Injection Molding Services: Things You May Want to Know

      In the world of plastic products, blow molding and injection molding are two major molding technologies, but their relationship is often misunderstood. Some people say that "blow molding is a type of injection molding", but in fact, the two are completely different! They are like a pair of "twin flowers". Although they belong to the field of plastic molding, they have their own advantages in principle, application and cost. This article will take you to explore the mysteries of these two processes in depth to help you find the most suitable solution.

Blow Molding

Core principle: Use compressed air to blow the softened plastic parison to fit the inner wall of the mold to form a hollow product.

Steps: Heat the plastic → Extrude the parison → Close the mold and blow air → Cool and shape → Demold and trim.

Key equipment: Blow molding machine, mold, air compressor.

Typical applications: Hollow products such as plastic bottles, fuel tanks, and medical device containers.

Injection Molding

Core principle: Inject molten plastic into a closed mold at high pressure, and form solid or complex structural parts after cooling.

Steps: molten plastic → high pressure injection → cooling and solidification → demoulding.

Key equipment: injection molding machine, precision mold.

Typical applications: solid or high-precision parts such as mobile phone cases, gears, and auto parts.

Blow molding relies on air molding and is suitable for "hollow"; injection molding relies on high-pressure filling and specializes in "solid".

Injection molds require extremely high precision (such as CNC processing), while blow molding molds are more flexible, but are prone to problems such as flash.

Material selection

Blow molding: commonly used soft plastics (such as HDPE, LDPE, PP), suitable for containers with high flexibility requirements.

Injection molding: compatible with hard plastics (such as ABS, PC, nylon), can add glass fiber to enhance performance.

Design freedom

Blow molding is limited to hollow structures, but can achieve complex curved surfaces (such as bottles with handles).

Injection molding can produce precision parts with details such as threads and bosses, and even multi-color one-piece molding.

Case highlights:

The "star product" of blow molding: carbonated beverage bottles (stretch blow molding technology improves pressure resistance).

The "masterpiece" of injection molding: foldable safety seats (complex structure + high-strength materials).

Initial investment

Blow molding molds are low cost (only one side mold is needed), and the machine price is more affordable.

Injection molds are expensive (precision machining), but suitable for mass production to dilute the cost.

Production cycle

Blow molding is fast for single-piece molding, but post-processing (such as trimming) may increase the time consumption.

Injection molding cycle is stable, and multi-cavity molds can produce dozens of parts at the same time, which is more efficient.

Economic recommendations:

Choose blow molding for small batches of hollow parts (such as customized containers);

Choose injection molding for complex solid parts (such as electronic component housings).

Blow molding technology:

Multi-layer coextrusion: Improve barrier properties (such as anti-oxidation layer of food packaging).

Biodegradable materials: Respond to environmental protection needs, such as PLA blow molding bottles. 

Injection molding technology:

Micro injection molding: Produce millimeter-level precision parts (such as medical catheters).

Smart mold: Integrated sensors monitor pressure and temperature in real time

  Comparison Items

  Blow molding

  Injection molding

  Applicable Products

 Hollow containers (bottles, barrels)

 Solid parts (gears, housings)

  Material Type

 Soft plastics (HDPE, PP)

 Hard plastic (ABS, PC)

  Mold Cost

 Low

 High

  Design complexity

 Medium (limited to hollow structures)

 High (supports fine details)

  Production batch

 Small and medium batches

 Large quantities

     In the development of plastic products, choosing between 3D printing and injection molding requires comprehensive consideration of factors such as cost, precision, batch size, and design complexity. The following is an analysis of the two processes from the two core dimensions of cost optimization and high-precision requirements, combined with the characteristics of the two processes, and provides a basis for decision-making.

 1) Production batches determine the core cost structure

3D printing: suitable for small batches (usually <1000 pieces) or single-piece production. It does not require mold costs, has low material loss (only the support structure may waste a small amount of material), and is flexible in iteration. For example, when using FDM technology to print a prototype, the cost per piece may be only 1/10 of that of injection molding.

Injection molding: more cost-effective in large-scale (>1000 pieces) production. Although the mold development cost is high (thousands to tens of thousands of yuan), the cost per piece decreases significantly as the batch increases. For example, in one case, the injection mold cost $10,000, but the cost per piece was only $0.1 when producing 100,000 pieces.

 2)Design and iteration cost comparison

3D printing: CAD models can be directly printed after modification, without additional costs, suitable for the prototype stage where the design is frequently adjusted. For example, a company shortened the R&D cycle from 4 weeks to 48 hours by using 3D printing molds.

Injection molding: Mold modification costs are high (especially metal molds), suitable for mass production after the design is finalized. If the mold structure needs to be adjusted, it may be necessary to re-open the mold, which will increase the cost by tens of thousands of yuan.

 3)Material And Post-Processing Costs

3D printing: limited material types (such as PLA, nylon, resin, etc.), some high-performance materials (such as PEEK) are expensive; post-processing usually only requires grinding or sandblasting.

Injection molding: wide selection of materials (such as ABS, PP, PC, etc.), lower prices; but post-processing such as mold polishing and electroplating may increase costs.

Decision suggestions:

Small batch/prototype: choose 3D printing (FDM, SLA or SLS);

Large batch/finalized product: choose injection molding.

