Dalian injection molding and compression molding are two different plastic processing techniques, with significant differences in principles, equipment, applicable materials, and product characteristics. The specific differences are as follows:
1、 Forming principle and process
Principle: The granular or powdered plastic is heated and melted by an injection molding machine, and the molten material is injected into a closed mold cavity at high speed using the thrust of a screw or plunger. After cooling and solidification, the mold is opened to remove the product.
Key steps:
Plasticization: The raw materials are heated, melted, and uniformly mixed in the barrel;
Injection: Melt is injected into the mold cavity at high pressure and speed through a nozzle (pressure can reach tens to hundreds of megapascals);
Cooling and shaping: The melt inside the mold solidifies rapidly at low temperatures;
Demolding: The mold is opened and the ejection mechanism pushes out the finished product.
Features: High degree of automation, short molding cycle (seconds to minutes), suitable for large-scale production.
Compression Molding
Principle: Place plastic raw materials (powder, granular or prefabricated blanks) directly into an open mold cavity, close the mold and apply pressure (usually several tens of megapascals) to melt and flow the raw materials under high temperature and pressure, fill the cavity, and solidify before demolding.
Key steps:
Feeding: Put the raw materials into the mold cavity according to a certain amount;
Pressure heating: After the mold is closed, pressure is applied through a hydraulic press while heating to melt and shape the raw materials;
Curing: Maintain pressure and temperature until the material crosslinks or hardens upon cooling;
Demolding: When opening the mold to remove the product, it may be necessary to clean the burrs.
Features: Simple equipment, low mold cost, but long production cycle (several minutes to tens of minutes), low degree of automation.
2、 Applicable material types
Injection molding
Main materials:
Thermoplastic materials (accounting for the vast majority), such as polypropylene (PP), polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), nylon (PA), polycarbonate (PC), etc., are suitable for rapid cycle injection molding processes due to their ability to be repeatedly heated and melted.
Partial thermosetting plastics (requiring specialized injection molding machines): such as epoxy resin and phenolic resin, but temperature control is necessary to avoid premature curing.
Limitations: High filling materials (such as long glass fiber reinforced plastic) may wear down screws and barrels, requiring special design.
Compression Molding
Main materials:
Thermosetting plastics: such as phenolic resin (PF), urea formaldehyde resin (UF), melamine formaldehyde resin (MF), etc. These materials undergo cross-linking reactions under heating and pressure, and cannot be melted again after curing, making them suitable for one-time molding processes of compression molding.
Some thermoplastic materials (less commonly used), such as polytetrafluoroethylene (PTFE), are difficult to inject due to their high melt viscosity and require cold pressing sintering for molding.
Composite materials: such as glass fiber reinforced thermosetting plastics (used for automotive components, electrical enclosures).
Limitation: Not suitable for high flow thermoplastic (prone to overflow from the mold before pressurization).
3、 Mold structure and cost
Injection molding
Mold features:
The structure is complex, usually a multi cavity mold (which can simultaneously form multiple products), including a pouring system (main channel, branch channel, gate), a cooling system (pipeline circuit), an ejector mechanism, etc.
It needs to withstand high temperature and high pressure, and the material is mostly high-quality mold steel (such as H13, S136) that has undergone heat treatment, with high processing accuracy (tolerance ± 0.01mm).
Cost: Mold development costs are high (tens of thousands to millions of yuan), but it is suitable for mass production to share the cost.
Compression Molding
Mold features:
The structure is simple, mostly consisting of a single cavity or a small number of cavities, and can be divided into overflow molds (allowing a small amount of raw material to overflow), non overflow molds (strictly restricting the flow of raw materials), and semi overflow molds (between the two).
No complex pouring system is required, and the mold surface needs to be wear-resistant (due to possible friction between raw material particles during the compression molding process), but the accuracy requirement is lower than that of injection molds.
Cost: The mold cost is relatively low (several thousand to several hundred thousand yuan), suitable for small batch production or large-scale products (such as bathtubs, trays).
4、 Product characteristics and application scenarios
Injection molding
Product features:
Shape complexity: It can form products with complex structures, thin walls, and fine patterns or inserts (such as mobile phone cases, gears, medical catheters).
Dimensional accuracy: The tolerance can reach ± 0.05mm, suitable for high-precision parts such as electronic connectors and optical lenses.
Surface quality: After polishing the mold surface, the product can obtain a high smoothness surface (such as transparent parts, cosmetic packaging).
Application scenarios: Daily consumer goods (such as mineral water bottles, toys), automotive components (such as bumpers, instrument panels), electronic appliances (such as shells, plugs), etc., covering more than 90% of plastic products.
Compression Molding
Product features:
Shape complexity: Suitable for structurally simple, thick walled or large products, difficult to form complex internal cavities or small structures (such as threads, narrow gaps).
Dimensional accuracy: The tolerance is relatively large (± 0.1mm or more), and the edges of the product are prone to flash, requiring subsequent trimming.
Surface quality: The surface may have mold lines or indentations, suitable for scenarios where appearance requirements are not high.
Application scenarios: Electrical switch panels (phenolic molded products), tableware (melamine resin tableware), automotive brake pads (thermosetting composite materials), large industrial components (such as water tanks, trays), etc.
5、 Comparison of Production Efficiency and Cost
Production efficiency:
The injection molding cycle is short (typical cycle of 10-60 seconds), which can achieve fully automatic continuous production; The compression molding cycle is long (especially for thermosetting plastics, which require several minutes for cross-linking and curing), usually operated semi automatically or manually.
Cost effectiveness:
When producing in small batches, the cost of compression molding is lower (molds are cheaper and do not require expensive injection molding machines);
During mass production, the unit cost of injection molding is significantly lower than that of compression molding (with high efficiency and less waste).
Waste disposal:
Injection molded channel waste can be recycled and reused (thermoplastic materials); The flying edge waste from compression molding is difficult to recycle (especially thermosetting materials, which are non renewable after solidification).
Choosing injection molding or compression molding requires a comprehensive judgment based on material characteristics (thermoplastic/thermosetting), product complexity, production scale, and cost targets:
Prioritize injection molding: thermoplastic materials, complex shapes, high precision, and large-scale production scenarios.
Priority should be given to compression molding: thermosetting materials, simple shapes, small batches or large products, as well as scenarios that are sensitive to mold costs (such as traditional daily necessities and industrial components).
