HDPE Injection Molding

HDPE Injection Molding: The Ultimate Guide

Probably, you would like to learn more about HDPE injection molding machine.

A reason this guide covers all vital aspects of HDPE injection molding – from benefits, properties of HDPE, injection molding techniques and many more.

Keep reading to lean more.

1. What Is HDPE Injection Molding

Injection molding of HDPE allows you to produce plastic parts of different shapes and sizes. The HDPE injection molding process involves transferring HDPE plastic in moldable state into a specific mold cavity design.

HDPE Injection Molded Parts

2. Properties Of HDPE Plastic

High-density polyethylene (HDPE) refers to a thermoplastic polymer formed through ethylene polymerization. The plastic boasts of high strength-to-density ratio, which is the reason it is used in manufacturing of most everyday plastic products.

Typically, the melting temperature of HDPE ranges between 356-536 Degrees Fahrenheit while the molding temperature is 68-194 degrees Fahrenheit.

There are many positive properties of HDPE injection molding products. This is because they are made using tough, low density plastic able to endure high temperatures.

The key features of HDPE plastic include:

3. Benefits Of HDPE Injection Molding

Some of the advantages of injection molding for the production of HDPE parts include:

a) Greater Customization: You create the HDPE mold to the unique specifications of your project.

b) Lower Cost: The repeatability and automation of HDPE injection molding process minimizes the cost of labor whilst increasing output.

c) Efficiency: Manufacturing HDPE products applying injection molding is faster in comparison to traditional molding techniques. Moreover, the method enables you to produce thousands of parts using the same mold.

d) Higher Output: The repeatability of HDPE injection molding facilitates manufacturing of higher volume of parts at a faster speed.

Though HDPE injection molding requires a higher initial capital, the unit cost reduces the more parts you manufacture over time.

Therefore, injection molding is the preferred plastic manufacturing process when your project needs large volume of HDPE products.

4. HDPE Injection Molding Process

The typical process of HDPE injection molding can be summarized as follows:

1. Preheating

The HDPE raw material is heated to a temperature of 150-180 degrees Celsius. This preheating process takes place in a separate machine called an extruder.

The molten plastic is stored in a hopper from where it will be fed into the next step.

2. Injection

The molten HDPE is pushed into the mold cavity through the nozzle by a plunger and injected at high pressure.

The speed at which this process happens depends on how fast you want your product to be produced.

However, the injection speed is usually between 5 and 20 seconds per cycle (depending on size).

There are multiple nozzles that inject different colours and materials into one mold cavity at once. Therefore, if you want a multi-coloured product you can just mix different coloured HDPE in one mold cavity.

HDPE Injection Molding Machine

3. Cooling And Ejection

The cooling process is very important because it affects the mechanical properties of the plastic. The cooling rate of HDPE injection molded parts is directly related to the internal stress that is developed inside the part.

If no cooling is applied, there will be high internal stresses and consequently low strength. This is due to uneven heating of the HDPE during injection molding process.

Typically, you can cool the plastic by passing cold water or air over it at rates of 20 – 40 gallons per minute. The temperature of the water should be between 40°F and 50°F (4°C and 10°C).

Cooling time may vary between 5 seconds and 7 minutes depending on the thickness of the part you are molding.

4. Inspection

Inspection is an important procedure in the HDPE injection molding process.

An inspection system is used to determine the quality of the HDPE product. It is responsible for ensuring that the products are suitable for use in the market and meet customer standards.

There are different types of inspections which include:

• Visual Inspection

Visual inspection is conducted on every part to ensure that all features of the part are as per the design intent.

• Dimensional Inspection

Dimensional inspection checks dimensions such as length, width, thickness and volume etc.

• Electrical Inspection

The electrical inspection takes place when there are parts with electronic devices or wires inside them. The components must be tested before being sent off for assembly so that there are no faults after assembly.

• Mechanical Inspection

Mechanical inspections check the strength of parts.

• Chemical Inspection.

Chemical inspections check if any harmful substances exist within the HDPE part.

5. HDPE Injection Molding Techniques

The commonly applied methods of HDPE injection molding include:

Melt Compression Injection Molding (MCIM)

MCIM process involves injecting the molten HDPE into the mold under high pressure, and then cooling and solidifying it in the mold cavity.

The main advantages of employing melt compression injection molding are:

• Reduce the wall thickness
• lighten the weight of the HDPE product, and
• Improve the part’s mechanical performance.

Melt Pressure Injection Molding (MPIM)

This process is similar to MCIM. However, it uses a pressure pot instead of a hydraulic press machine to inject molten plastic material into the mold cavity.

The method has been widely used for producing injection parts with complex geometries.

Cold Runner Injection Molding

This technique uses a cold runner system to transfer hot HDPE plastic from the extruder die into the mold cavity.

