Insert molding Technology|AMSL

What is an insert-molding |definition and benefits

Insert molding is a manufacturing process where pre-manufactured components (inserts), such as threaded fittings,brass inserts or other parts are placed into a mold cavity. Molten material is then injected into the cavity, bonding with the inserts to form a single molded part.

Advantages:

Saving cost: The inserts are embed into the mold, and create a single molding part. Saved processing cost for secondary process, like heat staking and ultra sonic welding.

Precision and high quality: This process allows for accurate placement of inserts, we can achieve high quality plastic injection moldings with precision.

Durable components: The process creates a strong bond between the metal inserts and the molded material, this enhances the durability and longevity of the components.

What is nuts/inserts? And how to choose it?

Nuts/inserts Type

There are 2 main types of knurled nuts used in insert molding: diamond pattern and twill pattern.

Diamond pattern: Not suitable for heat staking but performs well in ultrasonic process;

Twill pattern: Features a self-guidance function when embedded and has good performance in torsion and tension.

It is better to choose the nuts/inserts with grooves between the knurling, the upper and lower knurling direction is opposite, the grooves can hold plastic, this structure has better tension and torsional properties.

when working with PC, ABS material, avoid inserts with too sharp patterns

Materials:

✅Standard brass (C3604/H59)

✅Low lead copper / bronze (CuZn38Pb2), which meets with ROHS standards.

✅Stainless steel: In general no need the surface treatment, only plating under the special circumstances.

What are the embedding methods for inserts/nuts

Heat staking

This is the most common an usual way of embedding, generally with hot melt machine and manual soldering iron embedded nuts. The quality of heat staking is better, and lower cost for equipments.

Insert molding/ injection molding nuts

This process is very strict to the nut holes diameter, which controls within 0.05mm.

Ultrasonic nut/insert

Ultrasonic embedding is a kind of ultrasonic vibration, so that the friction between the nut and the surface of the workpiece transfer to the interface temperature increase, when the temperature reaches the workpice’s own softening temperature, the nut will be embedded. It is not recommended to use ultrasonic, because the equipment is very expensive.

Case studies: Influence of plastic BOSS hole size

Standard Status: Proper nut embedding within plastic results in adequate torsion and tension. Shows as Figure 1

Issues and Solutions:

Undersized Nut/Overlarge Hole: Causes insufficient torsion due to inadequate plastic embrace.Shows as Figure 2
Oversized Nut/Undersized Hole: Leads to overflow issues, causing poor fit and potential cracking.Shows as Figure 3

Examples

Shallow BOSS Hole Depth:

Problem:Causes insufficient torsional tension.

Solution:Use double twill nuts or redesign the nut pattern to single twill BS1 to increase the torsion force.

Overflow and Cracking

Problem: Poor fit due to the mismatch in nut/inserts and hole dimensions.
Solution:Optimize nut design by adding a guiding positioning part (C) to improve fit and reduce overflow.

How to choose the inserts?

Insert Size and Plastic Part Size:

1. Base Size (d): Should be smaller than the plastic BOSS bore diameter (C) for proper positioning.

2. Outer Diameter (D):Generally 0.25 – 0.3 mm smaller than the plastic bore diameter (C).

3. Length (L):The plastic hole depth (Y) should be 0.5 – 1.0 mm deeper than the nut length for plastic storage.

4. Wall Thickness (W): Should be at least 0.8 – 1.0 mm or more, depending on the nut/insert size.

Insert Size Plastic size

The table below is the parameter for inserts and plastic size for design reference.

Thread	Outer diameter (D )	Length (L)	Plastic hole diameter	Plastic wall thickness (W)
Diameter (C )	Hole depth ( Y )
M1.2*0.25	2.3	2	2	3	0.8
2.5	3.5
3	4
3.5	4.5
M1.2*0.25	2.5	2	2.2	3	0.8
2.5	3.5
3	4
3.5	4.5
M1.4*0.3	2.3	1.8	2	2.8	0.8
2	3
2.5	3.5
3	4
M1.4*0.3	2.35	2	2.1	3	0.8
2.5	3.5
3	4
4	5
M1.4*0.3	2.5	2	2.2	3	0.8
2.5	3.5
3	4
3.5	4.5
M1.4*0.3	2.7	2	2.3	3	0.8
2.5	3.5
3	4
3.5	4.5
M1.6*0.35	2.5	1.8	2.2	2.8	1
2	3
2.5	3.5
3	4
3.5	4.5
4	5
M1.6*0.35	2.7	2	2.3	3	1
2.5	3.5
3	4
3.5	4.5
4	5
M1.6*0.35	3	2	2.6	3	1
2.5	3.5
3	4
3.5	4.5
4	5
M1.7*0.35	3	2	2.6	3	1.2
2.5	3.5
3	4
3.5	4.5
4	5
M1.8*0.35	3	2	2.6	3	1.2
2.5	3.5
3	4
3.5	4.5
4	5