quality Of Acrylic Injection Molding

The injection molding process is normally very simple and easy since most of the procedures are automated.

However, there are issues in every production process that must be looked into and this is not different with molding acrylic.

Such factors could be at different levels of the process; at the design stage, molding stage or during the application.

Design for the mold tool – Since it is a very important stage, the design must account for all the shapes and bends.

This is the step where the cooling needs of the created part are put into consideration.

Any mistakes at this stage will lead to the production of poor-quality end products.

This is also the stage where the production speed of the mold in put into place.

The acrylic material tolerances are considered at this stage.

The wall thickness of the acrylic part – If the wall of the product is thick it means it is stronger and of good quality.

However, a thicker wall could cause warping because of the long period it takes to cool which also means more shrinkage.

Therefore, reducing the thickness of the wall will ensure less material is used which is cost effective.

It is also important to reduce the thickness of the end product for the purposes of preventing excess shrinkage.

Shrinkage – The difference in the density and volume of acrylic at the solid and liquid state can lead to shrinkage.

In order to take care of this conversion process, the cooling system in the mold must be well designed.

It is important to avoid warping at all cost in order to come up with the best quality products.

Therefore, there is need to control the cooling time or use water for cooling.

How the acrylic part will behave after sometime during usage – The application the product will be used for is a factor that must be considered.

This will determine how long it will last when exposed to weathering, creep, excess strain, corrosive materials or impacts.

The best type of acrylic material to be used for such applications must be chemical resistant, strong and flame redundant.

5.  Acrylic Injection Molding Process

The process for acrylic injection molding is very simple since it is automated.

Below is a discussion of the step-by-step molding process.

Preparation of all the needed materials – This step requires the cleaning of all equipment to be used.

Put on all the protective equipment and gather all these items including the dry pieces of acrylic material.

The acrylic material is fed into the holding barrel – The feeding is done after drying and cutting the material into pieces.

acrylic injection molding process

Cutting is done to ensure that the pieces are small enough to fit into the barrel of the machine.

The molding process – When the machine is turned on, the screw in turn and create enough heat that will melt the material.

The machine also aids in mixing of the material.

Once the heating and mixing is done, the acrylic material is injected into the mold system.

The material will take the shape of the designed mold.

Cooling stage – The cooling process creates a solid hard product with the shape of the mold.

At this stage the mold is designed with a cooling system that can be controlled to avoid warping or excess shrinkage.

Water can be used to aid in faster cooling process before the product is ejected or removed from the mold.

6.  Choosing Mold For Acrylic Injection Molding

Mold selection for a specific design from a particular material requires the consideration of the below factors.

·         Under The Injection Unit Consider The Following

Be able to calculate the weight of the maximum short of the material.

Also consider the quality of melt of the material that can be injected into the mold.

The speed at which the material is injected and how it spreads into the mold should also be considered.

It is also important to check for the maximum injection pressure of the machine.

The specification should cater for the maximum injection power needed.

Find out how the machine you need to purchase will handle or influence the cycle time.

·         Under Clamping Unit Consider The Following:

Consider the following: make sure that your specific mold will accommodate all the molten material.

The machine should have the capability to eject thicker parts depending on the design.

Find out the minimum part height and find out if the molding machine can accomdate it.

Ensure the machine has the correct clamping force for your part.

·         Under Hydro Motor

Ensure that the torque requirement needed for viscosity of the melt is correct for the specific part.

Under drive power, ensure that the frequency of power supply suits the kind of design for your part.

The pump motor rating should also suit the molding machine.

Under controls, ensure that the molding machine has the right control system in place.

7.  Acrylic Injection Molding Defects

The possible defects in this process and their solutions are described below.

Formation Of Blisters

This happens during the molding process when the screw is rotated at a very high speed.

It can be prevented by increasing the back pressure, or by increasing the temperature of the mold.

Another way is to change the position of the gate or to make the plastic partcles smaller and dry.

Brittle Product

This normally happens when the temperature in the mold is a bit low.

It can be prevented by raising the temperature of the mold and using better cooling channels.

Another way is to use dry the pellets properly, this will eliminate porosity.

Bubbles

This can be prevented by using very dry pellets, improving on the venting system.

Another way to prevent this is to make sure that the sprue and nozzle sizes are appropriate.