The typical configuration consists of two cylindrical steel vessels connected by a tube or pipe, known as the runner. The molten HDPE flows down through this tube into the mold cavity where it solidifies.

The cold runner is connected to the mold cavity by means of a gate. The material then flows through the gate into the cavity and fills it completely.

There are two types of cold runners:

Single-Shot

In a single-shot system, only one shot is molded at a time. This is accomplished by using an ejector to push out a preceding part after it has cooled down enough for handling.

Multi-Shot

In a multi-shot system, multiple shots can be molded without having to remove any previous parts from the mold. Multi-shot systems are typically used when there are many cavities in the mold.

You can also apply the technique you are molding large parts. This is because because they allow for better utilization of available space in the tooling.

Hot Runner Injection Molding

The method uses hot runners to transport the molten HDPE plastic from the injection nozzle to the cavity and runner system.

The main advantage of hot runners is that they allow you to maintain plastic temperatures at lower level than in conventional injection molding.

Moreover, the hot and cold zones are eliminated, which reduces the thermal stresses on parts and molds.

This can result in improved part quality, reduced cycle time and lower energy consumption.

6. Why Wall Thickness Is Important During HDPE Injection Molding

Wall thickness is one of the main considerations when designing HDPE injection molding parts.

It affects many critical features of the HDPE part, including cost, mechanical performance and cosmetics.

Carefully considering the wall thickness will help you avoid costly delays in your plan, because of mold modifications and molding issues.

Although manufacturability is crucial, you should determine the nominal wall thickness through functional performance stipulations.

Consideration of allowable stress and anticipated lifespan of the HDPE part should aid in determining the minimum or nominal wall thickness.

Exceptionally thin walls may require excess plastic pressure or result in air traps in sections where the HDPE plastic does not fill wholly.

Conversely, very thick walls will cost more, because of more material usage and extended machine cycle period.

After determining the nominal wall thickness, it should be as even as practical throughout the HDPE injection molding part for these 2 reasons:

• The molten HDPE assumes the path with minimal resistance as the plastic flows across the mold. Parts with thicker walls lead to preferential flow via them, resulting in weld lines and air traps.
• Plastics being poor heat conductors, uniform thickness guarantees even cooling, leading to uniform shrinking of the HDDPE as it solidifies. However, differential wall thickness results in varying rates of shrinkage, resulting in warpage of the HDPE parts.

7. Solutions When You Cannot Maintain Uniform Thickness In HDPE Injection Molding Design

There are two alternative you can employ in case you cannot achieve uniform thickness during HDPE injection molding process which include:

Rounded Corners And Levelled Transitions

Rounded corners and levelled transitions restrict the strains on the walls of the HDPE part. It also reduces the variations in shrinkage rates during cooling of the material.

Sharp corners equally result in stress risers within the part, acting as a likely failure point.

They are a major cost driver for HDPE injection molded products, because they need costly EDM machining to develop the mold.

Rounded corners enable the molten HDPE plastic to flow effortlessly across the mold. Therefore, maintain regular thickness throughout the part during designing of the corners.

You should ensure that the interior radius is at the minimum half of thickness of the wall. Moreover, the external radius needs to be the wall thickness together with internal radius.

Rounded Corners In HDPE Injection Molding

Draft Angles

Shrinkage during cooling may make the part to stick to the mold during HDPE injection molding process.

For that matter, adding draft enables you to remove the part easily.

You may have cosmetic faults on the HDPE part in case you do not add adequate draft angle to the parts.

Referred to as drag marks, the cosmetic flaws make the parts adhere to the mold.

Adding draft can be easily done employing most CAD systems. You need to perform this at the final design stages to maintain minimum complexity.

Often, 1 to 2 degrees draft angle on every side is allowable.

Nevertheless, you can add 3-5 degrees on each side for more texture, and 5 degrees or beyond on every surface for heavy texture.

HDPE Injection Molding Draft Angles

8. HDPE Injection Processing Parameters

The key parameters to consider during HDPE injection molding process include:

9. Optimization Of HDPE Injection Molding Process

You can apply the following technique to establish the optimum HDPE injection molding parameters.

Follow these procedures after establishing the correct mold and melt temperatures:

i. Configure the mold heater such that the core and cavity surface temperature falls within the right range for the HDPE plastic.

Measure and note the mold surface temperature.

ii. Configure barrel heater systems to ramp profile. Ensure it is higher at front zone and lower at feed throat.

Subsequently, load the molten HDPE, make numerous air shots and then confirm melt temperature.

Adjust set-points of the barrel heater and fasten back pressure appropriately to attain a melt in the suitable range.