 1) Process accuracy comparison

3D printing:

SLA/DLP: accuracy of ±0.01 mm, smooth surface, suitable for precision medical or electronic parts.

SLS/MJF: accuracy of ±0.1 mm, suitable for complex structures but slightly rough surface.

FDM: lower accuracy (±0.2 mm), obvious layer pattern, need post-processing.

Injection molding:

accuracy is usually ±0.05 mm, high surface finish (Ra 0.8~1.6 μm), no additional processing required.

 2)Material strength and stability

3D printing: weak interlayer bonding, which may affect mechanical properties; easy to deform at high temperatures (such as PLA softening point is 55°C).

Injection molding: The material is dense, high in strength and isotropic, and has better temperature resistance (such as ABS can withstand 80~100°C).

 3) Complex structure adaptability

3D printing: It can manufacture complex structures that are difficult to achieve with traditional processes, such as hollowing and conformal water channels. For example, the curved cooling channel in the mold can improve the injection efficiency.

Injection molding: Due to the mold demolding requirements, the design must avoid internal right angles or too deep cavities, otherwise it will increase the difficulty of processing.

Decision suggestions:

High precision + complex design: choose SLA or metal 3D printing (such as SLM), but you need to accept higher costs;

High precision + large batch: injection molding combined with CNC precision mold to ensure dimensional stability.

Clear requirements: batch, budget, design complexity, precision level, material performance.

Cost accounting: compare mold costs, single-piece material costs and post-processing costs.

Technology matching:

If fast iteration or small batches are required, 3D printing is preferred;

If high strength or surface finish is required, injection molding is preferred.

Hybrid solution: For example, use 3D printing to make prototypes or conformal water channel molds, and then mass produce them through injection molding.

1) Pepsi bottle mold: By combining 3D printed inserts with traditional metal molds, the cost is reduced by 96%, and the production cycle is shortened from 4 weeks to 48 hours.

2) Medical implants: Use SLA to print high-precision prototypes, and then switch to injection molding for mass production after verification.

3) Shoe mold manufacturing: 3D printing can achieve complex patterns, replacing traditional CNC, and increasing efficiency by 50%.

Between low cost and high precision, a balance needs to be made according to specific scenarios:

3D printing: the first choice for small batches, complex designs, and rapid iterations;

Injection molding: an economical solution for large batches, high precision, and high-strength scenarios.

In the future, hybrid manufacturing (such as 3D printing molds + injection molding mass production) may become the mainstream direction for balancing cost and performance.

     In the field of plastic injection molding, the "tonnage" of an injection molding machine directly determines its capacity limit. Tonnage does not refer to the weight of the entire machine, but the size of its clamping force, measured in tons (T). This parameter represents the maximum closing force that the clamping system of the injection molding machine can exert, which is used to resist the huge expansion pressure generated when the molten plastic is injected into the mold. If the clamping force is insufficient, the mold will be stretched open, resulting in flash burrs; and excessive tonnage will cause energy waste and equipment loss.

The calculation of clamping force follows the core formula:

Clamping force (T) = product projection area (cm²) × mold pressure (kg/cm²) × safety factor / 1000

The mold pressure depends on the material properties:

Ordinary plastics (such as PP, PE): 250-350 kg/cm²

Engineering plastics (such as PC, nylon): 350-500 kg/cm²

The safety factor is usually 1.1-1.2.

Injection molding machines can be divided into four categories according to tonnage, which have significant differences in mechanical structure, application scenarios and technical requirements:

（1）Small machine (30-100 tons)

Structural features: mostly vertical or angular design, mainly fully electric drive, small footprint

Injection volume: usually ≤200 grams (based on PE)

Typical applications:

Electronic connectors (USB interface, SIM card tray)

Precision gears (clocks, micro motor parts)

Medical equipment (syringe needles, test tubes)

Advantages: high precision (repeat error ±0.01mm), energy saving (60% less power than hydraulic presses), no oil pollution

（2）Medium-sized machine (150-500 tons)

Mainstream machine: hydraulic or hydraulic-electric hybrid drive, horizontal structure

Injection volume: 300-2000 grams

Application scenarios:

Household appliance housing (rice cooker, vacuum cleaner housing)

Daily necessities (washbasin, chair, toy)

Automotive interior parts (dashboard frame, door handle)

Technical highlights: Can be equipped with multi-color injection molding system (such as P-type/L-type double shot) to achieve dual-material composite molding

（3）Large machine (600-1000 tons)

Structural features: two-plate clamping mechanism, enhanced rigidity and mold opening space

Injection volume: 2500-5000 grams

Typical products:

Logistics pallet (1200×800mm standard size)

Automobile bumper

Large trash can

Challenge: Need to cooperate with mold temperature control system to prevent shrinkage of thick-walled parts

（4）Ultra-large machine (>1000 tons)

Representative models: Haitian 1600T, Fuqiangxin 1900T three-color machine

Capacity indicators: up to 4000 tons, injection volume exceeds 20kg

Application areas:

Automobile instrument panel (one-piece molding)

Yacht deck parts

Large turnover box (volume>1000L)

Innovative design: horizontal two-color machine (H type) with overlapping mold technology, production capacity increased by 70%

Table: Comparison of injection molding machine tonnage and key parameters