The Low-Pressure Moulding Process
The Low-Pressure Moulding process (overmoulding) provides unparalleled adhesion between materials. But Low-pressure overmoulding requires proper planning. The designer must pick the right overmoulding material to ensure maximum adhesion and that it fulfils the required function. Increased design complexity directly implies more thought and skill is needed in the overmould design.
Low Pressure Overmoulding: Not the same as injection moulding!
LPM low-pressure injection moulding is a process in which a resin (usually a polyamide) is injected over a previously manufactured or prepared part to give it properties it doesn’t already possess, namely ruggedness, IP ratings, cosmetic appeal, vibration protection etc. It is not the same!
Design For Overmoulding
Design procedures for overmoulding are an important consideration, just as the overmould tool design is. The wide ranging applications of low pressure overmoulding, mean that knowing the final market of the product is essential. For example, if the plan is to overmould a water-resistant case for some electronics, then an IP rated water-proof seal is the product’s goal. For a good overmould design the function of the part needs to be fully considered. To be able to do this, there are some key points to think about:
- The function of the part
- The intended market and use of the moulded part
- Types of exposure that the moulded part may face
- Why overmoulding of the part has been chosen
- Whether volume production is going to be required
Material Considerations For Low Pressure Injection Moulding
Overmoulding material type, grade and characteristics need to be considered when planning a low pressure moulding project. Primarily, polyamides and hotmelts are used, but some other plastics can be used if their melting point and viscosity fit the operating parameters of the LPM process.
- What is the application of the overmoulded product?. E.g. is it on a cable / connector that is regularly pulled in and out of connection.
- Does the end product need to withstand vibration and /or absorb shock?
- Is there a requirement for ergonomics – like grip or comfort
- What temperature range is the end product going to operate under?
What Is The Environment The Product Is To Be Exposed To?
The final consideration before finalising the design is the environment in which the overmoulded product will be used.
If the part is to be sited outdoors, then it might be exposed to regular radiation in the form of UV light. If this is the case, specific polyamides can be used which withstand such conditions.
If overmoulded part is to be in used in stressful conditions, careful consideration needs to be given to the overmould tool design. For example, an overmoulding part meant for vibration dampening will have a thick wall. This requires more material, which in turn increases cost as well as time because there will be a longer cycle time needed to produce each unit.
The Advantages Of Using Overmoulding
LPM – Low Pressure Overmoulding can minimise production costs. This is because it can use lower cost aluminium tooling and by assembling multiple parts in a tool, a product can be made in one mould cavity in a single operation, quite unlike standard injection moulding.
Rapid Cycle Times
Low pressure moulding is fast compared to conventional potting which takes hours and is wasteful and messy. Usually cycle time from part loaded to part removed and ready for onward processing is around 1 minute.
Ideal for Low to Medium Volume Production
LPM overmoulding is a good choice for low to mid-volume production. Even though injection times may be longer than in traditional injection moulding, it doesn’t have the high operating costs associated with conventional high pressure injection mould tooling.
When prototypes of the product are needed for initial evaluation before full-scale production begins, LPM is the faster, better choice with 48 hour turnaround times, or even 24 hour turnaround when using a 3D printed tool. If redesign of the part is required, LPM’s aluminium tooling or 3D printed tooling, allows a fair degree of tool redesign and modification at lower costs than with steel mould tools.
Low Pressure Overmoulding To Achieve IP Rating
Producing an overmould for a product to enable an IP rating will often require a longer cycle time and more process steps (usually two layers of overmould). These seals usually have water-resistant properties and need to achieve maximum adhesion with the substrate. Before designing this type of part, it will be necessary to choose the overmoulding material that provides the best adhesion to the substrate, together with the hardness required and the best water-proofing capabilities.
Overmoulding For Vibration And Shock Management
Overmoulding for vibration dampening generally requires the most material, along with 2 or more layers of different materials with a different shore hardness. It can have a long TAKT time. The requirement to protect sensitive electronics from shock has been successfully overcome by the team at Overmould by using LPSM – low pressure silicon overmoulding. This radically new process enables shock protection using incredibly protective layers overmoulded over the electronics in minutes, before then overmoulding again with polyamides to provide the required outer layer protection.
When To Choose To Use Overmoulding
- If you want to improve the ergonomics/feel of a product.
- If you need to add a unique type of design to a ready-made product.
- If you need a cost-effective manufacturing process instead of potting.
- If you need to absorb shock, g forces and vibration.
- If you need high levels of IP protection.
- If you need your electronics hidden and hard to get at.
- If you want to create a single part from multiple components
Material Considerations For Overmoulding
Bonding between the polyamide (resin) and the already manufactured part (substrate) is a critical part of the overmoulding process. There are two primary ways in which this bonding occurs:
- Chemical Bonding
Chemical methods occur at the molecular level and involves quite a lot of factors. Even though it is stronger compared to the mechanical method, it is relatively harder to achieve. Examples of factors that affect the chemical bonding include the degree to which the resin can wet the substrate. The degree of wetting increases the level of contact and allows stronger bonding. Chemical bonding is the presence of additives, fillers, or certain surface treatments that affect the chemical bond’s strength.
- Mechanical Bonding
Mechanical bonding occurs at the physical interface of both materials. One method is to make holes, grooves, or cuts in the substrate which the resin can flow into. After the resin cools, it locks into these holes. To further strengthen the mechanical bond, you can wrap the overmoulded material around the substrate. Ensuring that the overmoulded material surfaces aren’t exposed reduces the chance of it peeling away or being picked off.
Properties Of Overmoulding Materials
There are many combinations of substrate and resin materials that are compatible, however if the requirement is to protect the part, the resin’s thickness is just as important as its softness. The best way to measure a material’s softness is the flexural modulus, which is the measure of a material’s resistance to bending. The softness of the material is inversely proportional to the flexural modulus of the material.
Whilst various overmoulded materials are available for overmoulding applications, some are not suitable for LPM. If a goal is to enhance a products grip and feel, the best way to choose a resin is by determining its coefficient of friction. The coefficient of friction of a resin is directly proportional to its tactility (the degree of grip it possesses).
Other Overmoulding Methods
The most common type of overmoulding used in manufacturing is single shot overmoulding – a single injection stage to turn the “device” into a product. However, there are other factors that can affect the decision on which method of overmoulding to use.
Two-shot moulding methods involve firstly overmoulding the part using a softer, more adhesive material and then overmoulding it again with another, usually harder polyamide material. The whole process generally uses two mould tools or two cavities in a single mould tool. Sometimes a soft silicon is used in the first stage and then a conventional hotmelt polyamide in the second.
Pick & Place Overmoulding
In Pick & Place overmoulding there are generally two moulds and several stages, often using a “shuttle table”. Firstly, the parts are assembled into the first tool cavity, or a custom made tool insert which is then placed into the mould tool. The second stage then involves injecting the plastic on the pre-assembled part in the other tool to produce the finished part whilst the first tool is being filled with the parts that are assembled. In essence the “shuttle table” enables the left tool to have parts assembled in it while the right tool is overmoulding.
If you need more information on the LPM process, don’t hesitate to contact the Overmould team before making the final decision as to whether Low Pressure Overmoulding is the optimum process for your product.
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