Measure and register the melt temperature.

iii. Set the pressure of injection to high limit in order not to impact injection speed in a bad way.

iv. Set holding time and pressure to zero.

v. And begin making short shots minus any cushion. Consequently, increase shot volume gradually till the HDPE part is roughly 95 percent full.

This acts as the transfer point, then start weighing the HDPE parts.

vi. Configure packing pressure 20 to 30 percent lower than the pressure of the first stage and go on with weighing of parts.

vii. Increase the shot volume till you develop a cushion. Gradually increase pack time till there is no increase in the weight of the HDPE part.

You have reached gate freeze-off at this point.

viii. Configure the cooling time to allow the ejection of the HDPE part without pushing of the ejector into the formed parts.

Make sure you record the HDPE injection molding process conditions and label the parts appropriately.

This is essential in correlating part dimensional stability and performance with a particular set of operation parameters.

10. Types of HDPE Injection Molding Gates

The 4 most popular types of gates used during HDPE injection molding include:

Types of Gates In HDPE Injection Molding

Edge Gates

As the name suggests, these gates are situated at the edge, and are the best choice for HDPE injection molding of flat parts.

Edge gates will develop a mark on parting line.

Subgates

Sub-gates are popular but need ejector pins for automatic trimming. These gate types have different versions like smiley gates, tunnel gates, and banana gates.

The different variations enable you to shift the gate from parting line.

This is essential if the location of your gate requires moving from the parting line in order to improve filling.

Hot Tip Gates

The location of hot tip gates is above the mold, often only in places with conical or round geometry.

You can only use these gate types with hot runner HDPE injection molds.

Sprue Gates

Also known as direct gates, they are applied for single-cavity molds, which are usually big and cylindrical.

Sprue gates are low-cost, the easiest to make and maintain.

Though the simplest of all types of gates, direct gates leave a massive mark at their contact point.

11. HDPE Injection Molding Products

You can use HDPE injection molding to manufacture the following products:

Consumer Product

• Toys
• Milk crates
• Storage containers
• Appliances
• Sporting goods

HDPE Injection Molded Crates

Industrial Products

• Pipe thread protectors
• Hardware
• Construction products

Moreover, HDPE injection molding allows you to manufacture standard, thick- and thin-walled molding.

This is in addition to custom blends having:

• Color concentrate
• UV stabilizers
• Liquid color

12. Why You Should Add Bosses Ribs To HDPE Injection Molded Parts

Bosses

Boss refers to an isolated, raised section useful in connecting and fastening parts.

Bosses may be essential means of strengthening your HDPE injection molded parts, without jeopardizing the manufacturability.

In HDPE injection molding, everything should be thin-walled, in order to enhance part quality, speed, mold life, among others.

This implies that the parts you will crate will not have the necessary structural support and strength.

For that matter, you will need to add bosses.

You put bosses where you want to incorporate more structural integrity within sections that need it, such as slots and screw holes.

HDPE Injection Molding Bosses

You can equally add bosses to HDPE injection molded parts for alignment reasons. It is possible to create bosses that may insert into each other for alignment, same as dowel pins.

The boss design is very crucial, and shrinkage is one of the major considerations in HDPE injection molding process.

When you create a boss design for screw hole, you must ensure a smaller diameter design to make up for shrinkage.

Moreover, the boss thickness is equally vital. To prevent shrinkage of sink marks, the boss thickness needs to generally not exceed 60 percent of the overall wall thickness.

a) Ribs

Ribs help in adding structural integrity to the HDPE product and enhance its load bearing capability. They are a basic concept to incorporate to apart that may get sophisticated with HDPE plastics.

HDPE injection molded parts need to have an even thickness, nonetheless for ribs, this rule is broken. Normally, the wall thickness ranges between 50 to 75 percent of the nominal wall thickness.

The rib bottom must as well include a fillet. Usually, the radius of the fillet needs to be about 0.25T to 0.5T, where T denotes the nominal wall thickness.

Additionally, the fillet radius ought not to less than 0.010 inches, and the ribs need to also have a ceiling to their height.

Generally, you should make sure the ribs are as short as practical and drafted just as you have drafted other walls. Essentially, the rib height ought not to be above 2.5T.

Typically, the draft angle applied in HDPE injection molding spans from 0.5 to 1 degree on every side.

Ribs may easily create sink marks. Therefore, you must greatly decrease the ribs wall thickness with respect to the HDPE part’s nominal wall thickness.

HDPE Injection Molding Ribs

13.  HDPE Injection Molding Disadvantages

The major cons of HDPE injection molding comprise of the following:

• Poor UV resistance
• High thermal expansion
• Flammable
• Poor temperature capability
• Susceptible to stress cracking
• Can appear cheap
• Difficult to bond

For all your HDPE injection molding requirements, contact INOX now.