Parts That Are Burnt

This is caused by clogged vents.

It can be prevented by changing the location of the gate, adjusting the speed of injection and also pressure.

The screw should also be checked for proper mixing and in the right speed.

Also adjust the size of the nozzle and gate.

Cracks

This can be avoided by increasing the injection speed and setting the appropriate injection pressure.

Dark Specks

This is as a result of contamination found in the feed.

It can be minimized by reducing the injection pressure if caused by high pressure.

It could be caused by poor venting, improving the venting system will help.

Short Short

This can be avoided by increasing the rate of feeding in, also increasing the injection pressure.

Reduce viscosity by increasing the temperature of the barrel.

8.  Acrylic Injection Molding Machine

There are several types of injection molding machines in the market.

Below are some of the factors you should consider when choosing an acrylic molding machine.

• Actuation – there are several options for this matter, could be manual, hydraulic, pneumatic or electric actuation.
• Orientation of the machine could be horizontal or vertical.
• Clamping force options will be chosen depending on the specific design of the part.
• Stroke of the machine is the distance covered by the moving platen during operation, this depends on the part.
• Distance that will allow fitting for different sizes of the material must be specified.
• Short size should also be specified when selecting a machine.
• Injection pressure of the machine is also a factor that must be considered.

Acrylic injection molding machine

9.  Types Of Acrylic Injection Molding Techniques

Thin wall molding – this is the technology that is designed to create plastic parts that have very thin walls.

Gas assisted injection molding – this is the technology designed to counter the warping with the thick-walled parts.

Liquid silicone injection molding – this technique is created specifically for creating parts that are used with applications that are exposed to high temperatures or chemicals.

3D Printing – this type of molding technique that function by creating prototypes of the molded part.

10.  Application Of Acrylic Injection Molding

They are used in:

• Medical, architectural and military fields.
• Making bullet proof glass used in automobiles and aircrafts.
• Make adverts on signage
• Making optical fibres
• Prepare instrumentation covers and dials
• Optical glass.
• Lighting equipment.

11.  Acrylic Material Grade For Injection Molding

Extruded Acrylic – it is easy to form through injection molding and can be formed in any length.

It can also be formed through bonding with solvent cements.

General purpose and clear acrylic– this can be used as a replacement for glass and it is very conspicuous.

It can be used to make arts, display, signs and lighting.

Abrasion resistant acrylic– These are also called acrylic sheet AR and can resist abrasion perfectly well.

Impact modified acrylic– it can used as a replacement for the traditional polycarbonate and it is less expensive.

It also has higher strength and maintains it clear colour even after being used for a very long time.

Light diffusing acrylic– this grade offers perfect optical properties.

Bullet resistant acrylic—this grade is light weight, and it does not need lamination and it is very clear.

It is used for making bullet proof equipment.

Non-glare acrylic– it comes with a matte finish and displays characteristics needed for making photo or art frames.

12.  Acrylic Injection Molding Vs Acrylic Thermoforming

Acrylic injection molding is the process of creating parts through the injection of liquid materials into a molding machine under pressure.

The molded part is released after cooling.

acrylic injection molding

Acrylic thermoforming is the process where the plastic is heat then formed to a mold’s surface.

In terms of the production volume that the two processes can handle, injection molding is designed for high volume production.

While acrylic thermoforming is designed for small quantities.

The injection molding process can create parts that do not need secondary finishes.

On the other hand, parts produced through thermoforming must undergo secondary finishes.

Injection molding can handle smaller complex parts while thermoforming is meant for larger parts with less complicated designs

acrylic thermoforming

13.  Limitations Of PMMA Injection Molding

Acrylic is incapable of resisting solvents.

It is very combustible which means it can burn easily when the temperatures are high.

The molds that are available for the molding acrylic are not flexible.

The material after molding can crack under high stretch.

PMMA injection molding

-The application temperature is limited to below 200 F degrees; beyond this it will melt.

14.  Secondary Operations On Acrylic Injection Molded Parts

Degating – this is the process of removing the protrusions and intrusions created due to the gating system of the mold.

Deflashing – this is the process that removes imperfections on the finished product after degating.

Cleaning – cleaning process removes lefteovers on the finished product.

Decorating – this is the process that makes more presentable and attractive.

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