1、 Disadvantages of multi-layer screws

1. 成本高昂

• Material cost：
  Multi layer screws require corrosion-resistant alloys (such as SACM-645, 316 stainless steel) or high-temperature alloys (such as H13 steel) as substrates, which are 3-5 times more expensive than ordinary carbon steel. For example, a multi-layer screw with a diameter of 80mm and a length to diameter ratio of 32:1 may have a substrate cost that is 2000-3000 yuan higher than that of a single-layer screw.
• processing cost：
  Multi layer structures require precise matching of each functional area through precision machining (such as CNC machine tools), and the coating process (such as tungsten carbide spraying, ceramic coating) is complex. The processing cost of a single screw can increase by 1500-4000 yuan.
• maintenance cost：
  If a certain layer of coating wears out or fails, the screw needs to be replaced as a whole (rather than repaired locally), resulting in significantly higher maintenance costs than single-layer screws.

2. High maintenance complexity

• Difficult to clean：
  High filling materials (such as fiberglass and mineral fillers) are prone to residue in the gaps between the screw edges of multi-layer screws or on the surface of functional layers, forming stubborn adhesion layers. Ordinary cleaning methods (such as high-pressure water guns) may damage the coating and require the use of specialized cleaning agents or ultrasonic cleaning. A single cleaning process takes 4-6 hours, which is 2-3 times longer than a single-layer screw.
• Strict lubrication management：
  Multi layer screws require the use of wear-resistant and high-temperature resistant special lubricating grease (such as synthetic grease containing polytetrafluoroethylene), and should be replaced every 500-800 hours. Improper lubrication may accelerate bearing or screw wear, increasing maintenance costs by 30% -50%.
• High requirements for wear monitoring：
  It is necessary to regularly detect the wear amount of each layer (such as plasticizing layer and mixing layer) through precision equipment such as laser caliper. If the wear of a certain layer exceeds the standard (>0.2mm), the screw should be replaced immediately, otherwise it may cause equipment failure or product quality problems.

3. Limited applicability

• Challenges in processing low melting point materials：
  When processing low melting point halogen-free materials such as TPE and EVA, if the temperature control accuracy of multi-layer screws is insufficient (such as ± 10 ℃ or above), it may lead to material degradation or uneven discharge. For example, when processing halogen-free TPE cable materials, temperature fluctuations may cause material burning, resulting in black spots or bubbles.
• Complex structure leads to an increase in failure rate：
  The functional layers of multi-layer screws (such as barrier type and pin type hybrid structures) may be obstructed in material flow due to improper design or processing errors, causing "dead zones" or local overheating, increasing the risk of equipment shutdown.

2、 Disadvantages of single-layer screws

1. Poor material adaptability

• High filling material processing bottleneck：
  Single layer screws usually adopt a single groove design (such as equidistant deep grooves), which has limited shear and dispersion ability for high proportion glass fibers (40% -60%) or mineral fillers. For example, when processing halogen-free PP containing 60% fiberglass, single-layer screws may cause fiberglass breakage or uneven distribution, reducing material strength.
• 腐蚀性物料耐受性低：
  Single layer screws made of ordinary carbon steel or low-alloy steel substrates are prone to surface peeling or accelerated wear due to chemical corrosion when processing halogen-free flame retardants such as phosphorus and nitrogen. For example, when processing flame-retardant PA66, the lifespan of single-layer screws may be shortened to less than 3 months.
• Insufficient flexibility in temperature control：
  Single layer screws cannot adjust local temperature according to different material characteristics (such as high-temperature plasticization required for high melting point halogen-free materials), which may lead to material degradation or uneven discharge. For example, when processing halogen-free PC, temperature fluctuations may cause material yellowing and reduce product weather resistance.

2. Insufficient machining accuracy and stability

• Poor mixing uniformity：
  The rib structure of single-layer screws is simple (such as single head threads), and their ability to disperse additives such as flame retardants and color masterbatch is limited, which may lead to crystal points, color differences, or performance fluctuations in the product. For example, when processing halogen-free flame retardant ABS, the single-layer screw may not meet the flame retardant level due to uneven mixing.
• Difficulty in cutting thermal management：
  When high filling materials flow in a single-layer screw, they are prone to local overheating due to shear heat concentration, leading to material degradation or burning. For example, when processing glass fiber reinforced halogen-free PA6, single-layer screws may cause glass fiber fracture and reduce material toughness due to excessive shear heat.
• Low discharge stability：
  The conveying capacity of a single-layer screw is limited by the depth and lead of the screw groove. If there is a large difference in material flowability (such as high filling and low filling formulas), it may cause discharge fluctuations (such as cable outer diameter exceeding ± 0.2mm).

3. Environmental and compliance risks

• Controversy over the Environmental Protection of Coating Materials：
  Some single-layer screws use metal coatings containing chromium and nickel (such as hard chromium plating) to enhance wear resistance, but may not comply with the restrictions on heavy metals in RoHS or REACH regulations. For example, when using single-layer screws coated with chromium to process halogen-free materials, it may lead to excessive heavy metal content in the product and be rejected by the EU market.
• High cost of waste disposal：
  If the single-layer screw that is worn or scrapped contains heavy metal coating, it needs to be treated as hazardous waste (the cost can reach 3-5 times that of ordinary scrap steel). For example, scrapping a single-layer screw with nickel coating may cost up to 2000 yuan, while a regular screw only costs 300 yuan.

3、 Summary: How to choose?